Durability of wind turbine materials in off-

shore environments

Paul Hogg

University of Manchester

What is the problem?

Materials degrade? Why

Thermodynamics

• Most materials are not in thermodynamic equilibrium with their surroundings

Gibbs free energy (G) is not at a minimum

Remember:

G= U + PV- TS Free energy = internal energy + work minus entropy

Reactions will occur if D G = -ve

Examples of what could cause reductions in G

• Chemical reactions – reduction in U

• Physical reactions – increase in TS

• Fracture – reduction in PV ( reduced strain energy)

And combinations of the above

Blade structure

Main shaft bearing Gearbox Brake

Control unit Cooling and Ventilation system

Tower, hub, stairs, doors Transition piece Boat landing

monopile

Key issues

• Moisture ingress into electrical/mechanical systems

• Corrosion of metalwork (tower, monopile etc)

• Weathering of composite blades

(Courtesy Sintef)

Nacelles

The approach is to try and eliminate the external marine environment as much as is feasible, rather than producing all components from corrosion resistant materials. This allows commonality between on-shore and off-shore systems ( mechanical and electrical) but requires well designed nacelles and efficient internal environmental control

Siemens SWT3.6 107

Corrosion Protection of Wind Turbine Electrical System The turbine nacelle is designed to inhibit the ingress of water but the environment implies strong winds with salt bearing moisture. Therefore we need additional measures to protect the electrical and instrument control systems. In particular Power Generation Units Electrical Distribution Panel, Instrument and Electrical Isolation and Junction Boxes Hydraulic Pump Drive Motors

Synthetic gear grease in nacelles and elsewhere

Power Generation Units, Electrical and Instrument Distribution Panels, Electrical motors. This may be protected from condensation and dampness by the fitting of anti-condensation heaters or de-humidifiers (keep humidity below 60% threshold for corrosion) The use of a Vapour Corrosion Inhibitor (VCI) is effective to protect the inside of electrical and instrument enclosures, coil windings, contacts, and transformers . They are supplied in self-adhesive strips which emit a vapour which forms a microscopic layer of anticorrosion protection on the metal surface.

Mechancial systems –gears shafts etc Pack with grease where feasible to provide lubrication and a barrier to moisture

Corrosion inhibitors – anodic and cathodic

Useful in closed environments

Corrosion inhibitors – adsorption

Splash zone problems (its wet)

Metallic components suffer from electrochemical corrosion

• Wet Corrosion is the degradation of a metal due to its electrochemical reaction with its environment. It occurs by loss of electron oxidation reaction

What is wet corrosion?

M Mz+ + ze- (z, number of electron = 1,2 or 3)

but different from ‘dry’ oxidation

when a metal reacts with oxygen to form a metal oxide

4Cu + O2 2Cu2O

1. Medium is conductive. Therefore transport of ionic reacting species is rapid 2. Water may form complex ions with metal ions. Hydration energy changes the basic thermodynamics of reaction. 3. The formation of reaction product may not occur at the reaction site.

It is different because:

• A pure metal immersed in water will react to release in solution hydrated cations. The electrons stay in the metal

M ↔ M+ (H2O) + e- H2O

Metal/electrolyte interface – Basic thermodynamics

Metal in the electrolyte has developed an Electrochemical potential

Met

al

+ - +

+

+

+

+

+ +

+ -

+

+

+

+

+

+ + +

+

+ -

-

-

- -

+

+

+

-

-

Helmholtz Layer (1)

Gouy-Chapman Layer (2) Bulk

solution

- - - - - - - - - -

Met

al

Distance 0

(1) (2)

Potential

e- e-

e- e-

M+

M

M+

M+

M+

M+

Bu

lk s

olu

tio

n

Ions in solution are organised in a Double Layer

An equilibrium is reached: M M+ + e- and M+ + e- M

Basic wet cell corrosion • If an other electrode with a different electrochemical potential is added to the

system, further corrosion will occur.

