NRL Memorandum Report 2444

Marine Corrosion Studies

The Effects of Dissimilar Metal Coupleso and Toxicants from Antifouling Paints on the CorrosionI of 5086 and 6061 Aluminum Alloys

and Their Response to Cathodic Protection

(Fourteenth Interim Report of Progress)

T. J. LENNOX, JR., M. H. PETERSON, J. A. SMITH,

AND R. F. GROOVER

Physical Metallurgy BranchMetallurgy Division

May 1972

-)F r j I, fAUq 3 1972

NATIONAL TECHNICALINFORMATION SERVICE

NAVAL RESEARCH LABORATORYWashington, D.C.

Approved toi public release; distribution unlimited. • b'

Interim Report Series on Marine Corrosion

Edited by T. J. Lennox, Jr. -

First Interim Report, NRL Memorandum Report 1549 (1 Jul 1964)&

Second Interim Report, NRL Memorandum Report 1574 (Nov 1964)cThird Interim Report, NRL Memorandum Report 1634 (Jul 1965)

Fourth Interim Report, NRL Memorandum Report 1711 (May 1966)

Fifth Interim Report, NRL Memorandum Report 1792 (May 1967)

Sixth Interim Report, NRL Memorandum Report 1948 (Nov 1969)

Seventh Interim Report, NRL Memorandum Report 1961 (Jan 1969)

Eighth Interim Report, NRL Memorandum Report 2183 (Oct 1970)Ninth Interim Report, NRL Memorandum Report 2187 (Nov 1970)

Tenth Interim Report, NRL Memorandum Report 2300 (Jun 1971)

Eleventh Interim Report, NRL Memorandum Report 2333 (Jul 1971)

Twelfth Interim Report, NRL Memorandum Report 2348 (Aug 1971)

Thirteenth Interim Report, NRL Memorandum Report 2374 (Oct 1971)

D G -' ,::ti•;

0

By

CItIUI~AYAiLca~izLi CESý67

UNCLASSIFIEDSecu ritv Classification

DOCUMENT CONTROL DATA - R & D(Security classilication of title, body of ob~tract and indexing, annotation ma,t be entered when Ilse overall repor, Is classified)

I ORIGINNTING ACTIVITY (Corporate author) 20. REPORT SECURITY CLASSIFICAIJON

Naval Research Laboratory UNCLASSIFIED

Washington, D.C. 20390 2/. GROUP

3. REPORT TITLE

MARINE CORROSION STUDIES - The Effects of Dissimilar Metal Couples and Toxicants fromAntifouling Paints on the Corrosion of 5086 and 6061 Aluminum Alloys and Their Response toCathodic Protection (Fourteenth Interim Report of Progress)

4 DESCRIPTIVE NOTES (7y'pe of report and inclusive dates)

This is an Interim Report.5, AU THOR(S) (First name, middle Initial, last name)

T. J. Lennox, Jr., M. H. PetCerson, J. A. Smith, and R. E. Groover

6. REPORT DATE 7a. TOTAL NO. OF PAGES 7b. NO. OF REFS

May 1972 84 4SO. CONTRACT OR GRANT NO, 90. ORIGINATOR'S REPORT NUM,)ER(S)

NRL Problem No. 63M04-02b. PROJECT NO NRL Memorandum Report 2444Task No. SF 51-542-602-12431

C. 9b. OTHER REPORT NO(S) (Any other numbers :hat may be asslinedthis report)

d.

I0. OISTRIBUTION STATEMENT

Approved for public release; distribution unlimited.

I1, SUPPLEMENTARY NOTES 112. SPONSORING MILITARY ACTIVITY

Naval Ship Systems CommandDepartment of the Navy

IWashington, D.C. 2036013, AUSTRACT

Aluminum alloy 5086-H32 when not coupled to dissimilar metals was observed to becorrosion resistant in seawater or in the Potomac River at Washington, D.C. Severe corrosionoccurred in seawater however when coupled to any of the following dissimilar metals: coppernickel, 10%; yellow brass; 304 stainless steel; or mild steel. This galvanic corrosion could notbe completely prevented by cathodic protection. A magnesium anode also caused severe corro-sion of the aluminum in seawater.

In the Potomac River at Washington, D.C. the dissimilar metal corrosion of 5086-H32aluminum was more severe than in seawater. Except for the mild steel couple, cathodic pro-tection did not significantly reduce the depth of corrosion caused by the dissimilar metals inthe Potomac River water, although the incidence of attack was reduced.

Cuprous oxide antifouling paint caused corrosion at bared areas of 5086-H32 aluminumeven when a vinyl anticorrosive barrier paint was applied beneath the antifouling paint. Thecorrosion caused by the cuprous oxide toxicant was more severe in the Potomac River than inseawater. The depth of corrosion on the cuprous oxide coated specimen that was cathodicallyprotected and exposed in the Potomac River was not significantly less than on the unprotectedspecimen which was coated writh the tributyltin oxide toxicant. If an antifouling paint is re-quired, the best overall corrosion mitigatioat system on coated 5086-1132 aluminum was ob-tained on a specimen coated with tributyltin oxide and cathodically protected.

(Continued)

D OR IPAGE N) 5 UNCLASSIFIEDS/N 0101-807.6801 Security Classification

UNCLASSIFTEDSecurity Classification

14, LINK A LINK B LINK CKEY WORDS

ROLE WT ROLE WT ROLE WT

5086-H32 aluminum6061-T6 aluminumGalvanic corrosionCathodic ProtectionAntifouling paintsCuprous oxideTributyltin oxideSeawaterPotomac RiverLake WaterWeldmentsDissimilar metalsMagnesium anodeZinc anodesAluminum anodesMarine fouling

KI

Aluminum alloy 6061-T6 was severely corroded when continously immersed inquiescent seawater or in the Potomac River. In seawater corrosion caused by thecuprous oxide toxicant was not as severe as that observed on uncoated and unprotec-ted 6061-T6 aluminum, but in the Potomac River the depth of corrosion was signifi-cantly Increased by the cuprous oxide antifouling coating. The depth of corrosionwas less on the unprotected specimens coated with the tributyltin oxide toxicant thanon the specimens coated with the cuprous oxide toxicant. The most effective methodof reducing the corrosion on 6061-T6 aluminum coated with antifouling toxicants N..the combination of the vinyl anticorrosive barrier, the tributyltin oxide toxicant, andcathodic protection.

