AD/A-002 234

CORROSION OF ALUMINUM ALLOYS IN EXFOLI-ATION-RESISTANT TEMPERS EXPOSED TO MARINEENVIRONMENTS FOR 2 YEARS

Ernest J. Czyryca, et al

Naval Ship Research and Development CenterAnnapolis, Maryland

November 1974

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4. TITLE (aid Subilsie) 5. TYPE OF REPORT & PERIOD COVERED

Corrosion of Aluminum Alloys inExfoliation-Resistant Tempers Exposedto Marine Envirorinents for 2 Years 6. PERFORMINGORG. REPORT NUMBER

7. AUTHOR(&) S. CONTRACT OR GRANT NUMBER(a)

Ernest J. Czyryca andHarv;ey P. Hack

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASKAREA & WORK UNIT NUMBERS

Naval Ship Research and Task Area SF 54 541 702Development Center Task 14626Annapolis, Maryland 21402 Work Unit 2814-143

It. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

Naval Ship Research and November 1974Development Center 13. NUMBER OF PAGES

Bethesda, Maryland 20084 234. MONITORING AGENCY NAME 6 ADORESS(II differen; from Controlling Office) IS. SECURITY CLASS. (of Lhi reporl)

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19. KEY WORDS (Continue on reverse side It neceeary and Identify by block number)

Corrosion behavior Splash and spray zoneAluminum alloys Marine atmosphereEnvironments SensitizedFully submerged PittingSea water Edge attack

20. ABSTRACT (Continue on reverse aide If necee bay and Identify by block nutmber)The corrosion behavior of 5086, 5083, and 5456 aluminum alloysin H116 and H117 tempers was evaluated after 2 years of exposureto three marine environments; namely, fully submerged in seawater, splash and spray zone, and marine atmosphere. The alloy,displayed good corrosion resistance with no exfoliation orpitting observed. Panels of the same alloys in a sensitized

(over)ropmJ

oJAN 1473 EDITION OF I NOV 65I, OBSOLETE U Lr.5/N 0102-014-6601 I __'

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.-onditiofl exposed to the same environments showed some pitting

and edge attack but did not indicate a long-term corrosion

problem.

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ADMINISTRATIVE iNFORMATION

This report was prepared under TOP 21, Task Area SF 54 541702, Task 14626 on Engineering Prcperties of Aluminum Alloys forLightweight Ship Structures, Work Unit 2114-143. The investiga-tion was sponsored by the NavAl Sea Systems Command (SEA 035).Mr. B. B. Rosenbaum (SEA 03523) is the program manager, and Mr.T. C. West, Naval Ship Engineeiing Center (SEC 6101.0), is t"etechnical agent.

AC KNOWLEDGMEN '

The authors wish to acknowledge the work of M. G. Vassilarosin specimen preparation and sensitization treatment.

4432 3

TABLE CF CONTENTS

Page

ADMINISTRATIVE INFORMATIONACKNOWLEDGMENTINTRODUCTIONMETHOD OF INVESTIGATIONRESULTS AND DISCUSSION 2

Microstructures 2Corrosion 4

CONCLUSIONS 8TECHNICAL REFERENCES 8LIST OF FIGURES

Figare 1 - Photomicrograph; Microstructure of5456-1321, 1/4-Inch Plate (50OX)

Figure 2 - Photomicrograph; Microstructure of5086-H116, 1/4-Inch Plate (500X)

Fiqure 3 - Photomicrograph; Microstructure of5036-11116, 3/4-Inch Plate (500X)

Figure 4 - Photomicrograph; Microstructure of5086-11117, 1/4-Inch Plate (500X)

Figure 5 - Photonicrograph; Microstructure of5083-H116, 1 /4-Inch Plate (50OX)

Figure 6 - Photomicrograph; Microstructure of5083-11116, 1/2-Inch Plate (50OX)

Figure 7 - Photomicrograph; Microstructure of5456-11116, 1/4-Inch Plate (500X)

u-gu-,u - - PhLOiLCu Mdph ici.ostructu.e of5456-11116, 1/2-Inch Plate (500X)