Zn Cu Zn Zn2+ + 2e- (oxidation - anode)

Cu2+ + 2e- Cu (reduction - cathode)

The anode will be corroded and the cathode protected. Sacrificial anode method

Practically the potential of metals is determined versus a Reference Electrode. The reference is the Standard Hydrogen Electrode (SHE) : ½ cell H+/H2 at 298 K and concentration of 1 mol.L-1

Li+ + e- = Li -3.04 Na+ + e- = Na -2.71 Mg+2 + 2e- = Mg -2.38 Al+3 + 3e- = Al -1.66 Zn+2 + 2e- = Zn -0.76 Cr+3 + 3e- = Cr -0.74 Fe+2 + 2e- = Fe -0.41 Cd+2 + 2e- = Cd -0.40 Ni+2 + 2e- = Ni -0.23 Sn+2 + 2e- = Sn -0.14 Pb+2 + 2e- = Pb -0.13 Fe+3 + 3e- = Fe -0.04 2H+ + e- = H2 -0.00 Sn+4 + 2e- = Sn+2 +0.15 Cu+2 + e- = Cu+ +0.16 Cu+2 + 2e- = Cu +0.34 I2(s) + 2e- = 2I- +0.54 Fe+3 + e- = Fe+2 +0.77 Ag+ + e- = Ag +0.80 Hg+2 + 2e- = Hg +0.85

Standard reduction potentials

Electrode reaction Eo (V vs SHE)

In practice, reference electrodes such as mercury (I) chloride (Hg2Cl2) and silver/silver chloride are used.

• In practice, on the same piece of metal anodic and cathodic reactions can take place

Consider iron corrosion: Fe + ½ O2 + H2O Fe(OH)2

Fe Fe2+ + 2e- anodic reaction (- 0.44 V)

½ O2 + H2O + 2e- 2(OH-) cathodic reaction (+ 0.44 V)

Eo(cell) = -0.84 V

e-

e-

e-

Iron

Fe

Fe2+

Fe2+

Fe2+

Icorr

O2 + H2O

OH-

- A corrosion cell is established

-Reference to the electrochemical series indicates the potential under equilibrium condition

-But when both anode and cathode are short circuited so that a current flows, equilibrium is destroyed

-During general corrosion, anodic and cathodic sites alternate. But under certain conditions, areas may adopt a fixed characteristic.

‘Auto’ corrosion

Corrosion in semi-immersed conditions – Salt drop experiment

The reaction: Fe + ½ O2 + H2O Fe(OH)2 . Involves O2

Fe Fe2+ + 2e- anodic reaction does not involve O2

½ O2 + H2O + 2e- 2(OH-) cathodic reaction involves O2

Initially

rust

Finally

Blue: potassium ferricyanide indicator of Fe2+

Pink: phenolphtalein indicator of OH-

Reaction occurs mainly at the edge of the drop where oxygen from the air is present (anode side corrode, O2 access is difficult)

rust

Short path for O2 O2 O2 OH-

OH-

Fe2+

Fe e- e-

long path for O2

• In non equilibrium condition the metal potential differs from the equilibrium potential by an amount known as the polarisation (, = E-Ecorr).

Kinetic of corrosion reactions – Determination of Icorr and Ecorr

• In practice, it is not possible to measure Icorr, but it is possible to measure a function of I. If the potential is moved from Eeq = E corr in the anodic direction (more negative) we can measure the current and vice versa in the cathodic direction

e- e-

e- e-

M+

A

M+

M+

M+

M+

e-

e-

M+

B

M+

electrons

current Po

ten

tial

EB

EA uncoupled

uncoupled

AB (coupled)

cathodic

anodic

Ecorr

Icorr

Evans diagram

They are 8 type of electrochemical corrosion: 1- General corrosion 2- Galvanic corrosion 3- Crevice corrosion 4- Pitting corrosion 5- Intergranular corrosion 6- Selective leaching 7- Erosion corrosion 8- Stress corrosion/corrosion fatigue

General corrosion is the situation where a uniform rate of corrosion is experienced over the surface of the metal with sites locally alternating in character between anodic and cathodic

The different electrochemical corrosion phenomena

• Corrosion resulting when two dissimilar metals are in electrical contact.

Galvanic corrosion

When two metallic materials are in contact in the presence of a corrosive electrolyte, They will form a basic wet corrosion cell. Corrosion will increase on the least noble material (the anode - base material ) and decrease on the noblest (the cathode – noble material ). The increase in corrosion is called galvanic corrosion.

