State of the Art Study on Materials and Solutions against...

State of the Art Study on Materials and Solutions against Corrosion in Offshore StructuresNorth Sea Solutions for Innovationin Corrosion for Energy

February

2018

nessieproject.com

The NeSSIE project (2017-2019) seeks to deliver new business and investment opportunities incorrosionsolutionsandnewmaterialsforoffshoreenergyinstallations.TheprojectaimstodrawonNorthSearegionalexpertiseintraditionaloffshoresectors(i.e.oilandgas,shipbuilding)inordertodevelopsolutionsforemergingopportunitiesinoffshorerenewableenergysources(wave,tidalandoffshorewindenergy).TheNeSSIEprojectiscofundedbytheEuropeanMaritimeandFisheriesFund(EMFF).www.nessieproject.com

©NeSSIE-NorthSeaSolutionsforInnovationinCorrosionforEnergy–2018

ScientificandPatentLiteratureReview ProjectNeSSIE

PUBLICATIONThisreporthasbeenproducedbytheNeSSIEProjectConsortium(Deliverable2.1).DISCLAIMERThispublicationreflectsonlytheauthors’viewsandtheEuropenUnion isnotresponsibleforanyusethatmaybemadeoftheinformationcontainedtherein.Reproductionisauthorisedprovidedthesourceisacknowledged.Source: NeSSIE Project – cofunded by the European Maritime and Fisheries Fund(EMFF)–www.nessieproject.comAUTHORSGiuliaMasi(UniversityofBologna)FrancescoMatteucci(Aster)JeroenTacq(Sirris)ACKNOWLEDGEMENTSThankstotheNeSSIEprojectpartnersfortheirreviewof,andcontributionto,thisreport.Wewouldparticularlyliketothankfortheirinputs:GuyClaus(Sirris)DiegoCocco(LICEnergy)PaulCutter(SPIPerformanceCoatingsLtd)MarkLaurie(UniversityofEdinburgh)LarsLichtenstein(DNVGL)MarcosSuarezGarcia(BasqueEnergyCluster)


TABLEOFCONTENTS

1 ListofFigures..........................................................................................................................2

2 ListofTables...........................................................................................................................2

3 AbbreviationsandAcronyms...................................................................................................3

4 ExecutiveSummary.................................................................................................................3

5 Introduction............................................................................................................................3

5.1 Reportaimsandoutline...........................................................................................................3

5.2 Definitions................................................................................................................................5

5.3 MaterialsusedinMarineEnvironments..................................................................................7

5.4 CorrosioninOffshoreWindStructures(OWS)casestudy.......................................................8

5.5 Currentchallengeswithoffshorewinddevelopment..............................................................9

5.6 CurrentchallengesinOceanEnergies(OE)development......................................................11

6 Typesofcorrosion..................................................................................................................13

6.1 Introduction............................................................................................................................13

6.2 Microbiologicallyinfluencedcorrosion(MIC).........................................................................15

6.3 Foulingandbiologicalgrowth................................................................................................16

6.4 Corrosionfatigue....................................................................................................................17

7 Useofcorrosionresistantmaterials.......................................................................................19

7.1 Materialsselection.................................................................................................................19

7.1.1 CarbonmanganeseSteel.................................................................................................19

7.1.2 Castiron...........................................................................................................................22

7.1.3 Stainlesssteel..................................................................................................................22

7.1.4 Non-ferrousalloys...........................................................................................................26

7.1.5 NickelAlloys.....................................................................................................................26

7.1.6 TitaniumAlloys................................................................................................................27

7.1.7 Copperandcopperalloys................................................................................................28

7.1.8 Aluminiumalloys.............................................................................................................28

7.2 Corrosionallowance...............................................................................................................30

8 Corrosionprotection–Technologyoverview..........................................................................31

8.1 Activecorrosionprotection:cathodiccorrosionprotection(CCP).........................................31

8.2 Passivecorrosionprotection..................................................................................................32

8.2.1 Barriercoatings................................................................................................................32


8.2.2 CompatibilitywithCathodicProtection...........................................................................33

8.2.3 Surfacepreparationandcleaning....................................................................................34

8.3 Metalliccoatings.....................................................................................................................36

8.3.1 HotDipGalvanizing..........................................................................................................36

8.3.2 ThermalSpraying.............................................................................................................38

8.3.3 Nickelcoatings.................................................................................................................42

8.4 Organiccoatings.....................................................................................................................42

8.4.1 Epoxyandpolyurethanecoatingsystems.......................................................................42

8.4.2 Polyureacoatings.............................................................................................................45

8.4.3 Powdercoatings..............................................................................................................46

8.4.4 Ongoingresearchonorganiccoatings.............................................................................47

8.5 Hybridandothercoatingsystems..........................................................................................48

8.5.1 Phosphateconversioncoatings.......................................................................................48

8.5.2 ChemicallyBondedPhosphateCeramiccoatings............................................................49

8.5.3 FilmGalvanizingSystems.................................................................................................50

8.5.4 Silanecoatings.................................................................................................................52

8.5.5 Sol-gelcoatings................................................................................................................52

8.6 Alternativecorrosioncontrolmeasures.................................................................................54

8.6.1 Generalprecautions........................................................................................................54

8.6.2 Protectivecapsandshields..............................................................................................55

8.6.3 Sheathing.........................................................................................................................55

8.6.4 Dehumidifiers..................................................................................................................55

8.6.5 Sealingsandjoints...........................................................................................................55

9 CorrosionprotectionofOffshoreWind(OW)structures.........................................................57

9.1 Coatingsystemscurrentlyinuse............................................................................................57

9.2 CorrosionissueonOffshorewindstructures.........................................................................60

9.2.1 Primarysteel....................................................................................................................60

9.2.2 Secondarysteel................................................................................................................61

9.2.3 Repairsolutions...............................................................................................................62

10 Normativestandardsandguidelines......................................................................................64

11 Conclusions...........................................................................................................................67

12 References.............................................................................................................................68


2

1 List of Figures Figure1: installedcapacityupto2016:(a)cumulativesharebycountry(MW);(b)cumulativesharebybasin(MW)[4]................................................................................................................................................................5Figure2:Schematicrepresentationoflevelsandzonesinseawaterenvironment[26].....................................13Figure3:Differentmaterialsusedingearedwindturbines................................................................................20Figure4:Lossofthicknessduetocorrosionandbendingmomentofsteelinsheetpiling................................22Figure5:PRENnumberfordifferenttypesofstainlesssteel..............................................................................24Figure6:Galvanicseriesofmetalsandalloysinseawater[68]..........................................................................27Figure7:Corrosionprotectionmeasuresforaluminiuminmarineenvironments[69]......................................29Figure8:Coatingconceptsforaluminiuminthemarineenvironment[69].......................................................29Figure9:SurfaceaspectsofcleanedsurfacesinfunctionoftherustgradeaccordingtoISO8501-1................35Figure10:ComparativestudyofZnandZnAlcoatedsteelwire,immersedinoceanwater.CourtesyofBekaertNV[90].................................................................................................................................................................37Figure11:Picturesof cut edgesof 4mmsteelwireswith Zinc,Benzinal andBenzinal 3000 coatings after3yearsofexposureinBelgium[91].......................................................................................................................38Figure12:ThicknessrangeofTSA(µm-mils)asfunctionoftheestimatedservicetimeandatmosphere[98]..40Figure13:Acompletecorrosionprotectionsystemforsteelsubstrates............................................................48Figure14:Comparativestudyonagougedsamplesubjectedtoacycliccorrosiontest[143]...........................50Figure15:Corrosionhealingmechanismofahybridsolgelcoatingwithnanoparticles[133]..........................53Figure16:Generaldesigndetailingofconstructiveelements............................................................................54Figure17:Protectionofboltedsteelconnectionsbyspecialclamps[150]........................................................55

2 List of Tables Table1:MaincriticaldifferencesbetweenoffshoreO&GplatformsandOWstructures[9].............................10Table2:Corrosionrates(mm/y)ofcarbonsteelinoffshoreservice[40]...........................................................14Table3:CorrosionzonesandformofcorrosioninOWS[18,19]........................................................................15Table4:CompositionofsteelgradesforpipesaccordingtoAPI........................................................................20Table 5: Chemical composition of unalloyed and low alloyed structural steel with improved corrosionresistance.............................................................................................................................................................21Table6:RiskfactorsusedinthecalculationofCorrosionResistanceFactors[65].............................................23Table7:DesignationofCRFvaluestoCorrosionResistanceClasses...................................................................24Table 8: Compositionof a selectednumber of stainless steel alloys, their PREN value and theCRC class forwhichtheyaresuited(*Note:Sea-CureandZeron100arecommercialnames,notAISIdesignations)............24Table9:Stainlesssteelselectionaccordingtosoilconditions[44].....................................................................25Table10:Compositionofnickelsuperalloysusedinmarineenvironments(wt%).............................................26Table11:Aluminiumalloysformarineapplications............................................................................................28Table12:OverviewofcoatingsystemsspecifiedforuseoncarbonsteelinstandardsISO12944-5,ISO20340andNORSOKM-501,fortheoutsideofOWstructures.Coatingsystemsaresubdividedaccordingtoexposurecategory...............................................................................................................................................................57Table13:OverviewofcoatingsystemsspecifiedforuseonmetallizedcarbonsteelinstandardsISO12944-5,ISO20340andNORSOKM-501,fortheoutsideofOWstructures.....................................................................58Table14:AlternativecoatingsystemswhichcouldbeusedonOWstructures,classifiedaccordingtoexposurezone.....................................................................................................................................................................59


3

3 Abbreviations and Acronyms CCP CorrosionCathodicProtectionCFRP CarbonFibreReinforcedPolymersDO DissolvedOxygenFRP FibreReinforcedPolymersLCoE LevelizedCostofEnergyMIC MicrobiologicallyInfluencedCorrosionOE OceanEnergiesO&G Oil&GasO&M Operations&MaintenanceOWE OceanWaveEnergyOW(S) OffshoreWind(Structures)SCC StressCorrosionCrackingTCE TidalCurrentEnergyTCT TidalCurrentTurbine

4 Executive Summary OperationandMaintenance(O&M)costsaretypicallyaround15-30%ofthetotallife-cyclecostofanOffshoreWind(OW)farm.WithintheO&Mbudget,alargeelementisduetoreplacementpartscaused by corrosion problems to metal structures. This document (deliverable 2.1) will map thecurrentactivities,research,standardsandnormsoncorrosionsolutionsandmaterialscurrentlyusedin the offshore energy field and consider in more detail the impact of corrosion on offshorerenewable energy structures in wave, tidal and offshore wind. This deliverable will be used as acurrentstateoftheartstudyintheoffshoreindustryandwillformthebaselineforfurtherworkinWP2,WP3andWP4.Thereportwillsetthecontextbylistinganddefiningthemaintechnicalconceptsandmaterialsthatareused inmarineenvironmentsandthemainchallengesof theoffshorerenewableenergy field.This is followed by a description of the main corrosion phenomena occurring in offshoreenvironmentswithattentiongiventotheirrelationshipwiththecharacteristicareasofanoffshorestructure.The reportwill then reviewthematerialsand innovativesolutionsadopted incorrosionprotection in the offshore sector and present them together with themain industrial protectionstrategiesforoffshorewindstructures.Inthelastchapters,internationalstandardsandguidelinesonthemethodsofdesignandoperationofthemainoffshorerenewableenergystructures,materialsandprotectionstrategiesfrommainlycorrosionandfatiguedamagesaresummarizedbeforesomeconclusionsaredrawn.

5 Introduction

5.1 Report aims and outline

Theaimof this report is to present the stateof art in termsof corrosion solutions and corrosionresistantmaterialssuitableforapplicationinoffshorerenewableenergystructures.Thereportisthe


4

resultofadesktopstudy(scientificliteratureandstandards)andismainlyfocusedonoffshorewindandtidalenergystructures.Thereportiscomplementedwithinputfromindustryexperts.Inthischapter, firstanumberoftermsusedthroughoutthereportaredefined. Insection5.3thematerials used in marine environments are summarized. Some attention is paid to compositematerials and concrete. However, these will not be discussed further in the report. This reportfocuses on corrosion issues and is therefore mainly concerned with metallic materials. Theintroductory chapter continueswith a short exploration of corrosion and other challenges in theoffshorewindandoceanenergysector.The main corrosion phenomena occurring in offshore environments are described in detail inchapter 6. A distinction is made between the various areas of an offshore structure, in whichdifferent intensity of corrosion attack takes place. A review of corrosion resistant materials andinnovative as well as commercially applied protection strategies for offshore wind structures ispresentedinchapters7and8. Inchapter9anoverviewofcurrentcorrosionmitigationstrategies,usedinoffshorewindstructuresisgiven.Thisiscomplementedwithremainingcorrosionissuesthatwereidentifiedandpossibleinnovativeanti-corrosionstrategies.Finally,internationalstandardsandguidelinesrelevantforthedesignandoperationofoffshorerenewableenergystructures,materialsandprotectionstrategiesforcorrosionandfatiguemitigationarepresentedinchapter10.Chapter11collectsthemainconclusiveremarksofthisreport.According to current projections of the world electricity demand, electricity is set to remain thefastest-growing final form of energy worldwide, growing by 2.1% per year over the 2012-2040period.Totalglobaldemandisexpectedtoreach34900TWhin2040,from19400TWhbackin2012.Inthisfast-growingmovement,offshorerenewablesareestimatedtoplayamajorrole,drivenbyboth political and economic factors. The renewables-based electricity generation is projected totripleover2013-2040,overtakingcoaltobecomethelargestsourceofelectricity.Accordingtothenewpoliciesscenario,33%oftheworldelectricitygenerationbysourcewillcomefromrenewablesin 2040 (IEA, 2014) [1]. A significant part of this renewable energy comes fromwind turbines, ofwhich a large share of is located offshore. This has advantages in terms of predictability andavailability of wind, as well as availability of development sites. Ocean wave and tidal energygeneration are, by their very nature, also located offshore. The aim of this report is therefore tocontributetothedevelopmentofrenewableenergybyfocusingontheseoffshorestructures.Forecasts from2009 indicated thatby2020 the total installedoffshorewindcapacity couldbe40GW,meeting4%of theEU’s total electricity consumption.However, at theendof2015 the totalinstalledcapacity reachedonlyabout11GW,covering1.5%of theEU’s totalenergyconsumption[2].ThismaybeexplainedbythecomplexityinvolvedintheOffshoreWindsector,andthefactthatsupply chains are not yet established, leading to higher costs of energy generation than othermethodsofgeneratinggreenelectricity.Offshorewindhasanaverageproductioncostof110-130€/MWh,nonetheless,therealreadyareparksthatwillbecommissionedafter2020thathaveclosedprices ranging 65-95€/MWh [3]. To continue the development of offshore renewable energy, thecostofinstallingandmaintainingthesestructuresneedstobedecreasedfurther.Asignificantpartof thecostsare the resultof theharshconditions inwhich thesestructuresneed tooperateand,morespecifically,theresultingcorrosion.Bypresentinganoverviewofnewandexistingcorrosionmitigation strategies, this reportwillprepare theway to find innovative solutions thatwill reducethecostofrenewableenergy.TheUKhasthelargestamountofinstalledoffshorewindcapacityinEuroperepresenting40.8%of


5

all installations, as reported in Figure 1a. Germany follows with 32.5%. Despite no additionalcapacityin2016,Denmarkremainsthethirdlargestmarketwith10.1%andtheNetherlands(8.8%)displacesBelgium(5.6%)tohavethefourthlargestshareinEurope,ashighlightedinFigure1a[4].More in general, installation in the North Sea account for 72% of all offshore wind capacity inEurope,asreportedinFigure1b.Indetail,theIrishSeahas16.4%ofinstalledcapacityfollowedbytheBalticSeawith11.5%[4].Incomparison,thedevelopmentofotherformsofrenewableoffshoreenergy (waveand tidal energy) is only in a start-up stage. Therefore, this desktop study ismainlyfocusedonoffshorewindstructuresintheNorthSeabasin.However,becausetheenvironmentandcorrosion phenomena are comparable, solutions for offshore wind structures can also serve assolutionsforwaveandtidalenergydevices.

(a)

(b)

Figure1: installedcapacityupto2016:(a)cumulativesharebycountry(MW);(b)cumulativesharebybasin(MW)[4]

5.2 Definitions

Inthisparagraphallthetechnicalconceptsdiscussedinthisreportaredefinedasfollows:MarineEnvironment:Theocean,seas,baysestuariesandothermajorwaterbodies,includingtheirsurfaceinterfaceandinteraction,withtheatmosphereandwiththelandseawardofthemeanhigh-watermark.Seasandoceansactasacoherentecosystem,withinwhichallspeciesandhabitatsareactive and essential components. In addition, an enhanced appreciation on the interconnectionbetweenmarineecosystemandhumancommunitiesisincreasing.Inparticular,humanshavebeenoperating with and within marine environment for millennia, causing change through complexinteractions. The consequences of human activities are now so profound that there are negativeimpacts on the structure and functionofmarine ecosystems around the globe.At the same time


6

exploitationoftheseascontinuestogrow[5].Offshorerenewableenergy:whichcanbeextractedbytides,waveandwinds.Inparticular,offshorewind structure (OWS) uses energy from the wind to drive a turbine to generate electricity.Renewable ocean energymainly consists of tidal and wave energy. Tidal current energy (TDE) isderived from twice daily changes in water flow. Ocean wave energy (OWE) devices generateelectricitybyusingwatermotioncausedbywindsattheseasurface.Asworldpopulationgrowsatan average rate of 0.9% per year to an estimated 8.7 billion population in 2035, the energyconsumption will sharply increase when more peoples move to urban areas.[6]. Future energydemandsarehardtomeetwithoutburningfossilfuelsordependingonnuclearpower.Explorationofrenewableenergyisoneoftheexpectedsolutionstoachievethesustainabledevelopment[7].Corrosion: “Physiochemical interactionbetweenametal and its environmentwhich results in thechangesinthepropertiesofthemetalandwhichmayoftenleadtoimpairmentofthefunctionofthemetal,theenvironmentorthetechnicalsystemofwhichtheseformapart”[8].Corrosioncanbegenerallydescribedconsideringthecorrosionsystem,athreesystemphases:medium,materialand interphase. The predominant media acting in offshore structures are air and seawater. Inaddition, condensation water in internal spaces and soil on foundations can be considered asadditional media. As for offshore energy sector, the main ambition is to facilitate to Europeancompaniesthebestpartnershipsinordertoaddressandsolvespecifictechnicalchallenge,corrosioninseawaterisafundamentalissuetotakeintoconsiderationinthedifferenttechnologicalprocesssteps. In particular, for offshoreOil&Gas one of themain challenge includes operation in deeperseas,higherpressuresandtemperatureswithanincreasedlevelofcorrosiveanderosivematerials,while for offshore wind energy the main challenge is to mitigate corrosion of towers andfoundations, with a resultant increase in logistical challenges for installation and operation andmaintenance[1].Soachievinghighqualityresultsinanoffshoreenvironmentisamajorchallenge.Corrosiononoffshorestructureshighlydependsonsite-specificfactorssuchaswatertemperature,salinity,chlorinity,waterdepthandcurrentspeed.Theapplicationprocessandthespecificityofthecorrosionprotectionsystemareextremely importantandshouldbesuitable for thesubstrateandtheenvironment.Operation: refers to activities contributing to the high-level management of the asset such asremotemonitoring,environmentalmonitoring,electricitysales,marketing,administrationandotherback office tasks. Operations represent a very small proportion of Operations and Maintenance(O&M)expenditure,thevastmajorityofwhichisaccountedfordirectlybythewindfarmownerorthesupplierofthewindturbines.Maintenance: accounts for by far the largest portion of O&M effort, cost and risk.Maintenanceactivity is the up-keep and repair of the physical plant and systems. It can be divided into: i)preventive (proactive repair to, or replacement of, known wear components based on routineinspections or information from conditionmonitoring systems also including routine surveys andinspections) and corrective (reactive repair or replacement of failed or damaged components)maintenanceandii)remote/unmannedorwithpersonnel.


7

5.3 Materials used in Marine Environments

Awiderangeofmaterialsisappliedinoffshoresystems.Inparticular,differentmetalsareused[9]:

• Unalloyedandlow-alloyedsteel;• Unalloyedandlow-alloyedcaststeel;• Castiron;• Stainlesssteel;• Stainlesscaststeel;• Copperandcopperalloys;• Aluminiumalloys.

AllthesemetallicmaterialsareregulatedbythestandardDNVGL-OS-B101whichprovidestechnicalrequirements, production techniques and guidance for metallic materials to be used in thefabrication of offshore structures and equipment [10].Metallicmaterials,mainly steel alloys, areoftenprotectedfromtheharshenvironmentoftheseawaterbycoatings.Insubmergedandburiedsectionsofoffshorestructurescathodiccorrosionprotection(CCP)canalsobeapplied.Compositematerials havealso attracted lotsof attention in theoffshore sector, due toexcellentmechanicalanddurabilityproperties.Compositematerialsaremadeofanassemblageofdifferentpartswithpeculiarmechanicalandchemicalproperties inordertoproduceanewmaterialwhoseperformanceissuperiortothatoftheindividualpartstakenseparately[11].Composite materials are usually classified in fibre composites, particle composite and laminatedcomposites. Inparticular fibrereinforcedpolymers (FRP)havebeenusedextensivelyworldwide inboth onshore and offshore applications, applied on secondary structures such as railing gratingwalkway,cabletray,butalsoforturbinebladesforbothwindandoceanenergy[12].Inaddition,compositematerialsshowpotentialasfireprotectorsinoffshorestructure.Carbonfibrereinforced polymers (CFRP) can also be used to repair steel structures. Galvanic corrosion couldoccurbetweenthecarbonandthesteel.Topreventthis,alayerofE-GlassFRPmaterialcanbeusedtoelectricallyinsulatethetwomaterials[11].Finally,theadhesivethatshowsgooddurabilityundermarine environment condition is the epoxy resin and bonding is enhanced when the surface istreatedwithsilane[13].Particularlytheoceanenergyfield isnaturallyturningtousecompositematerialsbecauseoftheirperceivednon-corrosiveproperties in theharshmarine environment aswell as their high specificstrength and stiffness. However, the durability of this type of materials is considerably lower tometalsintermsoferosion.Thisespeciallyimportantfortidalturbines[7].DNVGL-ST-C501 [14]providesan internationalacceptablestandard for safedesignwith respect tostrengthandperformancebydefiningminimumrequirementsfordesign,materials,fabricationandinstallation of load-carrying Fibre Reinforced Plastic (FRP) laminates and sandwich structures andcomponents.Offshore foundation structures,mainly in the O&G, aremade of reinforced concrete due to lowinstallationcostandgooddurability.Thesemaybefloatingorgravitybasedstructures.DNVGL-ST-C502regulatestheprinciples,technicalrequirementsandguidelinesforthedesign,constructionandin service inspection of offshore concrete structures [15]. However, in the aggressive chloridecontaminated marine environment, susceptibility of the steel-reinforced concrete to corrosion


8

attackcanoccur.Inparticular,chlorideingressintotheconcreteaffectsconcretedurabilitythroughthe destruction of the thin passive oxide layer protecting the concrete steel-reinforcement fromcorrosiondegradation[16].Rusts,theby-productfromchlorideattacksofconcretesteel-rebar,areexpansive within the concrete structure leading to cracks, spalling, delamination and loss ofstructural integrityof concrete. Theuseof corrosion inhibitor admixtures in concrete foroffshorestructureshasbeen identifiedasapotential solution,not just for improvingstrength,butalso forimproving its corrosion resistance. Well-known substances for inhibiting steel rebar corrosion inchloride contaminated environment include compounds of chromate and of nitrites, as well asinnovative,non-toxicandenvironmentally-friendlyadmixtures[16].Ascorrosionisthemainissueformetallicalloysrelatedtodurabilityinoffshoreenvironmentsthisreportismainlyfocusedoncorrosionmechanismofmetallicalloysaswellascorrosionsolutionsinordertooptimizethedurabilityofoffshorestructures.

5.4 Corrosion in Offshore Wind Structures (OWS) case study

Anoffshorewind(OW)structureconsistsofastation-keepingsystem,foundation,foundation/towerinterface structure, tower, nacelle, rotor blades and any other secondary steel structures (boatlandings,platforms,walkways,…)[2].Currently,offshorewindstructuresareparticularlyabundantcomparedtoothertypesofmarinebaseddevices,suchaswavesorcurrentenergyconverters[17].In general, OW farms are planned for operational lives exceeding 20 years. For this reason, theymust be designed tomeet or exceed the operational and environmental loads expected to occurduringthedesignlifeofthefarm.Inparticular[18,19]:

• OWSshouldbedesigned tooperate for stateddesign lifewithout requirements for large-scalerepairs,replacementorrefurbishment;

• OWS should be designed to resist mechanical damage, physical and environmental loadsandchemicaldeteriorationwhileaimingatminimizingthetotallifecostofthestructure;

• inspection, maintenance and repair (O&M) should be performed on a schedule inaccordancewithprojectspecifications;

• structure and structural components should be designed with ductile resistance toapplicableloads;

• structuralconnectionsshouldbedesignedastominimizestressconcentrationsandcomplexstressesflowpatterns;

• Thestructureshouldbedesignedconsideringseveral loadingtypes,suchaswindandtidalloads,hydrodynamic loads inducedbywavesand currents, snowand ice loads, impactbyfloatingiceandothermechanicalloads.

Offshorewindstructuresareexposedtoaharshenvironment,including[20]:

• Corrosion(acceleratedbythepresenceofchloridesandmicroorganismsinseawater);• Physicalloadsandimpact(seeabove);• ExposuretoUV-radiation;• Extremesubzerotemperatures;• Marinegrowthandbiofouling;• Birddroppings(canchemicallydegradecoatings).


9

OneofthemostprobablefailureanddegradationmechanismsforOWsteelstructuresarecorrosionand fatigue.Corrosioncanbedescribedvery clearly througha corrosion system, consistingof thethree systemphases:medium,material and interphase. In offshorewindenergy (OW) structures,the main media are air and seawater. Seawater differs in composition depending on thegeographical location; however generally it is considered to be composed of 3.5 wt% of sodiumchloride(NaCl)anditspHrangesfrom7.8to8.3andisconsideredasmildlyalkaline[9].Inaddition,asgeographicallocationsinducedifferentvaluesofcorrosionrates[9],difficultiesinprovidingmeanvaluesofcorrosionratesinthedifferentgeographicallocationsexists.TwocorrosivitycategoriesmustbeconsideredforOWSaccordingtoISO12944-5:2007.Firstly,C5-Mis formarine,coastalandoffshoreareaswithhighsalinityfortidal,splashandatmosphericzones.Secondly,Im2isforthezonespermanentlysubmergedinseawater[2].Finally, regarding fatigue damage due to rapid development in the wind sector, offshore steelmanufacturers now use standardized materials and fabrication techniques for the production ofthickerplates formonopilesandother typesof supportstructures.Therefore, thebehaviorof theinnovative materials with respect to the area of application needs to be understood. Also,fabrication techniques, such as the types of welding process employed nowadays, significantlyinfluencecrackgrowthbehaviourinthematerialsinairandseawaterenvironments.Thisisbecauseofpossiblechanges inthemicrostructureoftheweldmaterialsand levelofweld inducedresidualstresses as a result of material thickness, material type, welding input parameters, and levels ofrestraintsemployedparticularlyduringwelding[21].Different types of corrosion attack, relevant for ORE-structures, are described in chapter 6. Foroffshorewind structures, amoredetailedoverviewof specific corrosion issues remaining today ispresentedinsectionErrore.L'origineriferimentononèstatatrovata..