'The uncoaled aluminum alloys were heavily fouled with marine growth in sea-water, but in the Potomac River only slime developed on their surfaces. After 1264days in seawater moderate fouling was observed on the cuprous oxide coated speci-mens and moderate to heavy fouling with a few barnacles on the tributyltin oxidecoated specimens. Cathodic protection had little or no effect on the degree of foulingobserved on the specimens immersed for 1264 days in seawater.

DD FORV ,- (J BACK) , UNCLASSIFIED(r[A(P [ 2) Security Chvla ifiatIon

* ,~m m ~ mm •Iwlm••• i

Contents

Abstract ................ iiProblem Status .......... iiiAuthorization ........ .. iii

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

PROCEDURES .............................. 2

EXPERIMENTAL RESULTS ..................... 3

SUMMARY .................................. li

A CKNOWLEDGMENT ..................... ...... 3

REFERENCES ................... ............ 3

i.

ABSTRACT

Aluminum alloy 5086-H32 when not coupled to dissimilarmetals was observed to be corrosion resistant in seawateror in the Potomac River at Washington, D.C. Severecorrosion occurred in seawater however when coupled to anyof the following dissimilar metals: copper nickel, I0%;yellow brass; 304 stainless steel; or mild steel. Thisgalvanic corrosion could not be completely prevented bycathodic protection. A magnesium anode also caused severecorrosion of the aluminum in seawater.

In the Potomac River at Washington, D. C. the dissimilarmetal corrosion of 5086-H32 aluminum was more severe thanin seawater. Except for the mild steel couple, cathodicprotection did not significantly reduce the depth ofcorrosion caused by the dissimilar metals in the PotomacRiver water, although the incidence of attack was reduced.

Cuprous oxide antifouling paint caused corrosion at baredareas of 5086-H32 aluminum even when a vinyl anticorrosivebarrier paint was applied beneath the antifouling paint.The corrosion caused by the cuprous oxide toxicant was moresevere in the Potomac River than in seawater. The depth ofcorrosion on the cuprous oxide coated specimen that wascathodically protected and exposed in the Potomac River wasnot significantly less than on the unprotected specimenwhich was coated with the tributyltin oxide toxicant. Ifan antifouling paint is required, the best overall corrosionmitigation system on coated 5086-H32 aluminum was obtainedon a specimen coated with tributyltin oxide and cathodicallyprotected.

Aluminum alloy 6061-T6 was severely corroded whencontinuously immersed in quiescent seawater or in thePotomac River. In seawater corrosion caused by the cuprousoxide toxicant was not as severe as that observed on un-coated and unprotected 6061-T6 aluminum, but in the PotomacRiver the depth of corrosion was significantly increased bythe cuprous oxide antifouling coating. The depth ofcorrosion was less on the unprotected specimens coated withthe tributyltin oxide toxicant than on the specimens coatedwith the cuprous oxide toxicant. The most effective methodof reducing the corrosion on 6061-T6 aluminum coated withantifouling toxicants was the combination of the vinylanticorrosive barrier, the tributyltin oxide toxicant. andcathodic protection.

The uncoated aluminum alloys were heavily fouled withmarine growth in seawater, but in the Potomac River onlyslime developed on their surfaces. After 1264 days in

ii

seawater moderate fouling was observed on the cuprous oxidecoated specimens and moderate to heavy fouling with a fewbarnacles on the tributyltin oxide coated specimens.Cathodic protection had little or no effect on the degreeof fouling observed on the specimens immersed for 1264 daysin seawater.

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

INTRODUCTION

Aluminum alloys have been used over the past several yearsfor some ship hulls, other naval structures and oceano-graphic instrument packages. Alloys 6061-T6 and 5086-H32have been of particular interest for use in seawater. Theformer alloy is weldable and heat treatable. The latteralloy is also weldable, but its strength is realized throughstrain hardening.

Marine structures are usually designed with strength require-ments as the primary parameter governing the choice ofmaterials. It may also be necessary to use more than onemetal in the structure to obtain the desired mechanicalstrength. Because alloys with less than optimum corrosionresistance maybe used, and because of the acceleratedcorrosion that will be catqed by dissimilar metal couples,it is prudent to control the corrosion to acceptable l.imitsthrough the use of protective coatings and cathodicprotection.

, ,ntifouling coatings are also often required to limit weightincreases and to minimize the additional drag created bymarine organisms growing on the structure. The standardNavy antifouling coating is Formula No. 121 of MIL-P-15931which contains cuprous oxide as the toxicant. At one timethis antifouling toxicant was considered tolerable onaluminum provided there was a 6 mil barrier-layer of pore-free vinyl anticorrosive paint between the cuprous oxideand the aluminum. This undoubtedly would be satisfactoryif the barrier film remained intact during service, butalmost invariably the paint coating is damaged in serviceand bare metal is exposed at some areas.

The present study was initiated to determine the corrosioncharacteristics of 5086-H32 and 6061-T6 aluminum alloys inseawater and fresh water. Another objective was to determinewhether cathodic protection could prevent the corrosion ofaluminum alloys caused by dissimilar metals or the toxicantsin antifouling paints. A comparison of the corrosive effectsof cuprous oxide (Cu 2 0) and tributyltin oxide (TBTO) anti-fouling paint toxicants on the aluminum alloys and theresponse to cathodic protection was also included in thisStudy.

Interim results have been previously reported (1,2,3, and 4).

The present report includes data from specimens exposed forlonger time periods and correlates data for several timeperiods and different exposure locations.

