NASA Technical Paper 3545

NASA-TP-3545 19950019257

The Corrosion Protection of AISITM 1010 Steel byOrganic and Inorganic Zinc-Rich PrimersM.D. Danford and M.J. Mendrek

i!i,iii!:iLIBRARYCOPY

!{,"" 2 1995,% [,/I,NGLEYRESEARCHCENTER;'_ LIBRARYNASA

HAMPTON,VIRGINIA

March 1995

https://ntrs.nasa.gov/search.jsp?R=19950019257 2020-05-25T19:30:17+00:00Z

IJJl_iiiiiiiiii_iii3 1176 01419 3990 NASA Technical Paper 3545

The Corrosion Protection of AISITM 1010 Steel byOrganic and Inorganic Zinc-Rich PrimersM.D. Danford and M.]. MendrekMarshallSpaceFlightCenter• MSFC, Alabama

National Aeronautics and Space AdministrationMarshall Space Flight Center • MSFC, Alabama 35812

IMarch 1995

TABLE OF CONTENTS

Page

1INTRODUCTION.........................................................................................................................

EXPERIMENTALPROCEDURE................................................................................................ 1

RESULTSANDDISCUSSION.................................................................................................... 3

OrganicPrimer................................•.................................................................................. 33InorganicPrimer ................................................................................................................GalvanicCurrentMeasurements........................................................................................ 3Applicationto SRM/SRB................................................................................................... 4

4CONCLUSIONS/RECOMMENDATIONS..................................................................................

6REFERENCES...............................................................................................................................

°oo

III

LIST OF ILLUSTRATIONS

Figure Title Page

1. Equivalentcircuitmodelusedfor analysisofEISdata................................................. 7

2. Chargetransferresistance,organicprimer.................................................................... 8

3. Poreresistance,organicprimer...................................................................................... 8

4. ICORR,organicprimer..................................................................................................... 9

5. Coatingcapacitance,organicprimer............................................................................. 9

6. Chargetransfercapacitance,organicprimer................................................................. 10

7. Chargetransferresistance,inorganicprimer........................i........................................ 10

8. Poreresistance,inorganicprimer.................................................................................. 11

9. ICORR,inorganicprimer................................................................................................. 11

10. Coatingcapacitance,inorganicprimer.......................................................................... 12

11. Chargetransfercapacitance,inorganicprimer.............................................................. 12

12. ComparisonofICORRcurvesfor organicandinorganicprimers................................... 13

13. Comparisonof poreresistancesfor organicandinorganicprimers.............................. 13

iv

TECHNICAL PAPER

THE CORROSION PROTECTION OF AIS1TM1010 STEEL BY ORGANIC ANDINORGANIC ZINC-RICH PRIMERS

_TRODUCTION

The SpaceTransportationSystem(STS)solidrocketboosters(SRB's) consist of reusablesolidrocket motors (SRM's) that provide the major sourceof thrust duringthe first 2 min of launch.Afterseparationand parachutedeployment,the motorssplashdownin the AtlanticOceanand are recovered.Recoveryand towbackoperationsusually take24 to 36 h. Duringthis period, the aggressiveseawaterenvironmentcausesseverecorrosionof exposedbaremetalhardware.

Steel componentson the SRB's includethe motorcases and certain componentsof the nozzlethat are made of forged D6AC steel and the externaltankattach (ETA)ring and kick ring made from4130 steel. Thesecomponentsare protectedfromseacoastatmosphericand seawaterimmersioncorro-sion by the applicationof an organicprotectionsystemconsistingof a zinc-richepoxy primerand anepoxytopcoat,manufacturedby Rust-OleumTM. Thissystemhas providedadequateperformancefor thisapplicationsinceinceptionof theshuttleprogram.

Pendingenvironmentallegislation,however,threatensthe continueduse of this primer/topcoatsystem. The volatile organiccontent (VOC)of the Rust-OleumTM primer and topcoat is 470 and 535gm/L,respectively.Legislationthat implementsthe provisionsof the 1990Clean Air Act (CAA)for theaerospace industry is contained in the National EmissionsStandard for Hazardous Air Pollutants(NESHAP).A specificNESHAPfor the aerospaceindustrysets limits of 350 and420 g/L for primersandtopcoats,respectively.Withoutspecialexemption,the useof coatingsystemsexceedingtheselimitswillrequireemissioncontrol.The costof emissioncontrolprovidestheprimaryimpetusfor replacementof theRust-OleumTM system.