Driving force = potential established between the 2 metals

The more reactive metal undergoes accelerated corrosion

Factors influencing Galvanic corrosion rates

• Distance effect

Accelerated corrosion due to galvanic effects usually occurs near the junction

Resistance of solution and metals may reduce current away from the corrosion point. Therefore for poor electrolytes (e.g water), galvanic corrosion may be very localised (e.g sharp groove).

• Area effect

For a given cell (I.e. a given current) the current density is greater for a small electrode than a large electrode.

Current density = corrosion rate

Therefore:

Large cathode / small anode is a dangerous combination Total corrosion of the anode

Large anode / small cathode is better galvanic corrosion may be insignificant

Positive uses of Galvanic corrosion

• Can use galvanic couple to protect metal component

Examples

a) galvanised iron - Zn coating on iron

b) sacrificial anodes - Zn/Mg coupled to steel structure

c) impressed current to oppose corrosion current

Crevice

Intensive localised corrosion frequently occurs within crevices or shielded areas Accelerated corrosion in confined space (gap between two surfaces)

Usually involves small volume of stagnant solutions.

Reasons for accelerated corrosion:

• extra time of wetness in wet/dry cycles

• more corrosive environment -concentration of salts in capillaries

• small anode, large cathode

• difference in oxygen concentration gives potential difference

Crevice

Crevice - Mechanism of formation – concentration cell

Inside the Crevice Outside the Crevice

O2 depleted O2 readily available

Anodic reaction cathodic reaction

High [Mz+] concentration High [OH-] concentration

Overall positive Overall negative

High [Cl-] concentration Normal [Cl-] concentration

Cl- and OH- diffuse into the crevice to maintain a minimum potential energy. =>Metal chloride is formed. Hydrolysis of metal chloride lowers pH=> MCln + nH2O = M(OH)n + nHCl

Inside the Crevice Outside the Crevice

Low pH Normal pH

High [Cl-] concentration Normal [Cl-] concentration

High [Mz+] concentration Low [Mz+] concentration

Pitting corrosion

Characteristics – extremely localised attack producing holes in metal

Pits usually grow in the direction of gravity - often require a long initiation period

Pitting corrosion – Concentration cell

The reaction of pitting is:

• autocatalytic • stimulated by Cl-

• its mechanism is very similar to crevice corrosion (centre of pitting O2 depleted)

Gravity effect = dense concentrated solution necessary keep reaction proceeding

Pits cathodically protect the rest of the metal

Key facts

• General corrosion is not (really) a problem – we can calculate the rate (of material removal) and allow for it.

• Unexpected and localised corrosion is a big problem – all of the material removal sustained by a cathodic reaction is focused on one spot –rupture/failure/leakage etc. – Big cathode-small anode – Galvanic corrosion – Crevice corrosion – Pitting corrosion

Atmospheric Area -The area above the Splash Zone which includes the turbine tower

The submerged area

The Splash Zone

Corrosion Protection of Wind Turbine Support Structures There are three distinct areas of the support structure which usually consists of a monopod and tower that require protection

Fouling can induce localised corrosion

Courtesy TNO

Corrosion prevention: A summary of the main approaches used in the control

of corrosion.

Galvanic corrosion: The galvanic current, Ig, flows as electronic current through the metallic contact between the base and noble metals, and as ionic current through the

electrolyte in which they are both immersed

Sacrificial surface coatings: Zinc on steel is anodic and even at a defect where the basis metal is exposed the sacrificial coating will corrode, leaving the cathodic steel protected. In the atmosphere the corrosive surface films of moisture may be very thin, and protective current cannot flow very much more than 1 mm or so over a bare surface from the edge of the zinc coating on “galvanised” steel.

Noble surface coatings: Nickel is the most commonly used noble coating on steel, being kept bright by a thin layer of chromium on its outer surface. The coating has good corrosion resistance but at any pore or defect the iron underneath acts as an anode in a galvanic cell, and the coating is undercut

Passivity and film breakdown: A surface film, often an oxide or hydrated oxide of the metal, can give very efficient protection by conferring passivity. Aggressive agents such as chloride ions can cause localised breakdown and rapid corrosion by pitting

Cathodic protection: A system using sacrificial anodes. Special aluminium alloys are also often used as anodes. The electrons shown as travelling from the anodes to the steel structure pass

along the metallic connection which attaches the anode, and not through the seawater electrolyte.