1

PROCEDURES

Most of the study was conducted in seawater at the NRLMarine Corrosion Research Laboratory, Key West, Florida,and in the Potomac River at the Naval Research Laboratory,Washington, D.C,

Each aluminum specimen measured 12 x 12 inches and wassheared from larger plates. The 5086-H32 specimens were1/8-inch thick except for the welded specimens which were3/16-in. thick. Welded specimens were fabricated using5356 filler metal. The 6061-T6 specimens were all 1/16-in.thick.The dissimilar metals or galvanic anodes that were coupled

to the aluminum specimens measured 1/2-in. x 1 1/4-in. x6-in. with an area ratio of the dissimilar metal or cathodicprotection anodes to the aluminum specimer of approximately1:18. The dissimilar metals included copper nickel, 10%;yellow brass; 304 stainless steel;and mild steel. Thegalvanic anodes used for the cathodic protection phase ofthis study included proprietary aluminum anodes, zincanodes (MIL-A-18001H), and magnesium anodes (MIL-A-21412A).The dissimilar metals and anodes were drilled and tappedand fastened to the aluminum by stainless steel boltsthrough the specimensý. No insulating barrier was usedbetween the faying surfaces of the aluminum specimen andthe dissimilar metal (or anode), but the bolt heads andthe area around the through bolts were sealed with epoxyresin. The metallic resistance between the aluminumspecimens and the coupled metals (or anodes) was less than0.001 ohm before and after these immersion studies.

The aluminum specimens were solvent-wiped prior to coatingand the following anticorrosive paint system was applied:

(a) one coat (0.5 mil) Wash Primer-Formula No. 117 ofMIL-P-15328

(b) six coats (6 mils) zinc chromate primer, Vinyl-Formula No. 120 of MIL-P-15930.

The antifouling paint toxicants were applied over the anti-corrosive paint system and included the following two types:

(a) (Cu 0) - Two coats (4 mils) Antifouling Vinyl(red) - Formula No. 121 of MIL-P-15931

(b) (TBTO) - Two coats (4 mils) Antifouling (proprietaryvinyl formulation, copper-free but pigmented to adark red color),

2

A circular window (bared area) of approximately one squareinch was left uncoated at the center of one surface of eachpainted specimen. This window simulated a damaged area onthe coating, permitted the study of any e.ffects on thealuminum caused by the toxicants leached irom the antifoulingcoatings, and provided for the evaluation of the responseto cathodic protection from the galvanic anodes.

Specimens were mounted on reinforced phenolic rods andspacers with the flat surfaces of the specimens paralleland were exposed in the totally immersed condition.Individual specimens were spaced approximately 4 inchesapart. This mounting method provided an intentional creviceand reduced the number of support racks required for theexperiment. A typical exposure rack is shown in Fig. 1.

The aluminum alloy specimens and coupled metals were in-dividually weighed to within one gram prior to assembly andat the completion of the experiment. All )f the barespecimens were cleaned of fouling after removal fromexposure, rinsed with fresh water, dried, and sprayed withan inhibited spray before returning them to NRL fordetailed evaluation.

Final chemical cleaning of the uncoated specimens consistedof immersion with alternate brushing in a solution of 2 wt.% chromic acid - 5 wt. % H3 PO4 at 80-85' C (175-185' F).This was followed by a fresh water rinse and drying. Thepaint on the coated specimens was removed by repeatedimmersion in acetone and brushing, The stripped specimenswere then cleaned as noted above for the uncoated specimens.

EXPERIMENTAL RESULTS

Uncoated 5086-H32 Aluminum - Seawater Performance - Effectsof Weldments, Dissimilar Metal Couples and CathodicProtection. Exposure Periods Up to 809 Days (26W. Months).

The depth of corrosion and the response to cathodicprotection for various areas on the welded specimens of5086-H32 aluminum are shown in Fig. 2. These data showedthat the deepest corrosion on any of the welded specimensin seawater was 13 mils and that cathodic protectioneffectively eeduced the depth of corrosion except at thearea immediately adjacent to the weld on the circular weldedspecimen. There was evidence of cracks in the welds, notablyon the circular welded specimen. The weld cracks wereevident after exposure whether or not the specimen wascathodically protected by an aluminum anode, but it was notcertain whether these cracks were present prior to theexposure. The weld area and areas adjacent to the weld on

4 3

the unprotected specimen are shown in F '. 3. A similararea of the protected specimen is shown in Fig. 4.

The depth of corrosion at various locations on the 5086-H32aluminum caused by the dissimilar metals and the responseto cathodic protection in preventing this corrosion aresummarized in Figs. 5 thru 8.

Each of the following dissimilar metals: copper nickel, 10%;yellow brass; 304 stainless steel; and mild steel causedaccelerated corrosion at some location on the 5086-H32aluminum. Of all the dissimilar metals studied the coppernickel caused the deepest corrosion beneath the dissimilarmetal and on the general areas of the specimen. The 304stainless steel accelerated the corrosion of the aluminumthe most at the crevice formed by the phenolic mountingspacers. Edge corrosion of the aluminum was also causedby each of the dissimilar metals.

Cathodic protection from the aluminum anodes reduced thedepth of corrosion on the 5086-H132 aluminum caused by thedissimilar metals, but only in the case of the 304 stainlesssteel couple (Fig. 7) was the depth of corrosion reducedto that observed on the bare, uncoupled and unprotected5086-H32 aluminum. The depth of corrosion on bare 5086-H32aluminum (with and without cathodic protection) and notcoupled to a dissimilar metal has been shown on Fig. 9.The data for the maximum corrosi•,n depth for bare and un-protected 5086-H32 aluminum hav-. ben shown on theappropriate graphs for compar.ative purposes.

The weight loss data have ili~o been included in Figs. 5 thru9 for comparative purposes 31though the weight loss datawere not indicative of the severity of the corrosion on thealuminum caused by the dissimilar metals. However, weightloss data as well as depth of corrosion data did show theaccelerated corrosion on 5086-H32 aluminum when cathodicprotection was attempted from a magnesium anode (Fig. 9).There was also some indication that the depth of corrosionon the general areas of the specimen may have been slightlyaccelerated by cathodic protection from a zinc anode.Cathodic protection from aluminum anodes prevented the edgecorrosion on tile aluminum alloy specimens that were coupledto the dissimilar metals.

