Stress corrosion resistance is increased by distorting surface grain-boundary structure and by interrupting the corrosion and stress corrosion. The first is accomplished by machining or shot peening and the second by removal from and later reexposure to the corrosive environment

Brummer, S. B.

Cocks, F. H.

English

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NASA TECH BRIEF 
September 1970	 Brief 70-10396 
NASA Tech Briefs announce new technology derived from the U .S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
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be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Increased Resistance to Stress Corrosion of Aluminum Alloys 
.
3000
0 15-Hr Drying 
30-Min Drying 
1000 
ilel.: 
a) 
U-
0 
Co 
w 10
0 100 200 300 400 800 1200 
Minutes of Prior Corrosion 
Time to Failure Versus Time of Prior Corrosion for 
Specimens Dried After Prior Corrosion 
A method of increasing the stress corrosion resist-

ance of high strength aluminum alloys has been de-

veloped. The method is based on two observations: 
First, surface treatment such as machining or shot 
peening distorts the surface grain-boundary structure.
Since stress corrosion of such alloys (specifically 
aluminum-7075) occurs along grain boundaries, it 
cannot begin before the distorted surface layer is pene-
trated by pure corrosion. 
Second, both pure corrosion and stress corrosion 
start initially at particular sites, beginning with the 
most favorable ones. If the corrosion and stress cor-
rosion are interrupted, as by removal of the corrosive 
environment, and the aluminum is later reexposed, 
the corrosion and stress corrosion are not reinitiated 
at the earlier sites but at completely new ones. These 
new sites are less favorable than the original sites, 
so that corrosion and stress corrosion now proceed 
at rates lower than their original rates; thus final fail-
ure of the aluminum occurs later than if the corrosion 
had not been interrupted. 
Unstressed specimens of aluminum-7075-1651 were 
corroded for various times in a 1M solution of NaCI 
buffered to pH 4.7 with sodium-acetate and acetic 
acid; the density of the anodic current applied was 
0.3 mA/cm 2. After corrosion the specimens were re-
moved and dried in a partial vacuum for either 30 mm 
or 14 hr. When the 30-min specimens were reexposed 
to corrosion under stress, the time to failure was 
shorter than if the corrosion had not been interrupted. 
When, however, the drying was very thorough (14 hr) 
the time to failure was greatly increased (see fig.). Thus 
the anodic treatment increased the aluminum's resist-
ance to stress corrosion. 
Notes: 
1. The aircraft, chemical, and boatbuilding industries, 
metallurgists, oceanographers, and designers of 
high-strength aluminum structures may be in-
terested.
(continued overleaf) 
NEW
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2. Requests for further information may be directed	 Patent status: 
to:	 Inquiries about obtaining rights for the commer-
Technology Utilization Officer	 cial use of this invention may be made to NASA, 
Code A&TS-TU	 Code GP, Washington, D.C. 20546. 
Marshall Space Flight Center	 Source: S.B. Brummer and F. H. Cocks of 
Huntsville, Alabama 35812 	 Tyco Laboratories, Inc. 
Reference: TSP70-10396	 under contract to 
Marshall Space Flight Center 
(MFS-20788) 
Brief 70-10396	 Category 04


Increased resistance to stress corrosion of aluminum alloys

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