Candidatesfor replacementof the currentcoating systemhave not been thoroughlyidentified,screened,and tested for applicabilityto SRBrequirements.However,oneclass of coatingsystemstandsoutas the most likelyalternative;inorganiczinc-richprimerandinorganictopcoat.Thesesystemshavebeen extensivelytested for their ability to protectstructuralsteel in the KennedySpaceCenter (KSC)seacoastenvironmentandhave beenfoundto provideoutstandingservice.Inorganicsystemsare avail-ablewith compliantVOClevels.SomesystemscontainzeroVOC.

In this work, the current Rust-OleumTM organiczinc-rich primeris comparedto an inorganiczinc-richprimerformulatedby AMERONTM for its efficacyin protectingsteel in adverseenvironments.UnhardenedAISITM 1010steel was chosenfor thisstudy becauseit was readily availableandis highlycorrosive.A studyof properlyhardenedD6ACsteel,the materialcomprisingtheboostercasings,will beundertakenat a later time.

EXPERIMENTAL PROCEDURE

Flat plates, 10.2by 15.2cm (4 by6 in),of AISITM 1010steelwere coatedwith zinc-richprimer,one with the organic type manufacturedby the Rust-OleumTM Corporation and the other with aninorganic type manufacturedby the AmeronTM Company.Before coating, the plates were grit blasted

andcleanedwith alcoholandacetone.Eachof the primer-coatedplateswereclampedinto flat corrosioncellsmanufacturedby EG&G-PARCandexposedto a 3.5-percentsolutionof sodiumchloride(Na-C1).Corrosiondata were obtainedover a period of 21 days, with silver/silverchloridereference electrodesbeingusedin both cases.

Both the alternatecurrent(ac) electrochemicalimpedancespectroscopy(EIS)and the directcur-rent(dc) polarizationresistance(PR) techniqueswereemployedin this investigation.Both the EIS andthe PR methods were used for study of the inorganicprimer. However,the organic primer was notamenableto studyby the PRmethoddueto thelowcorrosioncurrentsgeneratedandonly EISwas usedin thatcase.

The EG&G-PARCmodel378 impedancesystemwas usedfor the collectionof EIS data.For theEIS measurements,data were takenin threesections.The first twosections,beginningat 0.001 and 0.1Hz, respectively,were obtainedusing thefastFouriertransform(FFT)technique.The thirddatasection,rangingfrom 6.28 to 40,000Hz, was collectedusing the lock-inamplifier technique.The sequencingwas performedautomaticallyby a computerusingthe autoexecuteprocedure,with all datamergedto asingleset for eachrun. Thesedatawere subsequentlyprocessedand analyzedusingthe modelshowninfigure 1.The samecomputerwas usedto continuetheexperiment.

Data for the PR technique were collected using the same type of instrumentation with theEG&G-PARCmodel352 software,whichwas developedespeciallyfor dc measurements.Instrumenta-tion developedby EG&G-PARCautomaticallycorrected the data for IR drop during the scan. Thepotentialapplied to the specimenduringthe scan variedfrom -20 to +20 mV on either side of the cor-rosionpotentialECORR,withdatapoints(currentandpotential)beingrecordedin l/4-mVincrements.

For the EIS data,valuesfor eachof thecircuitcomponentsin figure1were treatedas parametersin the nonlinear ORGLS1 least-squaresprogram, which automaticallyadjusted these parameters toobtaina best fit to the observedBodemagnitudedata(logimpedanceversuslog co,whereco= 27r× fre-quency). Corrosioncurrents and, hence,corrosionrates were obtainedusing the PR techniquefor theinorganicprimer.Estimatesof thecorrosioncurrentsfor the organicprimerwereobtainedfrom EISdatausing the Stem-Geary2_ equationfor chargetransfercontrolusing estimatedTafel constants (50 mVeachfor ba andbe)and (Rt+R$) as the totalchargetransferresistance:

babc 1 (1)ICORR= 2.303 (ba+b c) × (Rt+R f)

Experiencehas shownthat the Tafel constantsobtainedby the PR methodare usually in the neighbor-hoodof 50 mV.