The photographs in Figs. 10 thru 18 depict the corrosion ofthe 5086-H32 aluminum caused by the dissimilar metals andthe response to cathodic protection. In most instances theareas shown were under and immediately adjacent to thelocation of the dispirailar metals on the speci.mens. Figure19 shows the excellent corrosion resistance of bare

4

unprotected 5086-H32 aluminum after 809 days in seawaterwhen not coupled to a dissimilar metal.

Comparative data showing the depth of corrosion on 5086-H32aluminum and the effectiveness of cathodic protection incounteracting the detrimental effects of the dissimilarmetals are shown in Figs. 20 thru 23. These data wereobtained for two time periods from exposures at Key West,and two time periods from exposures at Ft. Amador, CanalZone. The depth of corrosion on unprotected specimens wasgenerally greater at Key West than at Ft. Amador. This wasevident except when copper nickel was the dissimilar metaland the exposure time at Key West was relatively short.

The Ft. Amador experiments did not indicate an increase ofcorrosion depth with time. The 540 days Ft. Amador dataand the 319 days Key West data also indicated that cathodicprotection from an aluminum anode was completely effectivein preventing corrosion caused by the dissimilar metals.The longer term (809 days) Key West data indicated, however,that a prolonged incubation period existed before tiiedetrimental effects of the dissimilar metals copper nickel,10% and 304 stainless steel became evident (Figs. 20 and 22).The longer term Key West data also showed that cathodicprotection was not completely effective in counteracting thedetrimental effects on the aluminum caused by any of thedissimilar metals studied.

Uncoated 5086-H32 Aluminum - Lake and River Water Performance.Effects of Weldments, Dissimilar Metal Couples and CathodicProtection. Exposure Periods up to 675 days (22 Months).

The data for the depth of corrosion on the welded specimensof 5086-H32 aluminum exposed for 675 days in the PotomacRiver are summarized in Fig. 24. The 5086-H32 aluminumcorroded to a depth of 56 mils adjacent to the weld and to33 mils on general areas of the specimen. The deepestcorrosion occurred on the circular welded specimen whichalso showed slight cracks in the weld, but it was notcertain whether these cracks were present prior to exposure.The weld and adjacent areas on this specimen are shown inFig. 25,

A comparison of the data in Figs. 2 and 24, and 3 and 25,indicates that the corrosion of the unprotected welded specimenwas more severe in the Potomac River than in seawater atKey West. The response to cathodic protection in preventingthe corrosion on the welded specimens in the Potomac Riveris not known because these specimens were lost when thedock from which the specimens were suspended was removed bya demolition crew unbeknownst to the authors.

5

The depth of corrosion produced on the 5086-H32 aluminumby the dissimilar metal couples in the Potomac River and theresponse to cathodic protection have been shown in Figs. 26thru 29. Each of the dissimilar metals: copper nickel, 10%;yellow brass; 304 stainless steel; and mild steel causedsevere corrosion of the 5086-H32 aluminum. On the generalareas of the aluminum the corrosion in the Potomac Riverwas l1 to 2 times deeper than on similar specimens exposedin seawater at Key West. (Compare Figs. 5,6,7,8 with Figs.26,27,28, and 29, respectively).

Cathodic protection from the aluminum anodes did notsignificantly reduce the depth of corrosion caused by thedissimilar metals; copper nickel, 10%; yellow brass; or 304stainless steel in the Potomac River exposures except increvice areas formed by the mounting spacers. Only in thecrevice areas was the depth of corrosion reduced to thatobserved on bare, unprotected 5086-H32 aluminum not coupledto dissimilar metals, Figs. 26,27,28, and 30.

In the case of the mild steel couple in the Potoma: Rivercathodic protection was effective and significantly reducedthe depth of corrosion at all locations on the 5086-H32aluminum, Fig. 29.

Figures 31 to 40 show the appearance of the specimens withand without cathodic protection from the aluminum anodes.On the 5086-H32 aluminum specimens coupled to the dissimilarmetals, copper nickel, 10%; yellow brass; and 304 stainlesssteel, Figs. 3] through 36, the incidence of corrosion wasreduced by cathodic protection, but as indicated previouslythe depth of corrosion was not significantly reduced exceptAt the crevice areas. There was sufficient weight loss onthe unprotected specimens of 5086-H32 aluminum coupled tocopper nickel, 1C%; and yellow brass, (Figs. 26 and 27) toindicate that cathodic protection from the aluminum anodesalso reduced the incidence of corrosion. A comparison ofFigs. 37 and 38 shows that essentially complete protectionwas obtained by cathodic protection on the 5086-H32aluminum coupled to mild steel. Figures 39 and 40 show theexcellent corrosion resistance of bare 5086-H32 aluminumnot coupled to a dissimilar metal after 675 days in thePotomac River.

Figure 41 shows the area on 5086-1132 aluminum beneath amagnesium anode used for cathodic protection in the PotomacRiver. This photograph and the data in Fig. 30 indicatethat the magnesium anode caused some corrosion on the 5086-1132 aluminum, but it was much less severe than the acceleratedcorrosion observed in seawater (compare Figs. 9 and 30).

6

Figures 42 through 45 compare the depth of corrosion after540 days in Gatun Lake, Canal Zone and after 675 days in thePotomac River. The depth of corrosion data from specimensat these two locations indicate the severe detrimental effectscaused by the copper nickel, 10%; yellow brass; and 304stainless steel at both sites, and the ineffectiveness ofcathodic protection from an aluminum anode in counteractingthis corrosion, Figs. 42 thru 44. The detrimental effectof the mild steel couple was not significantly reduced bycathodic protection in the Gatun Lake although cathodicprotection was effective in the Potomac River, Fig. 45.

Coated 5086-H32 Aluminum (with and without weldments) Sea-water Performance - Effects of Cu 2 0 and TBTO antifoulingcoatings and cathodic protection. Exposure Periods up to1264 days (41J months).

