NRL Memorandum Report 1961

Marine Corrosion Studies

Characterization of the Corrosion Behaviorand Response to Cathodic Protection of

Nineteen Aluminum Alloys in Sea Water

(Seventh Interim Report of Progress)

R. E. GROOVER. T. J. LENNOX. JR.. .AND M. H. PFFERsom

Ph ical Metallurgy BranchMetallurgy Ditiion

January '1969

! "-D D '.

NAVAL RESEARCH LABORATOR 'Washington, D.C.

This document hau bemn apimwed for public release and sale; it* distribution ia unlimited.

CONTENTS

Abstract ........................................ iiProblem Status ................................. . .... iiAuthorization ................................... i

INTRODUCTION ......................... .......... 1PROCEDURES ..... . . . . . . . . . . . ............ 1

CORROSION alARACrERISTICS ................. ...... 3

Criteria for Corrosion Resistance ............... 3

Depth of Attack on Aluminum Alloys in 368Days in Quiescent Sea Water .......................

ELECTRODE POTENTIALS ......... ................. 5

CORRELATION BETWEEN ELECTROCHEMICAL POTENTIALAND CORROSION RESISTANCE ............................ 6

SUMMARY AND CONCLUSIONS ... ............................. 7

ACKNOWLEDGMENT .. .. ... . .. . .. . . .. . .. . . ... 8

REFERENCES ...................................... 8

ABSTRACT

The corrosion characteristics of nineteen structuralaluminum alloys were studied in quiescent sea water. Somealloys were exposed with and without cladding, anJ othersat more than one strength level for a total ol twenty-sixmaterial conditions. The response to cathodic protectionof all the alloys studied was also determined.

The electrochemical potentials of the alloys weremonitored during the experiment and a relationship wasdeveloped between the electrode potential and the corro-sion characteristics of the alloys. These mean potentialvalucs were found to differ significzntly in many instancesfrom the electrochemical potential values determined byother investigators from laboratory measurements in sodiumchloride-hydrogen peroxide solution.

Cathodic protection from galvanic anodes was effectivein reducing the corrosion damage to acceptable limitsexcept edge attack was not eliminated on one of the highstrength 7XXX series aluminum alloys.

PROBLEM STATUS

This report completes one phase of the task; work iscontinuing on other phases.

AUTHORIZATION

NRL Problem No. 63M04-02Task No. SF 51-542-602-12431

ii

f

INTRODUCTION

Aluminum alloys are finding increased use in marineand ocean engineering applications because of their favor-able strength-to-weight ratio. Although a number ofinvestigators (1-7) have reported on the corrosion behaviorof aluminum, only limited information is available on thebehavior in quiescent sea water of the higher strengthalloys hitherto used principally for aircraft applications,and almost no quantitative information is available as tothe effectiveness of cathodic protection.

It was also desired to establish reliable electro-chemical potential data for aluminum alloys in sea water.Most of the previously reported electroche ical poten-tials (8) were measured in short-term lab tory experi-ments in sodium chloride solutions contaii ; hydrogenperoxide.

Electrochemical potential data are required forseveral reasons: The values so measured (1) establishthe potential for a given alloy and any galvanic anodeused for cathodic protection must have a more negativepotential, (2) offer an initial guide to the selectionof compatible alloys, i.e., alloys none of which willsuffer accelerated corrosion when coupled in a multi-alloy structure, and (3) provide a basis for developinga means for predicting long-term corrosion behaviorfrom short-term electrochemical measurements.

For these reasons a broad-spectrum study of alumi-num alloys was initiated. Nineteen structural alloyswere included in this study. Some alloys were availablewith Alcladding or in several hardness levels. A totalof twenty-six material conditions were studied (Table 1).In addition, one proprietary aluminum alloy anode with aknown stable electrochemical potential was included as acontrol.

