Stressed alloy specimens are immersed in a salt-dichromate solution at 60 degrees C. Because of the minimal general corrosion of these alloys in this solution, stress corrosion failures are detected by low-power microscopic examination

Helfrich, W. J.

English

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December 1968	 Brief 68-10536 
.
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
A Rapid Stress-Corrosion Test for Aluminum Alloys 
S
Alternate immersion in aqueous 3.5% NaCl solution 
has been an effective stress-corrosion cracking test for 
most aluminum alloys. Unfortunately, the alternate 
immersion test also promotes excessive general cor-
rosion of those alloys which contain copper as a major 
alloy addition (2000- and copper-bearing 7000- ser-
ies alloys). The general corrosion attack of these 
alloys can mask fine cracks and, therefore, materially 
increase the difficulty of determining the cause of 
failure. Supplemental metallographic examinations 
are often helpful in determining if stress-corrosion 
cracking did or did not occur, but these examinations 
are expensive and time consuming. 
Continuous immersion in an aqueous (common 
salt—potassium dichromate) l% NaCl-2% K2Cr2O7 
solution at pH 4 and temperature of 60°C has proved 
to be an effective stress-corrosion test for aluminum 
alloys such as 2024, 2014, 2219, and 7075, which are 
most prone to general corrosion in 3.5% NaCl alter-
nate immersion testing. Because of the minimal gener-
al corrosion of these alloys in this salt-dichromate 
solution, stress corrosion failures are readily detected 
by low-power microscopic examination of the test spe-
cimen. The salt-dichromàte solution test is particu-
larly well-suited to the tesk of rapidly separating 
various tempers of 2024, 2219, and 7075 aluminum 
alloys on the basis of their relative susceptibilities to 
stress-corrosion cracking. 
Stressed alloy specimens, e.g., C-rings, tensile 
rounds, etc., are immersed in the salt-dichromate 
solution at a temperature of 60°C. The solution is 
prepared on a weight basis (l% salt-2% dichromate) 
using reagent grade chemicals and distilled water. As 
prepared, the solution should exhibit a pH in the
range of 3.7 to 4.0. If necessary, the pH should be 
brought within this range with additions of normal 
HCI or NaOH solution. 
The test solution must be contained in an inert 
vessel, preferably glass, which will withstand the test 
temperature of 60°C. Provisions should be made for 
maintaining the temperature within ±1°C of 60°C 
for the best reproducibility of test results. It is not 
necessary to reflux the solution; but the vessel should 
be suitably covered to minimize evaporation losses. 
Distilled water must be added periodically to com-
pensate for any evaporation losses. 
The recommended immersion period in the salt-
dichromate solution is seven days, but this period 
may be extended for several weeks without danger of 
excessive general corrosion attack of the specimens. 
Failure of highly stressed susceptible alloys and 
tempers generally occurs during the first few hours of 
testing. Therefore, specimens should be inspected at 
least once during the first four hours' exposure and 
twice a day thereafter. 
Because of the minimal general corrosion attack in 
the salt-dichromate solution, chemical etching of the 
unstressed specimens is a necessary preliminary step 
for reproducible response in this test. Specimens 
should be immersed for 30 seconds in a 5% NaOH 
solution at 75°C, soaked in cold concentrated HNO3 
for 15 to 30 seconds to remove smut, and finally rinsed 
in hot tap water. The use of the etch does not compli-
cate the test. In fact, it may reduce the variability 
associated with most stress-corrosion tests in assur-
ing complete removal of oils, residual oxides, and 
smeared metal introduced during the machining of the 
specimens.
(continued o'erIeaf) 
Now
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Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000492 2020-03-12T06:31:12+00:00Z
Note:	 Patent status: 
Documentation is available from:	 Inquiries about obtaining rights for the commercial 
Clearinghouse for Federal Scientific 	 use of this invention may be made to NASA, Code 
and Technical Information	 GP, Washington, D.C. 20546. 
Springfield, Virginia 22151	 Source: W. J. Helfrich 
Price $3.00
	 of Kaiser Aluminum and Chemical Corporation 
Reference: B68-10536	 under contract to

Marshall Space Flight Center 
(M FS-20 175)
S 
Brief 68-10536	 Category 03


A rapid stress-corrosion test for aluminum alloys

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