In the PR methodusedfor the inorganicprimer,curvesof potentialversuscurrentwere obtainedand the datawere analyzedusing the programPOLCURR.5 The theory for the PR techniquehas beendescribedpreviously.2-4Allcorrosioncurrentsobtainedin this workare believedto be largelydueto thecorrosionof zinc.Valuesfor the corrosionrates in mils/year(mpy)maybe obtainedby the relation:

0.1288E ICORR (2)CorrosionRate(mpy)= d

In equation(2), E is the equivalentweightof the corrodingmetalin grams,ICORRis the corrosioncur-rent density(#A/cm2),andd is the densityof the metal.

RESULTS AND DISCUSSION

Organic Primer

The charge transferresistance(Rt)-timecurvefor thisprimeris shownin figure 2, and it showsthat this parameteris increasingin valuewithtime.The poreresistance(Rp)-timecurveis shownin fig-ure 3 anddecreaseswith time.The corrosioncurrent(lcoRR)-timecurve,withvaluesestimatedfrom EISmeasurements,is shownin figure4, and it indicatesthat this parametergenerallydecreaseswith time.The coatingcapacitance(Cc)-timecurve is shownin figure5, andthe chargetransfercapacitance(Cat)-time curve is shownin figure 6. The Cc-timecurvein figure5 is oscillatoryin nature,but is showingasharpriseat the end,whilethe Cat-timecurveriseswithtime.

The risingvalueofRt andalsoof Ccare compensatoryin natureas far ascontributionto the totalimpedanceis concerned.However,the averagevalueof the chargetransferresistanceRt is 38.1 kohms,whichis quite high. The decliningvalue of the Rp-timecurvein figure3 contributesto a lower overallimpedance,a trendusuallyobservedin coatedmetalsystems.This indicatesthat the poresin the primerare not being filled to a great extent.The averagevalue of Rp is 1.57kohms,a rather high value. Theaveragevalue of ICORRwas 0.303/tA/cm2, a rather low value consideringthat zinc is the most likelysource of metal corrosion. The decliningvalue of ICORRin figure 4 is contrary to the trend usuallyobservedfor coatedmetal systems,and indicatesthat the corrosionprotectionaffordedthe steel by zincgraduallydecreaseswith time.

Inorganic Primer

The Rt-timecurve,shownin figure7, riseswithtime.The averagevalueof Rt in this caseis 0.27kohms,a very lowvalue,althoughthetrendis thesameas thatfor the organicprimer.TheRp-timecurveis shown in figure 8, showing a steadilyrisingtrend. This trend is contrary to that observedfor theorganicprimer,with the averagevalue beingonly0.094kohms.This value is very low comparedwithan averagevalue of 1.57kohmsfor the organicprimer.Thislowvalueindicatesthat the porosityis veryhigh. The ICORRvalues obtainedfrom PR measurementsdecreaserapidly with time duringthe first 5daysand decreaseslowlythereafter(fig.9). The averagevaluewas 9.0/.tAJcm2, very muchhigherthanthat for the organicprimer(0.303/.tA/cm2).Diffusionof the mediumhad occurredto a radiusof about5cm (2 in) fromthe point of exposure,whichconsistedof a totalarea ofonly 1cm2.Thus,it is likely thatthe corrosioncurrentarose froman areamuchlargerthanthe exposedarea,verifyingthat diffusionwasindeedrampant.In general,thevery high-averagevalueofICORRpointsto muchbettercorrosionprotec-tion of the steel by the inorganicprimerthan that for the organicprimer.The coating capacitance,Co,was quite largeat the beginning(63,999kohms),butdroppedrapidlyafter 5 days.The Cccurvefor theperiod5 to 21 daysis shownin figure 10.The Cat-timecurveis shownin figure11,showinga graduallyincreasingtrend,similarto thatfor theorganicprimer.The valuesfor thiscurvewere alsocomparabletothosefor the organicprimer.Figures12 and 13showcomparisonof the IcoRR-timeandRp-timecurves,respectively,for the organicandinorganicprimers.