PROCEDURES

The experiment was conducted at the Naval ResearchLaboratory's Marine Corrosion Research Laboratory,

Key West, Florida. The exposure racks were suspended undera pier in quiescent, but not stagnant, sea water. The watercharacteristics during the period of interest were asfollows:

Temperature: 16.5 to 32.50C (61.7-90.5*F)Resistivity: 16 to 21 ohm-centimeterspH: 8.2Oxygen: Not measured, but assumed to be saturated

The specimens used in the study measured 6 x 12 in.except for the 5052-H32 panels which were 5-in. wide, andthe 7079-T6 panels which were 4-in. wide. Specimens weiecut from commercially supplied stock which varied in thick-ness from 0.050 to 1 in. To expedite the experiment mate-rials were exposed in the as-rolled thickness. Specimenswere attached to painted aluminum racks with nylon bolts,nuts, washers, and insulating strips. The artificialcrevices formed at the attachment points were utilizedto assess the susceptibility of the aluminum alloys tocrevice corrosion. Duplicate specimens were exposed foreach material condition. One specimen was exposed un-protected and the second specimen was provided with a1/2 x 1 1/4 x 6-in. aluminum alloy anode. An anode ofthis size was sufficient to polarize the protected speci-men to approximately minus 1.1 volts to a Ag/AgCl referenceelectrode. The crevices formed at the interfaces of theanodes and the aluminum specimens were sealed to preventa build-up of corrosion products benind the anodes whichcould hb.ve caused mechanical detachment of the anodes.

An insulated electrical test lead was attached toeach specimen to allow the measurement of the electro-chemical potential relative to a remote Ag/AgCl referenceelectrode. The potential of each specimen was measuredweekly for the first 63 days. The potentials of selectedspecimens were measured for longer times, in some instancesup to 360 days of the 368-day experiment.

At the conclusion of the exposure period, the grossmarine fouling was removed from the specimens with ahigh-pressure water jet, and the specimens were chemicallycleaned in a 2 percent chromic acid-5 percent phosphoric

2

acid solution maintained at 80-850 C. The corrosion on allspecimens was characterized by visual inspection and bymeasurement of the depth of attack with a di,-l gage

I ?micrometer.

CORROSION CHARACTERISTICS

Criteria for Corrosion Resistance

The choice of the most meaningful parameter to charac-terize the corrosion behavior of an alloy is in large partdependent upon the end use contemplated for the material.Although weight loss or "average corrosion rates" ininches penetration per year might be quite useful inestimating the life of a massive pier structure constructedof mild steel, the same type of data would be meaninglessif used to estimate the life of an aluminum instrumentpackage in which the most likely mode of failure would beperforation due to preferential attack or pitting.

In many instances it is, therefore, meaningless toreport data based on weight loss for raterials which donot corrode uniformly, because such data are of little orno value to the designer and might readily mislead anunwary novice into a poor material choice.

Three parameters were used to characterize the corro-sion attack on the aluminum alloys of the present study.They were:

1. The deepest attack in the crevice formed bythe plastic mounting attachments.

2. The deepest attack on the surface of thespecimen not associated with any knowncrevice.

3. The mean value of the five deepest pointsof attack regardless of location.

In addition, some of the alloys studied developed edgecracking in the form of exfoliation or delamination. The

3

depth of this type of corrosion could not be accuratelydetermined, but the presence of such attack has been noted.

Depth of Attack on Aluminum Alloys in 368 Days in QuiescentSea Water

Quantitative depth of attack data for each material areshown on Fig. 1-5. These data are arranged using TheAluminum Association's designation for the alloys.The figures show not only the depth of attack on the un-protected alloy, but also the degree of reduction of attackobtainable through the use of cathodic protection.

Alloys X7002-T6, 7178-T6, and Alcad 7178-T6 developededge cracking during the exposure period. These data arenoted on Figs. 4 and 5, but no quantitative estimate of thedegree of attack was feasible. Edge cracking of the X7002-T6alloy was eliminated by either Alcladding or cathodic pro-tection (Fig. 6), but neither of these techniques was ade-quate to completely eliminate this type of attack on the7178-T6 alloy (Fig. 7).

