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PicklingandPassivatingofCorrodedTanksonChemicalShipNo4/2011
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Thechemicaltankerdestinedformineralacidtransport
started exploitation in February 2000. The cargo acid
tankshavebeenconstructedfromduplexstainlesssteel
2205.Thecargoes thathavebeenshippedwerenearly
exclusivelysulphuricacidandphosphoricacids.Voyages
were mostly along the western American coast. Ship
designingwas
not
satisfactory.
Trace
weld
leakages
in
two tanks, cracks ofwelds connectingmain deckwith
transversebulkheadofthemostcorrodedtank,acrack
in one tank bottom and small openings at transversal
bulkhead/bottom weld in one tank were discovered
within four years of exploitation. The cargo acid tanks
gotwithintheseyearsexternalreinforcementsofsome
transverse bulkheads and also internal stiffeners.
Mineralacid
passivation
was
performed
by
the
constructing company before exploitation started and
aftereachoftworepairs.Duringrepairsheavygrinding
worktoremoverustwasdone.
Corrosion damages found before pickling/passivationworkstartedAprimary inspectionwasperformedduringashortstay
ofthe
tanker
in
Morehead
City
harbor
in
the
USA.
At the very beginning active electrochemical potential
valuesweremeasuredwith 175 True RMSMultimeter
FLUKEof certainnumberof rust coveredplates at the
top part of some tanks giving results around 500 mV
against zinc/zinc sulphate electrode. The measured
values suggested that some plates were made from
different steel than duplex 2205 and the steel quality
waslower
compared
to
duplex
2205
which
had
typically
potentialvaluesover900mV.
Some of corrosion damages were serious ones that
could bring catastrophic effects. Several fragments of
theweldswerecompletelyrusted;someofthemjoining
platesatthetankceiling,theotherjoiningwallswiththe
ceiling.Severalfragmentsoftheweldswerecompletely
coveredwithrustoranumberofrustspotswereoveror
aroundthe
welds
between
the
tank
ceiling
and
the
walls. Corrosion of platewelds is themost dangerous
problem as regards tank tightness. Rust was located
mostlyaround theweldsatsomedistancebut insome
cases close to the welds. When electrochemical
potentialswere checked itwas found that a fewweld
segments with the potentials after passivation below
600 mV presented improper welding material quality
(Fig.1).
Fig. 1.Rust left afterpassivation shows improperweld
fragments
Insomecasesrustfromtheweldswasdispersedaround
by liquid cargo on over walls or ceilings. This could
happenwhen cargohaddissolved ironand ironoxides
(rust)wereformedafterdrying.Someweldsjoiningthe
ceilingplateswerecompletelycoveredwithheavyrust
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and the surrounding surfaceof theplateswas covered
withsuperficialrust.
Corrosionaroundweldsofeyepads,ruststainsovertop
ofthewallsandovertheceilingsinalltanks(15%ofthe
surface covered with rust in the most corroded tank)
and some small pitswere present. Single shallow pits
were under rust spots in places that were heavily
ground. Overheated strips after welding supporting
elements from external side of transverse corrugated
bulkheadsstartedtorust.Thickruststripsattheinternal
bulkheadswereupto100mmwide(Fig.23).
Fig.2.Stripsofrustfromcomingfromoutsidesupports
welding
Fig. 3. Rust over weld and strip of rust from outside
welding
The edge sides of nearly all vertical, bottom or ceiling
stiffeners were covered with heavy rust spots (Fig. 4)
being a result of improper cutting technology. These
edgesunderrustweresmooth. Inmanycasesrustwas
concentratedatoneedgesideofthestiffeners.Atsome
stiffenersweldslugswereoverwelds.Thelargesurfaces
ofthestiffenerswerewithnorustatall.
Fig.4.Rustededgesofsupports
Mostof the ladder stepswere coveredwith small rust
spots coming obviously from imbedded iron external
contamination that came through open manholes. All
therailings
were
covered
with
small
rust
spots
(Fig.
5).
Fig. 5. Rust on ladder steps and railings coming from
embeddediron
Both of them steps and vertical angles had potentials
over 1000 mV against zinc/zinc sulphate electrode.
There was neither rust on the ladder sides (angels
havingwidesurface),noroverladderwelds.
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Heavy rust was situated within and around the
inspectionmanhole.Thenutsandboltheadsatasingle
manholewerepartlycoveredwithrust.
Theslopepartsoflongitudinalstarboardsidebulkheads
were at some cases covered with rust spots. A single
fillingpipewascoveredwithrustspots.
The bad state of steel surfaces showed that latest
pickling/passivating process but also exploitation were
notproperlyperformed.
Thesurveyonpickling/passivatingbyANCORACo.Anactionon rust removalwasundertakenbyANCORA
Co. five years after the tanker was constructed. The
pickling/passivationwork
by
Ancora
Co.
was
done
in
the
AstillerosBraswell InternationalS.AShipyard inPanama
(Fig.6.)duringgeneralrepairperiod.
Fig.6.Thetanker inBraswell InternationalS.AShipyard
inPanama
The cleaning and pickling/passivation were performed
by hand spraying of Avesta Welding chemicals
transportedfromtheproducerinSwedenbyair(Fig.7).
Fig. 7. Chemicals for cleaning, pickling and passivation
deliveredfromAvestaCo.byair
Pressure spraying done with the usage of mobile
scaffoldingwasnoteasybecauseoftheslopesofsome
walls above bottoms that restricted an access to
corroded constructions, mainly at the corners. The
problem was solved by constructing a small platform
stickingoutfromthescaffolding.
Time period for a surveywas limited according to the
periodofmechanicalrepairs.Atmosphericconditionsto
carry out work were difficult as during the days
temperatureinthetankswasincreasingtoaround60oC
becauseof
asunny
weather.
The
decision
was
taken
to
workwithin tanksduringnights,whereasvisualcontrol
ofeffectsandpotentialmeasurementswereperformed
duringdays.
Thetanksandpipingsystemswerepassivatedwithafull
success,findingapositiveapprovalfromboth,theship
ownerrepresentativeandthetechnicalinspector,
ManagerofMaritimeConsultants,NYUSA.Thestainless
steelsurfaces
passivated
by
Ancora
were
in
very
good
conditionswhencontrolledyearslater.
Author:DrZbigniewKlenowicz,consultanttoAncoraCo.
email:[email protected]