5.5 Current challenges with offshore wind development

Arapiddevelopmentofoffshorewindstructuresoccurredfrom2000to2015duetothetargetssetby theEuropeanUnion (EU),butalsoduetoOWSunitsof largercapacitybeing installed in largerfarms [22]. Inparticular, in2015windenergycontributedtomeeting11.4%of theEU’selectricitydemand, against only the 2.4% calculated in 2000 [22]. Since 2011, the United Kingdom has thelargestamountofinstalledoffshorewindpoweredstructuresinEurope,followedbyGermanywithboththesecountriesshowingasignificantincreasefrom2011to2015.Denmarkisthethirdlargestproducer of offshore wind power in the EU [2]. In addition, or offshore wind energy the mainchallenges include increased water depths, more remote and distant site locations, corrosion oftowers and foundations and larger size of components, with a resultant increase in logistical

challengesforinstallation,operationandmaintenance.Onshore wind farms have shown an outstanding level of development in the last few decades;howeverthishasledtoadecreasedamountofavailable,highenergyonshoresitesremaining[23].Offshorewindstructureshaveavailablephysicalspacewithdeepoceanwatersrepresenting70%oftheworld’s area [22]. In addition, OWS can be also placed at greater distance from the coast toreduce onshore visual impact and reduce social planning conflicts. Even though OWS costs arehigher comparedwithonshore,OWSareplaced in locationswherewind speeds aremuchhigherthanonshorelocations.Forallofthesereasons,offshorewindenergyisapromisingoptionforclean


10

energyproductioninEurope.OWS are subjected to several structural damage mechanisms including corrosion and fatigue;protectivestrategiesshouldbeconsideredbecausetheyareessentialtoreachtheexpectedservicelife for which a structure was designed. Different protection systems can be used to delay andmitigate corrosion initiation and its related consequences such as safety, structural integrity andservice life [2].Asimpleapproach forprotectingoffshorewindstructurescanbe toadaptcoatingsystemsdevelopedforoffshoreOil&Gas(O&G)platform.Thecoating industryhas,overtheyears,developedspecial coatingsystems toprotectoffshorestructures fromcorrosion.Thisapproachoftransferring the technology used for O&G industry would also allow the use of existing standardassessments schemes developed by the industry and regulatory bodies [9]. However, there arecritical differences to be considered between oil platforms and wind towers. In particular, fewsystematic relationships between companies and organizations across these industrial sectors arecurrently set. In addition, considering conventional offshore O&G structures, some technicaldifferencesarereportedinTable1.

Table1:MaincriticaldifferencesbetweenoffshoreO&GplatformsandOWstructures[9]O&Gplatforms OWstructuresCorrosion protection systems generally underpermanentinspection

Unmanned structures with highly restrictedaccess(accessibility<60%)[24]

Areasofdeterioratedcoatingcanberecognisedandrepaired

Deterioratedcoatingrepairsarenotfeasible

No strong influence of loading in corrosionphenomena

Structuraldesignandcorrosionfatigueascrucialissue

Corrosion protection is always the last step during a production process. When a job is behindschedule,thereisoftenpressureonthepaintshopandthesolutiontocompromisetheapplicationpaintingprocesstosolvethesetime-relatedproblemscancreateanextremelyexpensivesituation[25]. In fact, the costs for the repair of corrosion protection systems of offshore wind energystructuresare5-10timesashighastheinitialapplicationcosts.IfrepairmustbecarriedoutdirectlyinOWstructure,costsareassumedtobe50timeshigherthanthecostsconsideredduringtheinitialapplication of the corrosion protection systems during the production of the tower [9]. For thisreason, corrosion protection systems must show a higher reliability considering the harshenvironmentandmustbecapabletoprotectthestructureforasufficientperiodoftimeevenifthecorrosionprotectionsystemismechanicallydamaged(expectedservice life forOWstructure is25years).Finally,an internationally acceptable level of safety shouldbedefined inaccordancewiththeminimumrequirementsforcorrosionprotectionsystems,assuggestedby[26].Offshorerepairofcoatingsismoreexpensivethanpaintingonshore,duetologisticofgettingmenandmaterials to the jobsiteandthe limitedaccess to thestructurescreatedbyoffshoreweatherconditions[25].Themaindifferencesbetweenonshoreandoffshoreexposurearethat inonshoreconditions, usually there is cyclic due/condensationwith or withoutminor salinity andmoderateexposuretosunlight,resultinginmoderatecorrosion,whereasinoffshoreenvironmentalong-termexposure to humidity with high salinity, intensive influence of UV light, wave action and thepresence of splash zone area occur and high corrosivity stress give rise to dramatic and very fastcorrosion [25]. Moreover, achieving high quality results in an offshore environment is a majorchallenge.Corrosiononoffshorestructuresishighlydependentonsite-specificfactorssuchaswatertemperature, salinity, chlorinity,waterdepth, and current speed. Theapplicationprocess and the


11

specificityofthecorrosionprotectionsystemareextremelyimportantandshouldbesuitableforthesubstrate and the environment. Effective, unambiguous, feasible and achievable specificationsshouldbepreparedbyexpertswithagoodunderstandingofthetechnologyinvolvedinprotectivecoatingsystems.Expertjudgmentisprimarilyimportantwhencoatingssystemsareappliedinveryspecificconditionssuchasharshoffshoreenvironment[2].Inparticular,inthescenarioofoffshoreapplicationsinNorthSearegionprotectivesystemswillfaceagreaternumberofchallenges,mainlycausedbytheharshenvironment[27].Theenvironment ischaracterisedbyviolentwind,highwaves,verylowairtemperatures,infraredradiation,floatingandpushing ice, rime and snow. It is known that steel corrosion will not be accelerated in low-temperature seawater or low-temperature atmosphere [28], although the water may showincreasedoxygencontents. It isnotanincreaseincorrosivity,butratherthequestionhowsurfaceprotectivecoatingswillrespondtotheharshenvironment,thatwilldeterminetheperformanceoforganic coatings, including thecorrosionprotectioncapacity, icingandde-icingbehaviourand theresponse to mechanical loads [27,29]. In terms of corrosion protection capacity, the low airtemperaturesmay be a special challenge to the coatings. Temperatures as low as −60 °C can beexpected in North Sea region [27,29]. Standard testing scenarios for offshore coatings [30,31]requireairtemperaturesupto−20°Conly,anditisnotknownhoworganiccoatingsmayperformatlower temperatures.The response tomechanical loads,namely to impact,will alsobeaffectedatlowtemperatures.Itisknownthattheresponseoforganicmaterialschangesfromplasticresponsetoelastic,orelastic–plastic,response[32],andtohigherrigiditymodulusatlowtemperatures.

5.6 Current challenges in Ocean Energies (OE) development

Oceanshaveatremendousnaturalresources,whichareabletomakesignificantcontributiontoourfutureenergydemands [7]. Inparticular,oceanenergy (OE)offersa renewable resourcewith theadvantage of being predictable several days in advance, consistent throughout the day and nightandsignificantlyhigherinitsenergydensitycomparedtowindandsolarenergies[33].Severaltypesof ocean sources have been defined as potential sources to generate electricity, including tidalbarrage,tidalcurrentenergy,waveenergy,oceanthermalenergy,andsalinitygradientenergy[34].Thechallengeofoceanenergyglobalmarketis337GWofinstalledcapacityby2050andinEuropetheaspirationstatedbytheoceanenergysectoristoinstallupto100GW[1].Sooceanenergyhassignificant potential, but related technologies require further improvement to drive down costs.Investments in research activities forwave and tidal developers showa clear interest tomobiliseresources for strategic markets. However, the cost of tidal and wave energy generation is lesscompetitive than other methods of generating green electricity. In particular, tidal energy hasproductioncostsaround350€/MWhandwaveenergy,hashighercostsatsome450€/MWh[35],though these figures are roughestimates, due to theemergingnatureof these technologies. TheOceanenergiesmarketsrepresentforEuropeaprogresstowardsaresource-efficienteconomy,withgreatprojectionintermsofeconomicgrowthandjobcreation.However,theyarestillfarfromtheforecastedscaleandcommercialdeployment.ThisincreaseintheOceanenergyinstalledpowerwillonly be possible if a dramatic reduction of the LCoE of these renewable sources is achieved,estimatedtobeataround130-150€/MWhforwaveandtidalgenerationwithin2020.Tidalcurrentenergy(TDE)isoneofthemostadvantageousresources,whichcanbeextractedfromtheriseandfallofsealevelscausedbythegravitationalforceexertedbythemoonandsunandtherotationof theearth[7]. It ismorepredictableandeasier tobequantifiedcomparedtowindand


12

wave energies. For this technology, extreme sea conditions have to be considered for thesurvivabilityoftidalcurrentturbines(TCT),whichisthebiggestcurrentchallengeforoceanenergy[1,7]. In particular, sea environment is harsh due to the intrinsic nature of the sea state. Theunderwater conditions are more predictable and calm compared to the water surface in sea.Atmospherichurricanedoesnotexistunderwater.Lessactionneedtobetakentothetidalcurrentpowercomparedtothewindpowertechnologies.However,extremeeventsoccurfrequently,suchas hurricane, typhoon, tsunami and storm, and bring along the extreme wave and strong wind,whichcouldhavesevereimpactsonthesurvivabilityofTCTs.So,thesurfacewavegeneratedbytheextremeeventsmayhavenegativeinfluencesontheperformanceofTCTs.Tidalcurrent turbinesarevulnerable to thedamageof seabedscour, fatigue failureofbladesandcorrosionfailureduetosalinewaterattackandthehydrodynamicfailureofbefoulingattheblades[7].Inparticular,TheTCTsfacecorrosionissueintheirlifetimeandthecorrosiveenvironmentbringuncertaintytothereliabilityofTCTs.TCTsoperate insuchenvironmentwiththepresenceof largesuspendedsolidswhichmaypossibly lead toerosivedamageover lifetimeof thedevice [36].Thelossofmaterialson theoverallTCTssystemmaycauseseverenegativeeffecton thedurabilityofTCTs.SomeoftheTCTsarepartiallysubmerged,suchasSeaflowandSeaGen.TheupperpartoftheTCTs is affected by aggressive atmospheric environment which contain high content of chloride,oxygenandothercorrosiveminerals.Inaddition,theseawaterarisingfromwaveeffectscouldsprayon the upper part of TCTs. Salts may be detected in the air due to salt spray blown by wind.Moreover,carbondioxide(CO2),hydrogensulphide(H2S),sulphurdioxide(SO2),andsulphurtrioxide(SO3)are thegasescontained in theair.Theyaccelerate thecorrosion rate throughactivating the

thinlayerofelectrolyte[37].Ontheotherhand,thesubmergedstructureofTCTscouldexperienceimmersioncorrosion.The ocean energy field is naturally turning to use composite materials as their perceived non-corrosive properties in the harshmarine environment as well as their high specific strength andstiffness.Forexample,Seaflow(300kW)andSeaGen(1.2MW)tidalturbinesaretheoceanenergydevicespartiallymadebycompositematerial[38].However,thedurabilityofcompositematerialisconsiderably inferiortometals intermsoferosion [7,36]. Inparticular, thebladesofturbinesmayoperate in turbulent slurry flows. The cavitation may interact with corrosion processes. Plasticdeformation of the surface or puncture of the corrosion-resistant passive films on unprotectedmetallic surface may take place. The particles in the flow can even strip protective paints frommetallicsurfaces [7,36].The lossofmaterialsonthestructuralcomponentsofTCTsmay influencetheirstructurecapacity.So,thepotentialsitesforTCTshavefastflow,whichcouldwashawaythecorrosion-resistantpassivefilmsonthemetallicsurfaceofTCTs'components.ThepaintingagainstcorrosionandfoulingonthesurfaceofTCTs'componentsalsofacethesameissue.ThedegradationofcoatingsofTCTsshouldbeinvestigatedinordertoknowtheoptimumscheduleformaintenance.Also marine fouling could increase the hydrodynamic loading significantly due to increasing ofmarinestructure'ssurfaceroughnessanddimensionsofsubmergedpartsofmarinestructures[39].Furthermore,underwateroperationandinspectioncouldbeimpededbytheassemblageofmarineorganisms due to obscuration of underlying substratum. It affects further maintenance andmonitoringofTCTs.


13

6 Types of corrosion

6.1 Introduction

Several zones exposed to different environmental conditions are usually considered in offshorerenewable energy structure. In particular, a schematic representation of the five typical zones ofOWSisshowninFigure2andthetypicalzonescanbedefined,asfollows:

1. Buried zone that includesany structuralpartsburied in sea floor sedimentsor coveredbydisposedsolids.

2. Submerged zone, where the structure is permanently submerged. Corrosion cathodicprotection(CCP)isgenerallyusedinthiszone,ofteninconjunctionwithcoating.

3. Tidalzoneorintermediatezone,betweentheminimumandthemaximumleveloftidesandgovernedby their variations. This region is subjected towetting anddrying cycles. In thisarea,degradationoccurredduetochemicalattack,abrasiveactionofwavesandtheattackofmicroorganisms.Thiszoneisoftencoated.

4. Splashzone,immediatelyabovethemaximumleveloftheintermediatezone.Inthiszone,thesurfaceisdirectlyaffectedbywatersplash.Theheightofthesplashzoneisafunctionofthewaveheight, aswell as of the speed andwinddirection and is subjected to cycles ofwetting and drying. This zone is often coated using a multi-layer scheme involving glassflakes polymer to help protect against mechanical damage. Corrosion becomes moresignificantaswaterevaporatesandsaltsremainsonthesurfaceofthesubstrate.

5. Atmospheric zone, above the splash zone where the steel tower and topside structuresuffers actions frommarine aerosol, however, unlike the splash zone the structure is notdirectlyattackedbywatersplashes.Thewindscarrythesaltsintheformofsolidparticlesorasdropletsofsalinesolution.Thequantityofsaltpresentdecreasesasafunctionofheightdistancefromthemeanwaterline.Alsothiszoneistypicallycoated[2,7,9,20,26].

Figure2:Schematicrepresentationof levelsandzonesinseawaterenvironment[26]

As carbon steel is usually applied for both OWS and OE industry (especially for constructing thesupporting structureofTCTs), someauthors report that thecorrosion ratesof carbonsteel in theharshoffshoreenvironmentcanbegreaterthan2.5mmperyear,dependingonthelocation[40].In


14

addition,corrosionrateofcarbonsteelincreaseswithincreaseofwatervelocityatanexponentiallydecreasingrate[7].Inparticular,forNorthSearegionacorrosionratemeanvalueof0.2mm/yforcarbon steel (one of themost usedmaterials in OWS) wasmeasured from experimental studies[9,41]. However, from experimental studies, corrosion rates were measured with mean valuesreportedinTable2andcorrosionratesofsteelarethehighestincorrespondenceofthesplashzoneofoffshoreconstruction,asreportedinTable2[20].So,splashzonerequiresparticularattentionforcorrosionprotection.

Table2:Corrosionrates(mm/y)ofcarbonsteel inoffshoreservice[40]

Environmentalzone Corrosionrate(mm/y)Buriedinsoil 0.06-0.10Submergedzone 0.10-0.20Intermediatezone 0.05-0.25Splashzone 0.20-0.40Atmosphericzone 0.050-0.075Inaddition,forthedesignpracticeDNVGL-RP-0416stateshigherminimumvaluesforcorrosionratesonprimarystructuralpartsinsplashzones.Inparticular,fortemperateclimateregion(annualmeansurface temperature of seawater ≤ 12 °C) corrosion rate should be considered as 0.30mm/y forexternalsurfaces,whileforsubtropicalandtropicalclimateregion0.4mm/yistheminimumvalue[26].Corrosioncanbeclassifiedaccordingtheappearanceofthecorrosiondamageorthemechanismofattackas[2,42]:

• Uniformorgeneralcorrosion:accountsforthegreaterproportionofmetaldeteriorationintermsofmassandofmetalconvertedtocorrosionproducts.Itispredictableonthebasisoflaboratoryandfieldtesting;

• Pitting corrosion: localised corrosion attack in which only small areas of the metal arecorroded, whilst the remainder is largely unaffected. Pitting is insidious, as it frequentlyresults in perforation of the metal with consequently loss in physical and mechanicalperformance;

• Crevice corrosion: intense localised corrosion than can occurwithin narrow crevices thatmay be formed by the geometry of the structure (e.g. riveted plates,welded fabricationsandthreadedjoints),bycontactofmetalwithnon-metallicsolidsorbydepositofsand,dirtorpermeablecorrosionproductsonthemetallicsurface;

• Filiform corrosion: characterisedby the formationof anetworkof threadlike filamentsofcorrosionproductsonthemetallicsurfaceusuallycoated,asaresultofexposuretoahumidatmosphere;

• Galvaniccorrosion:occurswhendissimilarmetalsareindirectelectricalcontactincorrosivesolutionsoratmospheres;

• Erosion-corrosion: includes all the formsof acceleratedattack that induce the removalofmetallicmaterialsduetotheabrasiveactionofmovementofafluid (liquidorgas)athighvelocity;

• Intergranularcorrosion:wheretheregionimmediatelyadjacenttothegrainboundariesarepreferentiallyattacked;

• De-alloying: selective removal of one metal can occur in certain alloys under certainenvironmentalconditionsandcanresultinperforationorinamoreuniformattack;


15

• Environmental assisted cracking, including stress corrosion cracking (SCC), corrosionfatigue and hydrogen damage: combined effect of the chemical attack and mechanicalstresses(e.g.corrosionfatigue,tensilestresswithspecificcorrosiveenvironments).

Table3showsthecorrosionzonespreviouslydefinedofanOWstructureandthemostlylikedformofcorrosionforeachzone.

Table3:CorrosionzonesandformofcorrosioninOWS[18,19]

Corrosionzones MainformofcorrosionAtmosphericzone

Externalandinternalareasofsteelstructure Uniformanderosion-corrosion,Stress-corrosioncracking(SCC)

Internalsurfaceswithoutcontrolofhumidity UniformandpittingcorrosionInternalsurfacesofstructuralparts Crevice,pittingandgalvaniccorrosion,SCC

Splashandtidalzone Externalandinternalareasofsteelstructure Uniform,crevice,pittingcorrosion,SCC

Internalsurfacesofcriticalstructures Uniform,crevice,pittingcorrosionComponentsbelowmeanwaterlevel(MWL)

Uniformcorrosion,MICComponentsbelow1.0waterleveloftheMWLExternalsurfacesinthesplashzonebelowMWL

Submergedzone Externalandinternalareasofsteelstructure Uniformcorrosionanderosion-corrosion,MIC

Internalsurfacesofsteelstructure Uniform,creviceandpittingcorrosion,MICCriticalstructuresandcomponents Uniformand/orpittingcorrosion,MIC,SCC

In addition, considering the deep-sea environment additional considerations should be stated. Inparticular, this environment is verydifferent from the sea at the surface, as it is characterisedbytotalabsenceofsunlight,highhydrostaticpressure(increaseof1atmforeach10mindepth)andalowwater temperature of about 3 °C [43]. As the business community increasingly request greatperformances for structure materials (e.g. metals and alloys) in deep-seawater, laboratoryexperimentswereperformedundercontrolledconditions.Itwasfoundthattheavailableliteratureon this topic is scarce, deep-sea experiments are expensive and technically demanded and theresults comparison is difficult as they are influenced by experimental features and local effects.Howeversomemainissuescanbehighlighted:concentrationofdissolvedoxygen(DO)insea-water(oxygenlevelincreasesathigherdepthof100mmduetocold,dense,oxygenatedwatersinkinginpolar regions), low sea-water temperature (usually about 3 °C) and microbial communities(composition of bacteria present in deep-sea biofilm appears to be as complex as the coastalbacteriabiofilmcommunityandverydiverseevenatgreatdepth)[43].

6.2 Microbiologically influenced corrosion (MIC)

Microbiologicallyinfluencedcorrosion(MIC)occursinseawaterandinsoils.Itisoftenseenaspittingattack.AgreatvarietyoforganismscanbeassociatedwithMIC.Somecanbeeasilyobservedsuchas barnacles, algae, mussels and clams while others are microscopic (bacteria). Thesemicroorganisms tend toattach toandgrowon thesurfaceof structuralmaterials, resulting in theformationofabiofilm.Themicroorganismsandbacteriamaydirectlycausecorrosion,orinfluenceother corrosion processes of metallic materials [2]. In the case of OWSs, heavy encrustations of


16

hard-shelled fouling organisms form on the surface of the metallic structure. These organismschange the environmental variables including oxidizing power, temperature and concentration.Therefore,thevalueofagivenparameteronthemetallic/seawaterinterfaceunderthebiofilmcanbedifferentfromthatinthebulkelectrolyteawayfromtheinterface.Thisdifferencemayresultincorrosion initiation under conditions in which there would be none if the film was not present.Presenceofthebiofilmmayalsoyieldchangesintheformofcorrosion(fromuniformtolocalized)oranincreaseinthecorrosionrate[2].MIC refers to corrosion that is induced by the presence and activities ofmicroorganisms and theproductsproducedintheirmetabolisminanaerobicconditionsandstagnantfluids.Themechanismby which sulphate reducers (SRB) accelerate metal corrosion is the topic of various researchprojects,butdetailsof theprocessarestill inadequatelyunderstood. Inaddition toSRBalso Iron-reducingbacteria(IRB)orIron-oxidizingbacteria(IOB)canplayinrelevantroleinMIC.Especiallythesulfatereducingbacteria(SRB),whichoccurunderanaerobicconditionscanplayamajorpartinsoilcorrosion.ThereactionproductofSRBmetabolismissulfideions,whichreactwiththemetalsurfaceallowing corrosion. The environmental conditions under which these microorganisms normallyoperatearetemperaturesfrom20to30°CandpHfrom6to8[44].This is a very specific site problem depending verymuch on the environmental conditions and ishard to predict or even detect. Biofilm morphology plays a critical role in MIC pitting [45]. MICusually takes place around the mud line, outside and inside the foundation structure and isinfluenced strongly by thenutrients levels in thewater [46]. Theprobability forMIC is increasingwiththeavailabilityoforganicmatterinthesoil.MICisalsopossibleinanoxygenfreeenvironment,makingtheconceptofairtightsubstructuresevenmorequestionable.MIC could be detected using coupons as test samples in the suspected areas to monitor thecorrosionprocessesandexaminingtheseduringtheinspectionprogram.ChangesinthepHvalueofthe internal water column (mainly localized at the bacteria/metal interface) can also indicaterelevantbacteriagrowth[47].ObjectiveevidenceontheimpactofthecorrosionprocessesandtheintegrityofthestructurewillonlybeattainedbylocalinspectionandwallthicknessmeasurementsbyRemotelyOperatedVehicle(ROV)ordiver[48].

6.3 Fouling and biological growth

Marine biological fouling, usually called marine biofouling, can be defined as the undesirableaccumulation of microorganisms, plants, and animals on artificial surfaces immersed in seawater[49].Theadverseeffectscausedbythisbiologicalsettlementarethefollowing:

• Highfrictionalresistance,duetogeneratedroughness,whichleadstoanincreaseofweight;• Influenceonincreaseddragandhydrodynamicloading;• Deteriorationofthecoatingsothatcorrosion,discolouration,andalterationoftheelectrical

conductivityofthematerialarefavoured[50];• Increase of the O&M costs mainly linked to the regular operations of removal of this

biologicalsettlement.The imminentbanofenvironmentallyharmful tributyltin (TBT)-basedpaintproductshasbeen thecause of a major change in the antifouling paint industry. In the past decade, several tin-freeproducts have reached the commercial market and claimed their effectiveness as regards thepreventionofmarinebiofoulingonshipsinanenvironmentallyfriendlymanner.From the analysis of the factors affecting the biofouling process, the interference with the


17

settlementandattachmentmechanismsisthemostpromisingenvironmentallybenignoption.Thiscan be accomplished in two main ways: imitation of the natural antifouling processes andmodificationof thecharacteristicsof thesubstrate.Thepotentialdevelopmentofbroad-spectrumefficient coatings based on natural anti-foulants is far from commercialisation. However,exploitation of aweakening of biofouling adhesion bymeans of the non-stick and fouling-releaseconceptsisataratheradvancedstageofdevelopment[49].

6.4 Corrosion fatigue

Fatigue consists of a process of localized, cumulative and permanent damage resulting from theactionofcyclicloading.Repeatedloadingandunloadingofastructure,mainlyintensioncanresultinfailure.Thepermanentactionofcyclicloadingcanleadtocrackinitiation.Fatiguestrengthmainlydependsonthenumberofcyclesofloading,therangeofserviceloadstressesandthepresenceofhigh stress concentrations. In addition, the main parameters influencing the fatigue life of astructure are the materials properties, the geometry and properties of the element, theenvironmentaleffectandtheloading[2].Thecorrosioncrackgrowthmechanismcanbeamplifiedunderfatigueloadingduetothesynergeticinteractionof theappliedcyclic loadsand the influenceof thecorrosiveenvironments.This is thecase of the mechanisms may occur in the offshore structures due to the cyclic loading on thestructurefromharshoffshoreenvironments[21].Fatiguebehaviourcanbesignificantly influencedby variation in temperature, pH, dissolved oxygen and salinity and in particular by seawatercomposition.Forthisreason,significantdifferencesintestconditionsmayalsoresultsinappreciablevariationsincrackgrowthbehaviourofthematerials.Fatigue cracks can progress from existing defects that may be introduced by manufacturing,fabrication,transportationandinstallation.Inparticular,weldedpartsareverysusceptibletofatiguecracking generally due to stress concentration and residual stresses, as well as microstructuraldefectsandheterogeneitiesinducedbydiscontinuitieslinkedtoweldingexecution.Inaddition,poordesigncanincreasestressconcentrationincreasingitssusceptibilitytofatiguecrackingalsoincaseof simultaneous actions of fatigue and corrosion. For these reasons, defects or cracks in offshorestructuresneedtobereliablyinspectedandmonitoredtoensurethatthestructuremeettheservicedesignlife[21].In addition, fatigue design is considered for offshore steel structures, in accordance with therecommended practice DNVGL-RP-C203 [51]. This standard is valid for Carbon Manganese steelmaterialsinairwithyieldstrengthlessthan960MPa,whileforthecarbonandlowalloymachinedforgings for subseaapplications theS-Ncurvesarevalid for steelswith tensile strengthup to862MPainairenvironment. Inaddition,forsteel(C-Mn) inseawaterwithcathodicprotectionorsteelwith free corrosion, the recommendedpractice is validup to690MPa. Finally,DNVGL-RP-C203 isalsovalidforboltsinairenvironmentorwithprotectioncorrespondingtothatconditionofgradesupto10.9,ASTMA490orequivalent[51].Considerationofthefatiguelimitstateisamajorpartofthedesignofoffshorewindfoundations,aswind turbines induce high fatigue loading into the foundations. The resistance against fatigue isnormallygivenintermsofanS-NcurvethatreportsthenumberofcyclesNversusthestressrangeS. Inparticular, thedesign fatigue life forstructuralcomponentsshouldbebasedonthespecifiedservicelifeofthestructure.TheS-Ncurveareprovidedforthefollowingenvironmentalconditions:“in air” for surface with coating, “in seawater” for surfaces with cathodic protection, “freecorrosion”forsurfaceprotectedbyonlycorrosionallowance[48].Incorrespondenceoftidalzone,


18

theS-NcurvesapproachrevealsomeuncertaintyintheinternalmonopileduetothefactthattheseareasthatarenotconstantlysubmergeddonotreceiveadequateCCP.