Figure 9 - Photomicrograph; Microstructure of5456-11117, 1/4-Inch Plate (500X)

Figure 10 - Photomicrograph; Microstructure of5456-H117, 1/2-Inch Plate (500X)

Figure 11- Photographs; Comparison of SurfaceAttack on As-Received and Sensitized5456-11117 1/4-Inch Plate Exposed toSplash and Spray and MarineAtmosphere

Figure 12 - Photoinicrograph; De] ami nation UnderBlisterincq on Sensitized 5456-H1171/4-Inch Plate After Splash andSpray Exposure (250X)

Figure i - Photograph; Wide, Shallow Pitting inSensitized 5083-11116 I/4.-Inch PlateAfter Fully Immersed Exposure

Figure 14 - Photographs; Edge Corrosioni; -,n 5083-1116 Panels Exposed to Flowing Sea Water

Figure 15 - Photographs; Edge Corrosion on 5456.-11116 Panels Exposed to Flowing Sea Water

INITIAL DISTRIBUTION

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INTRODUCTION

The aluminum-magnesium alloys, 5086, 5083, and 5456, areattractive materials for naval construction because of theirmarine corrosion resistance, high strength-to-weight ratio, andweldability. The alloys have been used by the Navy for light-weight superstructures and as the primary structural metal forhigh-speed, high-performance ships and craft. The most desirablecombination of properties in sheet and plate was achieved in amildly cold-worked temper (quarter-hard) designated as 5086-H32,5083-H321, and 5456-H321. However, under certain conditions,exfoliation corrosion problems were experienced with the standardtemper. Exioliation or lamellar corrosion is a type of inter-granular corrosion causing delamination in thin plate material.

To solve the exfoliation problem, the aluminum industrydeveloped rolling procedures for tempers meeting the mechanicalproperty requirements specified for the H32 and H321 tempers andhaving a metallurgical structure resistant to exfoliation-typecorrosion.1 '2 These tempers for sheet and plate high-Mg, 5000-series alloys are designated H116, developed by Reynolds MetalsCompany, and H117, developed by Alcoa. These exfoliation-resistant tempers are included in Interim Federal SpecificationsQQ-A-00250/19 and QQ-A-00250/20 for 5086 and 5456 alloys,respectively.

The purpose of this investigation was to study the corrosionbehavior of the three high-strength,Al-Mg alloys, 5086, 5083, and5456, in the exfoliation-resistant tempers when exposed to threemarine environments. This report summarizes the results forexposure durations of 6 months, 1 year, and 2 years.

METHOD OF INVESTIGATION

Panels were cut from plates of 5086-H116, 5086-H117, 5083-H116, 5456-H116, and 5456-H117 alloys. The test panel size,thickness, and source are listed in table 1. Specimen panelswere exposed with the as-rolled surface finish and with saw-cutor sheared edges.

Panels were tested in two conditions, viz, as-rolled andsensitized. The sensitizing treatment, 1 week at 100' C* in alaboratory oven, is an accelerated aging process to simulate theworst possible condition of the alloy microstructure after long-.erm service. Such a condition might occur in deck and super-structure applications where exposure to the sun and tropicaltemn)eratures are experienced.

2nnerscripts refer to similarly numbered entries in the Techri-cdil References at the end oi the text.*Ab1hrviations used in this text are from the GPO Style Manual,1/3, uniless otherwise noted.

TABLE 1MATERIALS UNDER INVESTIGATION

Plate NSRDC* TestAlloy Thickness Code Panel Size Sourceand Temper inch Letters inch

5086-H116 1/4 EST 12 x 3 Reynolds Metals Company5086-H116 3/4 ESX 12 x 3 Reynolds Metals Company5086-H117 1/4 ETJ 8 x 3 Alcoa5083-H116 1/4 ETC 12 x 3 Reynolds Metals Company5083-HI16 1/2 ETD 12 x 3 Reynolds Metals Company5456-H116 1/4 ESY 12 x 3 Reynolds Metals Company5456-H116 1/2 ESZ 12 x 3 Reynolds Metals Company5456-H117 1/4 ETK 8 x 3 Alcoa5456-H117 1/2 ETF 12 x 3 Alcoa

*NSRDC - Naval Ship Research and Development Center

Panels in both conditions were exposed to three differentmarine environments at the Francis L. LaQue Corrosion Laboratory,Wrightsville Beach, North Carolina. The three environments were:

e Completely submerged in a trough of slowing flowingsea water (2 to 3 ft/s).

