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Development of Test Specimens for Evaluating SCC Behaviorof Marine Aluminum Alloys and Assessment of the Severity

of Various Marine Exposure Conditions

Francine BovardAlcoa Technical Center

100 Technical DriveAlcoa Center, PA, 15069

Christine HenonAlcan Centre de Recherche de Voreppe

BP27, Parc Economique Centr AlpF-38341, Voreppe, France

Rebecca WyssAlcoa Technical Center

100 Technical DriveAlcoa Center, PA, 15069

Catherine Wong, Ph D

NAVSEA1333 Isaac Hull Avenue SE

Washington, DC 20376

Jerry CurranASRC Aerospace

NASA CorrosionTechnology LaboratoryKennedy Space Center, Florida, 32899

Jim TowersKvichak Marine Industries, Inc.

496 NW Bowdoin PlaceSeattle Washington, 98107

Jacob PadrulAlcan GATI

9399 West Higgins Rd, Suite 820Rosemont, IL 60018

ABSTRACT

An evaluation of the relevance of the mass loss in ASTM G671to the in-service stress corrosioncracking (SCC) performance of high magnesium aluminum alloys is needed to validate the mass lossrequirements in ASTM B928/B928M.2However, uniaxial SCC testing of thin gauge aluminum alloys in

orientations other than short transverse (ST) cannot reliably predict SCC performance for in-serviceapplications with an ST stress component.3The objectives of this study were to identify an SCC testspecimen and evaluate various exposure conditions. The suitability of U-Bend specimens (per ASTMG30)4to study stress corrosion cracking was established using 5083-H321 material that was thermallytreated to intentionally create a sensitized condition. The testing, which was conducted at NASAKennedy Space Center Beach Corrosion Test Site (BCTS), demonstrates the relative severity ofseacoast atmosphere, full seawater immersion, seawater spray zone, and tidal exposures. Results ofthis study will be used to select the environmental conditions for additional testing of 5xxx materials with

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varying degrees of intentional sensitization to validate the mass loss requirements specified in ASTMB928/B928M.

Key words: 5083-H321, ASTM B928/B928M, intergranular corrosion, stress corrosion cracking,seacoast atmosphere, full immersion, spray, seawater, tidal, sensitization, U-bend, pre-form.

INTRODUCTION

Although 5xxx series wrought aluminum alloys generally exhibit excellent corrosion resistance,exposure to moderately elevated temperature can cause sensitization due to precipitation of the -phase (Mg2Al3) on the grain boundaries. The susceptibility to intergranular forms of corrosion includingintergranular stress corrosion cracking (IGSCC) increases with increased continuity of the grainboundary precipitation.3,5,6

Cracking incidences in service environments and in laboratory tests have demonstrated that the loadingmode is also important in the SCC performance of these materials. Previous testing7 hasdemonstrated that severely sensitized 5xxx materials exposed while stressed to 75% of the tensile yieldstrength in longitudinal (L) or long transverse (LT) orientation did not fail even after exposure for up toone year in the 3.5% NaCl alternate immersion environment. The same severely sensitized materialsfailed rapidly when stressed in the ST orientation at stress levels as low as 25% of the tensile yield

strength. Because of the particularly strong dependence of SCC susceptibility on the stress orientation,it is clear that uniaxial SCC testing of thin gauge aluminum alloys in orientations other than shorttransverse (ST) cannot reliably predict SCC performance for in-service applications with an ST stresscomponent.3

The ASTM B928/B928M material specification for marine sheet and plate alloys has lot acceptancerequirements that invoke the use of nitric acid mass loss testing (per ASTM G67) to verify that the as-produced material is resistant to intergranular forms of corrosion. Prior to the publication of ASTMB928/B928M in 2004, testing per ASTM G67 was not a requirement for these materials. The ASTMG67 test method provides guidelines regarding interpretation of the mass loss data, which wereadopted in ASTM B928/B928M. An evaluation of the relevance of the mass loss in ASTM G67 to thein-service performance of high magnesium aluminum alloys is still needed to validate the mass loss

requirements in ASTM B928/B928M.

Testing of sensitized material was conducted at NASA Kennedy Space Center Beach Corrosion TestSite (BCTS), to determine the relative severity of seacoast atmosphere, full seawater immersion,seawater spray zone, and tidal exposures. Results of this study will be used to select theenvironmental conditions for additional testing of 5xxx materials with varying degrees of intentionalsensitization to validate the mass loss requirements specified in ASTM B928/B928M.