Sacrificial Anodes on ships

Wave buffeting removes protective paints

Atmospheric Area

This area is above the splash zone and includes the turbine tower. It is normally protected by the following applications, 1. Zinc primer applied preferably as a thermal spray 2. A silicon epoxy resin sealant 3. A coating of two-part liquid epoxy coating 4. A final coat consisting of polyurethane which consist of one or two- part and applied by brush or spray and are normally moisture curing and drying if solvent free.

Protection by paints: Although paints form a barrier between the basis metal and any corrosive environment, most oil-based paint films are permeable to some extent. Corrosive agents may penetrate the film, but if a pigment which contains corrosion-inhibiting chemicals is present these may be released and will reduce the likelihood of attack.

Splash Zone Protection This area requires treatment due to its contact with the seawater combined with the

salt-laden air of the marine environment. The offshore oil and gas industry has historically used rubber coatings or aluminium spray as a corrosion protection method. However, because of the simple structure design of a monopod, compared to the intricate oil and gas platform steel jacket supports, it doesn’t really warrant these expensive applications. It can be adequately protected by using an identical anticorrosion procedure as that of the atmospheric area. Alternately a relatively new concept in corrosion attack is to apply two layers of polyester based powder which should attain at least a 1000um thick coating.

Faults – single layer of epoxy results in porosity and blistering

Internal stresses in thick layer results in cracks

Impact of landing boats flakes off paint

The submerged area (Monopod Type)

This can be protected by the application of a coating of two-part epoxy compound, being applied at the construction yard. A method of cathodic protection using zinc anodes should also be used to compliment the epoxy coating. The anodes can be either fitted at the construction phase or an underwater sledge arrangement used. Both methods have their merits, the onshore fixing of the anodes to the structure being quicker and more economical, but difficult to replace under water. An underwater sledge consists of a frame which contains numerous anodes welded into it. The frame is then positioned close to the support on the seabed and connected to the structure by heavy copper wire cables. This method allows for easy replacement of anodes but is expensive due to its installation to the location and, connection to the structure which requires underwater welding by divers.

A specialized solution - In 2004-2005, Deepwater was commissioned to put 20 years of experience with the offshore oil and gas industry into designing a cathodic protection system to overcome these problems. The environmentally friendly, buried impressed-current VSE Anode(Vaulted Seawater Envelope) was the result. Designed specifically for shallow-water wind farms, the VSE allows turbine operators to retrofit new cathodic protection to existing turbines. The unit is powered by the turbine itself and provides corrosion protection to all subsea and buried metal components of the structure. Monopile Interiors are at risk - Many offshore wind turbines have uncoated monopile interiors. In several instances where the seal has leaked, these turbines have filled with water, causing a severe and immediate risk to the integrity of the structure. Luckily, with minor modifications to the structure, sacrificial anodes can be installed inside to protect the uncoated steel. RetroLink anodes can protect this internal area from corrosion without underwater intervention, keeping emergency installation costs as low as possible.

Cathodic protection: An impressed current system

Relies on conduction through the soil for buried structures

Blade materials don’t rust!

What are the problems with composites? Glass fibres and carbon fibres Epoxy resins and polyester resins

What about the blades?

Basic issues Resins:- all resins will degrade to some extent in moist environments. Polyesters and epoxies are susceptible to chemical attack by water. This is not significant or even noticeable at room temperatures ( 20-25oC) even after extended time periods. Resins do absorb water and act as a semi-permeable membrane.

Swelling with water is only significant at higher temperatures but the main effect is to toughen the resin slightly (plasticisation) and slightly reduce the glass transition temperature. At higher temperatures> 40oC, polyester and vinyl ester based resins exhibit increasing levels of hydrolysis of the ester linkages. This ultimately leads to shrinkages ( and leaching) and cracking of the materials. Epoxies are largely inert unless tested at high temperatures>100oC and are attacked by alkali’s.

fibres Glass fibres ARE attacked by water - but again this is slow. At high temperatures >80oC the fibres are weakened and pitted. This is accelerated in alkali environments. In acids E-glass exhibits a dramatic stress corrosion failure. Carbon fibres are largely inert. Glass fibre composites suffer from water attack by the resin becoming plasticised and water migrating to the glass-matrix interface where debonding occurs resulting in a general weakening of the structure's

No exposure, good bonding Water exposure, poor bonding

What are the key issues for composites....