The corrosion behavior of the aluminum alloys and theirresponse to cathodic protection are summarized in Table 1where the alloys have been separated into groups accordingto the mean depth of attack. The group of alloys showingthe greatest inherent corrosion resistance includes manyof the 5XXX series alloys. However, the X7005-T63 and7106-T63 alloys were more corrosion resistant than hadbeen expected and the data from this study place themin the most corrosion resistant group.

The effect of the hardness condition of aluminumalloys on their corrosion characteristics is demonstratedby the 6061 alloy. In the T651 condition, this alloy wasamong the most corrosion resistant studied with a meandepth of attack of less than 1 mil and with no evidenceof crevice corrosin. In contrast, in the more commonT-6 condition the mean depth of the five deepest pointsof attack was 8 mils with a maximum depth of attack ofover 10 mils. These same effects were not evident in thestrain hardened alloys, i.e., the 1100 and 5XXX seriesalloys.

4

Cathodic protection virtually eliminated the localized

corrosion on all the aluminum alloys studied with theexception of 7178-T6 where it did not eliminate the edgecorrosion. For example the mean depth of attack on 7079-T6was reduced from over 15 mils to less than 1 mil. For2024-T351, the mean attack was reduced from 40 mils to3 mils, and for the alloy for which cathodic protectionwas least effective (7178-T6) all corrosion except foredge cracking was reduced to a maximum of 4 mils.

ELECTRODE POTENTIALS

This phase of the investigation included the deter-mination of the open circuit electrochemical potential ofeach specimen when immersed in quiescent sea water. Withinthe scope of this study it was not feasible to investigatethe complex factors on which the electrochemical potentialsof passive metals are dependent. For a detailed discussionof factors which affect the electrochemical potentials offilm-pore type electrodes, the reader is referred to thepapers by Akimov (9,10).

The electrochemical potential of each alloy (negativeto a Ad/AgCl reference electrode) and the values reportedby other investigators (8,11,12) are shown in Figs. 8-10.In addition to the maximum (most electropositive), minimum(most electronegative), and mean values of the observedpotential of each unprotected specimen, the potentialrange observed on the cathodically protected specimen isalso shown.

It will be noted that the potential values measuredin this study are in most cases significantly electro-negative to the values reported for sodium chloridesolutions containing hydrogen peroxide (8) or those forhighly aerated sea water flowing L-t 13 feet per secondreported by LaQue (11). However, more recent data fromINCO (12), also developed under velocity conditions, arein most instances more compatible with those data de-veloped in the present study.

The potential data summarized in Table 2 are themean values observed during the indicated time periods.

5

This table can be considered a galvanic series for the meanvalue open-circuit potentials of aluminum alloys in quiescentsea water. The potentials of analuminum-5% zinc anode, azinc anode (MIL-A-18001G), and a high potential proprietaryaluminum anode have been added for reference purposes.

The mean open-circuit potential of aluminum alloysranged from 0.69 volts to 1.24 volts negative to a Ag/AgClreference electrode. It is evident that an initial poten-tial difference of approximately 0.5 volt could exist ifalloys froin the two extremes of the table were coupled andimmersed in sea water. Thus, accelerated corrosion is tobe expected if certain combinations of aluminum alloys areelectrically bonded in a marine structure.

Table 2 also shows that some of the structural alumi-num alloys are more electronegative than the common gal-vanic anodes normally used in sea water. While this couldpresent a serious problem, the more electronegative struc-tural alloys in this study all demonstrated excellentcorrosion resistance, and there was no indication thatcoupling these alloys to the aluminum alloy "anode" usedin this experiment (minus 1.11 volts to Ag/AgCl) accel-erated corrosion damage of the structural alloy. However,considerable caution snould be used in coupling the moreelectronegative structural alloys to galvanic anode mate-rials such as zinc (minus 1.05 volts) or aluminum-5% zinc(minus 0.95 volts).