19

7 Use of corrosion resistant materials Corrosion is primarily and issue formetallicmaterials. Although concrete, polymers and polymercomposites also suffer from degradation under influence of environmental conditions, thesematerialclasseswillnotbeconsideredinthisreport.Theaimofthisreportistoprovideabasistoreduce costs due to corrosion in offshore energy structures. These costs are mainly linked tostructuresforwhichtheuseofmetallicmaterialsisrequired.

7.1 Materials selection

A notable number of materials are being utilized for offshore structures, including the followingmaterials[9].Inparticular,forOWstructuresdifferentcorrosionratevaluesweremeasuredatanintheNorthSea fordifferentmaterials. Inparticular, zinc showsanannual corrosion rateof0.0031mm,whilecopperandsteelannualcorrosionratevaluesof0.0053mmand0.119mm,respectively[9].However for cost reasons,OWstructuresaremainlymadeof corrosion sensitive constructionsteels. For this reason, the corrosion protection strategies play a strong role in the service life ofoffshorewindenergystructures.TwocorrosivitycategoriesmustbeconsideredforOWSaccordingtoISO12944-5[52].Firstly,C5-Mis formarine,coastalandoffshoreareaswithhighsalinityfortidal,splashandatmosphericzones.Secondly,Im2isforthezonespermanentlysubmergedinseawater.Corrosionratesforconstructionsteel below sea level average 0.2 mm/y. In the tidal and splash zones, the corrosion rates mayfluctuatebetween0.4to1.2mm/y[41,53].Otherauthorshavereportedthatthecorrosionrateofconstructionsteelinoffshoreenvironmentscanbeashighas2.5mm/ydependingonthelocation[40].

7.1.1 CarbonmanganeseSteel

Carbonmanganese steel is not corrosion resistant andmust be protected by means of cathodicprotection, coatings or other systems. This type of steel is still included in the current review,because it is themost commonly usedmaterial for pipelines in the North Sea.More specifically,materialgradesX46,X52andX60havewidelybeenused.Latermorehighstrengthsteel typesasX65, X70 and higher were developed and used [54]. Table 4 shows the chemical composition ofthese unalloyed steels. The index refers to theminimal yield strength expressed in KSI (x 6.89 toconvert inMPa).Othermorecorrosionresistantsteelasduplexandsuperduplexsteeland13%Crstainlesssteelshavebeenusedaspipelinematerial.Duringthelastyearsafewpipelinesweremadeof C-Mn steel internally lined (mechanical bonding) or clad (metallurgical bonding)with corrosionresistantalloyssuchas316L,Incoloy825andInconel625.Forexternalprotectioncoatingsbasedonasphalt enamel or fusion bonded epoxy coveredwith other types of plastic such as PE or PP formechanical protection or heat insulation. Besides these coatings the corrosion is controlled bycathodicprotectionincaseofcoatingdamages.


20

Table4:CompositionofsteelgradesforpipesaccordingtoAPI

Grade Max%C Max%Si Max%Mn Max%P Max%Cr MinYield(MPa)API5LX46 0.16 0.45 1.5 0.025 0.3 316API5LX52 0.16 0.45 1.6 0.025 0.3 358API5LX56 0.16 0.45 1.6 0.025 0.3 385API5LX60 0.16 0.45 1.6 0.025 0.3 413API5LX65 0.16 0.45 1.65 0.025 0.3 447API5LX70 0.16 0.45 1.7 0.025 0.3 482

Figure3:Differentmaterialsusedingearedwindturbines

Inthe’60sand’70stheconstructionofoffshoreplatformsfortheproductionofoilandgas intheNorth Sea required heavy plate properties that significantly exceeded the requirements ofconventionalconstructionalsteels[55].Duetostringentrequirementsonthesafetyandreliabilityofthe platforms under very extreme external conditions (low temperatures, severe storms, highwaves,andcorrosionbyseawater),aswellasthenecessityofpartialassemblyon-siteatsea,heavysteelplateswithespeciallyhighductility,highresistanceagainstcrackgrowth,andgoodfabricationpropertieshadtobedeveloped.Thesesteels involvedsubstantialdevelopmentsfromfine-grainedsteelstospecialoffshoregrades.TheseoffshoresteelslikeS355G10+MaccordingtoEN10225wereintroduced.Theplatescanbedeliveredineitheranormalized(+N)orinthermomechanical-rolledcondition (+M)with thicknesses of up to 250mm. In particular, the TM-rolled heavy steelplatesshowverygoodtoughnesspropertiesafterweldingandcanbeprocessedinacost-effectivemanner


21

[55].Recentplatformconstructionsarecharacterizedbyahigheroverallheightduetodeepersealevels.When applying the classical S355 steel grade in these constructions, the necessity of largecross-sections considerably increases the total weight of the structure. Therefore, high-strengthsteelplates like the steel grade S460 [56] have gained recognition in the North Sea platformapplications.Theseplatesaredeliveredinthicknessesofupto120mmintheTM-rolledcondition.Plateswith greater thicknesses are normally used in the quenched and tempered condition (+Q).TestswithhigherstrengthsteellikeS690arecurrentlyunderinvestigation[56]toreducetheplatethickness but maintaining or increasing the bending moment of the tower. Figure 3 gives anoverviewofthedifferentmaterialsusedinmodernwindturbines[57].The tower ismainly composedofmild steel, aluminiumandplasticswhereas thenacelle containshighgradesteel,stainlesssteelandcastiron.Regardingtheuseofhighstrengthsteel(yieldstrength> 450 MPa) in offshore and marine environments, hydrogen embrittlement (HE) and corrosionfatigue is mentioned as problems [54,58]. The principal sources of hydrogen in steel are fromcorrosionreactionsandcathodicprotection(CCP).Weldingcanalsocausehighhydrogencontentsifforexampletheconsumablesarenotdryandwithoutapplyingpreheat.Ithasbeenshownthattheuptake of hydrogen by steel in the marine environment is strongly influenced by the combinedeffectsofCCPandsulfatereducingbacteria(SRB).CCPproduceshydrogenonthesteelsurfaceanditsabsorptionispromotedbythebiogenicsulfideproducedbythesulfidereducingbacteria(SRB).When sulfides are present, as in sour oil and gas environments, it is often termed sulfide stresscorrosioncracking(SSCC).Inbothcases,theseareformsofhydrogenembrittlement(HE).Theyarequitedistinctfromhydrogeninducedcracking(HIC),usuallyassociatedwithpipelinesteels,inwhichhydrogengeneratedbyinternalcorrosioninsouroilorgasisabsorbedbythesteelandcollectsatelongatedmanganese sulfide inclusionswhere it leads to stepwise cracking. Thementionedmildcarbonmanganese and high strength steels have no resistance against general, local and specificcorrosioninharshmarineenvironmentsandmustthereforebecoatedorpainted.Howeverspecialsteels or low-alloyed construction steels were developed containing small amounts of copper,chromiumandnickelwithincreasedcorrosionresistance.Theso-calledweatheringsteels(e.g.Cor-Tensteel)areanexampleofthisgroup[59].HowevermostoftheseCorTensteelsarenotsuitedforexposure in marine atmospheres because the presence high chlorine concentration will locallyattackthesurfaceanddestroythenaturalprotectionlayerasformedinurbanenvironments.Table5 shows the composition of low-alloyed construction steel that can be used against corrosion. Inmostcasesthesurfaceofthesesteelsmustbepaintedtoprotectthesteelagainstlocalcorrosion.Table5:Chemicalcompositionofunalloyedandlowalloyedstructuralsteelwithimproved

corrosionresistance

Steelgrade %C %Mn %Cr %Ni %Cu %N %Al %PX60(S420) <0.16 <1.60 - - - - - <0.025Cor-TenB <0.19 0.8-1.25 0.40-0.65 <0.4 0.25-0.40 - 0.02-0.06 0.035S355J0WP <0.12 <1.0 0.30-1.25 - 0.25-0.55 0.009 - 0.06-0.15ASTMA690 0.22 - - 0.75 0.50 - - 0.15AMLoCor 0.11 0.87 1.0 - - 0.007 0.65 -

A new construction steel i.e. AMLoCor was recently developed by the Arcelor Mittal Group inLuxemburg for sheet piling [60]. The composition of this steel can be found in the Table 5. The


22

mechanical properties of this steel are equivalent to the S355but its composition contains 0,8%chromium,0.4%aluminiumand0.011%nitrogen.Therearethreetypesavailabledependingonthestrengthlevel i.e.Blue320,Blue355andBlue390.ComparativecorrosiontestswithS355showedthatthenew(uncoated)steelhasalowercorrosionrateinthesplash-andimmersion-zones(Figure4).

Figure4:Lossofthicknessduetocorrosionandbendingmomentofsteel insheetpil ing

7.1.2 Castiron

Although most cast iron grades have a good corrosion resistance to water and atmosphericconditions,theyarenotresistanttochlorinecontainingenvironments.ThenodularcastirongradeEN-GJS-400-18-LTisforexamplementionedassubstratefornacellebedplates[57]etc.As for steel alloying cast iron with chromium and nickel increases the corrosion resistancesignificantly.NiResist cast iron type II containing3%C, 20%Ni and2%Crwasmentionedas amaterial used inmarineenvironments forpumps,pipesetc.Another reference [61]mentions theuseofhighsiliconcastironcontaining14%Siasamaterialforanodesinsuppressedanodiccurrentsystemsinsteadofgraphite,leadorotherexpensivemetalslikeplatinumetc.

7.1.3 Stainlesssteel

Stainlesssteelcanalsobeused inoffshorecomponentsalthoughthematerialcost ismuchhigherthan theunalloyedandalloyed steels thatare commonlyused for structuralandpiping solutions.Thestainlesssteelsarecorrosionresistantbecauseofthehighchromiumcontent(>12%Cr)besidesother elements like nickel and molybdenum etc. The chromium is especially responsible for thecorrosionresistanceofsteelbecauseoftheformationofathinbutadheringchromiumoxidelayerat its surface that reducescorrosion.Howevermostaustenitic stainless steels likeEN1.4301 (AISI304)haveamoderatecorrosionresistance,dependingontheenvironment.Twocasescanhappen:sensitization afterwelding and local corrosion (pitting) due to free chlorine and sulfate ions [44].Somestainless steels likeEN1.4307 (AISI304L)andEN1.4404 (AISI316L) canbeweldedwithoutsensitization but their resistance against chlorine is quite low. Steels with higher chromium andespeciallymolybdenumcontents,likeEN1.4539(AISI904L)haveahigherresistanceagainstchlorineattackandpitting.


23

Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pittingcorrosion resistanceofvarious typesof stainless steels,basedon their chemical compositions (EN1994-1).ThePREN(orPRE)numbersareusefulforrankingandcomparingthedifferentgrades,butcannotbeusedtopredictwhetherpittingwilloccurforaspecificcombinationofstainlesssteelandcorrosiveenvironment.Thehigherthenumberis,thebetterthepittingresistanceagainstchlorine.AnumberofslightlyvaryingformulasforthecalculationofPRENareinuse.Themostusedversionoftheformulais[62]:

PREN=Cr+3.3Mo+16NIn the tungsten bearing super-duplex types, for example 1.4501, tungsten is also included in themolybdenum-ratingfactortoacknowledgeitseffectonpittingresistance[62]:

PREN=Cr+3.3(Mo+0.5W)+16NPRENvaluesforaselectionofstainlesssteelsaregiveninTable8.The Euro Codes 1-4 (EN 1993-1-4) provide a procedure for selecting the appropriate grade ofstainlesssteelforacertainenvironment.ACorrosionResistanceFactor(CRF)[63]iscalculatedandused to asses the corrosive environment. The various stainless steel grades are classified inCorrosion Resistance Classes (CRC) I, I, III, IV & V, according to the CRF value forwhich they aresuited.TheCRF isthesumofthreefactors:riskofexposuretochlorides(F1),riskofexposuretoSO2(F2)andtheinfluenceofcleaningandrain(F3).ThevalueoftheCRFcanrangefrom1tobelow-20.AmorenegativeCRFvalueindicatesanenvironmentwithahighercorrosionrisk.TheriskfactorsusedintheCRFcalculationaredetailedinTable6.ThedesignationofCRFvaluestoCorrosionResistanceClassesisshowninTable7[64].CRCclassesforaselectionofstainlesssteelsaregiveninTable8.

Table6:RiskfactorsusedinthecalculationofCorrosionResistanceFactors[65]

F1:RiskofExposuretoChloridesNote:MisdistancefromtheseaandSisdistancefromroadswithdeicingsalts.1 Internal,ControlledEnvironment0 LowRiskofExposure:M>10kmorS>0.1km-3 MediumRiskofExposure:1km<M≤10kmor0.01km<S≤0.1km-7 HighRiskofExposure:0.25km<M≤1kmorS≤0.01km-10 VeryHighRiskofExposure:RoadTunnelswherede-icingsaltsareused.-10 VeryHighRiskofExposure:M≤0.25km.NorthseacoastofGermany&AllBalticCoastal

Areas.-15 VeryHighRiskofExposure:M≤0.25km.Incl.allotherEuropean/UKCoastalAreas.F2:RiskofExposuretoSO2Note:forEuropeancoastalenvironmentsthesulphurdioxidevalueisusuallylow.Forinlandenvironmentsthesulphurdioxidevalueiseitherlowormedium.Thehighclassificationisunusualandassociatedwithparticularlyheavyindustriallocationsorspecificenvironmentssuchasroadtunnels.SulphurdioxidedepositionmaybeevaluatedaccordingtothemethodinISO9225.0 LowRiskofExposure:10averagedeposition-5 MediumRiskofExposure:10-90averagedeposition-10 HighRiskofExposure:90-250averagedepositionF3:Influenceofcleaningandrain0 Fullyexposedtowashingbyrain


24

-2 Specifiedcleaningregime-7 Nowashingbyrainorspecifiedcleaningregime

Table7:DesignationofCRFvaluestoCorrosionResistanceClasses.

CRF CRC1 I

0≤CRF<-7 II-7≤CRF<-18 III-7≤CRF<-20 IVCRF≥-20 V

InFigure5threegroupsofstainlesssteelcanbedistinguished:ferritic,austeniticandduplexsteels[63].Abovethehorizontal lineofseawaterat20°Conly6alloyscanbeselectedwithPRENhigherthan30.Theonlyferriticsteelthatcompliesistheso-calledSea-CureproducedbyPlymouthUS[66].Most of the steels that have sufficient high corrosion resistance against seawater contain highchromium and nickel contents combined with high molybdenum and nitrogen contents. Thecompositionofaselectednumberofstainlesssteelalloys,alongwiththeirPRENvalueandtheCRCclassforwhichtheyaresuited,aregiveninTable8.

Figure5:PRENnumberfordifferenttypesofstainlesssteel

Table8:Compositionofaselectednumberofstainlesssteelalloys,theirPRENvalueandtheCRCclassforwhichtheyaresuited(*Note:Sea-CureandZeron100arecommercialnames,

notAISIdesignations) EN AISI/Name Symbolic Type C Cr Mo Ni N PREN CRC

1.4016 430 X6Cr17 F <0.12 16-18 - - - 16.0-18.0 I1.4113 434 X6CrMo17-1 F <0.12 16-18 0.9-1.4 - - 19.0-22.6 1.4512 409 X2CrTi12 F <0.08 10.5-11.75 - - - 10.5-12.0 I1.4521 444 X2CrMoTi18-2 F <0.03 17-20 1.8-2.5 - <0.03 23.0-28.7 1.4526 436 X6CrMoNb17-1 F <0.12 16-18 0.9-1.4 - - 19.0-22.6

Sea-Cure* F <0.03 25-28 3-4 1-3,5 <0.04 35.5-42.0 1.4301 304 X5CrNi18-10 A <0.08 17.5-19.5 - 8.0-10.0 <0.11 17.5-20.8 II1.4307 304L X2CrNi18-9 A <0.03 17.5-19.5 - 8.0-10.0 <0.11 17.5-20.8 II1.4311 304LN X2CrNiN18-10 A <0.03 17.5-19.5 - 8.0-10.0 0.12-0.22 19.4-23.0 II1.4401 316 X5CrNiMo17-12-2 A <0.08 16.5-18.5 2.0-2.5 10.0-14.0 <0.11 23.1-28.5 III1.4404 316L X5CrNiMo17-12-2 A <0.03 16.5-18.5 2.0-2.5 10.0-14.0 <0.11 23.1-28.5 III1.4406 316LN X2CrNiMoN17-11-2 A <0.03 16.5-18.5 2.0-2.5 10.0-14.0 0.12-0.22 25.0-30.3


25

1.4439 317LMN X3CrNiMoN17-13-5 A <0.03 16.5-20 3-4 10.5-15 0.10-0.22 28.0-36.7 IV1.4539 904L A <0.02 19-23 4-5 23-28 <0.15 32.2-39.9 IV1.4547 254SMO X1CrNiMoCu20-18-7 A <0.02 19.5-20.5 6.0-6.5 17.5-18.5 0.18-0.22 >40 V1.4362 2304 D <0.03 21.5-24.5 0.05-0.6 3.0-5.5 0.05-0.20 23.1-29.2 III1.4410 2507 X2CrNiMoN25-7-4 D <0.03 24-26 3-5 6-8 0.24-0.35 >40 V1.4462 2205 X2CrNiMoN22-5 D <0.03 21-23 2.5-3.5 4.5-6.5 0.10-0.22 30.8-38.1 IV1.4501 Zeron100* D <0.03 24-26 3-5 6.0-8.0 0.20-0.30 >40 V

TheFinnishcompanyOutokumpuclaims todeliver itsForta2205duplex steel (EN1.4462) for theconstructionof66windturbinesintheNorthSeaneartheGermancoast.‘Jacketed’tubescomposedofastainlesssteelsleeveoverunalloyedsteeltubeshavebeenreportedasanalternativesolutionforcorrosionofpipesandtubes.In the non-tidal zones the chloride concentrations are much higher than inland locations (up to1,500ppmCl)[44].Euro-InoxrecommendsfortheseaggressiveenvironmentstousemolybdenumcontainingstainlesssteelssuchasEN1.4401orEN1.4521. In thetidalzone, the2%molybdenumcontainingstainlesssteels(austeniticandferritic)areevaluatedasunsatisfactory.InsteadEN1.4547andEN1.4410canbeusedwithoutsignsofcorrosion.Concerning soil corrosion recommendations from Euro-Inox are based on the results of differentexperimentsinsoils.Inasoilforwhichtheresistivityislessthan1.000Ω·cm,thechloridecontentishigherthan500ppm,andthepHlessthan4.5shouldbeconsideredasaggressive.Table9summarizesthestainlesssteelselectioncriteriaandindicatesthetypesofstainlesssteelthatcanbeemployedwithoutanycoatingand/orcathodicprotectionintheabsenceofstraycurrent.

Table9:Stainlesssteelselectionaccordingtosoil conditions[44]Stainlesssteelgrade

SoilconditionsName Number

X5CrNi18-10 1.4301 Cl<500ppmResistivity>1000Ω.cm

pH>4.5X5CrNi17-12-2 1.4404X5CrMoTi18-2 1.4521X5CrNi17-12-2 1.4404 Cl<1500ppm

Resistivity>1000Ω.cmpH>4.5

X2CrMoTi18-2 1.4521

X2CrNiMoN25-7-4 1.4410 Cl<6000ppmResistivity>500Ω.cm

pH>4.5X1CrNiMoCu20-18-7 1.4547

TheresistanceofconstructionsteeltoMICcorrosionislowandseverecorrosionwilloccur.Stainlesssteelshoweverhaveahigher resistance that increaseswith theamountof thealloyingelements,whicharealsobeneficialforresistancetopittingcorrosionandcrevicecorrosion[67].Asforchloridecontamination,pittingandtunnellingisformedduringMICespeciallyaroundwelds.The standard austenitic steels like EN 1.4307 (AISI 304L) and EN 1.4404 (AISI 316L) are quitevulnerable tomicro-biologically influenced corrosion.Duplex steels like EN 1.4462 (AISI 2205) arereported as borderline materials, whereas hyper-duplex, super-duplex and high-alloy austeniticstainless steel grades like EN 1.4547 (254 SMO) can be regarded as immune tomicrobiologicallyinfluencedcorrosion(MIC)inseawater[67].


26

7.1.4 Non-ferrousalloys

Non-ferrous alloys like aluminium, copper-nickel, nickel and titanium can be used in offshoreconstructiondespitetheirlowermechanicalpropertiesandhighercostcomparedtosteel.

7.1.5 NickelAlloys

Nickelandnickel-basedalloyshavethebestcorrosionresistanceforalmosteverychemicalproduct,especially for sulfate and chloride containing environments. There exist several commercial nickelalloys, as thoseproducedby theHaynesCompany. Thesealloys arenot subject to chloride stresscorrosioncrackingascomparedtootherstainlesssteelsandtheirpittingresistanceisextremelyhighmakingthesealloyssuitableforfastenersandotherfixingequipment inextremeharshconditions.Thehighpriceofthesealloyshowevermakesthemratherunsuitableasasubstrateforconstructiveelementslikepillarsandgirders.Table10givessomealloysmentionedinliterature[68]formarineapplications.

Table10:Compositionofnickelsuperalloysusedinmarineenvironments(wt%).

Grade Ni Cr Mo Co W OtherInconel625 >77 20-23 - <1 - 3-4Nb/TaHastelloyC-276 57 16 16 <2.5 4 -Incoloy825 38-46 19-23.5 2.5-3.5 - - >22FeRene41 >50 18-20 9-10.5 10-12 - -Figure6showsthegalvanicseriesofdifferentmetalsinseawater[68].Thenickelalloysarelocatedatthebottomofthediagramnearinertmaterialslikeplatinumandgraphite.Fromthisfigureitcanbeseenthatmetalswithdifferentpotentialcangenerategalvaniccorrosionwhenconnectedtoeachother.Ref[69]explainsthatsometimesgalvaniccouplesarepermitted ifthesurfaceof theanodicmetal (lessnoble) is larger thanthesurfaceof thecathodicmetal (mostnoble)forexamplestainlesssteelboltsinaluminiumplates.Inothercases,galvanicseparationusingjointsandsealsareneededtopreventcorrosionofthelessnoblepartoftheconstruction.


27

Figure6:Galvanicseriesofmetalsandalloysinseawater[68]

7.1.6 TitaniumAlloys

Titaniumhasincreasingapplicationsinmarinestructuresoverthelastyearsdespiteitshighercostcompared to steel and stainless steel [70]. The four principal areas of application have been itsstrength per weight advantage (highest of all metals), its complete immunity to corrosion byseawater(0.01mm/year),itsheattransfercapability,anditshighcorrosionfatiguelimitsinbothlowand high cycle fatigue. Titanium alloys are lighter than steel and have excellent mechanicalpropertiesmakingthemusefulforconstructivepartswithhighcorrosivedemands.Theapplicationsin offshore are stress joints, pipes, water tanks, sleeves,manholes etc. The hardenable alloys Ti-3Al2.5V(grade9)andTi-6Al4V(grade23)aremostusedbutalsopureTi (grade2)canbeapplied[70].Theexcellentcorrosionperformance(seeFigure6)isduetoaverythinandhighlyprotectivesurfaceoxidefilm.Ifscratchedordamagedthesurfaceoxidewillimmediatelyrestoreitselfinthepresenceof air or seawater.Welds and castings of the common titaniummarine grades exhibit corrosionresistance comparable towroughtmaterials, eliminating a concern over heat affected zones or aneedtoupgradealloyinginweldmetalorcastings.The fatigue properties and toughness of commonmarine grade titaniumalloys are unaffected byseawaterexposure.Thealloysareimmunetoseawaterstresscorrosioncracking.Unalloyedtitanium(grade1 and2) is susceptible to crevice corrosionpitting in some severe seawater environments,suchasatgasketedmechanicaljoints[71]grade7(Ti-0.15Pd)cansolvethisproblem.


28

7.1.7 Copperandcopperalloys

AccordingtotheCopperDevelopmentAssociationcopperalloyslike90-10CuNiandMonel400(67-33NiCu)canbeusedforpipingandvalvesingasandoilplatforms[72,73].Corrosionratesfrom0.5to 1.5mm/year can be expected in seawater. Also copper-nickel-silicon alloy and naval brass (59%Cu–40%Zn–1%Sn)areusedasmaterialsinmarineenvironment.Thesemetalsarealsousedforsocalled‘sheathing’(protectionsheets)ofsteelpilesandshiphulls[73].Whereassheathingofshiphullsforcorrosionandbiofoulingresistanceisaconceptwaitingfora definitive demonstration and test, sheathing of a variety of offshore structures is a provenapplicationforcopper-nickelalloysandthenickel-copperalloylikeMonel400.

7.1.8 Aluminiumalloys

According to theNorwegiancompanyHydro, aluminiumalloys canbeused foroffshoreandwindplatformstoreducetotalweight[69].Theuseofaluminiumforanodesincathodicprotectionandforthermalspraycoatingsandcladding isoneof theapplicationsbesides itsstructuraluse.As forstainlesssteel,aluminiumisprotectedbyasuperficialandnaturaloxidelayer.Thislayercanalsobeappliedbyaspecialsurfacetreatmentcalled‘anodizing’.AnodizingisanelectrochemicalmethodofmakingaprotectivealuminiumoxidefilmAl2O3atthesurface.CladdingisamethodofcoatingofAl-alloysproductsbythinlayerofpureAlorAl-alloys.IfthecladdinglayerisanodicincomparisontothebaseAlalloy, theproductsarecalledalcladproducts.Alloy7072 isusedascladdingforAl-MgandAl-Mg-SialloysPure aluminium (1000 series) and aluminium-magnesium alloys (5000 series) have good generalcorrosion resistance and is used for sheet and plates. The hardenable alloy 6082 has also bereported[74]asalloyforextrusionsinshipbuilding.Thepresenceofhigh chlorine concentrations canhoweverdestroy theprotective layerandcauseintergranularandstresscorrosion.Thereforecoatingaluminiumwithpaintswill alsobenecessaryagainst severe corrosion attack depending on the location to the sea level. Table 11 show thecompositionofcorrosionresistantnon-ferrousalloysforoffshoreapplications.Figure7showssomeprotectionmeasuresforaluminiumcomponentsusedinmarineatmospheres[69].