The depth of corrosion and response to cathodic protectionat the intentionally bared areas and under the paint on thewelded 5086-H32 aluminum specimens exposed for 1264 days inqu&escent seawater at Key West are shown in Fig. 46. Thesedata and the photographs in Fig. 47 show that the Cu 2 0 anti-fouling coating caused accelerated corrosion of the aluminumeven with the vinyl anticorrosive barrier coating betweenthe metal and the antifouling coating. Figs. 46 and 47 alsoindicate that cathodic protection from the aluminum anodewas effective in preventing the detrimental effects of theCu 2 0 antifouling coating. The data (Fig. 46) also showthat the TBTO antifouling coating was not detrimental tothe 5086-H32 welded aluminum and that cathodic protectionwas completely effective in preventing any corrosion of thealuminum when TBTO was the toxicant in the antifouling paint.Figure 48 shows the weldea specimens coated with TBTO, withand without cathodic protection, and the one area on theunprotected specimen which showed some corrosion.

The accelerated corrosion caused by the Cu 2 0 in the anti-fouling paint on unwelded 5086-H32 aluminum and theeffectiveness of cathodic protection in preventing thiscorrosion are shown in Figs. 49 and 50. Cathodic protectionreduced the corrosion on the specimen coated with the Cu 2 0antifouling paint but the least corrosion was observed onthe cathodically protected specimen with the TBTO as thetoxicant. (Figs. 49 and 51.)

The depth of corrosion data after 809 days exposure inquiescent seawater at Key West have also been included inFig. 49. The 809 day data would appear to indicate thEpcathodic protection was less effective in reducing thecorrosion caused by the Cu 2 0 and that TBTO antifouling paint

7

plus cathodic protection was only slightly better to controlcorrosion on 5086-H32 aluminum than Cu 2 0 plus cathodicprotection.

However, the 1264 day data confirmed an earlier suspicionthat the procedure used to remove the coatings from thespecimens after 809 days exposure caused an artifact in thedata. Cleaning of the coated specimens after 809 daysexposure was on a mass-production type basis in which thestripping solutions normally used by a commercial paintshop to remove old paint from aluminum prior to repaintingwere used. The anomalies in the data caused by thiscleaning procedure confirm that extreme caution must beexercised in removing paint from corrosion specimens. Undueexpedience in any phase of a corrosion study is generallyunwise especially when considerable exposure time is involved.

Coated 5086-H32 Aluminum - Potomac River Water Performance.Effects of Cu 2 0 and TBTO antifouling coatings anid cathodicprotection. Exposure period of 675 days (22 months).

The depth of corrosion and response to cathodic protectiondata at the intentionally bared window areas and under thepaint on the 5086-H32 aluminum exposed for 675 days in thePotomac River are shown in Fig. 52. Figure 53 is a photographof the severe corrosion that occurred at the window area ofthe specimen coated with the Cu 2 0 antifouling paint. Asnoted with the dissimilar metal couples, the corrosionobserved on the Cu 2 0 antifouling paint coated 5086-H32aluminum was more severe in the Potomac River than in sea-water. Cathodic protection of either the Cu20 or TBTOcoated specimens did not reduce the corrosion significantlybelow the corrosion depth observed on the unprotected TBTOcoated specimen. Figure 54 is a photograph showing thecondition at the window area of the cathodically protectedCu 2O coated specimen. Figure 55 shows the appearance atthe window area of unprotected 5086-H32 aluminum coatedwith TBTO, and Fig. 56 shows a similar specimen that had beencathodically protected.

Coated 6061-T6 Aluminum - Seawater Performance. Effects ofCu2 0 and TBTO antifouling coatings and cathodic protection.Exposure Periods up to 1264 days (41J months).

The dep, h of corrosion and response to cathodic protectiondata at the bared window areas and under the paint on the6061-T6 aluminum exposed for 1264 days in quiescent sea-water at Key West are summarized in Fig. 57. The un-protected specimen coated with Cu 2O antifouling paint wasseverely corroded. The depth of corrosion was not as greatas on the uncoated specimen, but was approximately twice as

8

deep as on the unprotected specimen coated with TBTO. Al-though cathodic-protection reduced the depth of corrosion onthe Cu 0 coated specimen, the most effective combinationfor regucing corrosion on the antifouling coated specimensof 6061-T6 aluminum was the TBTO antifouling paint andcathodic protection.

The 809 day data on the coated 6061-T6 aluminum alloy showedsimilar trends although the effectiveness, of the TBTO anti-fouling paint system plus the cathodic protection from analuminum anode was less evident. This is believed to be aresult of the previously discussed artifact caused by themethod used to remove the coating from these specimens.Figures 58 and 59 are photographs of 6061-T6 aluminumspecimens cozated with Cu 2 0 and TBTO antifouling coatings,respectively. Selected areas on the cathodically protectedspecimens are also shown.

Coated 6061-T6 Aluminum - Potomac -River Water Performance.Effects of Cu 2 0 and TBTO antifouling coatings and cathodicprotection. Exposure Period of 675 days (22 months).

The data for the depth of corrosion and response to cathodicprotection at the bared window areas and under the paint onthe 6061-T6 aluminum exposed for 675 days in the PotomacRiver are summarized in Fig. 60. Uncoated and unprotected6061-T6 aluminum corroded to a depth of 20 mils. The datain Fig. 60 and the photograph in Fig. 61 show that theCu 2 0 antifouling paint increased the depth of corrosion.

The data in Figure 60 and a comparison of Figs. 61 and 62show that cathodic protection from an aluminum anodesignificantly reduced the depth of corrosion caused by theCu 0 antifouling paint. As indicated in other phases ofthis study, the TBTO antifouling paint caused less severecorrosion on the aluminum alloy, and the combination of TBTOand cathodic protection from an aluminum anode resulted inthe greatest corrosion protection. Photographs of the corrosionon the unprotected and protected specimens of 6061-T6 aluminumcoated with TBTO are shown in Figs. 63 and 64, respectively.

Effectiveness of Antifouling Coatings and Response toCathodic Protection.

A qualitative summary of the corrosion of 5086-H32 and 6061-T6 aluminum alloys, the corrosion caused by antifouling

PN paint toxicants, the response to cathodic protection, andthe order of ranking of each material in the specificenvironment are shown in Table 1. This should be a usefulguide in appraising the relative performance of these alloyswhen bare or coated with Cu 2 0 or TBTO type antifouling paints.