9 Exposure in a "splash and spray" zone which is thelevel of wave breaking.

* Exposure to a marine atmosphere 80 feet from theshore.

Where sufficient material was available, duplicate specimens weretested for every exposure condition. A total of 282 specimenswas included in the program. Specimens were removed after expo-sures of 6 months, 1 year, and 2 years. Note that specimens werenot cleaned, inspected, and reexposed; rather, each exposure wasfor the total duration, and specimens were left undisturbed duringthe entire period of the specific ttst.

RESULTS AND DISCUSSION

MICROSTRUCTURES

Exf, liation or lamellar corrosion is a specific type ofintergranular corrosion attack which can occur along grainboundaries parallel to the metal surface of aluminum productshaving an elongated grain structure, such as light gage plate orextruded shapes. The generation of corrosion products forces the

2 2

uncorroded layers apart and causes the metal to swell and delami-nate or flake apart. For those AI-Mg alloys containing greaterthan 3% Mg, susceptibility to exfoliation is dependent on theamount of cold work introduced.3 Plate and sheet having beenseverely rolled show a striated grain structure with precipitatein the grain boundaries as a continuous line (figure 1). Theprecipitate (Mg2AI3 ) is anodic to the solid solution in the grainbodies and corrodes preferentially.3 It is the continuity of theprecipitate which makes the structure liable to exfoliationattack.

The H32 and H321 tempers apply to products which are strainhardened and then stabilized by a low-temperature heat treatmentto slightly lower the strength and to increase ductility andstress-corrosion resistance. This process can result in a micro-structure in which the precipitate is present in a continuous line.Microstructures of as-received 5456-H321 plate and the samematerial sensitized (I week at 1000 C) are shown in figure 1.With heavy lamellar precipitate already present in the as-receivedmicrostructure, sensitizing had little effect in furthering theprecipitation.

The new tempers, 1116 and H117, apply to products which arestrain hardened less than quarter-hard and do not undergo astabilizing heat treatment. The objective of these tempers is toprovide material having a metallurgical structure with a discon-tinuous network of precipitate. Such a structure should not besusceptible to exfoliation. Figures 2 through 10 show the micro-structures of the new tempers for the alloys and plate thicknessesunder the present investigation in both as-received and sensitizedconditions. Note the discontinuous network of the precipitate inthe as-received plates.

Interim Federal Specifications QQ-A-00250/19 (Navy-Ships)for 5086 plate and sheet and QQ-A-00250/20 (Navy-Ships) for 5456plate and sheet in the H116 and H117 tempers require, as part ofthe material qualification procedure, that samples of productionlots be examined metallographically. The examination must show amicrostructure predominantly free of a continuous grain boundarynetwork of Al-Mg precipitate. Ihe as-received microstructures ofplates of all alloys and thicknesses for the 11116 and H117 tempersin this study meet the requirement (figures 2 through 10).

The H116 and HI17 tempers represent two different approachesfor providing a discontinuous precipitate network. In the H116temper, the precipitate is dispersed throughout the metal in dis-,onnccted paths, while the 11117 process prevents the formation ofpaths of precipitate along the grain boundaries. It was observedthat in the as-received condition, the alloys in H117 temper(figures 4, 9, and 10) had a sparse population of precipitates in':oittparison to those in H1.16 temper. However, after the sensitiz-ing treatment, the 11117 tempers show heavy, continuous "stringers"(,r prf:cip tate, whereas the alloys in the 11116 temper show only aI i'jht incr(eaise in precipitate density and continuity. It seems,

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therefore, that inherent in the H116 tempering procedure is astabilizing process which inhibits further growth of precipitate.The H117 tempers appear to hold much Mg in solution. Conse-quently, this less stable condition is more responsive to thesensitizing treatment.

