Stress corrosion in copper–nickel alloys: Influence of ammonia

Abstract

The test results from an earlier investigation by the author on the mechanical behaviour of a low nickel nickel-copper alloy exposed to a solution of 3·5%NaCl + 10%MgCl₂ with and without ammonia indicated that the presence of 1% ammonia alters the failure mode and affects the load bearing capacity of the material. In the present investigation, further tests have been conducted on the same alloy to study its mechanical/corrosion behaviour on exposure to an aqueous solution of ammonia in comparison with exposure to a similar solution of ammonia containing 3·5%NaCl + 10%MgCl₂. A constant displacement rate of 9·6 × 10^-3 μ m s^-1 was used and the concentration of ammonia was maintained at 5%. On exposure to the 5% aqueous ammonia solution, the mechanical properties of the alloy were degraded significantly and the material exhibited a brittle stress corrosion type of failure. However, on exposure to 5% ammonia in the presence of 3·5%NaCl + 10%MgCl₂, there was less change in failure mode relative to that in air and a significant improvement in the mechanical properties of the material compared with those in aqueous ammonia. This indicates that the presence of MgCl₂ and/or NaCl can be beneficial when low nickel copper alloys are exposed to ammonia. This paper presents the experimental results and observations from the recent tests, and discusses the relative roles of ammonia and NaCl/MgCl₂ in determining the mechanical properties and modes of failure in a low nickel content copper-nickel alloy.