Table11:Aluminiumalloysformarineapplications

Alloy %Mg %Cu %Mn %Fe %Zn %Si1050 <0.05 <0.05 <0.05 <0.4 <0.05 <0.15083 4-4.9 <0.1 0.4-1 <0.4 <0.1 <0.16082 0.6-1.2 <0.1 0.4-1 <0.5 <0.2 0.7-1.37072 <0.1 <0.1 <0.1 <0.1 0.8-1.3 <0.1


29

Figure7:Corrosionprotectionmeasuresforaluminiuminmarineenvironments[69]

Although aluminium alloys can be used in marine environments without coatings sometimes asupplementalprotectionisneeded(Figure8).Whenpaintingaluminiumstructures,pre-treatingthesurface isessential. Inadditiontodegreasing,theoxide layerhastoberemovedeitherchemically(pickling)ormechanically (blasting, sweepingor grinding)prior to coating. This is theonlyway topermanently prevent paint delamination in a marine atmosphere. Figure 8 gives two differentapproachesforpaintingaluminiumcomponents.

Figure8:Coatingconceptsforaluminiuminthemarineenvironment[69]


30

7.2 Corrosion allowance

Corrosionallowancesareactivecorrosionprotectionstrategiesandrepresentmaterialsectionsthatare allowed to corrode without compromising the function (safety, stability, strength) of theconstruction.Thisapproachcanbeconsideredwhengeneralcorrosion isexpectedtooccurandinparticular forOWstructures corrosionallowance is recommendedmainly for the splash zone (SZ)[9]. The design of the corrosion allowance value depends on the material, expected mechanicalloads, corrosivitycategoryandcorrosionzone.Generally it is recommended tocombinecorrosionallowancewithcoating,inordertohaveamorereliablestructurebutalsoforcostlyreasons.


31

8 Corrosion protection – Technology overview In chapter 7 an overview of corrosion resistant metal was presented. However, using corrosionresistantmaterialsisnotalwayspossible,notcostefficient.Evenwhencorrosionresistantmaterialscan be used, theymay still degrade or require additional protection as a safetymeasure. In thischapter,variousstrategiestopreventcorrosionarethereforediscussed.This chapter should be considered as an overview of what technologies for corrosion protectionexist. Not all of the presented solutions are currently used in offshore applications, although theauthorshaveaimedtoselectthosesolutionsthathavepotential intheoffshoresector.Thescopehasbeenkeptfairlywideonpurpose,andinvitesthereadertobeopentonewandinnovativeideasforcorrosionprotectionintheoffshoresector.Foranoverviewofwhatarecurrentlythemostusedcorrosionprotectionsolutionsinoffshorewindturbines,thereaderisreferredtochapter9.

8.1 Active corrosion protection: cathodic corrosion protection (CCP)

Cathodiccorrosionprotection(CCP)canbedefinedas“electrochemicalprotectionbydecreasingthecorrosion potential to a level at which the corrosion rate of the metal is significantly reduced”,reported in the International Standard ISO 8044 [8]. For carbon and low-alloy steels, cathodicprotection should be considered as a technique for corrosion control, rather than to provideimmunity [48]. CCP is a method for the protection of uncoated and coated sections which arepermanentlysubjecttoseawateractingaselectrolyte.WhencombiningCCPwithacoatingsystem,a coatingmust be selectedwhich is compatiblewith CCP (see section 8.2.2). Steel reinforcementbars inreinforcedconcretecanalsobeprotectedwithCCPforOWstructures.CathodicprotectioncanbeprovidedbyGalvanicAnodeCathodicProtection(GACP)usinganodestosacrificethemselvesin order to protect themain structure or Impressed Current Cathodic Protection (ICCP) using anexternalDCpowersource[9].AttentionmustbespenttonumerouscriteriawhendesigningCCPsystem,asprotectionpotentialinV and protection current density in A/cm2,whichmust select in accordancewith the operationalconditions. For this reason, DNVGL-RP-B401 [75] gives requirements and guidelines for cathodicprotection design, anode manufacturing and installation of galvanic anodes. For offshore windstructures, galvanic anodes (GACP) are generally preferred. Galvanic anodes must meet certaincriteriaintermsofmaterialsandcomposition.Galvanicanodesusuallyconsistofzincoraluminiumalloys.However,Magnesiaalloysarenotallowedformaritimeapplications.These installationsareveryrobustandreliableoncethe functionalityhasbeenestablished.Costlyoffshore maintenance can be expected to be minimized with these systems. However, therecommendationsintheDNVGL-RP-B401[75]arenotspecificallyprovidedfortherequirementsofoffshorewindfarmwithmonopilefoundations,butforjacketfoundations.Ingeneral,foracorrectlydesignedgalvanicanodeCCPsystem,theprotectionpotentialshallbeintherangeof-0.80to-1.10Vinaerobicconditions,whiletherangeof-0.90to-1.10Vinanaerobicconditionsisrequiredforthemainstructuralparts incarbonsteelorcast iron.JacketstructuresforOil&Gas industryhavethepossibilitytospreadtheanodesoverlargepartsofthestructure,butmonopolesthemselvescannotbeequippedwithanodeseasilyduetotheinstallationwithpiledrivingprocesses[48].Thereforetheanodes are frequently equipped to the transition piece (TP) and installed together with this TPstructure.Impressed Current Cathodic Protection (ICCP) is becoming more popular in the OW industry,particularly for external protection and therehavebeendiscussions about theuseof this typeofcathodic protection internally. In particular, design and installation of CCP systems working with


32

external current require special attention as it ismore susceptible to environmental damage andthird-partymechanicaldamagethangalvanicanodessystems,assuggestedinDNVGL-RP-0416[26].In addition, ICCP requires moremaintenance and inspections, which is costly to provide for OWstructures.

8.2 Passive corrosion protection

8.2.1 Barriercoatings

Passive corrosion protectionwith barrier coatingsworks by separating themetal to be protectedfromthecorrosiveenvironment.Barriercoatingsmusthaveexcellentbarrierproperties(topreventtheingressofcorrodingspeciessuchasoxygenandwater);sufficientadhesiontothemetalsurfacetoeffectivelyresistunder-filmmigrationofmoisture;besufficientlyductiletoresistcracking;havesufficientstrengthtoresistdamage;and,ifusedinconjunctionwithcathodicprotection(CP),haveproperties that are compatiblewith CP (see section 8.2.2). Additionally, if the external protectivecoating is an electrically insulating type, it must also have low moisture absorption and highelectrical resistance. In general, an electrically insulating protective coating with insulationresistanceof106Ω-m²isgood;and,dependingontheserviceconditions,acoatingwithaminimumresistance of 104 Ω-m² is acceptable [76]. Passive corrosion protection systems may consist ofseveral layers of different types of coatings, in that case, the compatibility between the coats(layers)mustbeensured.The coating process involves the application of non-metallic coatings, metallic coatings, or thecombinationof these twotypesofcoatingsonthesteel surface (namedduplexsystems).Metalliccoatings are generally composed of non-ferrousmetals, commonly zinc, aluminiumand its alloys.These metallic coatings provide protection to steel structures against corrosion by both galvanicactionandbarrier.Moreover,themetalliccoatingsprotectsteelsacrificiallyatdamagedareasoratsmallporesinthecoatings[2].Sprayedmetalsareusuallyappliedtoflangeconnections,framesandplatformrailings.Howevertheyarealsofrequentlyappliedtothewholetowersections(usually incombinationwithorganiccoatingsystems)[9].The ideal coatingsystemshouldassure theproperperformanceof thestructureduring its servicelife without requiring structural repairs. Themajor factors to be considered in the selection of acoatingsystemare:thetypeofstructureanditsimportance,environmentalconditions,servicelife,requireddurability,coatingperformance,andcostsincludingitsapplicationandsurfacepreparation.Foracoatingsystemtoachievetheoptimumperformance,thefollowingstepsshouldbefollowed[25]:

• Selectionofthemostsuitableprotectivesystemaccordingtotheparticularenvironmentalconditions.

• Coatingrequirements• Assessmentofthestructuredesigntooptimizecoatingsystemapplication• Detailclearlyandunequivocallythespecificationsofthesystem• Useadequateandsuitabletechniquesforcoatingdeposition• Respecttherequirementsofthecoatingsystem• Rigorousqualitycontrolofthespecifiedandsuppliedmaterials• Inspectionatallphasesduringcoatingsystemapplication.

SoforOWS, inparticular,thecoatingsshouldberesistanttohighcorrosivestressduetoelevated


33

saltconcentration inbothwaterandair, impact loadingdueto icedrift (particularly forNorthSearegion)orfloatingobjects,biologicalstress,namelyunderwater,notablevariationsintemperatureofbothwaterandair[77].Algae(plants),animalandbacterialifeonsitecausesbiologicalstressonthestructuralcomponentsinthesubmergedandinthesplashzones.Algaeandanimalgrowthaddsweight to the structural component and influences the geometry and the surface texture of thecomponent. The marine growth may therefore impact on the hydrodynamic loads, the dynamicresponse,theaccessibilityandthecorrosionrateofthestructure[7].Inspections,atallphases,areessential to make sure that all requirements of the coatings specification are satisfied. Anunambiguousandadequatequality control systemshouldbe implemented.Quality controlof theentirecoatingprocesswillensurethattheappliedsystemswillreachtheirfullpotential.Protectionofsteelbypaintingisgenerallyensuredbytheapplicationofseveralcoatsofdifferentpaints,eachhaving a specific role. Thedifferent types of coats are definedby theorder of applicationon thesubstrate,namely:Theprimer(firstcoat),undercoat(anycoatbetweentheprimerandthefinishingcoat)and the topcoat (finishingcoat). Thedifferent layers shouldhavedifferent colors toease itsidentification. Generally, the coating system is characterized by the number of layers (coats)involvedandisknownbythenameofthepaintbinderusedinthetopcoat.Therearealsosystemswithoutundercoats[2].Inadequate adhesion may promote failure of the coating and expose the substrate to theenvironment(aggressivespecies)andthereforecausecorrosion.Mostorganic-basedcoatingfailuressuch as cracking, delamination, fouling damage, mud cracking and dirt under paint can only beresolvedbysandblastingthesurfaceorremovingthecoatingmechanically,cleaningthesurfaceandapplyinganewcoat.Incaseoffoulingdamage,thedamagedpaintshouldbereplacedbyatougherand more adherent coating with antifouling properties. The primer that is applied on the steelsurface should provide adequate adhesion and anticorrosive protection.Undercoats are generallyusedtoincreasetheoverallthicknessofthecoatingsystem.ThetopcoatprotectsthelayersbelowfromenvironmentalagentssuchasUVlightfromthesunandprovidesprimaryabrasionresistanceanddecorationwhennecessary.Appropriate surface preparation is crucial for the performance of paint systems. In certain cases,surface preparation is very expensive and/or difficult to carry out leading to the development ofcoatingsknownassurface-tolerant[78–80].Thistypeofsystemconsistsof introducinghydrophilicsolvents or surface-active agents in the coating that when combined with the moisture on thesurfacewill causemoisture dispersion through the film paint. Nevertheless, this type of coatingsshouldonlybeusedaslastresort.

8.2.2 CompatibilitywithCathodicProtection

Cathodic protection and barrier coatings are often used simultaneously in order to provide anadditional degree of protection. The barrier coating provides the first line of defense.When thecoatingisdamagedordeteriorates(ages)underinfluenceofenvironmentalconditions,andthebaresteelisexposed,cathodicprotectionisprovidedtopreventcorrosion.When combining the use of cathodic protection (CP) and barrier coatings, the selected coatingshould be compatible with CP. Every coating system has finite life and eventually degrades or isdamaged,allowingoxygen,water,andchemicalstoreachthesubstrate.Atthispoint,twoproblemscanoccur:

1. Cathodicdisbondmentofthecoating.2. InadequateCPofthesteelduetoshielding.


34

Cathodicdisbondmentisthedestructionofadhesionbetweenacoatingandthecoatedsurfacedueto cathodic reaction products. In the absence of defects, cathodic disbondmentwill normally notoccur.However,ifthereisacoatingdefect,CPcurrentwillpassintothemetalatthecoatingdefect,resulting in a highly alkaline environment due to the formation of hydroxyl groups at the steelsurface.Thehighlyalkalineenvironmentcoupledwiththepolarizedpotentialcancausethecoatingto lose adhesion and disbond from the substrate. As the defect gets larger, the current flowincreases and more of the coating is pushed away from the metal. This can result in rapiddisbondment and coating breakdown. It is thus important to select coatings resistant to alkalineenvironmentsandCPcurrentflow[76].Anotherproblempresents itselfwhen the coating isdamagedorbecomespermeable (andallowscorrosive species to reach the steel substrate), but prevents the CP current to flow to the steelsubstrateatthelocationofthedefect.Thesteelwillthennotbeprotectedandcorrosionwilloccur.This is called ‘shielding’ and can be defined as “as high-resistance or nonconducting materialspreventing CP current from reaching the structure to be protected, or low-resistance materialdivertingthecurrentawayfromthestructuretobeprotected”[76].ForacoatingtobetrulycompatiblewithCP,thecoatingshouldallowtheCPtoprovideprotectiontothestructureifdisbondmentoccursandwaterpenetrates.TheCPcurrentshouldbeabletopassthroughacoatingwhichhasbecomepermeable.Additionally,thecoatingshouldnotpreventtheCPcurrentfromreachingthesteelsubstrateatdefectswithverysmalldimensionssuchaspinholesandhairline fractures.As such,whenCP-compatible coatingsdegradeorwater contacts the steel, thesurfacewillstillprotectedfromcorrosionandSCCbecausetheCPcurrentcanreachthesteel[81].

8.2.3 Surfacepreparationandcleaning

It is importantthatthesurfaceofanobjecttobecoatedagainstcorrosionisfreeofcontaminantssuch as oxide (rust), oil or grease and dust [82]. Any external contamination will decrease theadhesivestrengthbetweenthecoatingandthesubstrateandthecorrosiveprotectionwillfail.In ISO 8501andNACE several cleaningmethods are recommended. Themost effectivemethod isblasting and its effectiveness is expressed as Sa value ranging from1 to 3. The lowest value Sa 1correspondswithasurfacewhereloosecontaminantswereremovedbybrushing.InmostcasesSa2½isusedforpreparationofcoatingsforharshenvironment[83].Toobtainthiscleanlinessalmostalloxidesandoilareremovedbyblasting,whereasSa3 isasurfacethat iscompletelyfreeofoxidesandothercontaminants.Figure9showsthevisualaspectofthisSascale.Foroffshorestructuresitisrecommended [84] touse steelwithan initial rust gradenotworse thanB (NorsokM501)ornotworsethanC(ENISO12944).Theservicelifeofacoatingsystemdependsontherustgradeoftheinitialsteelandthedegreeofpreparation.


35

Figure9:SurfaceaspectsofcleanedsurfacesinfunctionoftherustgradeaccordingtoISO

8501-1

Besidesblastingusinggrid,shotorsodathecleaningofcontaminatedsurfacescanbeexecutedbyother methods like brushing, grinding even dry ice cleaning, high pressure waterjet and laserablationcanbeused [85].However, someof thesemethodsgeneratesomepractical issueswhenmetalpartsandhugeconstructionslikeoffshoreplatformsmustberepairedandtreatedinopenairagainstcorrosion.Theeasiestwayistousemanualbrushingandgrinding[84].Dustremovalisveryimportantbecauseloosenon-adherentparticleswillaffecttheadhesionofthecoating. Cleaning with solvents ismostly effective before painting or coating. Nevertheless somemetalparticlesandevensaltparticlescanstillbepresentonthesurfaceaftercleaning.Tomeasuretheamountofresidues(sumofallsolublesaltsexpressedasmg/m²)thesocalledBresletestwasdeveloped (EN ISO 8502-6).With this simple chemical test the amount of soluble salts includingchlorine is measured after removing them with a patch. Some contamination can be introducedduringblastingbecause thegrit canadsorb salt particles aswell [83]. Inoffshoreapplications theamountofchlorinehoweverisveryhighsothathighconcentrationsofsaltcanbeexpectedintheblastinggritasonthecleanedsurface.Theresidualsaltishygroscopicandwillgenerateanosmoticforceinthepaintlayerandblistering.Normallyamaximumconcentrationof20mg/m²isconsideredasacceptable[83].Another importantparameter for the surfacepreparation is thequality and the finishof edges inmetal parts and constructions. In EN 1090-2, ISO 8501-3 and ISO 12944-2 the specification forcorrosion environments higher C2 like CX (offshore environment) are described. For weldedstructures special attentionmustbepaid at the finish and imperfectionsof thewelds [2,84]. Thisspecification is expressed as preparation grades P1 (lowest), P2 and P3 (highest). For offshorestructurespreparationgradeP2andP3shouldbespecifieddependingontherelevantdetailsoftheconstruction.ForP2nosharpedgesareallowedandforP3roundingwitharadiusofgreaterthan2mmisneededbeforecoatingorpainting.


36

8.3 Metallic coatings

Twomaingroupsofcoatingscanbeappliedonoffshoreandoilandgasconstructions:hotdipzinccoatingsandthermalspraycoatings[82,86–88].

8.3.1 HotDipGalvanizing

Steel constructions can be galvanized onshore after assembly and welding according to theconventionalhotdipgalvanizingpractice.Thismeansthatthesteelstructureisdippedinamoltenzinc bath. Depending on the immersion time the zinc layer will grow to a certain thickness. Thethicknessofthegalvanizedcoatingisinfluencedbyvariousfactorsincludingthesizeandthicknessoftheworkpiece and the surface preparation of the steel. Thick steels and steels which have beenabrasiveblastcleanedtendtoproducerelativelythickcoatings.Additionally,thesteelcompositionhasaninfluenceonthecoatingproduced.Silicon and phosphorus can have amarked effect on the thickness, structure and appearance ofgalvanizedcoatings(socalledSandelineffect).Thethicknessofthecoatingislargelydependentonthe silicon contentof the steel and thebath immersion time. These thick coatings (circa200µm)sometimes have a dull dark grey appearance and can be susceptible tomechanical damage. Thegalvanizedlayeriscomposedofseveralsublayerswithincreasingironcontentandhardnesstowardsthesubstrate.Thispropertyisimportantfortheabrasiveresistancecomparedtoothercoatings.Since hot-dip galvanizing is a dipping process, there is obviously some limitation on the size ofcomponentswhichcanbegalvanized.Doubledippingcanoftenbeusedwhenthelengthorwidthofthe workpiece exceeds the size of the bath. Some aspects of the design of structural steelcomponentsneedtotakethegalvanizingprocessintoaccount,particularlywithregardtheeaseoffilling,ventinganddrainingandthelikelihoodofdistortion.Toenableasatisfactorycoating,suitableholesmustbeprovidedinhollowarticles(e.g.tubesandrectangularhollowsections)toallowaccessforthemoltenzinc,theventingofhotgasestopreventexplosions,andthesubsequentdrainingofzinc. Further guidance on the design of articles to be hot dip galvanized can be found in EN ISO14713[82].The Zinc Info Center Benelux [89] specifies a minimum thickness of 80 µm for moderateenvironments and > 120 µm for severe environments. According to Momber [9] the minimalthicknessdependsalsoonthethicknessofthesubstrateand85µmseemstobeaminimumvalueformassiveconstructions(>6mm)asusedinoffshore.Thebenefitof zinc is its anodicnature towards irongiving the so-called the sacrificialor cathodicprotection(CCP).Thismeansthatdamagesinthezinccoatingwillberestored(self-healingreaction)bytheformationofzinccorrosionproducts(whiterustformation)protectingthefreesteelagainstredrustcorrosion.Inmostcasesgalvanizedsteelstructuresarepainted(duplexsystems)toincreasetheirresistance inasynergeticway.Forrepairthezinc layermustbecleanedofwhiterustbeforeapplyinganewpaint.Sometimesadhesionproblemswiththepaintcanoccurbecauseofremainingwhiterust.Zincphosphatingofgalvanizedsurfacesisalsoamethodtoimprovethecoatingadhesionofpaintlayers.Hotdipmetallizing isnotonlyperformedwithpurezinc.Hotdip layerscontainingAluminiumandMagnesiumarealsoapplied.Thisprovidesadvantagesboth intermsofmechanicalpropertiesandcorrosionresistance.TheFeZnintermetallicwhichformswhenzincandironreactisahard,butbrittlelayer.Thiscanlead


37

to cracking of the galvanized layer. With the addition of aluminium, a more ductile, non-brittleFeZnAllayerisformedinstead.TheadditionofAlandMgalsoincreasesthecorrosionprotectioninharshenvironments.Evidenceforthiscanbefoundincorrosiontestdata,courtesyofBekaertNV[90].Steelwirewascoatedwithpurezinc,Benzinal®(ZN,5%Al),Benzinal2000®(Zn,10%Al)orBenzinal3000®(Zn,10%Al,<1%Mg)and immersed in real ocean water. The percentage of red and black rust was monitored as afunctionoftime.Atthetimeofwriting,thetestwasstillongoing,butitisclearthatafter20monthsof exposure, the Benzinal® coated wires performs drastically better, with virtually no rustobservation.

Figure10:ComparativestudyofZnandZnAlcoatedsteelwire, immersedinoceanwater.

CourtesyofBekaertNV[90]It shouldberemarkedthat thecoating thicknessesused in the testdescribedabove (80g/m²≈11µmZn)areverysmall.Thiswasrequired inorder toseedifferencesbetweenthevariouscoatingswithin a reasonable time frame. For offshore applications, coating thicknesses on steel wire aregenerallyintherangeof30-50µm(250g/m²).The addition of aluminium andmagnesium also increases the cathodic protection of the steel inareaswherethegalvanizinglayerisdamaged.Thisisillustratedonsteelwirebymakingacleancutandexposingthewirecross-sectiontotheatmosphere.ItisclearthattheZnAlMgcoatingprovidesthebestcathodicprotection[91].


38

Figure11:Picturesofcutedgesof4mmsteelwireswithZinc,BenzinalandBenzinal3000coatingsafter3yearsofexposureinBelgium[91]

Galvanized steel wire is used in many marine and offshore applications, including fishing ropes,offshore hoisting ropes, wire and rope for suspended structures and ROV cables. Evenwhen thewiresandropesaresheathedinpolymer,theyareoftengalvanizedinordertoprovideanadditionallayerofprotectionincasethepolymersheathingisdamaged.

8.3.2 ThermalSpraying

The most applied technique for the application of metallic coatings on steel structures in theoffshoresector,isthermalspraying[84].Itisparticularlysuitedforprotectingarticleswhicharetoolargetobedippedinagalvanizingbathandforrepair[82].Incaseofsprayedmetals,itisalsocalled‘metallizing’. This technique described in ISO 2063 sprays metal particles that are heated totemperaturesnearorabovethemeltingpoint,ontoasurface.Thecoatingparticlesareheatedbyelectrical(plasmaorarc)orchemicalmeans(combustionflame)andprojectedathighvelocitybyairorothergasses.Thermalsprayingcanprovidethickcoatingsrangingfrom20micrometerstoseveralmm,dependingontheprocessandfeedstock(powderorwire).Itcancoveralargeareaathigherdepositionratesthanothercoatingprocessessuchascladding.Thematerials suitable for thermal spraying includepuremetals,alloys,ceramics,evenplasticsandcomposites.The materials are fed in powder or wire form, heated to a molten or semi-molten state andacceleratedtowardssubstratesintheformofmicrometer-sizeparticles.Combustionorelectricalarcdischarge is usually used as the source of energy for thermal spraying. The resulting coatings aremadebytheaccumulationofnumeroussprayedparticles.Thermalsprayedcoatingsoftencontainporosityduetoparticleimpingement,insufficientlyheatedandundeformedparticles,gas inclusion, formationofoxideetc.The surfaceof the substratemaynotheatupsignificantly,sothatthemechanicalpropertiesofthesubstrateisnotinfluencedbytheheatinput.Generally,thecoatingqualityincreaseswithincreasingparticlevelocities.The most common thermal spray methods [87] areWire Arc and Plasma Spraying. However, anumberofothermethodsalsoexist:

• Detonationspraying• PlasmaTransferredArc(PTA)• Flamespraying• Highvelocityoxy-fuelcoatingspraying(HVOF)• Highvelocityairfuel(HVAF)• Coldspraying

Inclassicalusedprocessessuchas flamesprayingandwirearcspraying, theparticlevelocitiesaregenerallylow(<150m/s),andrawmaterialsmustbemoltentobedeposited.Plasmasprayingusesahigh-temperatureplasma jetgeneratedbyarcdischargewithextremehightemperatures,whichmakesitpossibletosprayrefractorymaterialssuchasoxides,molybdenumetc.Coldsprayingisamore‘exotic’typeofthermalspraying,andinfactthemetalpowderusedasarawmaterial is not heated, but accelerated towards the surface at very high velocities. Although itscurrentusesareratherlimited,coldsprayinghasanumberofinterestingproperties.Coldsprayinganditspossibleapplicationinoffshoreisdiscussedinsomewhatmoredetailbelow.


39

Intermezzo:Coldsprayforoffshoreapplications[92–94]Coldspray(CS)canbeusedtodepositionpowdermaterialontoasubstrate.Thepowderparticlesare not molten, but accelerated to supersonic speeds (Mach 4, four times the speed of sound)throughanozzle.Whentheparticleshitasubstratelayer,theybehaveasaliquid,rapidlycoolandformanatomicfusionbondwith it.Thegasused,primarilynitrogenorhelium, isathighpressureandtemperature–upto70barand1100°C.Although it isacoldprocess, it is still classifiedasathermalsprayprocess.A largevarietyofmaterial typescanbedeposited, includingmetals (Al,Cu,Ni,Ti,Ag,Zn,Ta,Nb),refractorymetals (Zr,W, Ta) and alloys (steels,Ni alloys,MCrAlYs, Al-alloys). CS has a number ofadvantages over ‘traditional’ thermal spraying. The most relevant once for possible offshorecorrosionpreventionapplications:

• There is no thermal degradation or partial oxidation of the coatingmaterial. This can beespeciallyrelevantforoxidationsensitivematerials.

• Due to the plastic deformation, compressive stresses are introduced into the coatingmaterial, instead of tensile stresses present after ‘traditional’ thermal spraying (shrinkageuponcoolingofthecoating).

• Verydensecoatings(lowporosity)canbeproduced.Thecoldsprayprocesscouldbeusedforthemetallizationofoffshorestructures.Thehigherdensityofthemetalliclayerreducesthedemandsforsealingofpores.Coatings can be applied on-site with a handheld device. CS could potentially be used for on-site(coating) repair jobs. Because cold spraying does not introduce high amounts of heat, acoating/metallization layer could be restored without damaging existing coatings in adjacentregions.TheEU-fundedprojectHydrobond[95]hasmadeadvancesinsuperhydrophobiccoatingsforwindturbinebladesusinganalternativecoldgassprayprocess.TheHydrobondteamhassucceeded inspraying ceramicmaterials (among others) onto awhole range of substrates –metallic and non-metallic.Thesenewcoatingscanbenefit theoffshorewind industry,by increasingblade lifebyatleastfiveyearsmorethanaconventionalblade.