The sensitization treatment may be more severe than naturalaging. Sensitization treatments may be considered a means ofobtaining a conservative evaluation of long-term corrosion per-formance when long-term data are not available, and the longdelay involved in complete evaluation under conditions of naturalaging and exposure is unreasonable.

CORROSION

The results of exposure to the three environments are summa-rized in table 2 for panels exposed to marine atmosphere, intable 3 for panels exposed to splash and spray, and in table 4for the fully submerged panels. The results are given in termsof corrosion rates in mils per year, based on weight loss andexposed surface area. In general, the calculated rates for anyexposure and condition were less than 1 mil/yr and decreased withtime, i.e., the initial'attack is highest.

The panels exposed to marine atmosphere and to splash andspray showe d a scattered light pitting on surfaces, with aslightly greater intensity in the latter exposure. The pittingcan best be described as "pinpoint" and of insignificant depth.The one exception to this general behavior was that of the sensi-tized, 1/4-inch-thick 5456-H117 panels. In both environments,the attack on these panels appeared as a minor scattered surfaceblistering. The attack was first noted after 1 year of exposure,being the only instance of increased corrosion rate at that time;however, the rates after 2 years of exposure show the attack tohave virtually ceased.

Surface attack on the as-received and the sensitized 5456-H117 i/4-inch panels after 2 years of exposure to splash andspray and marine atmosphere is compared in figure 11. Whitecorrosion product was present under the blisters, and an areaabout 1/2 x 1/8 inch near the edge of the sensitized panelexposed to splash and spray showed evidence of delamination(figure 12). Corrosion rates based on weight losses of the 1/4-inch panels ranged from nil to 0.10 mil/yr in both the splash andspray exposure and marine atmosphere.

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TABLE 2CORROSION TEST RESULTS FOR ALUMINUM ALLOYS IN

MARINE ATMOSPHERE 80 FEET FROM OCFAW

Corrosion RateAlo an late Mi /Yr

Alloy and T es/ Corrosion DescriptionTemper inch 6 1 2 After 2 Years

As-Rolled Condition

5086-H116 1/4 0.10 0.09 Nil5086-11117 1/4 0.10 0.13 Nil5083-11116 1/4 Nil 0.11 Nil5456-H116 1/4 0.10 0.11 Nil

5456-H!17 1/4 O.10 0.13 0.10 Scattered light pitting

5083-11116 1/2 3.30 0.13 0.105456-H116 1/2 0.35 0.13 0.055086-11116 3/4 0.50 0.12 0.101

Sensitized Condition

5086-HI16 1/4 Nil 0.17 Nil Scattered light pitting5086-H117 1/4 0.20 n.17 Nil Scattered light pitting5083-11116 1/4 Nil 0.33 Nil Scattered light pitting5456-11117 1/4 0.3u 0.65 Nil Moderate light blistering5083-1111b 1/2 0.40 0.31 0.05 Scattered light pitting5456-11116 1/2 0.40 0.23 0.10 Scattered light pitting5086-H116 3/4 0.50 0.26 0.10 Scattered light pitting

TABLE 3CORROSION TEST RESULTS FOR ALUMINUM ALLOYS IN

SPLASH AND SPRAY ZONE

Cor rosizon Rate:F-

12 n Plate ml,,r Corroson Description

After 2 Years

Amu -n. i tc L r.t ion

%~~~~~~ I;bH1 n/ )! .,j,,

1 -L16l, 1 /4 N .I Ni1 I4i -1111. 1 4 1 k.I0 Scattered light pitting

1 i 1 4 13 '. 10

4 4 I ' 1.h

1 Sc tt re . ii-ht pon

1 - .1 I' I Scattcred light pitting, -,-ii ; 1 . ' , I1 :ii. Sattt red light pitting* >, -UH'], 1.4 ,,2 .1 N;i Scattered light pitting

..- HII, 1.4 M.o . . I erate light blistering<.,-!ti1 , 1;2 ,. .1i f,.11, Scattered light pitting

,. ' 1 2.1. '.)1K Scittercd light pitting<, ~ ~ ~ ~ ~ ~ ~ ~ ~ ) .... :! ' ; .. ' '. - I f;l, Sc,,.ttc:red light pitting