9

Bare 5086-H32 aluminum was highly corrosion resistant whencontinuously immersed in either quiescent seawater or inthe Potomac River even without cathodic protection. Froma corrosion viewpoint TBTO is preferred over Cu2 0 as anantifouling toxicant on 5086-H32 aluminum. When TBTO wasused in conjunction with cathodic protection corrosion ofthe aluminum was at a minimum. In the Potomac River bare5086-H32 aluminum either with or without cathodic protectionwas least corroded.

Table 1 also shows that bare 6061-T6 aluminum was severelycorroded in both seawater and Potomac River water when notcathodically protected. In either environment the mostsatisfactory corrosion protection was obtained with theTBTO type antifouling paint and cathodic protection from analuminum anode.

Marine Growth (Fouling) Characteristics

Uncoated (bare) 5086-H32 and 6061-T6 aluminum fouledseverely in seawater. In the Potomac River the only foulingobserved was slime.

The Cu 2 0 and TBTO antifouling paints resisted marine growthequally well for approximately 10 months in seawater. At10J months the first growth of barnacles was noted on someof the TBTO coated specimens. The Cu 2 0 coated specimenswere free of fouling during the same time period.

After 809 days (261 months) in seawater a difference in theeffectiveness of the antifouling coatings was more evident.The Cu 2 0 coated specimens were still essentially free ofmarine fouling, but the TBTO coated specimens were fouledover approximately 20 percent of the surface. There wasalso a slight discernible difference in the foulingcharacteristics attributable to the cathodic protectionfrom the aluminum and zinc anodes. The slight reductionin antifouling properties attributable to cathodicprotection was evident only on the specimens coated withthe TBTO antifouling paint.

The characteristics of the antifouling paints on 5086-H32and 6061-T6 aluminum after 1264 days in seawater aresummarized in Table 2. These data show that moderatefouling was observed on the Cu 2 0 coated specimens andmoderate to heavy fouling with a few barnacles on the TBTOcoated specimens. Essentially no difference in the degreeof fouling was observed on the protected and unprotectedspecimens after 1264 days in seawater.

10

There was a marked difference between the color retentionon the Cu 2 0 and TBTO coated specimens. The Cu20 anti-fouling paint changed from a deep red color to that of theusual green patina on copper, while the TBTO paintessentially retained its original reddish color.

In the Potomac River, specimens coated with TBTO initiallyseemed to have less slime on the bared window areas, butthis was only a temporary effect, i.e., for a time periodmuch less than the 675 days exposure period. Slime wasthe principle type of fouling observed on the specimensexposed in the Potomac River.

SUMMARY

1. Bare. uncoupled, and unprotected 5086-H32 aluminum wasinherently corrosion resistant in quiescent seawater, inlake watel; and in the Potomac River.

2. The following dissimilar metals: copper nickel, 10%/o;yellow brass; 304 stainless steel; and mild steel,accelerated the corrosion of 5086-H32 aluminum in seawater.Cathodic protection from aluminum anodes reduced the depthof corrosion caused by the dissimilar metals, but was notcompletely effective. There were indications that a pro-longed incubation period existed before the acceleratedcorrosion, caused by the copper nickel, 10%; and 304stainless steel, became evident on the aluminum. Cathodicprotection from a magnesium anode also caused acceleratedcorrosion of the aluminum.

3. Cracks were noted on the circular welded uncoatedspecimens of 5086-H32 alumninum in seawater even whencathodically protected, but they may have been presentprior to exposure.

4. The depth of corrosion on 5086-H32 aluminum caused bythe dissimilar metals was greater in the Potomac Riverthan in seawater. Each of the dissimilar metals, coppernickel, 10%; yellow brass; 304 stainless steel; and mildsteel, caused accelerated corrosion of the aluminum exposedin the Potomac River. Cathodic protection did notsignificantly reduce the depth of corrosion caused bycopper nickel, 10%; yellow brass; and 304 stainless steelexcept in crevices formed by the mounting spacers. Cathodicprotection was effective,however, and did significantlyreduce the depth of corrosion on 5086-H32 aluminum causedby the mild steel in the Potomac River exposure, but: notin the freshl water lake exposure. The incidence ofcorrosion on the 5086-H32 aluminum caused by the dissimilarmetals was reduced by cathodic protection.

11

5. Cracks were evident on the welded specimens of 5086-H32aluminum exposed in the Potomac River but the response tocathodic protection could not be established becausespecimens were lost due to circumstances beyond our control.These cracks may have been present prior to exposure.

6. Cu2 0 antifouling paint applied over a vinyl anti-corrosive barrier coating accelerated corrosion at baredareas on 5086-H32 aluminum in seawater. Cathodic protectionreduced the corrosion caused by the Cu2 0 toxicant in theantifouling coating, but did not eliminate it.

7. TBTO antifouling paint did not accelerate the corrosionof 5086-H32 aluminum in seawater even at bared areas in thevinyl anticorrosion barrier coating. The best corrosionmitigation on the coated aluminum specimens was obtained,however, when TBTO was the toxicant in the antifoulingpaint and the specimens were cathodically protected.

8. Severe corrosion was caused by the Cu2 0 antifoulingpaint at bared areas on 5086-H32 aluminum exposed in thePotomac River despite the vinyl anticorrosive barriercoating. The corrosion caused by the Cu2 0 was more severein the river exposure than in seawater. TBTO antifouling,paint did not cause as severe corrosion as the Cu2 0 anti-fouling paint in the river exposure. Cathodic protectionon either the Cu2 0 or TBTO coated specimens did not reducethe depth of corrosion below that observed on the un-protected TBTO coated specimens.