In the Offshore sector, pure zinc, zinc-aluminium alloy (15% Al), pure aluminium (TSA) andaluminium-magnesium (AlMg5) alloys are applied by Thermal Spraying [96]. Grit blasting is thepreferred surfacepretreatment formost TS coatingbecause it creates a higher roughness on thesubstrate(gradeP3andSa3).Specialattentionshouldbepaidtothetreatmentofweldareaswherefluxresiduesandsharpedgescanoccur[82].ThesprayingequipmentgivessomephysicallimitstotheapplicationofTScoatings.Anoperatorcanuse a brush for painting in difficult to access areas but for TS there is no alternative evenwith aspraygunwithanangle-headnozzle.ThismeansthatthedesignmustbeadaptedsothatTScanbeapplied(ENISO14713).Zinc-basedTScoatingsaretraditionallysealedtofilltheporeseitherwiththinorganic(e.g.DichtolfromDiamantCo)orsiliconesealants,orbypaintingovertheTScoating [82].This isnecessarytoincreasecorrosionprotectionduration.Withoutthissealing,thezincmetalcorrodesatanelevated


40

rate, and once all sacrificial zinc has oxidized, the steel below will no longer be protected. TheinclusionofAlandMgcanresult inself-sealingoftheTScoating,bythecorrosionproductswhichformduringexposure.SomeDanishwindfarmsof15to22yearsoldwerecoatedwithzinc(resp.60and80µm)followedbyaHempelPaintsystemandarestillingoodcondition[86].Otherreferences[82,97]indicate100-150µmthicknessforzincand150-200µmforaluminium.Ref[96]indicatesthatacoatingthicknesslessthan150µmsignsofrustcanoccursothatthisvaluecanbestatedasanabsoluteminimum.

Figure12:ThicknessrangeofTSA(µm-mils)asfunctionoftheestimatedservicetimeand

atmosphere[98] AccordingtoPraxair[97]andCanadoil[98],thethicknessof150µmmentionedinFigure12istoolowtoguaranteeaservicelifeof20years inmarineenvironmentsandisdefinitely inadequateforimmersioninseawater.Theminimumthicknessshouldbeatleast250µmwhennoadditionalpaintlayerisapplied.Asmentionedwithhotdipzincmostthermalsprayzinclayersarealsopaintedproducinga longerlifethanthatofthesumoftheindividuallifetimesofzincandpaint.Theduplex lifetimeinmarineareaisestimatedaccordingtoref[86]at1.8-2timesthesumofthelifetimesofzincandpaintand1.5 to 1.6 times the sum for immersion in seawater. The time to firstmaintenanceof theduplexcoatingforextremeenvironmentsisbetween7and23years.AstudybySINTEF inNorwayshowedthatthe life-timecostathree-layerpaintsystemwashigherthanthatofasystemconsistingofaTSzinccoatingplusathree-layerpaintsystem,duetoreducedmaintenancecostrequiredoveraperiodof30years.AstudyattheAachenUniversitypointedoutthatathree-layerpaintsystem(primer,middleandtopcoat)onzincwasevenbetterthanafour-layerpaintwithout zinc. FrankGoodwin [99]developedanexcel based life-cycle cost comparatorCorrWindforcalculatingthemaintenancecostsofconstructionsforalifecycleof20years.Fora3.6MW wind turbine the savings are estimated at 0.8 Eurocent per kWh produced during theconsideredlifetime.

CaseStudy:TS(x)AcoatingsforoffshorecorrosionprotectionThermallySprayedAluminium(TSA)iswidelyusedintheoffshoreoilandgasindustry,onplatforms,inpipelineapplications,ontensionlegelementsandonproductionrisersindeepwater.UnpaintedTSA coatings can be used for atmospheric and submerged areas reducing the need of sacrificialanodes [96]. TSA provides an alternative to the use of conventional coatings and anodes onfoundations[100,101].Inoffshorewind,TSAhassofarprimarilybeenusedascorrosionprotection


41

for smaller steel components underwater or for larger components abovewater, for example inoffshoresubstations[102].However,ithasrecently(2017)beenusedforthefirsttimeinEuropeanwaterstoprotectthefoundationsofOffshoreWindEnergy(OWE)devicesoftheArkonaWindfarm[103] and has also been used in China [104]. At the time of writing, TSA is still not a standardsolution for corrosion prevention of OWE foundations. The CROWN-project therefore worked onremoving some barriers keeping TSA from being applied on OWE foundations, on a larger scale[100,102,103,105]. TheWelding Institute (TWI) also performed significant research in the area ofTSAcoatings[101].Arobotwitharcspraygunsdepositsa350µmthicklayerofmoltenaluminiumontothefoundations[103]. In the permanently submerged zone (monopile/jacket foundation), TSA provides cathodicprotection.Thecorrosionprotectionistwofold–itfirstlyactsasabarriertotheseawaterbetweenthesteelandseawater.However,thecoatingisslightlyporousandsowheresteelisexposed(onamicro-scale), the coating acts sacrificially to provide cathodic protection. This cathodic protectiveactionresultsintheformationofaluminiumoxidesandthedepositionofcalcareousmineralsfromthe seawater, which acts to seal the pores of the coating. Therefore sealant is not necessarilyrequired.However,ifsealantisused(forexampleontransitionpiecestoprovidecolour), itshouldbeappliedinsuchamannertonotinterferewiththecathodicprotectionfunctionoftheunderlyingTSA. Previouswork has shown poor performancewhen thick epoxy top-coats have been used inconjunctionwithTSAduetounfavourableelectrochemicalinteractions[105,106].SomebarriersneedtobeovercometofacilitatethelargescaleapplicationofTSA.Uncertaintiesinthe application process and electrochemistry, pairedwithmisleading testing of epoxy coated TSAcreatedmixperceptionofitsoffshoreperformances. Inaddition,thereisnostandardcoveringtheDESIGNofaCPsystembasedontheuseofTSA.Theperceivedriskandlackofstandardizationmaketherisktoohighforinvestors[105].TheCROWNprojecthasalreadytakenimportantstepstoremovingsomeofthesebarriers:

• Some light has been shed on electrochemistry and like-for-like performances againstmostcommonlyusedepoxycoatingsandcathodicprotectionsystems(corrosiontestingandmodelling).

• Theabilityof TSA coatings toprotectbare steel at locationswhere theTSA coating isdamagedhasbeenfoundtobeverygood[107].

• TSAcostefficiencyhasbeenmodelledandcomparedtosacrificialanodesandICCP(life-cyclecostmodelling).

• The process efficiency of TSA has been increased, resulting in a significant costreduction.

• The best way to spray the nodes of jacket foundations was investigated (UniversalCoatings).

Withrespecttostandardizationandcertification,theCROWNprojectteamisworkingtoaddtheuseof the TS(x)A in the list of possible corrosion protection solution in their corrosion standard. Thestandardisexpectedtobereadybythesecondquarterof2018[105].WithintheCROWNproject,threemetalsystemsweretested:pureAl,AlMg,ZnAl.ItwasfoundthatZnAl is highly protective of exposed steel but consumed at a relatively high rate when used inseawater[105].IncontrastthecorrosionratesofAlandAlMgaremuchlower,yetthecoatingsare


42

stillabletoprovideexcellentprotectiontoexposedsteel.WhileTSAcanbeappliedon-site, it is currentlynotbeingconsideredasa retrofitting solution forexistingwindturbines.Abovethewaterline,retrofittingistechnicallyfeasible,buttherecoatingofexternalsurfacesischallengingduetotheirexposuretotheelements.

8.3.3 Nickelcoatings

As for nickel alloys, nickel based coatings have an excellent corrosion resistance to differentchemicals.Theuseofnickelcoatings inoffshorehowever isratherrestrictedtosmallcomponentslikevalves,heatexchangersandpumps[108].Electroless nickel plating is a process for depositing a nickel alloy from aqueous solutions onto asubstratewithouttheuseofelectriccurrent.Itdiffers,therefore,fromelectroplatingwhichdependsonanexternalsourceofdirectcurrenttoreducenickelionsintheelectrolytetonickelmetalonthesubstrate. Electrolessnickel plating is a chemical process,which reducesnickel ions in solution tonickelmetalbychemicalreduction.There are major differences between electrodeposited nickel and electroless nickel coatings. Forinstance, theuniformityof thedepositof electrolessnickel ishigher thanwithelectro-nickel. Thepurity of electrodeposited nickel is typically greater than 99%butwhen sodiumhypophosphite isusedasa reducingagent inelectrolessnickelplating,a typical composition for thedeposit is92%nickeland8%phosphorus.Thephosphoruscontenthasagreateffectondepositpropertiesanditcan be varied over a wide range, typically 3 to 12%. The industry normally identifies electrolessnickelcoatingsaccordingtotheirphosphoruscontent,e.g.:

• Lowphosphorus2-5%P.• Mediumphosphorus6-9%P.• HighPhosphorus10-13%P.

Therearedistinctdifferences in thecorrosion resistanceandhardnesspropertiesof lowandhighphosphorus deposits. The structure of electroless nickel is responsible for some of its uniqueproperties. It differs greatly from the crystalline structure of electro-deposited nickel and it cannormally be described as having an amorphous structure. The absence of a well-defined crystalstructureeliminatesthepossibilityofintergranularcorrosionthatcanbeaproblemwithcrystallinedeposits, suchaselectrolyticnickel.Electrolessnickel, therefore,providesamoreeffectivebarriercoatinginprotectingasubstratefromcorrosiveattack.Heat treatment of nickel-phosphorus deposits at about 400°C causes significant changes inpropertiesespeciallyitshardness(upto1000HV)duetotheprecipitationofnickelphosphide(Ni3P)makingthecoatingresistanttoabrasiveenvironments.

8.4 Organic coatings

8.4.1 Epoxyandpolyurethanecoatingsystems

Epoxy and polyurethane (PUR) based paint systems are the most frequently used anti-corrosionsolutioninmanyindustries,includingtheoffshoresector.Typically,amultilayersystemconsistingofaprimer,2-3intermediatecoatsandatopcoatisapplied.Theprimercanbepaintbased,butoften


43

metallizationisalsousedasa‘primer’layer.The primer is applied directly onto the cleaned steel surface. Its purpose is to wet the surface,provide adhesion for subsequently applied coats, corrosion inhibition. Undercoats (intermediatecoats)areapplied to ‘build’ the total film thicknessof thesystem,possiblywithmultiple layersofpaint. A higher total dry film thickness of the coating system (DFT) generally results in a longerprotection life. Finally, a topcoat can be applied to provide the required appearance and surfaceresistance to the coating system. The topcoat is also the first line of defences against weather,sunlight, corrosion,mechanical damage andbiological influences (bacteria, fungi, algae, etc.). Thevarioussuperimposedcoatswithinapaintingsystemhavetobecompatiblewithoneanother[82].As already mentioned, the total DFT of the coating system has an important influence on thedurability of the corrosion protection system. Paints are not completely impervious to oxygen,moistureandsaltpenetration.Thesecorrosivemediadiffusethroughtheappliedpaintbarrierveryslowly.Thethickerthecoating,thelongerittakesforthecorrosivemediatoreachthesteelbelowthepaint.This isalsooneofthereasonswhyaprimerwithgoodcorrosion inhibitiveproperties isrequired.NexttoDFT,alsoahighercoatingdensityanddegreeofcross-linkingcanslowdownthediffusionprocessandprolongtheprotectionlifetime.Paintsarepigmentedwithavarietyofinhibitiveandnon-inhibitivepigments.Pigmentscanbeusedto give thepaint a specific colour, but there aremanyother typesof pigments such as corrosioninhibitors (for example zinc phosphate, a mild inhibitor, or metallic zinc dust), fillers, abrasionresistance and hardness enhancing pigments, pigments decreasing permeability to oxygen andwater,pigmentsgivingthepaintanti-foulingandanti-bacterialproperties,etc.Glass/ceramicflakescan be added to increase abrasion resistance. Care must be taken when adding these types ofparticles,asimpropergrindingcanadverselyaffectthecorrosionprotection[109].Theincorporationoflaminarpigments,suchasmicaceousironoxide,reducesordelaysmoisturepenetrationinhumidatmospheres and improves tensile strength. This illustrates that a large variety of epoxy andPURbasedpaints isavailableonthemarket,withexact formulationsbeingthepropertyof thecoatingmanufacturers.The twomost frequently used primers are epoxy primers and zinc-epoxy primers.Epoxy primers[82]are two-packmaterialsutilizingepoxy resinsandusuallyhaveeitherpolyamideorpolyaminecuringagents.Zincphosphateepoxyprimersarethemostfrequentlyencounteredandgivethebestdurability within the group. Zinc phosphate refers to the type of corrosion inhibitor used in theprimer.Zinc-epoxy primers [82] canbeeither zinc-richor reduced zinc types,wheremetallic zinc(dust)isusedasacorrosioninhibitor.Zinc-richprimersproducefilmswhichcontainatleast80%byweightofmetallic zincpowder [110]while corresponding figures for the reduced zinc typeareaslowas55%byweight.Epoxy based paints are typically used for the intermediate coats. They have good abrasion andchemicalresistanceandareverygoodbarrierlayers.EpoxypaintsaregenerallyalsolessexpensivethanthePURpaints,whichareusedastopcoats.PURpaintshaveabetterUVresistancethanepoxypaints,making themgood topcoats foroffshoreenvironmentswithhighUVexposure.PURpaintsalsohaveexcellent abrasionand chemical resistance,which is typically a littlebetter than thatofepoxypaints.


44

Inearly2016,sixorganiccoatingsystemswere investigatedaccordingtotheirperformanceunderArcticoffshoreconditions[27,29].Thestudiedcoatingsystemswereepoxyandpolyurethanebased,but differed in coating material, hardener, number of layers, dry film thickness and applicationmethod.ThecorrosionperformanceofthecoatingswasassessedundertestconditionsadaptedtoArcticoffshoreconditions[27,29].Theresultsindicatedthatifexposedtoverylowtemperatures(-60°C),thecoatingschangetheirresponsetocorrosiveandmechanicalimpactloads:

• Corrosionprotectionresistancedecreased.• Coatingadhesionincreased(pull-offstrength).• Impactresistanceandabrasionresistancedecreased.

Hoarfrostaccretionchangeswithcoatingtype. Improvedbehaviourwasobtainedforathree-layersystemwith high thickness (1400μm), consisting of two glass-flake reinforced epoxy coats and apolyurethanetopcoat[27,29].Underpressureofenvironmentalregulations, there isatrendtosearchforpaintswith lowerVOCcontents (VolatileOrganic Compound),which are considered as air pollutants. There are basicallythree possible products: a high-solids solvent-borne product, a solvent-free product or a water-borneproduct.Careshouldbetakenintheapplicationoftheseproducts,astheycanbedifficulttoapplyatthespecifiednominaldryfilmthicknesses.Moreinformationregardingtheuseoflow-VOCpaintscanbefoundintheInternationalStandardISO12944,part5[110].Singlelayer,solventfreeepoxycoating[111]Solvent free epoxy systems are environmentally friendly coating solutions. Some of these solventfreeepoxiesexistassinglelayercoatingsystems.Suchcoatingscanbeapplieddirectlytothebaresteelanddonot requireaprimer.Asa result, theapplication time is shortand therearenowaittimesbetweensubsequentcoats.This isabigadvantageformaintenancework,astheshut-downperiodisdrasticallyshortened.Single layer, solvent freeepoxycoatingsareused tostopunderwaterheavymarinecorrosionandcan withstand very aggressive environments, including Accelerated LowWater Corrosion (ALWC)andMIC.Thesolvent freecharacteristicof thecoatingplaysan important role in this.As solventsevaporate from traditional paints, they leave microscopic porosity, through which MIC inducingorganisms can find their way to the steel. Anti-fouling formulations of single layer, solvent freeepoxycoatingsalsoexist.The cost per liter of single layer, solvent free epoxy coatings is generally higher than that fortraditionalcoatingsystems,butreducedapplicationandmaintenancecostscanresultinareductionof the total cost of ownership. This type of coating is currently used on tidal turbines in theNetherlands.Asmaintenanceoftidalturbinesisverychallenging,thehighdurabilityofthecoatingcanoutweighthehighercost.OncesuchpaintisHumidur®,atwo-component,solventfreeepoxycoatings.ThespecificchemistryoftheHumidur®coatingmakesthatitscorrosionresistanceisnotdegradedunderinfluenceofUV.Thismeans that a topcoat is not required to provide adequate corrosion protection. The paint isprimer,intermediateandtopcoat,allinone.However,thecolourofthecoatingcandegradeunderUV.Therefore,forapplicationswherecolourcodingisimportant,atopcoatmaybenecessary.Thesolventfreeepoxycancureunderwater,whichisanaddedadvantageformaintenanceworks.


45

The repaired zone doesn’t have to be shielded during curing and can even be submerged. As aresult,thewindowofopportunity(intermsofweatherconditions)forrepairworkisenlarged.Thecoatingcanalsobeappliedandcureatsubzerotemperatures.A further advantage with respect to maintenance work is the surface tolerance of the coating.Humidur® performs well on surfaces prepared by water jetting or by means of hand tool ormechanicaltoolcleaning.Thegritblastingstepcanbeomitted.Inoffshoreenvironments,caremuststillbetakentoremovesaltsfromthesteelsurface.The higher viscosity of the paint gives it good edge protection properties. The coating thickness(singlelayer)canrangefrom400to2000µm(onsharpedges).Theproductlifetimeeffectivenessis20-50years.Single layer, solvent free epoxy coatings can be used both in submerged and atmospheric zones.They are currently used in a large number of applications. Some relevant applications for theOffshoreEnergySectorare:

• Construction of waterway walls with sheet piles. The sheet piles are first coated andsubsequentlyhammeredintotheground.Ithasbeenshownthattheappliedcoatingisnotdamagedandalsoprovidescorrosionprotectionbelowthemudline.

• Corrosion protection ofmaritime steel,with proven resistance after 30 years of exposure(jettypiles,mooringpiles,tubularpiles).

• Corrosion protection of offshore structures (complete Oil&Gas platforms, FPSO’s,maintenanceofoffshorewindturbines,supportstructures,coatingrepairofships).

• Asacoatingonconcretestructures.• ApplicationsintheGreatLakes(USA),exposedtofloatingice.• Petrochemistrysector:usedinmaintenancecampaigns.

8.4.2 Polyureacoatings

Polyurea coatings combine exceptional physical properties such as high hardness, very goodflexibility,goodtearstrength,tensilestrength,chemicalandwaterresistance.Polyureasystemsarevery tough, combining high elasticity with high surface hardness, resulting in very good abrasionresistance.Theyalsohaveverygoodbarrierpropertiesduetothehighdensityofpolyureacoatings.This makes that polyurea coatings combine a very good corrosion protection with excellentweatheringandabrasionresistance[112].Polyureacoatingscanbeappliedtothicknessesofupto6mmforapplicationsexposedtoextremeabrasion.Itshouldberemarkedthat‘Polyurea’isnotamaterial,butatechnology[113].Thereisawiderangeofdifferenttypesofpolyureacoatings, fromroofingtoblastprotectioncoatings,overcoatingsforthebackofatruck,carparkdecksandbridges.ItisalsousedinOffshoreApplications(shipping,oilplatforms) on decks, superstructures, ballast tank liners, chain wells, etc. These are all areassubjectedtoimpactandheavyabrasion,whichareexposedtoharshchemicalagentssuchassalts[114].Polyureacoatingscanbeappliedtoawiderangeofsubstrates,includingconcrete,steelandevenpolymers.The high abrasion resistance of polyurea coatingsmakes that they could be used on for example


46

wind turbine boat landings, ladders, rails, walkways, etc. Certain polyurea systems can also beapplied on polymers [113]. They could therefore be used on the blades of tidal turbines, whereerosionplaysanimportantrole.Additionally,thehighflexibilityandelongationofthecoatingsmakethemsuitableforthebridgingof dynamic cracks. The coatings can also expand and contract with the underlying structure astemperaturechanges(duetoflexibility),whichcouldbebeneficialforoffshoreapplications.Next to its physical properties, also its extreme application properties make it interesting foroffshore applications. Polyurea coatings are applied by spraying and have a high tolerance forhumidity,both fromtheenvironmentand fromthe substrate,and temperature (will cureevenattemperatures well below 0°C). They cure very rapidly to a solid surface, in a matter of seconds[112,114].Thefastcuringcanforexamplebeanadvantageforpierprotection,whichneedstobefullycuredbeforehightide.Combinedwiththetolerancetohumidityandtemperature,thiscouldmakepolyureacoatingssuitableforon-siterepairjobs.Polyurea coatings are considered to be a modern technology. They have been commerciallydeveloped in theUSandAsia since1990. It is only since2000 that commercial developmentalsofocusedonEurope[115].Inthepast15years,thetechnologyhasknownarapiddevelopment[114].

8.4.3 Powdercoatings

Powder coating is an organic coating that is applied as a free-flowing, dry powder. The maindifference between a conventional liquid paint and a powder coating is that the powder coatingdoesnotrequireasolventtokeepthebinderandfillerparts ina liquidsuspensionform.Thishasseveral environmental benefits, as no VOC’s are required. The powder is typically depositedelectrostaticallyandthencuredunderheat(150-200°C).Powder coatings offer a number of benefits compared to liquid coatings. In many cases, theyperformbetter and last longer than traditionalwet paints. Powder coatings are verydurable andcorrosionresistant.Powdercoatingsprovidecorrosionprotectionbybeingverygoodbarrierlayers.Comparedtoliquidcoatings,mostpowdercoatingsarealsomoreresistanttochips,scratches,wearandfading,andtheyretaintheirbrightnessandvibrancylonger[116].Thepowdercanbeeitherathermosetorathermoplasticmaterial.Thermosetsaremostlyinsolubleand will not melt, making them suited for high temperature service and chemical resistance.However,theycannotberepairedandaredifficulttomaintain.Thermoplasticscanbemeltedandremelted.Theycanberepairedandthethicknessofthecoatingmaybeincreasedwiththeadditionofmorematerial. Therearenodelaminationproblems,as sometimesassociatedwith liquidpaintsystems.Themostcommonpolymersusedarepolyester,polyurethane,polyester-epoxy(knownashybrid),straightepoxy(fusionbondedepoxy)andacrylics[117].Powder coating is also suitable for coating structures that end up in the offshore environment.Offshore powder coatings are found for example on pipelines and components on floatingproductionunits,fixedplatforms,mobilerigsanddrillships.Theuseofpowdercoatingsistraditionallylimitedbecausethecoatingisappliedintheshop(duetothe need for oven curing). However, technological development in the last 20 years hasmade itpossible toapplypowder coatingswitha typeof thermal sprayprocess. Thismakes itpossible todeposit and cure powder coatings simultaneously, eliminating the application restrictions oftraditionalpowdercoatings[117,118].


47

Fusion-bondedepoxycoatingsforoffshoreapplications[119].The most common powder coating in offshore applications is fusion-bonded epoxy coating (FBEcoating),whichissuitableforusealoneorinadualpowdercoatingsystem.TheFBEcoatingsystemisathermosettingresinforexternalsurfacesof,forexample,offshorepipelines.TheseFBEcoatingsprovide external metal surfaces with extremely strong corrosion protection. A typical coatingthickness is in the range of 400-600 µm. Very good adhesion is achieved, even in humidenvironments.Thesecoatingscomewithatleast20yearwarranty.Highperformancefusionbondedepoxypowderisalsosuitableforuseinathree-layersystemwhichprovidesevenbettercorrosionresistancethantheFBEalone:Layer1:FusionbondedepoxycoatingLayer2:Co-polymeradhesiveLayer3:Polypropylene(suitableforoperatingtemperaturesbetween-40°Cand110°C)Thelastlayeristheonethatdefinesthethicknessofthecoatingsystem.

8.4.4 Ongoingresearchonorganiccoatings

In the last few decades, several types of organic-based coatings have been developed [120],including anti-fouling paints [121–126], composites and nano-coatings [127,128], self-healingcoatings [129–132], and hybrid sol-gelmaterials [130,133,134] among others. However, very fewstudieswereperformedonOWstructuresusingself-healingandhybridsol-gelcoatings.Thismaybeexplainedby the fact that these types of coatings are notwell established in themarket and themost are still under optimization studies. Frei et al. [135] in 2013, reported that the TRL ofmostapplicationsinself-healingtopicsisinclasses2to4.Inrecentyears,newsystemsofpaintshavealsobeendeveloped inwhich theanticorrosive functionandother requiredpropertiesareensuredbythe same paint, thus reducing the number of coats applied and the costs associated with itsapplication[136].For corrosion protection coating systems, the research focus is on new environmentally friendlycoating systems with the ability to behave and adapt in response to environmental demands[123,126,137,138].Coatingswithpre-emptivehealingabilitiesmaybecomeoneofthemaintargetsin the coating industry [129,132]. Combined systems (multilayers) that associate different layerswith distinct functions will be improved and/or developed in order to achieve themost efficientprotectionpossibleagainstcorrosion[9,25,133,134].Thesecoatingsmaybecomprisedofmetal-richcoatings,containingzincormagnesiumparticlesorotherinhibitorparticles.There are many epoxy coatings commercially available for corrosionmitigation, but research forcontemporaneous polymers or hybrid materials have been gaining market niches. Additionally,several coatings with anti-fouling properties have also been studied. Recently, Palanichamy andSubramanian reported a study where bacteriocin incorporated epoxy-based paint exhibited anantifoulingpropertyinnaturalseawater[125].Azemaretal.recentlyreportedthedevelopmentofhybrid antifouling paints [124]. The authors reported the use of a copolymer binder, producing apaint thatassociateshydrophobicityandbiodegradableproperties.The reportedpaintspreventedfouling settlement and proliferation by erosion, biocides release, and high and constanthydrophobicity. Carteau et al. [123] reported the development of environmentally friendlyantifouling paints using biodegradable polymer and lower toxic substances showing that it ispossibletoobtainantifoulingactivitywith lowertoxicsubstances.Studiesusingcompositecoating


48

materialshavealsobeenperformed[127].

8.5 Hybrid and other coating systems

8.5.1 Phosphateconversioncoatings

Phosphateconversioncoatingsareextensivelyusedasprimer layers inmany industries,especiallytheautomotive industry, to improvetheadhesionoforganiccoatingsto ferrousmetals [139,140].There are three types of phosphate conversion systems: amorphous iron phosphate, crystallinemanganeseandzincphosphate.Themostusedconversioncoatingsforhighdemandingcorrosionprotectionarethezincphosphatebasedcoatings(tri-cationphosphatesystems).Thesecoatingscanbeappliedeitherbysprayorbyimmersionandrequireseveralprocessstages.Noreferenceswerefoundregardingtheuseofzincphosphate intheoffshoresector.However, intheory new offshore constructions could be phosphated by using spray systems. This phosphateconversionlayerwouldthanserveasareplacementforthetraditionalprimersusedtoday.Traditional organic primers can contain zinc particles or zinc phosphate as a pigment. Thesephosphatepigmentsalsoactascorrosioninhibitors,butdonotgivethesamebondstrengthtothesteel substrateasobtainedwithphosphate conversioncoatings. The latter result froma chemicalreactionwiththesteelsurfaceandarethuschemicallybondedtothesteelsubstrate.

