tb pickling and passivating of corroded tanks

7/30/2019 TB Pickling and Passivating of Corroded Tanks

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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]

tb pickling and passivating of corroded tanks

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