EXPERIMENTAL PROCEDURE

The material used for this study was 0.19 (4.8mm) thick 5083-H321 sheet. Pieces of the 5083-H321were given thermal treatments of 28 days at 120C (248F), 6 days at 150C (302F), or 6 days at

175C (347F) to intentionally sensitize the material. Mass losses for the materials were measured perASTM G67.

Longitudinal (L) and long transverse (LT) oriented U-bend and pre-form specimens were prepared perASTM G30. The appearance of the U-bend and pre-form specimens is shown in Figure 1 and thedimensions of the specimens are provided in Table 1. Laboratory SCC testing of both types of

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specimens was conducted on triplicate L and LT specimens of each thermal exposure condition byalternate immersion per ASTM G44 in synthetic seawater (ASTM D1141).8

The material thermally treated 28 days at 120C was used to prepare LT oriented U-bend specimens fortesting in marine environments at the NASA Kennedy Space Centers Beach Corrosion Test Site. Fivereplicate specimens were tested in each four different exposure conditions: seacoast atmosphere, fullseawater immersion seawater spray zone, and tidal exposures. Images of the U-bend specimens in thevarious testing environments are shown in Figure 2. Specimens were inspected weekly for the firstmonth and bi-weekly for months 2-6.

Table 1. Dimensions in inches of U-bend and pre-formspecimens as defined in Figure 2 of ASTM G30

SPECIMEN L M W T D X Y R R/T

Ubend 12 11 0.8 0.19 0.375 3.5 3.88 1.75 9.21

Preform 12 11 0.8 0.19 0.375 3.5 4.25 1.75 9.21

RESULTS

Figure 1: Schematic illustration of U-bend and pre-form specimens with dimension IDs.

Y=3.88"

Y= 4.25"

L

M

D (hole diameter centered at W/2)

R

X

W

T

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The environmental conditions for the exposures at NASAs Beach Corrosion Test Site were as follows:

1. Seacoast Atmosphere (Beach front) The U-bend specimens were exposed to a naturalseacoast environment. The specimens were fixed to racks that are oriented toward the AtlanticOcean. The racks were attached to stands that were positioned approximately 150 feet from thehigh tide line, and remained in that location throughout the duration of exposure.

2. Seawater spray/simulated splash (Splash zone) - This procedure exposed the U- bendspecimens to the effects of a seawater spray/splash environment. The seawater spray/splash

test utilizes a 1 hour cycle that includes a 10 minute period of exposure to seawater that issprayed onto the surface of the specimens, followed by a 50 minute drying period. A freshsupply of natural seawater drawn from the base of the Atlantic Ocean is used for all testing.This cycle is repeated 24 hours a day for the duration of exposure, except for brief durationsthat were required to evaluate and photo-document the specimens. The alternating duration ofexposure to seawater spray and drying is similar to the ASTM G44 laboratory testing.

3. Full Immersion Exposure This experimental protocol mimics the effects for a material thatremains immersed in seawater for extended periods of time. These conditions were met by fullyimmersing the U-bend specimens in a constantly replenished supply of natural seawater. Thespecimens remained immersed in the seawater for the duration of exposure, except for briefperiods to evaluate and photo-document the specimens. These specimens are the bottom row

of specimens in the photo marked Full Immersion/Tidal in Figure 2.

4. Tidal Exposure - Tidal conditions occur when a material is exposed to alternating periods ofimmersion and drying. These conditions were met by immersing the U-bend specimens in aconstantly replenished supply of natural seawater for 6 hours, followed by an additional 6 hourswhen the specimens are allowed to dry. The 6 hour cycles were repeated throughout the sixmonth duration of exposure. These specimens are the top row of specimens in the photomarked Full Immersion/Tidal in Figure 2.

RESULTS

Mass Loss and Laboratory SCC TestingThe results of the mass loss testing and the results of the laboratory SCC testing are summarized inTable 2. With no thermal treatment the 5083-H321 had a low mass loss of only 3 mg/cm2and did notfail in over a year of exposure in the laboratory testing. All of the thermally treated specimens had masslosses of 47-50mg/cm2. Thermally treated (i.e. sensitized) specimens with LT orientation all failed (i.e.cracking was observed) in 14 days or less. In the L orientation 8 of the 18 sensitized specimens failedbut the other 10 did not fail in more than a year of the alternate immersion exposure.