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TABLE 4CORROSION TEST RESULTS FOR ALUMINUM ALLOYS

FULLY SUBMERGED IN FLOWING SEA WATER

Corrosion Rate

Alloy and Plate mi!/yr Corrosion Description Edge AttackTemper Thickness After 2 Years After 2 YearsTemer inch 6 1 2

Months YearlYears

As-Rolled Condition

5086-H116 1/4 C.80 0.53 0.45 Light uniform attack, Moderatemoderate slight pitting

5086-HI17 ±/.: 0.90 0.52 0.40 Light uniform attack, Noneno pitting

5083-H116 1/4 0.75 0.47 0.40 Light uniform attack, Very slight and localminor incipient pitting

5456-H116 1/4 0.80 0.48 0.40 Light uniform attack, Noneno pitting

5456-H117 1/4 0.70 0.48 0.30 Light uniform attack, Noneno pitting

5083-H116 1/2 0.85 0147 0.50 Light uniform attack, Moderate and localminor incipient pitting

5456-11116 1/2 0.70 0.50 0.45 Light uniform attack, Moderate and localminor incipient pitting

5456-EI17 1/2 0.60 0.50 - Light uniform attack. None*no pitting*

5086-H116 3/4 1.00 0.43 0.30 Light uniform attack, Slight and localno pitting

Sensitized Condition

5086-H116 1/4 0.70 0.57 0.40 Light uniform attack, Slightno pitting

5086-H117 1/4 0.80 0.52 0.40 Light uniform attack, Noneno pitting

5083-EI116 1/4 0.70 0.51 0.80 Light uniform attack, Severesevere shallow pitting

5456-11117 1/4 0.70 0.56 0.50 Light uniform attack, Noneno pitting

5083-H116 1/2 1.15 0.56 0.40 Light uniform attack, Moderateminor shallow pitting

5456-H!16 1/2 1.85 1.02 1.15 Light uniform attack, Severeminor incipient pitting

5456-HI17 1/2 0.80 0.85 - Light uniform attack, Severe*

no pitting*

5086-1116 3/4 1.10 0.55 0.40 Liqht uniform attack, Moderateno pitting

*After I ycar in test.

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The highest corrosion rates resulted from the fully sub-merged exposure where the surface generally showed a light,uniform corrosion with minor pitting in some cases. Severe edgeattack occurred in most alloys and thicknesses in both theas-received and sensitized conditions. Based on weight loss, theaverage corrosion rate for the 1/4-inch-thick panels whichexperienced no edge attack (5086-H117, 5456-H116, and 5456-H137)was 0.80 mil/yr in the first 6 months (i.e., 0.40 mil of metalloss in 6 months). The average corrosion rate after the firstyear was 0.52 mil/yr (0.52 mil of metal loss) and 0.39 mil/yrafter the second year (0.78 mil of total metal loss). Thus, theactual corrosion rate in the second 6 months was 0.24 mil/yr (or0.12 mil of metal loss in the second 6 months) and 0.26 mil/yrin the second year (or 0.26 mil of metal loss in the second year).This analysis indicates that the general corrosion rate reducesto a low, uniform rate after the initial attack of the first 6months.

The pitting attack on the fully immersed panel surfaces wasminor and shallow in the alloys of H116 temper; alloys of theH117 temper experienced no significant local surface attack.Severe shallow pitting was experienced, however, in the sensi-tized 5083-H116, 1/4-inch plate, as shown in figure 13.

Edge attack generally occurred in the same panels thatexhibited some form of pitting attack. Attack was most severe inthe sensitized 5083-H116, 1/4-inch plate and sensitized 5456-H116,1/2-inch plate. Figures 14 and 15 show the attack in these speci-mens compared to the as-received condition and different thick-nesses. Although the attack occurred in some of the as-receivedplates, sensitized specimens showed the more severe attack. Thealloys in the 8117 temper did not exhibit edge corrosion exceptfor the sensitized 1/2-inch-thick 5456-H117. Both the massivepitting and the severe edge attack were found to be conventionalintergranular corrosion. By this process, corrosion of the grainboundaries tends to spread out in all directions, removing wholegrains, and causing an area of intense local attack.