CORRELATION BETWEEN ELECTROCHEMICAL POTENTIAL AND CORROSIONRES I STANCE

Analysis of the data showed that alloys with relativelyelectropositive potentials, i.e., minus 0.69 to 0.89 voltsversus the Ag/AgCl reference electrode, were more susceptibleto severe localized corrosion than alloys with more electro-negative potentials. In Fig. 11 the mean depth of the fivedeepest pits is plotted as a function of the observed ele --

trochemical potential. The marked correlation is readilyevident.

The galvanic series for structural aluminum alloys inquiescent sea water (Table 2) has been divided into two

6

columns with the less electronegative alloys on the left.These alloys with potentials ranging from minus 0.69 tominus 0.89 volts suffered relatively severe localizedcorrosion and would not normally be suitable for use insea water without cathodic protection or unless protectedby a high quality coating.

In contrast, the alloys in the right-hand column, withpotentials from minus 0.92 to minus 1.24 volts were essen-tially unattacked during the experiment. The Alcad 7178-T6is an exception to the rule because it suffered edge crack-ing. In this case, however, the electrochemical potentialdetermined was that of the cladding and not of the basemetal. Considered in this light, the behavior of theAlclad 7178-T6 is more reasonably viewed as a failure ofthe cathodic protection provided by the electronegativecladding.

SUMMARY AND CONCLUSIONS

The corrosion characteristics of the aluminum alloysstudied varied widely but the behavior can be convenientlysummarized by classifying the alloys by the observed elec-trochemical potential. The more electronegative alloys(minus 0.92 to minus 1.24 volts to the Ag/AgCl reference)were inherently resistant to both surface pitting andcrevice corrosion.

Alloys less electronegative than minus 0.89 voltssuffered severe corrosion. In general, the severity ofthe corrosion problem increased for the more electro-positive (less electronegative) alloys. Alloys in thisgroup should not be used in sea water without some formof supplementary protection.

Cathodic protection either from an external aluminumanode or from Alcladding was effective in reducing thesurface pitting and crevice corrosion of the alloysstudied to tolerable limits. However, while cathodicprotection eliminated the edge cracking observed onunprotected X7002-T6 alloy, it did not completely elimi-nate this phenomena in the 7178-T6 alloy.

7

A CKNOWLEDGMENT

The authors are indebted to Messrs. C.W. Billow andW.M. Lazier of the NRL Marine Corrosion Research Laboratory,Key West, Florida, for their assistance in conducting thisstudy. This research was supported by the Naval Ship SystemsCommand of the Navy Department.

REFERENCES

1. H. P. Godard, "The Corrosion Behavior of Aluminum,"Corrosion, 11, No. 12, pp. 542t-552t (1955) December.

2. Staff Manual. "Wrought Aluminum and Its Alloys,"Materials in Design Engineering, 16, No. 6, pp. 127-129(1965) June.

3. D. 0. Sprowls and H. C. Rutemi]ler, "Susceptibilityof Aluminum Alloys to Stress Corrosion." MaterialsProtection, 2, No. 6, pp. 62-65 (1963) June.

4. C. R. Southwell, A. L. Alexander. and C. W. Hummer, Jr.."Corrosion of Metals in Tropical Environments -Aluminum and Magnesium," Materials Protection, 4, No. 12,pp. 30-35 (1965) December.

5. R. W. Judy, Jr. and R. J. Goode, "MetallurgicalCharacteristics of High Strength Structural Materials(Tenth Quarterly Report)," pp. 75-80, NRL Report 6454,April. 1966.

6, R. W. Judy, Jr. and R. J. Goode, "MetallurgicalCharacteristics of High Strength Structural Materials(Eleventh Quarterly Report)," pp. 67-71, NRL Report6513, August, 1966.

