Effect of ammonia concentration on environment-assisted failures in a low-nickel copper alloy

Abstract

Copper-nickel (Cu-Ni) alloys are the materials of choice for many piping applications; however, a number of premature field failures in copper-nickel pipes exposed to the marine environment have been observed by the author. A majority of these failures occurred at the bends in long-span and branched pipes located near water closets and/or bilges that were frequently filled with stagnated water. Failure analysis investigation revealed that the nickel content of the failed pipes was typically less than the specified value. The operative mechanism(s) causing the premature field failures invariably involved corrosion-assisted material cracking. The environment to which the failed pipes were exposed contained magnesium chloride (MgCl₂) and sodium chloride (NaCl), along with biodegradable materials capable of releasing ammoniacal byproducts. Many complex material-environment interactions are possible in marine piping systems, and the operative interaction depends on the stresses encountered and the chemistry and temperature of the exposure environment. While a failure analysis can usually identify the overall operative modes and mechanisms that cause the failures, an understanding of the material-environment interactions is needed to develop the corrective measures necessary to avoid premature field failures in real life applications.

A commercial Cu-5.37% Ni alloy very similar to the composition of the field failure pipes was studied under slow strain conditions in air and in solutions containing 3.5 wt.% NaCl + 10.0 wt.% MgCl₂ + either 1.0 wt.% ammonia or no ammonia additions. No deterioration of the mechanical properties of the alloy was observed in the tests conducted in the NaCl/MgCl₂ solution without ammonia. However, with the introduction of 1.0% ammonia, there was a reduction in the mechanical strength of the alloy, and the mode of failure changed from ductile rupture to brittle fracture. The slow strain rate tests of the alloy were conducted in aqueous ammonia at various concentration levels. The failures observed in aqueous ammonia showed a significant loss of strength with increasing ammonia concentration. The failures were predominantly brittle, exhibiting both intergranular and transgranular fracture paths. In general, the propensity for crack formation increased with increasing ammonia. The failures observed in aqueous ammonia were more severe and different than those observed previously in samples tested in ammonia containing 3.5% NaCl + 10.0% MgCl₂ solutions. This paper discusses the aqueous ammonia failures in detail.