The slight pitting and edge attack in the as-received plateswere insignificant and would probably be prevented in service bypaint; however, buttering of exposed edges with weld metal belowthe waterline is suggested for com.alete immunity to edge attack.Although alloys in a sensitized condition showed an increasedseverity of pitting and edge attack, none of the observationsindicates a corrosion problem beyond routine maintenance in thelong-term usage of these alloys in marine applications.

In a previoats investigation,' the same alloys in severalsheet gages in standard tempers (0, -H14, -H34, and -H321) werepartially immersed in sea water for exposure times up to 7 years.General corrosion damage was mild, characterized by shallowpitting. The present study confirms the excellent marine corro-sion resistance of the commercial 5000-series ai±oy, in the new[11(, and 11117 tempers.

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CONCLUS IONS

The results of the investigation reported herein may besummarized as follows:

* Metallographic analysis of as-received plates ofAl-Mg alloys 5086, 5083, and 5456 in 111$ and H117 tempers inthicknesses from 1/4 to 3/4 inch showed structures with discon-tinuous or randomly dispersed precipitate network ecessary forexfoliation resistance.

* The alloys in H117 temper showed continuous precipi-tate network after a sensitizing treatment. The same alloys inH116 temper realized only a slight increase in precipitatedensity and continuity, suggesting that the H16 temper producesmaterial less susceptible to natural aging.

* The results of 2 years of exposure to marine environ-ments (marine atmosphere, splash and spray, and fully submergedin sea water) indicate that alloys 5086, 5083, and 5456 in the11116 and H117 tempers have good corrosion resistancc. No exfoli-ation attack was evident on any test -anei.

TECHNICAL REFERENCES

1 - Brooks, C. L., "Aluminum-Magnesium Alloys 5066 and 5456-H116," Naval Engineers Journal, Vol. 82, No. 4, pp. 29-32(Aug 1970)

2 - Wood, C., Jr., 'Selecting Wrought Aluminum Alloys for MarineUse," Aluminuii Company of America (June 1969)

3 - Binger, W. W., et al, "Resistance to Corrosion and StressCorrosion," Aluminum, Vol. 1, Chapter 7, Amer. Soc. forMetals (1967)

4 -- Niederberger, P. B., et al, "Corrosion and Stress Corrosionof 5000-Series Al Alloys in Marine Environments," Corrosion,Vol. 22, No. 3, pp. 68-73 (Mar 1966)

4432 8

As Received Sensitized 1 Week at 1000 C

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S 4 s*.. 7r

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Figure1Microstructure of 5456-11321

1/4-Inch Plate (SOOX)

As Received Sensitized 1 Week at 1000 c

44

'e Figure 2

Liiic-Lostucture of 50b6 8116l/4-Inch Plate (5OOX)

4322

As Received Sensitized 1 Week at 10cr C

-* * ._ . , ,0 r ., .---. -

Fiur 3&

• - • , * • " .*

- *. -- . --. - .

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Figure 3Microstructure of 5086-11116

3/'i-inch Plate (50OX)

As Received Sensitized 1 Week at 1000 C

.. .. - . .. **,, • _ r -

= . -.

r 4'T..... 4* "-.. * . :'2 . ' * - ;, . .

Figure 4Microstructure of 5086-11117

1/4-Inch Plate (500X)

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- *. *~ 4~.77

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Figure 5Microstructure of 5083-H116

1/4-Inch Plate (5OOX)

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AS AReceived

Figure 7Microstructure of 5456-H116

1/4-Inch Plate (5OOX)(Sensitized Condition Not Teste:-d)

As Received Sensitized 1 Week at 1000 C

Figure 8Micorostructure of 5456-H116

4432

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* . , .----- -

Nor,-

Mirsrutr of 5456-1111

As -. e t _

- *

Figure i0Microstructure of 5456-LIM7

i/2-Incn] Plate (t)OU).)

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Figure 13

Wide, Shallow Pitting inSensitized 5083-11116

1/4-Inch Plate After Fullyimmersed Exposure

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