Testing in Marine EnvironmentsThe results of the SCC testing in various seawater environmental conditions at NASA Kennedy SpaceCenter Beach Corrosion Test Site are summarized in Table 3. The tidal and splash/spray

environments were the most severe conditions with all 5 replicates failing within the first week of theexposures. In the seacoast atmosphere exposures there were no failures at the 2 week inspection butall 5 specimens had failed by the 3 week inspection. The full immersion exposure results were mixedwith 2 failures occurring between weeks 1 and 2, and 1 specimen failing between weeks 3 and 4. Theother two full immersion specimens did not fail during the 6 month exposure.

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Figure 2: Images of U-bend Specimens in test at NASA Kennedy Space Center

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Table 2. Mass loss and laboratory SCC test results.

U-bends Pre-forms

Temp.

(C)

Time

(days)

ASTM G67

Mass loss

(mg/cm2) F/N

Time to

Failure*

(days) F/N

Time to

Failure*

(days) Orientation

120 28 47 2/3 3,6,329 3/3 3,3,6 LT

150 6 49 3/3 6,7,12 3/3 3,6,6 LT

175 6 50 3/3 3,6,7 3/3 9,12,14 LT

120 28 47 3/3 5,5,5 3/3 5,5,47 L

150 6 49 1/3 31,T,T 0/3 T,T,T L

175 6 50 0/3 T,T,T 1/3 7,T,T L

None None 3 0/3 T,T,T 0/3 T,T,T LT* Notes F/N = Failures/Number of specimens exposed; T indicates unfailed specimen was removedfrom test after 9 months of exposure.

Table 3. NASA Kennedy Space Center SCC test results.

Test Environment Orientation F/N

Time to Failure

(weeks)

Seacoast Atmosphere LT 5/5 3,3,3,3,3

Splash/Spray LT 5/5 1,1,1,1,1

Full Immersion LT 3/5 2,2,4,R,R

Tidal LT 5/5 1,1,1,1,1

Notes: F/N = Failures/Number of specimens exposed; R = Removed from test

unfailed at 6 months exposure

CONCLUSIONS

The following conclusions and recommendations can be made from this study:1. U-bend and/or pre-form specimens with LT orientation are useful for evaluating the stress

corrosion susceptibility of sensitized 5xxx sheet and thin plate materials that cannot be tested

with uniaxial loading in the ST orientation.2. Tidal and splash/spray conditions have been shown to be more severe than seacoast

atmosphere or full immersion environment. In tidal and splash/spray environments, intentionallysensitized specimen displayed cracks after shorter time durations than when exposed inseacoast atmosphere or full immersion environments.

3. Tidal or splash/spray exposures would be suitable conditions for additional testing to validatethe ASTM G67 mass loss requirements

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ACKNOWLEDGEMENTS

We would like to thank the members of the ASTM B07 marine task group including Matthew Collette,Hormoz Ghaziary, William Golumbskie, Robert Kain, Sajjad Kazi, Michael Niedzinski, Jack Snodgrass,Tom Summerson, and Paul Wycliffe, for their support and guidance for this study. We would also like toexpress our appreciation to the NASA Kennedy Space Center for the access to the Beach CorrosionTest Site and the testing and documentation services that they provided.

REFERENCES

1. ASTM B928/B928M (latest version), Standard Specification for High Magnesium Aluminum-AlloySheet and Plate for Marine Service and Similar Environments, (West Conshohocken, PA: ASTM).

2. ASTMG67 (latest revision), Standard Test Method for Determining the Susceptibility to IntergranularCorrosion of 5xxx Series Aluminum Alloys by Mass Loss after Exposure to Nitric Acid (WestConshohocken, PA: ASTM).

3. F. Bovard, Sensitization and Environmental Cracking of 5XXX Aluminum Marine Sheet and Plate

Alloys, 198

th

Meeting of the Electrochemical Society, (Pennington, NJ: The Electrochemical Society,October 3-8, 2004), pp. 232-43.

4. ASTM G30 (latest revision), Standard Practice for Making and Using U-bend Specimen Stress-Corrosion Test Specimens (West Conshohocken, PA: ASTM).

5. D.O. Sprowls and R.H. Brown, International Conf. on Fundamental Aspects of Stress CorrosionCracking, Ohio State Univ., Columbus, OH, (1967).

6. E.H. Dix, Jr., W.A. Andersen, M.B. Shumaker, CORROSION, Vol. 15, No. 2, pp. 55t-62t, February,1959.

7. ASTM G44 (latest revision), Standard Practice for Exposure of Metals and Alloys by AlternateImmersion in Neutral 3.5% Sodium Chloride Solution (West Conshohocken, PA: ASTM).

8. ASTMD1141 (latest revision), Standard Practice for the Preparation of Substitute Ocean Water(West Conshohocken, PA: ASTM).