NBSIR 78-1464 (R)

Stress Corrosion Behavior of

Clad 7050-T76 Aluminum

Sheet

B. T. Sanderson and W. F. Gerhold

Chemical Stability and Corrosion Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

April 1978

Prepared for Naval Air Systems Command

Department of the Navy

Washington, D.C. 20360

NBSIR 78-1464 (R)

STRESS CORROSION BEHAVIOR OF

CLAD 7050-T76 ALUMINUM

SHEET

B. T. Sanderson and W. F. Gerhold

Chemical Stability and Corrosion Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D C. 20234

April 1978

For Official Distribution Only

Prepared for Naval Air Systems Command

Department of the Navy

Washington, D.C. 20360

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman. Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Emeat Ambler, Director
Stress Corrosion Tests

Stressed and unstressed specimens of both thicknesses were exposed

in the marine atmosphere at Kure Beach, N.C. [80 foot (24 m) lot]. A

system of weights and levers was used to obtain the desired stress on

the stress corrosion specimens. The specimens were exposed with an applied

stress approximately equivalent to 0, 50, and 75% of the yield strength of

the alloy as determined by NBS [67.4 Ksi (465 MPa)] for the 0.088 in. (2.24

mm) thick specimens, and [65.8 Ksi (454 MPa)] for the 0.123 in. (3.12

mm) thick specimens.

Results

None of the stress corrosion specimens had failed after 798 days

exposure in the marine atmosphere. Visual examination of the specimens

after exposure in the marine atmosphere revealed the presence of heavy,

adherent gray corrosion products with considerable localized pitting

corrosion. Companion unstressed specimens which had been exposed for

the same period of time were removed from exposure along with the stressed

samples. In order to obtain an indication of the effect of corrosion

attack on the material, a comparison was made of the tensile properties

of unexposed specimens vs. those of specimens exposed to the marine

environment. The values were then averaged and calculated as percent

loss in tensile strength due to exposure in the environments. The

results given in Table II revealed no significant loss in tensile strength

as a result of exposure.

Metallography

Transverse sections were obtained from the exposed samples for

metallographic examination (Fig. 1). The localized corrosion

pitting is visible in the cladding material. In general, the microstructure revealed a uniform grain size in both the clad and core material.

-2 -
Conclusions

The results obtained from stress-corrosion tests on clad 7050-T6

aluminum alloy sheet material indicate that the alloy is not susceptible

to stress corrosion cracking in a marine atmosphere environment. No

failures were observed on specimens stressed at approximately 50 and 75%

of the alloy's yield strength after exposure for 798 days. This is in

agreement with earlier tests conducted on X7050-T76351 aluminum alloy plate.

However, as with the X7050-T76531 alloy, the clad 7050-T76 alloy was

susceptible to shallow surface pitting corrosion. This corrosion did

not appear to affect the tensile properties of the clad 7050-T76 alloy

where the loss in tensile strength due to corrosion was negligible (less

than 2% of the original tensile strength on unstressed specimens) as

compared to that of the unclad X7050-T76351 alloy (approximately 8%).

The clad layer on the sheet material provided protection from corrosion

to the core material.

-3 -
Figure 1. Photomicrograph of clad 7050-T76 aluminum alloy showing

localized pitting corrosion in the cladding material and

typical microstructure. Etched, Kellers Etch. X50.

-4 -
TABLE I

Transverse Mechanical Properties of Clad 7050-T76

Aluminum Alloy Sheet

Sheet

Thickness

Tensile

Strength d)

Ksi ( 2 )

Yield

Strength d)

(0.2% offset)

Percent

Elongation

in 2 in. (5.08cm)

^

0.088 in

(2.24 mm)

76.2 ± 0.4 67.4 ± 0.6 11 ± 1.0

0.123 in

(3.12 mm)

74.8 ± 0.5 65.8 ± 0.1 12 ± 0.3

(1) Average for 3 specimens with standard deviations.

(2) 1 Ksi = 6.8948 MPa.

-5 -
TABLE II

Results Obtained from Tests on Transverse Specimens of Clad 7050-T76 Aluminum

Alloy Sheet after Exposure in a Marine Atmosphere at Kuire Beach, N.C. [80-foot

(24m) lot]

Sheet Exposure Stress Days Percent Loss in

Thickness Percent of

Ksi

(1)

Exposed Tensile Strength

Yield Strength (2)

0.088 in .0 798NF 1.2

(2.24 mm) 0 798NF 1.6

0 798NF 0.1 1 . 0 Ave

50 34.2 798NF 0.2

50 34.0 798NF 1.6

50 34.2 798NF 1.7 1 . 2 Ave

75 51.2 798NF 1.4

75 51.0 798NF 0.2

75 50.6 798NF 0.5 0 . 7 Ave

0.123 in 0 798NF 0

(3.12 mm) 0 798NF 1.6

0 798NF 0.1 0 . 6 Ave

50 32.5 798NF 0.5

50 32.5 798NF 0.4

50 32.5 798NF 0.4 0.4 Ave.

75 49.2 798NF 0.8

75 49.6 798NF 1.5

75 49.6 798NF 0.5 0.9 Ave.

(1) 1 Ksi = 6.8948 MPa.

(2) NF denotes no failure after exposure for number of days shown.

-6 -
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-1464

2. Gov’t Accession

No.

4. TITLE AND SUBTITLE

Stress Corrosion Behavior of Clad 7050-T76 Aluminum Alloy

7. AUTHOR(S)

B. T. Sanderson and W. F. Gerhold

9. PERFORMING ORGANIZATION NAME AND ADDRESS

3. Recipient’s Accession No.

5. Publication Date

6. Performing Organization Code

8. Performing Organ. Report No.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Work Unit No. 5610410

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Sfreef, City, State, ZIP

)

Naval Air Systems Command

Department of the Navy

Washington, D. C. 20360

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15.

SUPPLEMENTARY NOTES

16.

ABSTRACT (A 200-word or teas tactual summary of moat significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Transverse tensile specimens of clad 7050-T76 Aluminum alloy were stressed at

zero, fifty, and seventyfive percent of the alloy's yield strength and exposed in

a marine environment. After 798 days exposure, there were no failures. The clad

7050-T76 material studied appears to be resistant to stress corrosion cracking in

this environment.

17.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Alloy; aluminum; atmosphere; cladding; marine; mechanical

properties; sheet; stress corrosion

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