Figure13:Acompletecorrosionprotectionsystemforsteelsubstrates

In some iron phosphate coating applications, the cleaning and coating steps are combined (ironphosphatewashing).Thisprocessismostusedforpowdercoatingofsteelparts(seesection8.4.3).Otherwise each process step is typically separated by a rinse step to remove residual chemistry(Figure13).Ironphosphatecoatingscanbeappliedbyhandwiping,withahandheldspraywand,byimmersion,or a spraywasher. Thenumber and typeof process stages is directly dependent on finishedpartrequirements.Acleaner/coatercombinationfollowedbyarinseistheminimumchemicalcleaningand phosphating process used. The addition of stages in the process can provide enhancedperformance. Unlike the iron phosphate, a zinc phosphate coating cannot clean and coat


49

simultaneously.Inaconventionalthree-stageprocess,aseparatecleaningstageisrequiredbetweeneachpretreatmentstage[139].Wipeapplicationironphosphatingisthesimplestmethodofphosphating.Theeasyapplicationandits working ability at lower temperatures make wipe application iron phosphating very feasible.Metal parts canbewiped, brushedor cleanedwith a sponge. It is preferred for such largemetalpieceswhichdonotfitintophosphatingbathsandalsowhenworkingareaisnotenoughtosetupacontinuouslineorimmersionlineofphosphating[141].Henkeldevelopedanewdry-in-placephosphatesystemthatdoesnotneedafinalwaterrinse.ThecommercialproductsBonderite1455-W(Wipe)andBonderite7400accordingtothemanufacturershouldbeusedoneithersteel,galvanizedsteelorevenaluminiumsubstrates[140].After phosphating, a final post treatment canbe applied to passivate the phosphate layer. In thepastchromaterinsewasusedbutduetostringentenvironmental rulesotheralternativeproductsaredevelopedbasedonzirconiumoxide[142].

8.5.2 ChemicallyBondedPhosphateCeramiccoatings

ChemicallyBondedPhosphateCeramic(CBPC)coatingsarecorrosionprotectioncoatingsforcarbonandmildsteels.CBPCisatwolayersystemthatisappliedinasinglecoatingstep[143].Thecoatingisappliedbysprayingofanacid/basemixture.Theacidreactswiththesteeltoforma2µmthickironmagnesiumphosphatelayer,whichischemicallybonded(covalentbond)tothesteelsubstrate.Thisalloylayereffectivelypreventscorrosionofthesteel[144,145].The second layer of protection is a 300-900 µm thick ceramic shield that serves as a phosphatereservoir.Thehardnessoftheceramictoplayerprovidesgoodabrasionandimpactresistance.Theceramic layer ischemicallybondedtothe ironphosphatealloy layer. If theceramicshellandalloylayer belowarebreached, the ceramicwill bleedphosphate into thebreach and continuously re-passivating the steel to ensure that the alloy layer remains intact. This illustrates that the CBPCcoatingisnotabarriercoating,butaself-healingconversioncoating[144,146].Iftheareaofthebreachistoolarge,thephosphatemaynotbeabletoreachandre-alloytheentiredamaged area. However, because of the chemical bonding between the alloy layer and thesubstrate, and between the ceramic and the alloy layer, corrosion promoters like oxygen andhumiditycannotgetbehindthecoating.Damagetothecoatedsubstratewillnotspreadbecausethecarbon steel’s surface is turned into a layer of stable oxides [144,145]. Corrosion will thereforeeffectivelybeblockedatthestartofthecoatedarea.

Evidenceforthis isprovidedbyatestperformedbyEonCoataccordingtoNASAstandards:deeplygougedsamplesarecyclicallyexposedto4hoursofsaltspray,followedby4hoursofUVlight.Mosttested industrialcorrosionprotectioncoatingsfailed in45daysor less (samplesontherightside).TheEonCoatCR sample (sampleson the left side) doesnot suffer fromcorrosion, evenafter 120days. The EonCoat CR coating is expected to have a service lifetime of 30 years [143]. It hasundergoneasaltsprayASTMB117testformorethan10.000hours,withoutevidenceofcorrosion[145].


50

Figure14:Comparativestudyonagougedsamplesubjectedtoacycliccorrosiontest [143]

Apartfromitsoperationalproperties,alsoitsforgivingnesstowardsapplicationconditionsmakesitan interesting coating technology.Only aminimumsurfacepreparation is required.A commercialblast to SSPC-SP6/NACE3 is generally sufficient. In fact, the presence of slight surface oxidationimprovesthereactionsbetweenthecoatingandthesubstrate.EonCoatcaneasilybeappliedoveraflash-rustedsurface,makingblastingconditions lessstringent.TheCBPCcoatingcanbeappliedonhumid surfaces and is 100% salt tolerant. Thismakes that the coating can be applied on-site, inoffshoreconditions.Thecoatingdriesin10-15minutesafterapplicationandisservice-readyinonehour.CBPCcoatingscanalsobeusedforlocalrepairs.Nomaskingofadjacent,coatedareasisneeded.Ifsprayedoverexistingpaint,theCBPCcoatingcansimplybewashed-off.Importantwithrespecttoapplicationinoffshoreconditionsisalsotheslightlyalkalinesurfaceofthe100% inorganic coating. This makes the coating resistant to micro fouling. EonCoat is a whitecoating,butitcanbecoveredwithacolouredtopcoatifnecessary.EonCoat is currently used in a large number of applications such as submarine vessels, fuel oilstoragetanks(insideandoutside),coastalbridgestructuresandpipelines,andiscurrentlygainingafootholdinthecoatingofoffshorestructures[146].SpecificallyforOffshoreWindstructures,CBPCcoatingscouldpotentiallybeusedoneverypartofthestructure(MP,TPandtower,bothinsideandoutside).Forthecoatingofstructuresexposedtoheavy abrasionand impact (suchas theboat landing), aCBPC coating couldbeusedas aprimer,followedbyatoughpolyureatopcoat.

8.5.3 FilmGalvanizingSystems

Filmgalvanizingsystemscanbeusedtodepositametalliczinccoatingonsurfacesthatneedtobeprotectedfromcorrosion.Thezinclayercanbeappliedasapaint,withbrush,rollerorbyspraying(withaspraygun,orsimplyfromaspraycan).Theresulthowever, isametalliccoatingcontaining96%highpurityzincdustinitsdryfilm.Itisthusametalliccoating,providingcathodicprotectiontosteelsubstrates,ontopofthepassivebarrierprotectionitoffers.Commercialized under the name ZINGA® [147], this type of coating provides cathodic protection,similartohot-dipgalvanizing,butwiththeadvantagethatitcaneasilybeappliedtolargestructuresor on-site. It can thus not only be applied to new structures, but can also be used for repair


51

operations,forinstancefortherepairandreloadingofgalvanizedstructures.The resulting zinc-coatingconsistsof96%zinc, theother4%beinganorganicbinder.Theorganicbinder improves the coating-substrate adhesion. Additionally, the binder provides supplementarybarrier protection, reducing the depletion of zinc and increasing the protection lifetime incomparisonwithforexamplethermalsprayedzinccoatings.Asthecorrosionprocesscommences,insolublezincsaltsareformedwhichprovideanadditionaldegreeofprotection.An importantadvantageof the filmgalvanizingsystem, is itseaseofapplication. Itcanbeappliedwithbrush,rollerorspray(gunorspraycan).Thefilmistouchdryin10min,canbeovercoatedwitha second layer after 1hourorwith a compatible top coat after 4-6hours. Painting is possibleonsurfaceswithtemperaturesrangingfrom-15°Cto+40°Candinatmosphereswithhumidityupto95%.When painting on a new surface, high pressure wash-down, followed by blasting to Sa 2.5 isrequired. Recoating/reloading of previously galvanized surfaces requires a cleaning to removegreaseandothercontaminationsfollowedbyalightsweepblasttoremovezincsalts.Theremainingzinclayercansimplybeoverpaintedwiththefilmgalvanizingsystemtoreloadit.Typical zinc layer thickness is 80-180 µm, depending on the atmospheric conditions and requiredlifetime.Thegalvanizingfilmcanbeusedstand-aloneandhasalreadyproventoprovideadequatecorrosionprotection inmarineenvironments (seebelow).Thezinccoatinghasa lightgreycolour,butcanbecolouredbyapplyinganadditionaltopcoat.Thefilmgalvanizingsystemcanalsobeusedasaprimer,withathicknessof60µm,afterwhichcompatibleepoxyandPUcoatingscanbeappliedonthestructure.Propertiesandadvantages:

• Singlecomponentsystem• Unlimitedshelflife• AdegreeofcathodicprotectionwhichcannotbeachievedwithtraditionalZn-richpaints.As

aresultnoundercreepcorrosionispossible,evenatdamagesites.• Canbeappliedlikeapaint,bothonlargestructuresandforsmall,quickrepair jobs(brush

paintingorcanspraying).• A sealant is not required (which is the case for porous thermally zinc sprayed, which

depletesrelativelyfastifnosealantisapplied).Film galvanizing systems can be used both to coat new structures (where it can replace hot-dip,traditional paint systems or thermal spray zinc coatings) and for the repair of existing structures.Somecurrentusesare:

• Rechargingandrepairofgalvanizedstructures.• ReplacingcompletepaintsystemsonOil&Gasplatforms.• Local protection of nuts and bolts, corners, difficult to access areas, etc. where coating

applicationandinspectionaredifficultandriskfordamageandcorrosionislarger.• Marine:shipdecks,steelpierlegs(inusesince2000),offshorehighvoltagepylons,offshore

chargingcranes.• Offshorewind:usedfortherepairofgalvanizedcomponentsinturbinehousing.• Desalinationplants(completesteelstructure).


52

Itcouldbeusedonoffshorewindstructuresasaduplexsystem,i.e.withtopcoatsonthetransitionpieceandtower.Fortheinsideofthemonopileandaroundthemudlineastand-alonegalvanizingfilmcouldbeused.Testing isunderwaytoprooftheMICresistanceofthegalvanizingfilm,asthiscouldprovidesignificantaddedvaluetothecoating.

8.5.4 Silanecoatings

Other emerging pre-treatment products are the silane-based chemistries that uses ‘nanoscale’technology to provide good corrosion resistance and coating adhesion without heat [142]. Likezirconium-based pre-treatments, these formulations also save waste disposal and maintenancecostsbyreducingsludge.Theremayalsobesavingsinrinsewaterconsumption,becausetherinsefollowing the treatment is cleaner than the rinse following a phosphate treatment. Silane-basedproducts can be used in conventional three-stage pre-treatment systems (and not for on- siteapplications),andmaybeabletousetypicalcitywaterinsteadofdemineralizedwater.Morestagesand higher-purity water may be necessary, however, for finished products that require higherstandardsofcorrosionresistance.

8.5.5 Sol-gelcoatings

Sol-Geldoesnotrefertoaspecificmaterialtype,buttoaprocess.Materialisdepositedasaliquid,loaded with nano-scale solid particles (called a Sol), which solidifies after in-situ crosslinkingbetween the nano-particles. In case of coating deposition this means cross-linking happens afterdepositiononthesubstrate.Sol-Gel coatings canbepurely inorganic innature (forexamplepure Si-O coatings), but thesearelimited to very thin films (typically < 500nm). These films are very dense and provide excellentbarrierproperties.However,theyrequirehightemperaturecuringwhich,togetherwiththeirlimitedthickness and brittle nature, makes them unsuited for offshore corrosion protection applications[148].Hybrid Sol-Gel coatings on the other hand consist of alternating inorganic and organic polymericchainslinkedinasinglecovalentnetwork.Thesecoatingscanbebuiltuptolargerthicknesses(upto10µm)[92],butmoreimportantly,theyaremoreflexibleandabrasionandimpactresistant.Recentdevelopmentsmake itpossible todeposit these coatingselectrolytically, bybrushingandevenbyspraying.Curingofthesehybridcoatingscanbeperformedatlowertemperatures(80-150°C)orbyUV-light.Insomecasescuringcanevenbeomitted.Assuch,thepracticalbarriersforapplicationofthesecoatingsonlargestructurescanbeovercome[148].To the authors’ knowledge, there are currently no large-scale corrosion solutions commerciallyavailablethatarebasedontheSol-Geltechnique.However,theuseofhybridSol-Gelcoatingsdoesofferanumberofadvantagethatcouldresultinabreak-throughinthenearfuture[92,133,148]:

• Relativelyinsensitivetomoistureonthesubstrate(dependingoncoatingchemistry).• Formationofacovalentbondwiththesubstrate,resultinginexcellentadhesion.• Excellent adhesion with additional organic coating layers (for example epoxies). If the

organicpartofthehybridcoatingistunedcorrectly,acovalentbondwithorganicovercoatsispossible.


53

• Excellentbarrierpropertiesduetoahigherdensitythantraditionalorganicpaints.• Corrosion inhibitors such as zinc particles or certain chemicals can be incorporated in the

coating.• Sol-Gelcoatingswithself-healingpropertiescanbesynthesized.• ManySol-GelbasedcoatingscanberealizedbasedonVOC-freesolvents,whichmakesitan

environmentallyfriendlyprocess.Sol-Gel coatings can be applied to a wide range of substrates, including polymers, aluminium,stainless steel and construction steels. In the following, twopossible application exampleswill bedescribed.Self-healingSol-Gelcoatingonaluminiumcomponentsandstructures[133]To increase the corrosion resistance of aluminium, the surface can be coated by paints. Thealuminiumsurfacemustbepre-treatedtoensuregoodcoatingadhesionandcorrosionresistance,forwhichchromateshavelongsincebeenused.However,environmentalandworkersafetyissuesassociatedwithhexavalentchromehavedriventhemarkettolookforalternatives.One possible alternative is the use of thin (50-500nm) Sol-Gel films. According to the supplier, itdoesnotformacrystallinelayer,thereisnodwelltimeandrequireremovalofoxidesisnotrequiredprior to film deposition. By forming a covalent bond directlywith aluminium oxide, the chemicalproducesathin,hydrophobicbarriercoatingcapableofprotectingaluminiumfromfurtheroxidationbyblocking corrosivematerials to the surface [134]. Thepretreatmentmolecules aredesigned topenetratedeep intothenano-poresof thealuminiumoxideandself-assembleupto100 layersofhighly cross-linked polymers. Figure 15 shows a system under investigation for a so-called ‘self-healing’ sol gel coating on aluminium. Nano-particles doped with an inhibitor passivate exposedaluminiumwherethecoatingisdamaged.

Figure15:Corrosionhealingmechanismofahybridsolgelcoatingwithnanoparticles[133]

Sol-Gelbasedprimeronsteelstructures[148]Thegoodadhesionwhichcanbeachievedtoboththesubstrateandsubsequentpaintlayers,makesSol-Gelcoatingsgoodcandidatesforprimers.Ahybridepoxy-Si-Ofilm(1-10µm)canbesprayedonasteel substrateandcuredunderUV-light.By tuning thechemistryof thecoating,a covalentbondbetweenthesol-gelfilmandthesteelcanbeachieved.Theepoxychainsinthefilmshouldresultinagoodadhesionofasubsequentepoxybasedpaint.Nano-scalezincparticlescanbeincorporatedinthecoatingascorrosioninhibitors.It is expected that future developments will focus on the investigation of more environmental


54

friendly precursors/reagents, as well as VOC-free systems. Metal-rich sol-gel coatings, containingzincormagnesiumparticles,orcoatingsincludingotherinhibitorparticleswillalsobedevelopedinorder to further improve the corrosion protection. This should allow to combine the excellentbarrierpropertiesofsol-gellayerwithandcathodicprotectionofsacrificialparticles[133].

8.6 Alternative corrosion control measures

8.6.1 Generalprecautions

The design and detailing of a structure affects the corrosion durability of any protective coatingapplied on constructions [9]. Structures designedwithmany small components and fasteners aremoredifficulttoprotectthanthosewithlargeflatsurfaces.Thekeyissuestoconsiderinclude:

§ Accessforcoatingapplicationandmaintenance§ Avoidanceofmoistureanddebristraps§ Avoidanceorsealingofcrevices§ Drainageandventilationtominimizethe‘timeofwetness’§ Carefulmanagementofgalvaniccontactwithothermaterials

GeneralguidelinesforthepreventionofcorrosionbygooddesigndetailingcanbefoundinENISO12944-3,andsometypicaldo’sanddon’tsforsteelframedbuildingsareshowninFigure16.Mostoftheseguidelinesincludetheavoidanceofentrappedwater,dust(sand/salt).

Figure16:Generaldesigndetail ingofconstructiveelements

Bolts,rivetsandweldsareoftenusedinjointsforoffshoreconstructionsmadeofsteel.Usingdifferentmaterialscanleadtocrevicecorrosion,galvaniccorrosionaswellasstresscorrosioncracking.Inmostcasestheweldfillermaterialisselectedaccordingtothetypeofsteel.


55

Boltedsteelconnectionsarealwaysvulnerabletocorrosion.Boltsmadeofcarbonsteelcanbeusedbutoftenstainlesssteelboltsareused.Whenusingstainlesssteelboltsinflangesitisimportanttoprotect the bolts from corrosion. This can be done by applying paint layers or by the followinginnovativesolutions.

8.6.2 Protectivecapsandshields

BoltShield® developed metal-made protective caps in aluminium, stainless steel and zinc coatedsteelforboltsandnutsagainstcorrosion[149].Forboltedconnectionsbetweentubularsteelparts,thecompanyAlocitdevelopedspecialclampsmadeupfromtwohalvessurroundingtheconnection[150].Oneof these clamps is connected to the platformby a stub piece, they are hinged and/orboltedtogetheraroundthepile(Figure17).Byusingthesespecialclampscorrosioncanbesloweddownandevenstoppedafterusingsealingproducts.

Figure17:Protectionofboltedsteelconnectionsbyspecialclamps[150]

Ifweldingiscarriedoutaftertheapplicationofthemetalcoatingorwhenseveredamageoccurs,itisdifficulttoaccomplishthesamestandardofprotectioninthoseareascomparedtotherestofthestructure.Weldedjointsaregenerallycriticalbecauseoftheirsusceptibilitytostressconcentrationand residual stresses. Discontinuities linked to the execution of welding can induce defects andheterogeneitiesinthemicrostructure[2].

8.6.3 Sheathing

Protection of steel structures like platforms and ship hulls can also be obtained by the so called‘sheathing’ [73] componentswith thinmetal sheetsandplates.Theseplatescanbeconnectedbyweldingor adhesives. Special attention is needed to prevent crevice and galvanic corrosionwhenusingdifferentmaterials(seeabove).

8.6.4 Dehumidifiers

The companyCotespatented compact dehumidifiers and overpressure systems to be installed inoffshorewindtowerstokeeptheinsideofthenacelleandtowerdry[151].Bythisactionpenetratedseawaterorsaltwillberemovedfromtheinsideandcorrosionisprevented.

8.6.5 Sealingsandjoints


56

Seals are used inwind towers to prevent penetration of seawater. They aremostly consisting ofrubber-basedmaterials.Asmentioned above the galvanic contact between twometals like stainless steel and galvanizedsteelcancreatebimetallicorgalvaniccorrosiononthelessnoblepartofthejunction(anodiccell).Inadditioncrevicecorrosionbetweenthejunctioncanoccur.Typicalexamplesarebolts,screwsandnutsbutalsoweldscanreactasgalvaniccoupleswhentheweldingmaterial is different from the parentmaterial. To prevent this type corrosion for boltedconnections,isolationwithneoprenesealscanbeused[69].Coatingofcouplingswithpaintcanalsopreventthecorrosionofgalvaniccouples.


57

9 Corrosion protection of Offshore Wind (OW) structures Insection9.1thecoatingsystemswhicharecurrentlyusedmostoftenonOffshoreWindStructuresarepresented.Toalargeextend,thesecorrespondtothecoatingsystemsprescribedinnormsandstandards. Section Errore. L'origine riferimento non è stata trovata. lists a number of corrosionissuesthatremaintodayintheOffshoreRenewableEnergysector.Anattemptismadetolinksomeofthecorrosionprotectiontechnologiespresentedinthisreporttoremainingcorrosionissues.

9.1 Coating systems currently in use

TheselectionofcoatingsystemsforOWstructuresisnotstraightforward.Theapproachforcoatingsystemselection forOWSs is similar to that forotheroffshore structuresdesigned foroil andgasproduction. Procedures for selection and minimum paint system requirements are given inguidelines and standards [26,30,31,110], but often corporate experience and guidelines alsoinfluencethefinaldecision.Informationonthecoatingssystemsofspecificwindfarmsisdifficulttoobtain.However,itissafetosaythatmostoffshorewindturbineswillbecoatedaccordingtothespecificationsintherelevantstandards (NORSOKM-501, ISO12944-5, ISO20340,DNVGL-RP-0416). The limited informationonsite specific coating systems that were found, confirms this. Nevertheless, deviations from thestandards do occur, especially if construction of the wind farms is done in cooperation with forexampleOil&Gascompaniesthathaveexperienceintheoffshoresectorandapplythisexperienceintuning the coating system. Such information is generally kept confidential, as it can give animportant economic advantage. An overview of the coating systems specified for use on carbonsteelintheaforementionedstandards,fortheoutsideofOWstructures,isgiveninTable12.

Table12:Overviewofcoatingsystemsspecifiedforuseoncarbonsteel instandardsISO12944-5,ISO20340andNORSOKM-501,fortheoutsideofOWstructures.Coatingsystems

aresubdividedaccordingtoexposurecategory.Standard Primer Subsequent

coat(s)TotalPaintSystem

Type NDFT[µm] No.ofcoats NDFT[µm]CorrosioncategoryC5-MHighAtmosphericzone

ISO12944-5EP,PUR 80 EP,PUR 3-4 320EP,PUR 250 EP,PUR 2 500

EP,PUR(ZnR) 60 EP,PUR 4-5 320

ISO20340EP(ZnR) ≥40 EP ≥3 ≥280

EP ≥60 EP ≥3 ≥350NORSOKM-501 EP(ZnR) ≥60 EP ≥3 ≥280CombinedCorrosioncategoriesC5-MHighandIm2HighSplashandTidalzones(2)ISO12944-5 - - - - -

ISO20340EP,PUR(ZnR) ≥40 EP,PUR ≥3 ≥450

EP,PUR ≥60 EP,PUR ≥3 ≥450EP,PUR ≥200 EP,PUR ≥2 ≥600

NORSOKM-501 (1) (1) EP,PE ≥2 ≥600CorrosioncategoryIm2HighSubmergedzone(2)ISO12944-5 EP(ZnR) 60 EP,PUR 3-5 540


58

EP 80 EPGF,EP,PUR 3 500EP 800 - - 800EP 80 EPGF 3 800

ISO20340EP,PUR(ZnR) ≥40 EP,PUR ≥3 ≥350

EP 800 - - 800EP ≥150 EP,PUR ≥2 ≥350

NORSOKM-501 (1) (1) EP(3) ≥2 ≥350NOTESNDFT=NominalDryFilmThicknessEP=Epoxy;PUR=Polyurethane;EPGF=GlassFlakeEpoxy;PE=Polyester;ZnR=ZincRich(1)Notspecified(2) In the submerged, tidal and splash zone, a coating system is often used in combinationwith cathodicprotection(CP).Inthiscase,attentionshouldbepaidtoselectingacoatingsystemthatiscompatiblewiththeuseofCP.(3)CoatingsystemshallalwaysbeusedincombinationwithCP.The specification ‘High’, next to the corrosion category, indicates that the coating systems shouldhave a lifetime of at least 15 years in the environment under consideration.With respect to thesplashzone,DNVGLguideline(DNVGL-RP-0416)statesthat“forcoatingsystemsbasedonepoxyandmeeting the requirements for coating materials and quality control of surface preparation andcoatingapplicationinNORSOKM-501CoatingSystemNo.7A(min.DFT600μm)withausefullifeofupto15yearsmaybeassumed in thesplashzone.Foranequivalentsystembasedonglass-flakereinforced epoxyor polyester (min.DFT 700μm), the useful lifemaybe assumed to beup to 20yearsinthesplashzone.”InapresentationbySINTEF (2010) [152], itwaspointedout that apaint systemaccording to theNORSOKspecificationsfortheatmosphericzones(60µmEPZnR,150µmEP,70µmPU)generallyrequiresafirstmajormaintenanceafter10years.Inthesplashzonealifetimeof>20yearscanbeachievedwithatwocoatpolyestersystem(NDFT>1000µm).AclassicsystemthatmeetstherequirementsforOWstructuresintheatmosphericzoneconsistsofa zinc-rich, epoxy-based primer coating (60 μm), three successive epoxy mid-coats, and one PUtopcoat,withatotalnominaldryfilmthicknessof400μm[2].Theaboveonlyconsidersorganiccoatingsystemsanddoesnot includethedetailedapplicationofmetal coatings,which are not un-usual onOW structures [77]. Paint systems formetallized steelsubstrates are included in the standards ISO 12944-5, ISO 20340 andNORSOKM-501, albeit to alesserextent.AnoverviewisgiveninTable13.

Table13:Overviewofcoatingsystemsspecifiedforuseonmetall izedcarbonsteel instandardsISO12944-5,ISO20340andNORSOKM-501,fortheoutsideofOWstructures

Standard Sealercoat Subsequentcoat(s)

TotalPaintSystem

Type NDFT[µm] No.ofcoats NDFT[µm]

CorrosioncategoriesC5-MHighandIm2High

ISO12944-5 EP NA(1) EP 3 450

EP,PUR NA(1) EP 3 320

ISO20340 - - EP ≥2 ≥200

NORSOKM-501 (2) (2) (2) ≥2 ≥200(3)


59

NOTES(1)NA=notapplicable.Thedryfilmthicknessofthesealercoatwillnotsignificantlycontributetothetotaldryfilmthicknessofthesystem.(2)Notspecified.(3)NORSOKM-501specifies100µmofthermalsprayedzincorzincalloysasmetallization.Previously used coating systems included a Zn/Al-85/15-metallization (60-100µm), organic porefiller, two intermediate epoxy-based coats (2x 100-120µm) and a PU based topcoat (50-80µm)[2,77,84].However,itisstatedin[84]that,“duetothedemandsforlesstimespentonpainting,costreduction, and good experience over years with paint systems, wind turbine structures may bemetallized lessoften.Moreandmore,highqualitypaintsystemswithoutmetallizing(accordingtoDINENISO12944,C5-Marine),aregoingtobeusedforexternalprotection.”Environmental conditions inside the towers are less severe than on the outside. Therefore, purepaint systemscanbeused,according tocorrosionclassC4. In the lowerpartsof the tower, somespecificationsdorequiretheuseofmetallizationpluspaint[84].In principle, many chemical resistant coatings could be selected such as urethanes, epoxies,chlorinated rubber and vinyl polymers. In [2], a number of alternative coating systems arementionedthatareusedonOWStructures.TheseareindicatedinTable14,alongwithanumberofothercorrosioncontrolsystems. Itshouldbenotedthatthecurrentauthorsarenotawareofanycurrentusesof(chlorinated)rubberorneoprenecoatingsonOWstructuresintheNSB.