Galvanic Current Measurements

Galvanic current measurements were made with a flat cell especially designed for such purposesby EG&G-PARC. AISITM 1010 steel plates coated with each type of primer were clamped into one endof the cell, with a bare, grit-blasted steel plate clamped into the other end. The areas exposed to the

medium were 1 cm2 for both plates. Current measurementswere made individually on each primercoated plate, using the EG&G-PARCmodel 352 software, over a 24-h period. The mean galvaniccurrentfor eachprimerwas calculated.The meancurrentfor the platedcoatedwith organicprimerwas38.8/zA/cmz, whilethat for a plate coatedwith inorganicprimerwas 135.2/.tA/cmLBothcurrentswererelativelylargebecause of the zinc-richprimersthat wereactingas the anode.The potentialsdisplayedby bothprimerswerecloseto that reportedforpure zinc;namely,1,050mV (SCE), 6 althoughthe valuesare somewhatmorepositivefor the organicprimer.The veryhighvalueof the.galvaniccurrentobservedin the case of the inorganic primer indicates a very high degreeof cathodicprotection for the steel,althoughdiffusionof the mediummayhaveplayeda role in the high currentobserved.At the end of the24-htest, therewas no signof corrosionon the baresteelplatefor eitherprimer.

Application to SRM/SRB

Applicabilityof this work to the SRM/SRBlies in the need to replace the current zinc-richprimer/epoxytopcoat systemdue to environmentalconsiderations.Sincethe currentprimerhas a good20-yearservicerecord,the bestcandidatereplacementswouldbe materialsof similarcompositionwhichare environmentally compliant. The AMERONTM coating tested here fits both of these criteria.However, the AMERONTM coating is only one of several inorganic zinc-rich primers which havedemonstratedoutstandingperformancein the KSCseacoastenvironment,7so that morethanone systemmaybe qualified,eliminatingsolesourceproblems.

One restriction to the applicabilityof the inorganiczinc-richprimer to SRM hardwarelies inareasof hightensileresidualstress,suchasthe SRMstiffenerstubs.Compressiveyieldingin these areasat splashdownresultsin extremelyhigh residualtensilestresses.In addition,yieldingusuallyresultsindamageto the paintsystem,exposingboth baresteelandzinc-richprimer.The zinc-richprimerhas beenimplicated as a contributor to stress corrosion crackingin these areas. The mechanism of crackingassistedby the primerinvolveshigh tensilestress, seawaterimmersion,and exposedsteel and primer.Preferentialcorrosionof the primergenerateshydrogenthatmaybe absorbedby the adjacentsteelat anelectrochemicalpotential that is cathodic to the steel corrosionpotential. Since the inorganic primerprovidesa substantiallyhighercorrosioncurrent,it is expectedthat thiseffectwouldbe intensified,pro-ducingmore hydrogenand, hence, increasingthe probabilityof hydrogenassistedcracking and crackgrowth.Therefore,inorganiczinc-richprimersshouldnotbe usedin areasof high tensilestress. Investi-gationof alternateprimersfor motor stiffenerstubsshouldbe a separatestudy, concentratingon corro-sioninhibitingprimersrather thancathodicprotectionprimers,sinceinhibitingprimerswill not generatehydrogenand maystillbe able to protectthebaremetalexposedaftersplashdown.

CONCLUSIONS/RECOMMENDATIONS

This workhas demonstratedthe superiorityof the inorganiczinc-richprimer in protectingsteel.Electrochemicalgalvaniccorrosiontestingof thezinc-richprimerscoupledto A!SITM 1010steel showedthat the galvaniccurrentgeneratedwith the inorganicprimeras the anodeexceededthatgeneratedwiththe organicprimerby a factor of 3.5. Duringthe 21-dayEIS/PRstudy, the mean corrosioncurrentgen-eratedby the inorganiczinc-richprimerexceededthatby the organicprimerby a factor of 30. This fac-tor increasesto 77 whenonly the first 24 h of immersionareconsidered,whichis the time periodmostcriticalfor SRBapplicationsdue to the seawaterimmersionexperiencedduring towback.These resultsshowunequivocallythat the inorganicprimerprovidesbettercathodicprotection.

Individualequivalentcircuitparametervaluestaken fromnonlinearleast-squaresfit of EIS datafurther confirm these conclusions.In general, a differenceof two orders of magnitudewas observed

whencomparinglike resistancesin theearlystagesof immersion.A directcomparisonof the pore resis-tanceswas made in figure 13.The lowporeresistanceis indicativeof high porosity,leadingto a greaterapparentsurfaceareafor the inorganicprimer.