7. T. H. Rogers, "The Marine Corrosion Handbook,"McGraw-Hill, New York, New York, 1960.

8. Metals Handbook, Vol. 1. 8th Edition, American Societyfor Metals: Novelty, Ohio, 1961.

9. G. W. Akimov, "Electrode Potentials," Corrosion, 11,No. 11, pp. 477t-486t (1955) November.

8

10. G. W. Akimov, "Electrode Potentials," Corrosion, 11,No. 12, pp. 515t-534t (1955) December.

11. F. L. LaQue, "Theoretical Studies and LaboratoryTechniques in Sea Water Corrosion Testing Evaluation,"Corrosion, 13, No. 5, p. 314t (1957) May.

12. INCO. Private Communication, February, 1967.

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Fig. 6 - Edge cracking (delamination) ofX7002-T6 aluminum a 11 o y showing the

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Cathodic Protection.

_ 238

17

Fig. 7 - Edge cracldng(delamination)of7178-T6 aluminum alloy showing theineffectiveness of Alclad and cathodicprotection: (A) 7178-T6, (B) 7178-T6with Cathodic Protection, (C) Alclad7178-T6, (D) Alclad 7178-T6 with

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potentials in sea water at Key West, Florida.

22

UNCLASSIFIED

DOCUMENT CONTROL DATA -R & D

I 'a- L,, 'ING AC y~ ICarat, t .aaahar) 20. R~'OQT SECUITY CLA W ICA7ION

Naval Research Laboratory UNCLASSIFIED2b GROUP

Washington, D.C. 20390

CHARACTERIZATION OF THE CORROSION BEHAVIOR AND RESPONSE TO CATHODICPROTECTION OF NINETEEN ALUMINUM ALLOYS IN SEA WATER (Seventh Interim Reportof Progress)

4 OlrSCRIP VE NOTES (7)pe ofreporttand inlus ,e date&)

A progress report on the problem.5 AU THO iSj (Fir~st name, middle initial. Illst name)

R. E. Groover, T. J. Lennox, Jr., and M. H. Peterson

REPORT OaTE IS TOTAL NO OF PAGES b N OF REFS

January 1969 26 12Sa. CONTRA( I ORt GRANT 1O 90 OR'IONAVOR'S REPORtT e.,..VaCtSi

M04-02MO-.0 2O NRL Memorandum Report 1961

SF 51-542-602-12431a9t. 01 " E A RE POPRT NO(SI (Aaaf other numbers that ma be assigned

this report)

d

I^DSIRIOWTION STA'.ME%'

This document has been approved for public release and sale; its distribution is

unlimited.

11 5, FA '( 1 . N0'S I' SPOSCOR.O a..L-AR,*v ,

Department of the Navy (Office of NavalResearch) and Deep Submergence SystemsProject, Wshinzton, D.C. 20360.

The corrosion characteristics of nineteen structural aluminum allo s were studied

in quiescent sea water. Some alloys were exposed with and without cladding, and others atmore than one strength level for a total of twenty-six material conditions. The response tocathodic protection of all the alloys studied was also determined.

The electrochemical potentials of the alloys were monitored during the experimentand a relationship was developed between the electrode potential and the corrosion charac-teristics of the alloys. These mean potential values were found to differ significantly inmany instances from the electrochemical potential values determined by other investigatorsfrom laboratory measurements in sodium chloride-hydrogen peroxide solution.

Cathodic protection from galvanic anodes was effective in reducing the corrosion

damage to acceptable limits except edge attack was not eliminated on one of the high

strength 7XXX series aluminum alloys.

DD F.R..., (A( -1 23 _UNCLASSII'IED

% 0 . 7.(, ,

UNCLASSIFIEDSecurity Classification

14 LINK A LINK a LINK CKEY wOROS

WOL. WT ROLE WT ROLE WT

Marine corrosion

Aluminum alloysQuiescent sea waterElectrochemical potentialsCathodic protectionEdge corrosion

D i)D INO"....73- (BACK) 24 UNCLASSIFIED(PAGE- 2) Security Clssi Iication