Ammonia Test for Stress Corrosion Resistance of Copper Alloys

Within the ISO Committee TC 26/SC 2, Wrought Copper and Copper Alloys, a Working Group, WG4, has been set up to propose an ammonia test to evaluate the stress corrosion resistance of copper alloy products. To create a basis for such a proposal, the Swedish-Corrosion Institute has initiated a test program, which has been carried out by Metallverken in Sweden. The proposal is based on earlier experience with ammonia tests at Metallverken, including a method originating from F. Aebi and used in Switzerland, Great Britain, and the Federal Republic of Germany. The proposed test procedure involves a 24-h exposure of the test material at room temperature to an ammoniacal atmosphere in equilibrium with an ammonium chloride solution. The vapor pressure of ammonia in equilibrium with the solution depends on the pH value. So, by changing the pH value, the severity of the test can be regulated. For comparison, a selection of materials has been subjected to the proposed test, to other accelerated tests based on ammoniacal corrodants, to the mercurous nitrate test, and to a four-year field test in the urban atmosphere of Stockholm. The results have shown that the proposed method has advantages over other accelerated SCC tests hitherto used for production and acceptance control. The test has been described in ISO standard 6957.