Table14:AlternativecoatingsystemswhichcouldbeusedonOWstructures,classif iedaccordingtoexposurezone

AtmosphericzoneVinylsystems(3-4layers)Znphosphatepigmentedtwo-packepoxyprimer(1layer)Two-packepoxy(2layer)[2,20,27,29]Chlorinatedrubbersystem(3-4layers)SubmergedzoneThemaincontrolisCCP.Theuseofcoatingsystemsisoptional,generallyEPbasedcoatings,andtheseshouldbecompatiblewithCCP.WhencoatingsareusedfeweranodesarenecessaryandthecorrosionprotectionsystemisexpectedtolastlongerSplashandTidalzoneCoatingssimilartothosefortheatmosphericzoneareused.HigherfilmthicknessisemployedThesteelthicknessisincreased(toactascorrosionallowance)andiscoatedwiththesamecoatingsystemoftherestofthestructureThickrubberorneoprenecoatingupto15mmofthicknessPolymeric resins or glass-flake reinforcedpolyestermaterials are oftenused to protect againstmechanicaldamage[2,27,29]

Repairingthecoatingofstructuresinoffshoreenvironmentscanbeverydifficultandexpensive.Thesteel surface is likely to be moist or even wet, and contaminated with chloride ions during therepairingprocedure.Therefore,thecorrosionresistanceofpaintsystemsappliedoncontaminatedsteelsubstratesshouldbeinvestigated.In2011,Shietal.[153]reportedastudyontheinfluenceof


60

saltdepositionatthesteel/paintinterface.Theauthorsfoundcorrosionproductsunderthepaint.Itwassuggestedthattheexposuretimeofsteelsubstratebeforepaintapplicationshouldbeasshortaspossibletoavoiddegradationduetosaltdeposition.Nexttopropercoatingselectionandapplication,thecostofOWstructuresandtheirmaintenancecan be decreased by optimizing systems for Structural HealthMonitoring (SHM). This would alsoincreasetheoutputandreliabilityofcurrentwindenergysystemtechnologies.OptimizationofSHMcan lead to reducing labor costs of wind turbine inspection by the prevention of unnecessaryreplacement of components or early repairing interventions such as repainting of affectedcomponents.[22]

9.2 Corrosion issue on Offshore wind structures

Corrosion protection solutions have been in place since the installation of the first offshorewindturbines.However,corrosionofoffshorewindstructuresremainsachallengeandnewsolutionsarestillsoughtafter.Inthefollowing,anumberofthesecorrosionissuesarepresented.Theresultistheauthors’attempttodescribethecorrosionchallengesfacedintheconstructionandmaintenanceofoffshore wind structures. The list is compiled based on literature and a selected number ofinterviewswithindustryexpertsandisthereforenottobeconsideredasanexhaustiveaccount.Alongwithlistinganumberofissues,somesuggestionsaremadewhereinnovativesolutionscouldbeusedtotackletheidentifiedcorrosionproblems.Theauthors’suggestionsarenotanattempttopresentaconclusiveanswertotheremainingcorrosionissues,asthiswouldsurelyfail.Rather,itisan attempt to invite the reader to think aboutnew, innovative and creative solutions for existingproblems.Inthefollowing,adistinctionismadebetweenprimaryandsecondarysteelstructures.Primarysteelincludes structural components such as the foundation and tower. Failure of a primary steelcomponent could threaten integrity of the entire structure. Secondary steel structures arecomponents which are important, but whose failure will not result in a total collapse of thestructure.

9.2.1 Primarysteel

In the atmospheric zone, no significant corrosion issues have been reported. The currently usedcoatingsprovideadequateprotectionfortherequiredlifetime.Therearenosignsofoverallcoatingfailure.Inthesplashzone,coatingdamageandcorrosionisobserved.Ingeneral,thecoatingdamagesarenottheresultofcoatingfailure.Theusedcoatings(i.e.theproduct)areofhighquality,complywiththe relevant standards and provide the required protection to corrosion. If a coating is locallydamaged, this can almost invariably be linked to either mechanical damage (impacts by floatingobjectsorserviceboats)orimpropercoatingapplication.Ithasalsobeenobservedthatthecoatingthicknessinthesplashzonemaydecreaseovertime,asaresultofthegrindingactionofwaves.However,thiscaneasilybeentakenintoaccountinthedesignstage.Ascoatingdamageandcorrosioninthiszonearemainlyduetomechanicaldamage,amoreflexible


61

and damage tolerant coating could be a solution. Powder coatings or polyurea paints, whichgenerallyhavebettermechanicalproperties,couldbeused.Inbothcasesthecosteffectivenesshastobecomparedtoexistingsolutions(includingrepaircosts).Alternatively,formsofactiveprotectionlike film galvanizing, zinc thermal spraying or the use of chemically bonded phosphate ceramiccoatingscouldbeused,withacoloredtopcoat.ToreducetheLevelizedCostofEnergy(LCoE),attemptsaremadetoreducethecostofthecoatingsystem,without compromising the durability. Only reducing the coating thickness doesn’t reducethecostmuch,as the laborcost ismuchhigher than thepaintcost.Therefore,a reduction in thenumberof layers inthecoatsystemwouldbemuchmoreeffective.However,thisbringswith itaquestionof reliability. In amulti-layer system, a defect in an individual layer doesnot necessarilyresult ina throughcoatingdefect,making themulti-layer systemrelatively tolerant toapplicationdefects.Inasinglelayersystem,alayerdefectevidentlyresultsinathroughcoatingdefect.Movingtocoatingsystemswithlesslayersisthereforenotamatterofprovingthatthecoatingcanpreventcorrosion.Existingsinglelayersystemscanprovidethenecessarycertificatestoproofthis.Itis ratheramatterofquality control.Making theuseof single layer coatingsgenerally acceptable,would require the development of a control mechanism that allows to verify that the coating isdefectfreeoverthecompleteextendofthecoatedstructure(i.e.notjustspotchecks).In the submerged zone, the foundation structure is protected by cathodic protection (with orwithout an additional coating). If properly designed, this effectively prevents corrosion. There arehowevertwopointsthatshouldbehighlighted:

• ApresumedriskofMICaroundthemudline.• Corrosionontheinsideofmonopilefoundations.

It is generallybelieved thatMICaround themudline (up to1m into the soil) couldposea threat.However, no data has been found to clearly establish the severity ofMIC andwhether or not itindeedposesathreat. Investigationofdecommissionedfoundationscouldshedmore lightonthisissue.IfMICindeedposesathreat,itisalsounclearhowfoundationscouldbeprotectedagainstit.Thereis no conclusive evidence showingwhether or not ICCP helps to preventMIC. Coatingsmay be asolution. But foundations are hammered into the ground, often through the layer of scourprotection deposited on the sea bed. The question remains whether the coating integrity is notcompromisedaftersuchanaggressivetreatment.Corrosionon the insideofmonopile foundationswas first believednot to occur.Monopilesweresealedandconsideredairtight. Insuchanenclosure,oxygenwouldrapidlybeusedup,preventingcorrosion. However, it has been shown that fresh seawater and oxygen do penetrate into themonopile, necessitating some form of corrosion protection. This can be a coating, cathodicprotectionoracombinationofboth.Incasealuminumalloyanodesareusedforcathodicprotection,theseawaterpHcandroptoacidiclevels,making the cathodic protection ineffective. To avoid this, it should bemade sure that theseawaterinthemonopilecanberefreshedbytidalaction[154].Ascorrosioninsidethemonopiledoesoccur,specialattentionshouldbepaidtoweldswhereweldcrackingandpittingcanoccur[155].

9.2.2 Secondarysteel


62

Theimportanceofsecondarysteelcorrosionisoftenunderestimated.Nevertheless,secondarysteelstructuresaremoresusceptibletocorrosionandareconstantlybeingrepaired.A largepartofthecorrosionmaintenancecostscanbeallocatedtosecondarysteelstructures.Boatlanding Thecoatingscurrentlyusedonboatlandingscannotcopewiththeimpact

andmechanicalloadingduetoserviceboats.Asaresulttheboatlandingsshowseverecoatingdamageandcorrosion,makingconstantmaintenancenecessary.Theonlycoating solution is touseadamage tolerantcoating.Anexamplecouldbetheuseofpolyureacoatings,whicharealsousedinthe Offshore O&G sector for damage sensitive areas. As an additionalprecaution,achemicallybondedphosphateceramiccoatingcouldbeusedasaprimertoprovideamoreactiveprotection.Alternatively, corrosionprotectionwith a coating canbe abandoned andhighercorrosionallowancestakenintoaccount.

Railings,platformborders,hatches

Also thesecomponentsareoftensubject todamagedue to frequentusefor inspection andmaintenance activities, and hoisting of material. Alsoherepolyureacoatingsorpowdercoatingcouldbeconsidered.

Boltedconnections,cornersanddifficulttoreachareas

These difficult to coat areas are prone to early coating failure. As asolution,aformofactiveprotectioncouldbeused.Filmgalvanizingcouldbeperformedwithabrushorsprayasaformoflocalprotection.Certain types of flexible, thermoplastic anti-corrosion paints may alsoperformwellintheseareas.

Gratingfixation Gratings are often fixed to the support structure with bolts that arescrewed in stainless steel blocks, welded to the carbon steel supports.These welds are difficult to prepare in terms of surface roughness,resultinginpoorcoatingadhesion.Acoatingwithabetteradhesiontolessroughsurfaceswouldprovideasolution.Alternativeconnectionsolutionsarealsobeingused,suchasHilti®studs.Infixationswithnutsandbolts,contactcorrosionmaybeobserved.

Temporaryaccessories

Some secondary steel attachments are only used during installation (ex.cable pull-off). To reduce costs, less stringent corrosion protectionmeasures are often taken on these accessories. As the accessory is nolonger needed, corrosion of these structures may not be considered anissue.However,run-offcorrosionproductsthatcoverthecoatedstructurebelow may promote coating degradation of more important primary orsecondarysteelstructures.

Although often neglected in the wider corrosion debate, also electrical components suffer fromcorrosion.Thecostsdue to standstill and replacementpartscanbesignificant. Inorder to reducetheLCoE,alsothesecomponentsneedtobeconsidered.

9.2.3 Repairsolutions

No matter how performant the selected coating system, it will never be possible to completelypreventalldamages.Therefore,efficient coating repair solutionsarealso required.Mainy coating


63

productsspecifyverystrictsurfaceconditionsforapplications,includingsurfaceroughness,humidityand salt concentration. In the offshore environment in which repairs have to be conducted, it isoftenverydifficult, ifnot impossibletocomplywiththeserequirement(certainlyforhumidityandsalt). Coating systemswhich have a high tolerance to the surface condition have the potential tofacilitatemaintenancejobs.Commercial products such as ZINGA®, Humidur®, Alocit® and others claim properties that makethemsuitableforrepairjobs.Awiderangeofsuppliersofsuchproductsisalreadyavailableonthemarket,however,therearenorecommendationsorguidelinesspecifyingwhichproductsshouldbeusedinwhichsituation.


64

10 Normative standards and guidelines Severalnormativereferencesarecurrentlyusedfordesigningofoffshorestructureswiththemainfocusonoffshorewindandoceanenergy,selectionofmaterialsandcorrosionprotectionstrategiesinoffshoreconditions.Themainnormativestandardsandguidelinesarereportedasfollows:− Offshorestructures:

• DNVGL-OS-C401:“Fabricationandtestingofoffshorestructures”[156];• DNVGL-OS-E301:“Positionmooring”[157];• DNVGL-SE-047:“RiskBasedVerificationofoffshorestructures”[158];• DNV-OSS-121: “Classification Based on Performance Criteria Determined from Risk

AssessmentMethodology”[159];• DNVGL-ST-C502:“Offshoreconcretestructures”[15];• DNVGL-OS-D101:”Marineandmachinerysystemsandequipment”[160];• DNVGL-OS-D201:“Electricalinstallations”[161];• DNVClassificationnotesno.30.6“Structuralreliabilityanalysisofmarinestructures”[162];• DNVGL-SE-0420:“Certificationofmeteorologicalmasts”[163];• DNVGL-ST-0145:“Offshoresubstations”[164].

− Offshorewindfarms-methodsofdesignandoperation:

• DNV-OS-J201:“Offshoresubstationsforwindfarms”[19]• DNVGL-RP-B401:“Cathodicprotectiondesign”[75];• DNVGL-RP-0416:“Corrosionprotectionforwindturbines”[26];• DNVGL-RP-J101:“Useofremotesensingforwindenergyassessment”[165]• DNVGL-SE-0073:“ProjectcertificationofwindfarmsaccordingtoIEC61400-22”[166];• DNVGL-SE-0074: “Type and component certification of wind turbines according to IEC

61400-22”[167];• DNVGL-SE-0190:“Projectcertificationofwindpowerplants”[168];• DNVGL-SE-0441:“Typeandcomponentcertificationofwindturbines”[169];• DNVGL-ST-0126:“Designofsupportstructuresforwindturbines”[170];• DNVGL-ST-0361:“Machineryforwindturbines”[171];• EN50308:“Windturbines–Protectivemeasures–requirementsfordesign,operationand

maintenance”[172].− Oceanenergysystems-methodsofdesignandoperation:

• DNVGL-ST-0164: “Tidalturbines”[173];• DNV-OSS-312:“Certificationoftidalandwaveenergyconverters”[174];• DNVGL-SE-0163:“Certificationoftidalturbinesandarrays”[175];• IEC/TS62600-1:“Marineenergy-Wave,tidalandotherwatercurrentconverters-Part1:

Terminology”[176]; • IEC/TS62600-100:“Marineenergy-Wave,tidalandotherwatercurrentconverters-Part

100:Electricityproducingwaveenergyconverters-Powerperformanceassessment”[177];


65

• IEC/TS62600-101:“Marineenergy-Wave,tidalandotherwatercurrentconverters-Part101:Waveenergyresourceassessmentandcharacterization”[178];

• IEC/TS62600-200:“Marineenergy-Wave,tidalandotherwatercurrentconverters-Part200:Electricityproducingtidalenergyconverters-Powerperformanceassessment”[179];

• IEC/TS62600-201:“Marineenergy-Wave,tidalandotherwatercurrentconverters-Tidalenergyresourceassessmentandcharacterization”[180];

• IEC/TS62600-10:“Marineenergy-Wave,tidalandotherwatercurrentconverters-Part10:Assessmentofmooringsystemformarineenergyconverters(MECs)”[181].

− Materialsrequirements-metallicalloys,metalliccoatings,paintsandvarnishes:• EN 1090-1: “Execution of steel and aluminium structures – Part 1: Requirements for

conformityassessmentofstructuralcomponents”[182];• EN 1090-2: “Executionof steelandaluminiumstructures–Part2:Technical requirements

forsteelstructures”[183];• EN10204:“Metallicproducts–typesodinspectionsdocuments”[184];• EN 10225: “Weldable structural steels for fixed offshore structures – technical delivery

conditions”[185];• EN12473:“Generalprinciplesofcathodicprotectioninseawater”[186];• EN12495:“Cathodicprotectionoffixedoffshorestructures”[187];• ENISO12944:“Paintsandvarnishes–Corrosionprotectionofsteelstructuresbyprotective

paintsystems”[52];• ENISO14713:“Zinccoatings–Guidelinesandrecommendationsfortheprotectionagainst

corrosion of iron and steel structures – Part 1: general principles of design and corrosionresistance”[188];

• ISO8501:“Preparationofsteelsubstratesbeforeapplicationofpaintsandrelatedproducts–visualassessmentofsurfacecleanliness”[189];

• ISO8502:“Preparationofsteelsubstratesbeforeapplicationofpaintsandrelatedproducts–Testsfortheassessmentofsurfacecleanliness”[190];

• ISO8503:“Preparationofsteelsubstratesbeforeapplicationofpaintsandrelatedproducts–Surfaceroughnesscharacteristicsofblast-cleanedsteelsubstrates”[191];

• ISO20340:“Paintsandvarnishes–Performancerequirementsforprotectivepaintsystemsforoffshoreandrelatedstructures”[30];

• NORSOKM-501:“Surfacepreparationandprotectivecoating”[31];• DNVGL-OS-B101:“Metallicmaterials”[10];• DNVGL-OS-C101:“Designofoffshoresteelstructures,general-LRFDmethod”[192];• DNVGL-ST-C501:CompositeComponents[14].

Obligatorystandards for testingandselectionofcorrosionprotectionsystems,mainly forcoatingsand linings, foroffshorestructuresare in force forsomeyearsnow.This includes inparticular ISO20340andNorsokM-501[30,31].Thetestingprocedureconsistsofthreeindividualtests,cathodicdisbonment testandseawater immersiontest.Theageingresistance testprocedureconsistsof7-day cycle, whereby the coating system is exposed to three loading mechanisms, which must be


66

passed through25 times (4200h).Assessmentcriteria includeadhesionstrength (pull-off testing),coatingdeterioration,scribecorrosionanddelaminationduringthecathodicdisbondment.


67

11 Conclusions In this literature review a special attention was given to the presentation of the main corrosionmechanisms and forms that usually occur in offshore systems, material selection, surfacepreparationandmetallicandorganiccoatingsofconstructionsforoffshoresystems,mainlyfocusedonwind,tidalenergystructures.Themainconclusionscanbedrawnasfollows:

• Offshore renewables are estimated to play a major role, driven by both political andeconomic factors. The renewables-based electricity generation is projected to triple over2013-2040,overtakingcoaltobecomethelargestsourceofelectricity.Accordingtothenewpolicies scenario, 33% of the world electricity generation by source will come fromrenewablesin2040(IEA,2014).

• Offshore structures in general are subjected to several damage mechanisms includingcorrosionandfatigue;soprotectivestrategiesshouldbeconsideredasareessentialtoreachtheexpected service life forwhicha structurewasdesigned.Differentprotection systemscanbeusedtodelayandmitigatecorrosioninitiationanditsrelatedconsequencessuchassafety,structuralintegrityandservicelife.

• Cleaning of substrates before coating is still a very fundamental operation as it stronglyinfluencestheadhesionoftheprotectivelayersandcoatingsonthesubstrate.Grindingandblastingismostlyperformedbutothermethodslikehighpressurewaterjetcanbeused.

• Modern offshore constructions are more andmoremade of special alloys including highstrengthsteel,stainlesssteel,aluminiumandeventitaniumalloys.Lowandunalloyedsteelhowever must always be protected because of their low corrosion resistance to marineatmosphere. The best results are obtained by thick thermal spray aluminium or zinc-aluminium coatingswith orwithout additional paint layers. Smaller parts can bemadeofuncoated special stainless steels, nickel alloys etc. The number of innovative corrosionresistant alloys however is restricted: only the AMLoCorwas found to be a new alloy foroffshoreapplications.

• Finally,asurveyofthemainstandardsandguidelinesisreportedinordertocollectallthereferenceforthematerials,proceduresthatcanbeusedinoffshoresystems.


68

12 References [1] ADMAforEnergyRoadmap-ATechnologyRoadmapoftheVanguardInitiative’spilot

“AdvancedManufacturingforenergyrelatedapplicationsinharshenvironments,”2016.[2] S.Price,R.Figueira,CorrosionProtectionSystemsandFatigueCorrosioninOffshoreWind

Structures:CurrentStatusandFuturePerspectives,Coatings.7(2017)1–51.doi:10.3390/coatings7020025.

[3] WindEurope-UnleashingEurope’soffshorewindpotential-Anewresourceassessment,2017.

[4] TheEuropeanoffshorewindindustry-Keytrendsandstatistics2016,2017.[5] SOER2015-Europeanbriefings:Marineenvironment,EuropeanEnvironmentAgency,2015.[6] WorldEnergyOutlook2014,InternationalEnergyAgency,2014.[7] L.Chen,W.H.Lam,Areviewofsurvivabilityandremedialactionsoftidalcurrentturbines,

Renew.Sustain.EnergyRev.43(2014)891–900.doi:10.1016/j.rser.2014.11.071.[8] InternationalStandardBSENISO8044:2015Corrosionofmetalsandalloys.Basictermsand

definitions,2015.[9] A.Momber,Corrosionandcorrosionprotectionofsupportstructuresforoffshorewind

energydevices(OWEA),Mater.Corros.62(2011)391–404.doi:10.1002/maco.201005691.[10] DNVGL-OS-B101Metallicmaterials,2015.[11] M.R.Setvati,Z.Mustaffa,N.Shafiq,Z.I.Syed,AReviewonCompositeMaterialsforOffshore

Structures,in:Proc.33thInt.Conf.Ocean.OffshoreArticEng.OMAE2014,2014:pp.1–9.doi:10.1115/OMAE2014-23542.

[12] C.K.Lee,Corrosionandwear-corrosionresistancepropertiesofelectrolessNi-PcoatingsonGFRPcompositeinwindturbineblades,Surf.CoatingsTechnol.202(2008)4868–4874.doi:10.1016/j.surfcoat.2008.04.079.

[13] M.VSeica,J.A.Packer,FRPmaterialsfortherehabilitationoftubularsteelstructures,forunderwaterapplications,Compos.Struct.80(2007)440–450.doi:10.1016/j.compstruct.2006.05.029.

[14] DNVGL-ST-C501Compositecomponents,2017.[15] DNVGL-ST-C502Offshoreconcretestructures,2017.[16] J.O.Okeniyi,C.A.Loto,A.P.I.Popoola,Morindalucidaeffectsonsteel-reinforcedconcretein

3.5%NaCl:Implicationsforcorrosion-protectionofwind-energystructuresinsaline/marineenvironments,EnergyProcedia.50(2014)421–428.doi:10.1016/j.egypro.2014.06.051.

[17] L.Castro-Santos,V.Diaz-Casas,Sensitivityanalysisoffloatingoffshorewindfarms,EnergyConvers.Manag.101(2015)271–277.doi:http://dx.doi.org/10.1016/j.enconman.2015.05.032.

[18] DNV-OS-J101DesignofOffshoreWindTurbineStructures,2014.[19] DNV-OS-J201OffshoreSubstationsforWindFarms,2013.[20] A.W.Momber,P.Plagemann,V.Stenzel,Performanceandintegrityofprotectivecoating

systemsforoffshorewindpowerstructuresafterthreeyearsunderoffshoresiteconditions,Renew.Energy.74(2015)606–617.doi:10.1016/j.renene.2014.08.047.

[21] O.Adedipe,F.Brennan,A.Kolios,Reviewofcorrosionfatigueinoffshorestructures:Presentstatusandchallengesintheoffshorewindsector,Renew.Sustain.EnergyRev.61(2016)141–154.doi:10.1016/j.rser.2016.02.017.

[22] M.Martinez-Luengo,A.Kolios,L.Wang,Structuralhealthmonitoringofoffshorewind


69

turbines:AreviewthroughtheStatisticalPatternRecognitionParadigm,Renew.Sustain.EnergyRev.64(2016)91–105.doi:10.1016/j.rser.2016.05.085.

[23] A.Colmenar-Santos,J.Perera-Perez,D.Borge-Diez,C.Depalacio-Rodrìguez,Offshorewindenergy:Areviewofthecurrentstatus,challengesandfuturedevelopmentinSpain,Renew.Sustain.EnergyRev.64(2016)1–18.doi:10.1016/j.rser.2016.05.087.

[24] D.ToddGriffith,N.C.Yoder,B.Resor,J.White,J.Paquette,Structuralhealthandprognosticsmanagementfortheenhancementofoffshorewindturbineoperationsandmaintenancestrategies,WindEnergy.17(2014)1737–1751.doi:10.1002/we.

[25] K.Mühlberg,CorrosionProtectionofOffshoreWindTurbines:AChallengefortheSteelBuilderandPaintApplicator,J.Prot.CoatingsLinings.(2010).

[26] DNVGL-RP-0416Corrosionprotectionforwindturbines,2016.[27] A.W.Momber,M.Irmer,N.Gluck,Performancecharacteristicsofprotectivecoatingsunder

low-temperatureoffshoreconditions.Part2:Surfacestatus,hoarfrostaccretionandmechanicalproperties,ColdReg.Sci.Technol.127(2016)109–114.doi:10.1016/j.coldregions.2016.04.009.

[28] A.A.Mikhailov,P.VStrekalov,Y.M.Panchenko,Atmosphericcorrosionofmetalsinregionsofcoldandextremelycoldclimate(areview),Prot.Met.44(2008)644–659.doi:10.1134/S0033173208070023.

[29] A.W.Momber,M.Irmer,N.Gluck,Performancecharacteristicsofprotectivecoatingsunderlow-temperatureoffshoreconditions.Part1:Experimentalset-upandcorrosionprotectionperformance,ColdReg.Sci.Technol.127(2016)76–82.doi:10.1016/j.coldregions.2016.03.013.

[30] InternationalstandardISO20340Paintsandvarnishes-Performancerequirementsforprotectivepaintsystemsforoffshoreandrelatedstructures,2009.doi:10.1109/IEEESTD.2007.4288250.

[31] NORSOKM-501SurfacePreparationandProtectiveCoating,1999.[32] S.VHainsworth,P.J.Kilgallon,Temperature-variantscratchdeformationresponseof

automotivepaintsystems,Prog.Org.Coatings.62(2008)21–27.doi:10.1016/j.porgcoat.2007.09.006.

[33] M.Lehmann,F.Karimpour,C.A.Goudey,P.T.Jacobson,OceanwaveenergyintheUnitedStates:Currentstatusandfutureperspectives,Renew.Sustain.EnergyRev.74(2017)1300–1313.doi:10.1016/j.rser.2016.11.101.

[34] R.Pelc,R.M.Fujita,Renewableenergyfromtheocean,Mar.Policy.26(2002)471–479.[35] SIOcean-WaveandTidalEnergyMarketDeploymentStrategyforEurope,2014.

http://www.si-ocean.eu/en/Market-Deployment/Market-Deployment-Strategy/.[36] R.J.K.Wood,A.S.Bahaj,S.R.Turnock,L.Wang,M.Evans,Tribologicaldesignconstraintsof

marinerenewableenergysystems,Philos.Trans.R.Soc.A.368(2010)4807–4827.doi:10.1098/rsta.2010.0192.

[37] C.G.Soares,Y.Garbatov,A.Zayed,G.Wang,Influenceofenvironmentalfactorsoncorrosionofshipstructuresinmarineatmosphere,Corros.Sci.51(2009)2014–2026.doi:10.1016/j.corsci.2009.05.028.

[38] G.Marsh,Tidalturbinesharnessthepowerofthesea,Reinf.Plast.(2004)44–47.[39] A.Theophanatos,J.Wolfram,Hydrodynamicloadingonmacro-roughenedcylindersof

variousaspectratios,J.OffshoreMech.Arct.Eng.111(1989)214–222.[40] J.P.Ault,TheUseofCoatingsforCorrosionControlonOffshoreOilStructures,J.Prot.


70

CoatingsLinings.11(2006)42–46.[41] P.Higgins,A.Foley,Theevolutionofoffshorewindpowerintheunitedkingdom,Renew.