Fatigue – key design issue

Ultimate strength – key for overload Damage Tolerance- interaction of damage and static loading

Fatigue -glass Pultruded glass fibre ( UD and random)

Unidirectional glass fibre

Some evidence to suggest glass fibre composites are degraded in water/moist environments

Carbon fibre

Minimal differences between hygrothermal and as received in residual strength after fatigue

Base plate

specimen

Knife edgeplate

Back plate

Damage Tolerance – measured by compression after impact.

Theme one: Compression after impact of plates with and without environmental exposure

+ =

Temperatures = 40, 65, 93oC

0

10

20

30

40

50

Dam

ag

e w

idth

, m

m

0 5 10 15 20

Impact Energy, J

vinyl ester laminates

polyester/phenolic

laminates

Damage width, mm, as a function of impact energy

50

100

150

200

250

Com

pre

ssio

n a

fter

im

pac

t st

rength

, M

Pa

0 5 10 15 20

Impact energy , J

phenolic

polyester

vinylester

Impact but no environmental exposure

Compression strength, MPa

0

50

100

150

200

250

Co

mp

ress

ive

Str

eng

th (

MP

a)

0 50 100 150 200

TIME (Days)

polyester: no impact damage

40ÞC

65ÞC

93ÞC

0

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100

125

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Co

mp

ress

ion

str

eng

th (

MP

a)

0 100 200 300 400

Time (Days)

phenolic: no damage

93ÞC

65ÞC

40ÞC

0

50

100

150

200

250

Co

mp

ress

ive

Str

eng

th (

MP

a)

0 100 200 300 400

TIME (Days)

40ÞC

65ÞC

93ÞCENVIRONMENT BUT NO IMPACT

VINYL- ESTER

Compression strength, MPa

Compression after impact results for both VE and PE woven glass conform to previous data obtained for glass-epoxy prepreg laminates

assess behaviour on the basis of damage size

How does time and temperature influence the strength reducing effect of a defect - in this case a region of impact damage?

0

50

100

150

200

250

0 10 20 30 40

Damage width (mm)

vinylester

Exposure at 40oC

Exposure at 65oC

0

50

100

150

200

250

0 10 20 30 40

Damage width (mm)

Vinylester

Exposure at 93oC

0

50

100

150

200

250

0 10 20 30 40

Damage width (mm)

Vinylester

EFFECT OF IMPACTDAMAGE ONUNEXPOSED SPECIMENS

CAI

Damage width

1

EFFECT OFENVIRONMENTALEXPOSURE ONUNDAMAGEDSPECIMENS

EXPECTED SIMPLECUMULATIVEEFFECT OF BOTHENVIRONMENT ANDDAMAGE

Increasing temperatures and increasing times

CAI

Damage width

EFFECT OF IMPACT DAMAGE

ON UNEXPOSED SPECIMENS

CAI

Damage width

1

EFFECT OF

ENVIRONMENTAL

EXPOSURE ON

UNDAMAGED

SPECIMENS

EXPECTED SIMPLE

CUMULATIVE EFFECT OF BOTH

ENVIRONMENT AND DAMAGE

ADDITIONALTOUGHENING ACTIONOF ENVIRONMENT

Exp DATA POINT

Are there any other complications???????

What about the simultaneous effects of load

and environment

What about biaxial loading??

Biaxial tests

0

25

50

75

100

125

150

175

CA

I (M

Pa

)

A B C

A - environment + impact + CAI

B - Impact + environment + CAI

C - Environment and biaxial + impact + CAI

Impact energy= 5 Joules

Temperature=93 Deg C

Vinylester

Polyester

0

25

50

75

100

125

150

175

CA

I (M

Pa)

A B C

Vinylester

Polyester

A - environment + impact + CAI

B - Impact + environment + CAI

C - Environment and biaxial + impact + CAI

Impact energy= 5 Joules

Temperature=65 Deg C

Marine effects on composites

• At low temperatures these seem small.

• Fatigue is affected – but not by much and residual strengths look OK

• Damage tolerance is reduced

• Glass fibre composites are worse than carbon.