9. Bare, uncoupled and unprotected 6061-T6 aluminum wasseverely corroded in quiescent seawater or in the PotomacRiver.

10, Unprotected specimens of 6061-T6 aluminum coated withCu2 0 were severely corroded in seawater, but the depth ofcorrosion was less than on the uncoated specimen. Thedepth of corrosion on the unprotected specimen coated withTBTO antifouling paint was approximately one-half thatobserved on the unprotected specimen coated with Cu2 0.Cathodic protectioip reduced the depth of corrosion on theCu2 0 coated specimen, but the most effective anticorrosivesystem for 6061-T6 aluminum in seawater was TBTO anti-fouli'ng paint over a vinyl anticorrosive barrier andcathodic protection from an aluminum anode.

11. Cu2 0 antifouling paint significantly accelerated thedepth of corrosion on 6061-T6 aluminum in the PotomacRiver despite the vinyl anticorrosive barrier coating.Cathodic protection from an aluminum anode reduced thedepth of corrosion caused by the Cu2 0 toxicant but the

12

7

least corrosion occurred when the antifouling toxicant wasTBTO and the specimen was cathodically protected.

12. Bare 5086-H32 and 6061-T6 aluminum alloys were heavilyfouled by marine growth during exposure in quiescent sea-water. In the Potomac River these aluminum alloys developedonly slime on their surfaces.

13. After 1264 days in seawater moderate fouling wasobserved on the Cu2 0 coated specimens and moderate to heavyfouling with a few barnacles was observed on the TBTOcoated specimens. Cathodic protection had little or noeffect on the degree of fouling observed after 1264 daysin seawater.

14. There was a marked difference between the colorretention on the Cu 2 0 and TBTO coated specimens with theTBTO coated specimens essentially retaining their originalreddish color.

A CKNOWLEDGMENT

The assistance of Messrs. C.W. Billow and W. Lazier (retired)of the NRL Marine Corrosion Research Laboratory, Key West,Florida, in conducting the experimental phase of thisstudy is acknowledged. The coated aluminum specimens wereprepared by Mr. W.J. Francis of the Chemical Laboratory,Norfolk Naval Shipyard. This work was sponsored hy theNaval Ship Systems Command.

REFERENCES

1. B.F. Brown et al., "Marine Corrosion Studies (SecondInterim Report of Progress),' NRL Memorandum Report 1574,Nov 1964.

2. B.F. Brown et al., "Marine Corrosion Studies (ThirdInterim Report of Progress)," NRM Memorandum Report 1634,July 1965.

3. T.J. Lennox Jr., et al., "Marine Corrosion Studies(Fourth Interim Report of Progress)," NRL Memorandum Report1711, May 1966.

4. T.J. Lennox, Jr., M.H. Peterson,and R.E. Groover,"Corrosion of Aluminum Alloys by Antifouling Paint Toxicantsand Effects of Cathodic Protection," Proceedings NACE 24thConference, Cleveland, Ohio, March 18-22, 1968. NationalAssociation Corrosion Engineers, Houston, Texas, 1969.

13

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.0 DEEPEST CORROSION* AV 5 DEEPEST0 WEIGHT LOSS

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50-

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MC 391Fig. 2 - Welded 5086-H32 aluminum; with and without cathodic

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17

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'• ad wihoutcathdi."protection (CP), 809 days in seawater at•Key W~est, Florida.

60o DEEPEST CORROSION* AV 5 DEEPEST* WEIGHT LOSSI-SEVERE EDGE

CORROSION ON THE

50- UNPROTECTEDSPECIMEN

BENEATHDISSIMILARC,,

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Al MC395

Fig. 6 - 5086-H32 aluminum coupled to yellow brass; with andwithout cathodic protection (CP), 809 days in seawater atKey West, Florida.

21

600 DEEPEST CORROSION0 AV 5 DEEPESTM WEIGHT LOSSi-SEVERE EDGE

CORROSION ON THEUNPROTECTED50-MOUNTING SPECIMEN

CREVICEAREAS

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Fig. 7 - 5086-H32 aluminum coupled to 304 stainless steel; withand without cathodic protection (OP), 809 days in seawater atKey West, Florida.

22

600 DEEPEST CORROSION* AV 5 DEEPEST0 WEIGHT LOSS

1-CONSIDERABLE EDGECORROSION ON THE

50 UNPROTECTEDSPECIMEN

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(n)U,)o 40_j

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Fig. 8 - 5086-H32 aluminum coupled to mild steel; with andwithout cathodic protection (CP), 809 days in seawater atKey West, Florida.

23

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10 -ANODES CREVICEAREAS

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24

L

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Fig. 10 - 5086-1132 aluminum coupled to co ppe r nickel, 10%; uith-

out cathodic protection, 809 days in seawater at Key West, Florida.

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25

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Original magnification 1.2 X

30

V/

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MC405

Fig. 16 - 5086-1132 aluminum coupled to 304 stainless steel; withcathodic protection, 809 days in seawater at Key West, Florida.Original magnification 1.2 X

31

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Fig. 17 - 5086-1132 aluminum c o u p 1 e d to mild steel; withoutcathodic protection, 809 days in seawater at Key West, Florida.Original magniification 1.2 X

-ii

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IrL-

MC 407

Fig. 18- 5086-H32 aluminum coupled to mild steel; withcathodic protection, 809 days in seawater at Key West, Florida.Original magnification 1.2 X

33

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Fig. 19 - 5086-1132 aluminum; without cathodic pro-tection, 809 days in seawater at Key W~est, Florida.Original magnification 1.2 X

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0 AV 5 DEEPESTM WEIGHT LOSSi,-CRACKS EVIDENT IN

WELDCtR.

50 WELD 2-UNDERCUT ® WELDNO .3-SPECIMENS WITH CPCP WERE LOST WHEN

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U,)

o40

I" LINE0. WELD GENERAL

w AREAS3

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ciR. LINE NO NO CREVICEWELD WELD WELD WELD-3NO NO NO CPCP CP CP Al F CIR. LINE SfTWELD WELDT

NO NOCP CP MC413

01 T

Fig. 24 - Welded 5086-H32 aluminum after 675 days

in the Potomac River.