Sustain.EnergyRev.37(2014)599–612.doi:10.1016/j.rser.2014.05.058.[42] S.S.Shreir,Corrosion1Metal/Environmentreactions,Newnes-Butterworths,1976.[43] P.Traverso,E.Canepa,Areviewofstudiesoncorrosionofmetalsandalloysindeep-sea

environment,OceanEng.87(2014)10–15.doi:10.1016/j.oceaneng.2014.05.003.[44] P.J.Cunat,CorrosionResistanceofStainlessSteelsinSoilsandinConcrete,in:PlenaryAys

Comm.StudyPipeCorros.Prot.Ceocor,Biarritz,2002:pp.1–12.[45] I.B.Beech,J.Sunner,Biocorrosion:Towardsunderstandinginteractionsbetweenbiofilmsand

metals,Curr.Opin.Biotechnol.15(2004)181–186.doi:10.1016/j.copbio.2004.05.001.[46] R.E.Melchers,R.J.Jeffrey,Acceleratedlowwatercorrosionofsteelpilinginharbours,Corros.

Eng.Sci.Technol.48(2013)496–505.[47] M.Stipanicev,F.Turcu,L.Esnault,O.Rosas,R.Basseguy,M.Sztyler,etal.,Corrosionof

carbonsteelbybacteriafromNorthSeaoffshoreseawaterinjectionsystems:Laboratoryinvestigation,Bioelectrochemistry.97(2014)76–88.doi:10.1016/j.bioelechem.2013.09.006.

[48] L.Lichtenstein,DNVGLStandardHarmonizationRecommendedPracticeonCorrosionProtectionofOffshoreWindFarms,in:EUROCORR2016,Adv.Link.Sci.toEng.Eur.Fed.Corros.EventNo.390,Montpellier,Fr.Vol.Proc.(USBDevice),2016:pp.1–9.

[49] D.M.Yebra,S.Kiil,K.Dam-Johansen,Antifoulingtechnology-Past,presentandfuturestepstowardsefficientandenvironmentallyfriendlyantifoulingcoatings,Prog.Org.Coatings.50(2004)75–104.doi:10.1016/j.porgcoat.2003.06.001.

[50] J.J.Cooney,R.J.Tang,Quantifyingeffectsofantifoulingpaintsonmicrobialbiofilmformation,MethodsEnzymol.310(1999)637–644.doi:http://dx.doi.org/10.1016/S0076-6879(99)10049-1.

[51] DNVGL-RP-C203FatigueDesignofOffshoreSteelStructures,2016.[52] InternationalStandardENISO12944-1Paintsandvarnishes—Corrosionprotectionofsteel

structuresbyprotectivepaintsystems—Part1:Generalintroduction,(1998).[53] W.Dong,T.Moan,Z.Gao,Fatiguereliabilityanalysisofthejacketsupportstructurefor

offshorewindturbineconsideringtheeffectofcorrosionandinspection,Reliab.Eng.Syst.Saf.106(2012)11–27.doi:10.1016/j.ress.2012.06.011.

[54] TechnicalReport2006-3496DetNorskeVeritas.[55] P.Morada,Heavysteelplatesforeconomicalconstructionandoffshorestructures,

www.oakleysteel.co.uk,2015.[56] D.Buckthorpe,C.Fazio,L.Heikinheimo,W.Hoffelner,J.vanderLaan,K.F.Nilsson,etal.,

ScientificassessmentinsupportoftheMaterialsRoadmapenablingLowCarbonEnergyTechnologies,JRCReport,2011.doi:10.2790/41784.

[57] OffshoreWindmakingaMaterialDifference,ECJRC,2011.[58] J.Billingham,J.VSharp,J.Spurrier,P.J.Kilgallon,Reviewoftheperformanceofhighstrength

steelsusedoffshore,2003.[59] Cor-Tenstaalplaten,productoverzicht,ThyssenKruppChristonbrochure.[60] AMLoCor,sheetpiling.arcelormittal.com.[61] P.E.Francis,CATHODICPROTECTION.[62] BRITISHSTAINLESSSTEELASSOCIATION,accessedonJanuary2018.

https://www.bssa.org.uk/topics.php?article=111.[63] CorrosionResistanceofStainlessSteel,www.worldstainless.org.


71

[64] CalculationofCorrosionResistanceFactor(CRF),accessedonJanuary2018:http://www.aalco.co.uk/datasheets/Stainless-Steel-Stainless-Steel-for-Structural-Applications_417.ashx.

[65] CORROSIONRESISTANCEOFSTAINLESSSTEELS,SupportingpresentationforlecturersofArchitecture/CivilEngineering,accessedonJanuary2018:http://www.imoa.info/download_files/stainless-steel/issf/educational/Module_05_Corrosion_Resistance_of_Stainless.

[66] Sea-CureTechnicalbrochure,www.plymouth.com.[67] MIC,SandvikMaterialTechnology.[68] P.Ganesan,C.M.Renteria,J.R.Crum,VersatileCorrosionResistanceofINCONELAlloy625in

VariousAqueousandChemicalProcessingEnvironments,Superalloys.718(1991)663–680.[69] Alight,durableandeconomicalmaterialfortheoffshoreindustry,Sh.Offshore.6(2013)72–

76.[70] Titaniumforoffshoreandmarineapplications,www.titanium.org.sg.[71] M.R.S.JGBanker,TitaniumforSecondaryMarineStructures,Titan.ShipboardAppl.Int.

Titan.Assoc.(1996)1–10.[72] OffshoreOilandGasPlatformsandProcessing,www.copper.org.[73] CopperNickelAlloysinOffshoreOilandGasPlatformsandProcessing,www.copper.org.[74] E.Romhanji,M.Popovic,ProblemsandprospectofAl-Mgalloysapplicationinmarine

constructions,MJoMMetal.-J.Metall.[75] DNVGL-RP-B401CathodicProtectionDesign,2017.[76] K.RiggsLarsen,UsingPipelineCoatingswithCathodicProtection,Mater.Perform.(2016).[77] A.Momber,P.Plagemann,InvestigatingCorrosionProtectionofOffshoreWindTowers-Part

1:Backgroundandtestprogram,CoatingsandLinings.25(2008)30–43.[78] R.Jain,M.Wasnik,A.Sharma,M.KrBhadu,T.K.Rout,A.S.Khanna,DevelopmentofEpoxy

BasedSurfaceTolerantCoatingImprovisedwithZnDustandMIOonSteelSurfaces,J.Coatings.(2014)1–15.doi:10.1155/2014/574028.

[79] S.N.Roselli,B.delAmo,R.O.Carbonari,A.R.DiSarli,R.Romagnoli,Paintingrustedsteel:Theroleofaluminumphosphosilicate,Corros.Sci.74(2013)194–205.doi:10.1016/j.corsci.2013.04.043.

[80] D.D.N.Singh,D.Bhattacharya,Performanceandmechanismofactionofself-primingorganiccoatingonoxidecoveredsteelsurface,Prog.Org.Coatings.68(2010)62–69.doi:10.1016/j.porgcoat.2009.08.017.

[81] R.Norsoworthy,CoatingsUsedinConjuctionWithCathodicProtection–shieldingvsnon-shieldingpipelinecoatings,in:Int.NACECorros.2009-Conf.Expo,Pap.No.09043,2009:pp.1–11.

[82] Guidestogoodpracticeincorrosioncontrol-CoatingfortheProtectionSteelwork,NationalPhysicalLaboratory(NPL),2000.

[83] A.Black,P.Nielsen,Durabilityofcoatingrepairsystemsforoffshoreservice,FORCETechnology.

[84] K.Mühlberg,CorrosionProtectionofOffshoreWindTurbines,2010.[85] https://www.steelconstruction.info/Surface_preparation,Surf.Prep.[86] O.Knudsen,BenefitsofZincThermalSpraying,www.zinkinfo.org.[87] P.Fauchais,A.Vardelle,ThermalSprayedCoatingsUsedAgainstCorrosionandCorrosive

Wear,in:Adv.PlasmaSprayAppl.,Dr.Hamid,2012:pp.3–39.


72

[88] N.Ce,S.Paul,ThermallySprayedAluminumCoatingsfortheProtectionofSubseaRisersandPipelinesCarryingHotFluids,Coatings.(2016)1–11.doi:10.3390/coatings6040058.

[89] Hotdipgalvanizing,BrochureofZinkinfoBenelux,www.zinkinfobenelux.com.[90] CourtesyofBekaertNV,BartAllaert,Belgium.(www.bekaert.com).[91] B.Allaert,F.Rentmeister,Advancedcorrosionprotectionofstructuraltensionmembers,in:

Proc.FootBridg.2017,2017.[92] Contactwithexpert:WalterLauwerens,Sirris,November2017.[93] COLDSPRAYING:https://www.twi-global.com/capabilities/materials-and-corrosion-

management/surface-engineering-and-advanced-coatings/cold-spraying/.[94] https://3dprintingindustry.com/news/ge-combines-cold-spray-robotics-3d-print-bigger-

better-objects-126363/.[95] HYDROBONDProject:http://hydro-bond.eu/.[96] O.Doble,G.Pryde,UseofThermallySprayedAluminumintheNorwegianOffshoreIndustry,

Prot.CoatingsEur.2(1997).[97] ThermalSprayedAluminum(TSA)CorrosionProtectionSystemfortheOffshoreOilandGas

Industry,PraxiarSurfaceTechnologies,2010.[98] ThermalSprayAluminiumCoating,www.canadoilgroup.com.[99] F.Goodwin,Corrwind:ALife-cyclecostcomparator,www.zinc.org.[100] Consortiumtacklesbarrierstoadoptionofgame-changingcorrosioncontrolconcept:

http://www.owjonline.com/news/view,consortium-tackles-barriers-to-adoption-of-gamechanging-corrosion-control-concept_48693.htm.(2017).

[101] D.Harvey,IMPCOAT-ImprovedSplashandTidalZoneCoatingsfora40-yearDesignLife:https://www.twi-global.com/news-events/connect/2013/connect-november-december-2013/impcoat-improved-splash-and-tidal-zone-coatings-for-a-40-year-design-life/(2013).

[102] Arkona’sCorrosionProtectionGetsInnovationoftheYearAccolade:https://www.offshorewind.biz/2017/12/01/arkonas-corrosion-protection-gets-innovation-of-the-year-accolade/.

[103] E.ONKeepsArkonaFoundationsRustFreewithTSACoating:https://www.offshorewind.biz/2017/08/21/e-on-keeps-arkona-foundations-rust-free-with-tsa-coating/.

[104] S.Jeddere-Fisher,OffshorewindinChina:https://energyhub.theiet.org/users/77393-sam-jeddere-fisher/posts/28787-offshore-wind-in-china,(2017).

[105] Personalcontactswithexperts:DiegoCocco,LICenergy,CROWN-project.November-December2017.

[106] Personalcontactswithexperts:HenryBegg,TWI.January2017.[107] D.Cocco,AnumericalmodelforTSAlabcorrosiontesting:licenergy.co.uk.[108] R.Parkinson,Propertiesandapplicationsofelectrolessnickel,NickelDev.Inst.[109] Offshorewindfarms–successfulcorrosionprotectioncombinedwitheffectivequality

management,J.OdProt.Coatings&Linings.(2016).[110] BritishStandardsInstitution,nternationalStandardENISO12944-5Paintsandvarnishes—

Corrosionprotectionofsteelstructuresbyprotectivepaintsystems—Part5:Protectivepaintsystems,3(1998).

[111] Contactwithexperts:FrederikVanHaute,EvelyneDemol.Acotec(www.acotec.be).[112] M.Broekaert,Polyureaspraycoatings,Technol.LatestDev.Http//www.Huntsman.

Com/portal/page/portal/polyurethanes.(2002).


73

[113] DurabilityandDesignMagazine-InterviewwithDudleyPrimeauxofVersaFlexIncorporated.“EverythingYouNeedtoKnowaboutPolyureain3Minutes”.YouTubevideo,accessedon01/04/2017.

[114] R.LiningsAustralasia,FlexibleCoatingsHelpProtectMarineStructures:http://www.materialsperformance.com/articles/coating-linings/2016/05/flexible-coatings-help-protect-marine-structures,Mater.Perform.(2016).

[115] C.Godinich,Polyurea:amarketoverview,Eur.CoatingsJ.(2000)64–73.[116] PortablePlasticPowderCoatings:https://www.pcimag.com/articles/94755-portable-plastic-

powder-coatings,(2007).[117] J.Loustaunau,D.Horton,Fieldappliedpowdercoatingsarereplacinghighperformanceliquid

coatings,in:PowderCoat.’94Proc.,1994:pp.289–307.[118] BeyondPowderCoating!:http://resodyncoatings.com/.[119] Powdercoatingoffshore:http://www.coating.co.uk/powder-coating-offshore/.[120] S.B.Lyon,R.Bingham,D.J.Mills,Advancesincorrosionprotectionbyorganiccoatings:What

weknowandwhatwewouldliketoknow,Prog.Org.Coatings.102(2017)2–7.doi:10.1016/j.porgcoat.2016.04.030.

[121] E.Almeida,T.C.Diamantino,O.deSousa,Marinepaints:Theparticularcaseofantifoulingpaints,Prog.Org.Coatings.59(2007)2–20.doi:10.1016/j.porgcoat.2007.01.017.

[122] M.R.Detty,R.Ciriminna,F.VBright,M.Pagliaro,XerogelCoatingsProducedbytheSol-GelProcessasAnti-Fouling,Fouling-ReleaseSurfaces:FromLabBenchtoCommercialReality,ChemNanoMat.1(2015)148–154.doi:10.1002/cnma.201500056.

[123] D.Carteau,K.Vallée-Réhel,I.Linossier,F.Quiniou,R.Davy,C.Compère,etal.,Developmentofenvironmentallyfriendlyantifoulingpaintsusingbiodegradablepolymerandlowertoxicsubstances,Prog.Org.Coatings.77(2014)485–493.

[124] F.Azemar,F.Faÿ,K.Réhel,I.Linossier,Developmentofhybridantifoulingpaints,Prog.Org.Coatings.87(2015)10–19.

[125] S.Palanichamy,G.Subramanian,Antifoulingpropertiesofmarinebacteriocinincorporatedepoxybasedpaint,Prog.Org.Coatings.103(2017)33–39.

[126] M.R.Detty,R.Ciriminna,F.VBright,M.Pagliaro,Environmentallybenignsol–gelantifoulingandfoul-releasingcoatings,Acc.Chem.Res.47(2014)678–687.

[127] L.Telegdi,L.Trif,L.Románszki,Smartanti-biofoulingcompositecoatingsfornavalapplications,(2016).

[128] V.Torabinejad,M.Aliofkhazraei,S.Assareh,M.H.Allahyarzadeh,A.S.Rouhaghdam,ElectrodepositionofNi-Fealloys,composites,andnanocoatings–Areview,J.AlloysCompd.691(2017)841–859.

[129] S.H.Cho,S.R.White,P.VBraun,Self-healingpolymercoatings,Adv.Mater.21(2009)645–649.

[130] J.Carneiro,J.Tedim,S.C.M.Fernandes,C.S.R.Freire,A.J.D.Silvestre,A.Gandini,etal.,Chitosan-basedself-healingprotectivecoatingsdopedwithceriumnitrateforcorrosionprotectionofaluminumalloy2024,Prog.Org.Coatings.75(2012)8–13.

[131] G.Yoganandan,K.P.Premkumar,J.N.Balaraju,Evaluationofcorrosionresistanceandself-healingbehaviorofzirconium–ceriumconversioncoatingdevelopedonAA2024alloy,Surf.CoatingsTechnol.270(2015)249–258.

[132] K.K.Alaneme,M.O.Bodunrin,Self-healingusingmetallicmaterialsystems–Areview,Appl.Mater.Today.6(2017)9–15.


74

[133] R.B.Figueira,I.R.Fontinha,C.J.R.Silva,E.VPereira,HybridSol-GelCoatings:SmartandGreenMaterialsforCorrosionMitigation,Coatings.6(2016)12.

[134] R.B.Figueira,C.J.R.Silva,E.VPereira,Organic–inorganichybridsol–gelcoatingsformetalcorrosionprotection:areviewofrecentprogress,J.CoatingsTechnol.Res.12(2015)1–35.

[135] R.Frei,R.McWilliam,B.Derrick,A.Purvis,A.Tiwari,G.Serugendo,Self-healingandself-repairingtechnologies,Int.J.Adv.Manuf.Technol.69(2013)1033–1061.

[136] A.Mostafaei,F.Nasirpouri,Epoxy/polyaniline–ZnOnanorodshybridnanocompositecoatings:Synthesis,characterizationandcorrosionprotectionperformanceofconductingpaints,Prog.Org.Coatings.77(2014)146–159.

[137] P.Buskens,M.Wouters,C.Rentrop,Z.Vroon,Abriefreviewofenvironmentallybenignantifoulingandfoul-releasecoatingsformarineapplications,J.CoatingsTechnol.Res.10(2013)29–36.

[138] J.O.Iroh,D.Rajamani,Synthesisandstructureofenvironmentallyfriendlyhybridclay/organosilanenanocompositecoatings,J.Inorg.Organomet.Polym.Mater.22(2012)595–603.

[139] Phosphating,apowdercoatingpretreatment,www.powder-coater.com.[140] HenkelIndustrialSolutionsSelectorGuide-Adhesives,Sealants,Metalpretreatments,

COatings,Cleanersandmetalworkingfluids,http://www.henkel-adhesives.com/,2009.[141] www.phosphating.net.[142] R.Talbert,Pretreatments :theNextGeneration,Prod.Finish.Mag.1(2007).[143] EonCoatCrAlloyedCoatingforSteel.CommercialBrochure:

https://www.spiperformancecoatings.com/wp-content/uploads/2015/11/EonCoat-CR-Steel-Brochure-V4.1-PRINT.pdf.

[144] Corrosion:eatingoilindustryfrominsideout.https://pgjonline.com/2013/10/12/corrosion-eating-oil-industry-from-inside-out/,PipelineGasJ.(2013).

[145] D.Williams,Anewapproachtocorrosionmaydramaticallyextendaging,gas,water,electricity,aswellastelecomfacilityinfrastracture,Util.4Constr.(2017).

[146] Contactswithexpert:PaulCutter,SPiPerformanceCoatings.December2017-January2018(www.eoncoat.com).

[147] Contactwithexpert:ChristianVerbrugghe,ZINGA.January2018.(www.zinga.eu).[148] Personalcontactswithexperts:Prof.DeBuysserandDr.Watté,UniversityofGhent,Belgium,

December2017.[149] ProtectionofBoltsandNuts,www.boltprotection.com/en.[150] Acloserlookatriserclampprotection,rustisdead.blogspot.be/2015/07/a-closer-look-at-

riser-clamp-protection.html.[151] Lookingafteroffshorewindturbineinvestments,cotes.com/products/cotes-wind-

dehumidifiers.[152] A.Bjørgum,O.Ø.Knudsen,Corrosionprotectionofoffshorewindturbines,WindPowerR&D

Semin.(2010).http://www.sintef.no/project/Nowitech/Wind_presentations/Bjørgum,A.,SINTEFMC.pdfAccessed12January2011.

[153] H.Shi,F.Liu,E.H.Han,Thecorrosionbehaviorofzinc-richpaintsonsteel:Influenceofsimulatedsaltsdepositioninanoffshoreatmosphereatthesteel/paintinterface,Surf.CoatingsTechnol.205(2011)4532–4539.doi:10.1016/j.surfcoat.2011.03.118.

[154] K.RiggsLarsen,RetrofittingWindTurbineMonopileswithCathodicProtection,http://www.materialsperformance.com/articles/cathodic-protection/2017/10/retrofitting-


75

wind-turbine-monopiles-with-cathodic-protection,Mater.Perform.(2017).[155] M.Stephenson,CarbonTrust’sOffshoreWindAcceleratorlaunchessubseainspection

competition:https://www.carbontrust.com/news/2017/07/owa-launches-subsea-inspection-competition/,CarbonTrust.(2017).

[156] DNVGL-OS-C401Fabricationandtestingofoffshorestructures,2015.[157] DNVGL-OS-E301Positionmooring,2015.[158] DNVGL-SE-047Riskbasedverificationofoffshorestructures,2017.[159] DNV-OSS-121ClassificationBasedonPerformanceCriteriaDeterminedfromRiskAssessment

Methodology,2011.[160] DNVGL-OS-D101Marineandmachinerysystemsandequipment,2015.[161] DNVGL-OS-D201Electricalinstallations,2015.[162] DNVClassificationnotesno.30.6-Structuralreliabilityanalysisofmarinestructures,1992.[163] DNVGL-SE-0420Certificationofmeteorologicalmasts,2015.[164] DNVGL-ST-0145Offshoresubstations,2016.[165] DNV-RP-J101UseofRemoteSensingforWindEnergyAssessments,2011.[166] DNVGL-SE-0073ProjectcertificationofwindfarmsaccordingtoIEC61400-22,2014.[167] DNVGL-SE-0074TypeandcomponentcertificationofwindturbinesaccordingtoIEC61400-

22,2014.[168] DNVGL-SE-0190Projectcertificationofwindpowerplants,2015.[169] DNVGL-SE-0441Typeandcomponentcertificationofwindturbines,2016.[170] DNVGL-ST-0126Supportstructuresforwindturbines,2016.[171] DNVGL-ST-0361Machineryforwindturbines,(2016)1–106.[172] EN50308Windturbines-Protectivemeasures-Requirementsfordesign,operationand

maintenance,2004.[173] DNVGL-ST-0164Tidalturbines,2015.[174] DNV-OSS-312Certificationoftidalandwaveenergyconverters,2008.[175] DNVGL-SE-0163Certificationoftidalturbinesandarrays,2015.[176] IEC/TS62600-1:Marineenergy-Wave,tidalandotherwatercurrentconverters-Part1:

Terminology,2011.[177] IEC/TS62600-100:Marineenergy-Wave,tidalandotherwatercurrentconverters-Part100:

Electricityproducingwaveenergyconverters-Powerperformanceassessment,2012.[178] IEC/TS62600-101:Marineenergy-Wave,tidalandotherwatercurrentconverters-Part101:

Waveenergyresourceassessmentandcharacterization,(2015).[179] IEC/TS62600-200:Marineenergy-Wave,tidalandotherwatercurrentconverters-Part200:

Electricityproducingtidalenergyconverters-Powerperformanceassessment,(2013).[180] IEC/TS62600-201:Marineenergy-Wave,tidalandotherwatercurrentconverters-Tidal

energyresourceassessmentandcharacterization,(2015).[181] IEC/TS62600-10:Marineenergy-Wave,tidalandotherwatercurrentconverters-Part10:

Assessmentofmooringsystemformarineenergyconverters(MECs),2015.[182] EN1090-1Executionofsteelstructuresandaluminiumstructures-Part1:Requirementsfor

conformityassessmentofstructuralcomponents,2011.[183] EN1090-2Executionofsteelstructuresandaluminiumstructures-Part2:Technical

requirementsforsteelstructures,2011.[184] EN10204Metallicproducts-Typesofinspectiondocuments,2004.[185] EN10225Weldablestructuralsteelsforfixedoffshorestructures—Technicaldelivery


76

conditions,2009.[186] EN12473Generalprinciplesofcathodicprotectioninseawater,2014.[187] EN12495Cathodicprotectionforfixedsteeloffshorestructures,2000.[188] InternationalStandardENISO14713Zinccoatings—Guidelinesandrecommendationsfor

theprotectionagainstcorrosionofironandsteelinstructures-Part1:Generalprinciplesofdesignandcorrosionresistance,2017.

[189] InternationalStandardENISO8501Preparationofsteelsubstratesbeforeapplicationofpaintsandrelatedproducts-Visualassessmentofsurfacecleanliness,2006.doi:10.1109/IEEESTD.2007.4288250.

[190] InternationalStandardENISO8502Preparationofsteelsubstratesbeforeapplicationofpaintsandrelatedproducts-Testsfortheassessmentofsurfacecleanliness,2017.

[191] InternationalStandardENISO8503Preparationofsteelsubstratesbeforeapplicationofpaintsandrelatedproducts—Surfaceroughnesscharacteristicsofblast-cleanedsteelsubstrates,2017.

[192] DNVGL-OS-C101Designofoffshoresteelstructures,general-LRFDmethod,2016.


77

WHAT IS NESSIEThe NeSSIE project will tap into the existing knowledge of anti-corrosion technology / novel materials solutions in the maritime sector supply chain to develop demonstration projects for offshore renewables in the North Sea.The corrosion solutions, when developed and commercialised, will provide global growth and job creation opportunities across the European Union.

NeSSIE WILL• Develop and scope 3 offshore renewable energy

demostration projects relating to corrosion issues by drawing on existing maritime supply chain expertise.

• Accelerate the deployment and cost reduction of wave, tidal and offshore wind devices.

• Support the demonstration projects developed to access public and private investment.

• Create economic opportunities in the North Sea Basin.

FURTHER INFORMATION:

www.nessieproject.com

Innovative corrosion solutionsand new materials in wave,tidal and offshorewind energy sectors

3 DEMONSTRATIONCases

TOTAL BUDGET860.000 €

8 PARTNERS5 Countries

DURATION2017 - 2019

All partners are members of the Vanguard Initiative and involved in

the pilot “Advanced Manufacturing for Energy Related Applications

in Harsh Environment”

THE PROJECTIN NUMBERS

KEY ACTIVITIES

Assessment of innovative corrosion solutions in wave, tidal and offshore wind, through:• Technology roadmap• Supply chain analysis• Supporting consortia to scope and develop investable

demonstration projects

EXPECTED IMPACT• Greater collaboration within the value chain • Market focused demonstration projects on marine

renewables on offshore renewable• New opportunities for SMEs to create jobs and growth

in the Blue Economy

EU ADDED VALUE• Building a new model applicable to other cluster

partnerships• Fostering the transfer of technology solutions

to new sectors• Strengthening regional cooperation through

the Vanguard Initiative

Lead PartnerSCOTTISH ENTERPRISE

Mark Georgeson, Project ManagerTel. +44 (0) 300 013.38.58

Email [email protected]

SIRRIS - BelgiumBart TeerlinckTel. +32 498.919.394Email [email protected]

ASTER Società Consortileper Azioni - ItalyStefano ValentiniTel. +39 051 639.80.99Email [email protected]

LOMBARDY ENERGYCLEANTECH CLUSTER - ItalyCarmen DisantoTel. +39 02 58370810Email [email protected]

THE UNIVERSITY OF EDINBURGH - ScotlandHenry JeffreyTel. +44 (0)131 650.55.94Email [email protected]

CLÚSTER DE ENERGÍADEL PAÍS VASCO - SpainMarcos Suarez GarciaTel. +34 94 424.02.11Email [email protected]

FUNDACIÓN ASTURIANADE LA ENERGÍA - SpainIndalecio González FernándezTel. +34 985 467.180Email [email protected]

SVENSKT MARINTEKNISKT FORUM - SwedenJanne RydhTel. +46 70 052.47.50Email [email protected]

IF YOU WANT TO KNOW MORE ABOUT THE NESSIE PROJECT

PLEASE CONTACT

Other contacts - Project Partners

Learn Connect Demonstrate Commercialise

www.nessieproject.com

nessieproject.com

State of the Art Study on Materials and Solutions against...

Documents

Transcript of State of the Art Study on Materials and Solutions against...