Based on these results, it is recommendedthat testingto qualifya replacement for the currentRust-OleumTM primerbe initiated.Sincethe inorganiczinc-richprimershavebeen extensivelytested inKSCatmosphereandretestedin thisreport,additionalcorrosiontestingwouldnot be requiredfor appli-cationto case acreageor to the ETAring orkick ring,whicharenot consideredto be highstress areas.

REFERENCES

1. Busing, W.R., and Levy,H.A.: "GeneralNonlinearLeast SquaresProgramORGLS."OakRidgeNationalLaboratory,1958.

2. Stern,M., and Geary,A.L.: Journalof theElectrochemicalSociety,vol. 102,1955,p. 609.

3. Stern,M., andGeary,A.L.:Journalof theElectrochemicalSociety,vol. 104,1957,p. 56.

4. Stern,M.:Corrosion,vol. 14, 1958,p. 440t.

5. Gerchakov,S.M.,Udey,L.R., and Mansfeld,F.: "An ImprovedMethodfor Analysisof Polariza-_tion ResistanceData."Corrosion,vol.37, 1981,p. 696.

6. Kuster,C.A.,andCooper,G.H.:"ElectromotiveSeriesfor AerospaceMetalsand Alloys."Rocket-dyne finalreportMPTR7-177-1000,January1967.

7. KSC-STD-C-0001D:"Standard for Protective Coating of Carbon Steel, Stainless Steel, andAluminumon LaunchStructures,Facilities,andGroundSupportEquipment."

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Figure 1. Equivalentcircuitmodelusedfor analysisof EISdata.

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Figure2.Charge_sfer resistance,organicprimer.

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Figure3.Poreresis_nce,organicprimer.

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Figure 4. ICORR, organic primer.

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Figure7. Charge_ans_r resistance,inorganicprimer.

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Figure9. ICORR,inorganicprimer.

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Figure 12. Comparison of ICORR curves for organic and inorganic primers.

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Figure 13. Comparison of pore resistances for organic and inorganic palmers.

US. GOVERNM ENTP RINTING OFFIC E-1995-633-109 P20031 13

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March 1995 Technical Paper4. TITLEAND SUBTITLE S. FUNDING NUMBERS

The Corrosion Protection of AISITM 1010 Steel by Organic andInorganic Zinc-Rich Primers

6. AUTHORIS)

M.D. Danford and M.J. Mendrek

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) B. PERFORMING ORGANIZATION

George C. Marshall Space Flight Center REPORTNUMBERMarshall Space Flight Center, Alabama 35812 M--776

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National Aeronautics and Space Administration AGENCYREPORTNUMBERWashington, DC 20546-0001 NASATP - 3545

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Prepared by Materials and Processes Laboratory, Science and Engineering Directorate.

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13. ABSTRACT(Maximum 200 words)

Thebehaviorof zinc-richprimer-coatedAISrrM1010steelin 3.S-percentNa-C1wasinvestigatedusingelectrochemicaltechniques.The alternatingcurrent(ac)methodof electrochemicalimpedancespectroscopy(EIS),in the frequencyrangeof 0.001to 40,000Hz, andthe directcurrent(de)methodofpolarizationresistance(PR),were usedto evaluatethecharacteristicsof an organic,epoxyzinc-richprimerand an inorganic,ethylsilicatezinc-richprimer.A de electrochemicalgalvaniccorrosiontest wasalsousedtodeterminethecorrosioncurrentof eachzinc-richprimeranodecoupledto a 1010steelcathode.DurationoftheEIS/PRand galvanictestingwas21 daysand 24h, respectively.The galvanictest results demonstratedaveryhighcurrentbetweenthesteelcathodeand bothzinc-richprimeranodes(38.8and 135.2/tAJcm2 fortheorganicandinorganicprimers,respectively).Theresultsofcorrosionrate determinationsdemonstratedamuchhighercorrosionrate ofthe zincin the inorganicprimerthanin theorganicprimer,due primarilyto thehigherporosityin the former.EISequivalentcircuitparametersconftrmedthis conclusion.Basedon thisinvestigation,the inorganiczinc-richprimerappearsto providesuperiorgalvanicprotectionand isrecommendedfor additionalstudyforapplicationonsolidrocketboostersteelhardware.

14. SUBJECTTERMS 15. NUMBER OF PAGES

electrochernical techniques for corrosion, organic and inorganic primers on 17AISITM 1010 steel, corrosion protection of steel by zinc-rich primers 16.PRICECODE

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