39

In

c'i

-- 1 4

0 .0

p.- 1 df~~Ž. a Q .4)C

"Kf

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40

6460 61

INOI 0 DEEPEST CORROSIONICPI 0 AV 5 DEEPEST

I WEIGHT LOSScP I-SEVERE EDGEA) CORROSION ON THE

50- UNPROTECTED SPEC-IMEN. EDGE CORRO-SION NOT ELIMINATEDBY CATHODIC PRO-

BENEATH ,TECTION. S~DISSIMILARMETAL GENERAL SOME AREAS S•, • AREAS0- .0 MOUNTING BLISTER-LIKE040- CREVICE APPEARANCE WITH-J CPARS-A AREAS EXFOLIATION.NO•_ cPII

N.NO

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X 20-0

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Fig. 26 - 5086-H32 aluminum coupled to copper nickel, 10%;with and without cathodic protection (CP), 675 days in thePotomac River.

41L

60

0 DEEPEST CORROSION* AV 5 DEEPESTM WEIGHT LOSS

GENERAL I-SEVERE EDGE CORRO-AREAS SION ON THE UNPRO-50

TECTED SPECIMEN.NO cEDGE CORROSION NOTELIMINATED BY

CATHODIC PROTEC-TION.

o) BENEATH640- DISSIMILAR

METAL3, -<> MOUNTING-ruJ i CREVICE

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C0ZP

Al NOI0.,

CP

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0

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Fig. 27 - 5086-H23 aluminum coupled to yellow brass; withand without cathodic protection (CP), 675 days in thePotomac River.

42

600 DEEPEST CORROSION

* AV 5 DEEPEST0 WEIGHT LOSS_. SEVERE EDGE

CORROSION50-

GENERAL

40- AREAS04--r

N C MOUNTING3: CREVICES~AREAS

_BENEATH H~ AREAo30- DISSIMILAR I!

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10

NON C0 P c A

or)

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Fig. 28 - 5086-H32 aluminum coupled to 304 stainless steel;with and without c a t h o d i c protection (C P), 675 days in thePotomac River.

43

60 o DEEPEST CORROSION

0 AV 5 DEEPESTM WEIGHT LOSS

.- SOME EDGE CORRO-SION ON THE UNPRO-

50 TECTED SPECIMEN

640-

W GENERALCf,

_j N

S•30-a

I-

w MOUNTINGaN CREVICE

0 AREAS

ct 20-

0NO

BENEATHDISSIMILAR

METAL10

AlcP

Al

NO CP MAX. DEPTH BARE, C UNPROTECTEDCP A) Al MC 4180 0 1cm• I. .

Fig. 29 - 5086-H32 aluminum coupled to mild steel; withand without cathodic protection (CP), 675 days in thePotomac River.

44

600 DEEPEST CORROSION0 AV 5 DEEPESTG WEIGHT LOSS

i-SCATTED ETCH PITS

NOTE: EDGE CORROSION50 NOT OBSERVED ON

ANY OF THESPECIMENS

(o

N40

-J

~3:I--cr)

S30-

w"120z0

0'•20-

0:0

BENEATHCATHODIC PROTECTION MOUNTING

10 ANODES GENERAL AREASAREAS

Mg NO CP CP C CPA)Cp-C- CP CP + CCP CP CP AI Zn Zn Mg Z gZnM9Al CP Al Z Z

NO CP CP + mg NO CP CP + I , $ NO CP rl0cP Al zn _.n -R- CP Al z, z__ _Q rOl rOl rO cP _Al0O CMC 419

Fig. 30 - 5086-I132 aluminum; with and without cathodicprotection (CP), 675 days in the Potomac River.

45

/

Fig. 31 - 5086-1132 aluminum coupled to copper nickel, 10%r;without cathodic protection, 675 days in the Potomac River.Original magnification 0.5 X

46

Op/

Id

Orgia manfiain .

0

MC422

Vig. 33 - 5086-1132 aluminum coupled to yellow brass; with-out cathodic pro toe t ion, 675 days in the Potomac River.Original magnification 0.5 X

'48

I't

V"

MC40

Fig 34 58-12au iu ouldt-elwbaswith cahodic p-otcin 7 asi-tePtmc1 vr

Orignal aguiic,-ion . 5

AO

V,

At

MC424

Fig. 35 5086-H32 aluminum coupled to 30,4 stainless steel;without cathodic protection, 675 days in the Potomac River.Original mag'nification 0.5 X

50

' I' m

Fig. 36 - 5086-1132 aluminum coupled to 304 stainless steel;with cathodic protection, 675 days in the Potomac Iliver.Original magnification 0.5 X

51

NJe

0 :

E

0 V

U -

0

SI ''MC 426

Fig. 37 - 5086-1132 aluminum coupled to mild steel; with-out cathod ic protection, 675 days in the Potomac River.Original magnification 0.5 X

52

O/

10

t

TV MC 427.

Fig. 38 - 5086-H32 aluminum coupled to mild steel; withcathodic protection, 675 days in the Potomac River.Original magnification O.5X

53

S

4O

9108 sPY

Fig. 39 - 5086-1132 aluminum; without cathodic

protection, 675 days in the Potomac River.

Original magnification 0.5 X

54

/

~ I Est availabl~e copy.

Fig. 40 - 5086-H32 aluminum; with cathodicprotection, 675 days in the Potomac River.Original magnification 0.5 X

55

/..

-7-

7:

E 0

/7

MC430

Fig. 41 - 5086-1132 aluminum; with cathodic protectionfrom a magnesium anode, 675 days inl the Potomac Rtiver.Original magnification 1.2 X

56

I.-w

J o'J

czr qz 0 P

a CDb4 O0.O,~m

z Xa oCD 0o 10

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Fig. 50 - 5086-1132 aluminum coated with the Standard Navy VinylAnticorrosive Barrier and Cu 2 0 Antifouling Toxicant; with andwithouteathodic protection (CP), 1264 days in seawater atK1ey West, Florida, Figures reduced 10% in printing.

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Pig. 59 - 6061-T'G ,qaluminuni coated with the Standard Navy VinylAnticorrosive Barrier and TBTO Antifouling Toxicant; with andwithout c a t hi o d i c p r o t e c t i o n (C P), 1264 days in seawater atR~ey West, Florida. P igures reduced 15%• in printing.

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'79