Effect of cyclic stresses on stress corrosion cracking of Cu–Ni alloy

Abstract

The author has studied several failures in the field of bent and branched copper-nickel pipelines exposed to marine environments contaminated by ammoniacal byproducts. The cause of the failures was investigated by laboratory tests on the Cu-5.37 wt-%Ni alloy used for the failed pipes. The tests were conducted under slow strain rate testing (SSRT) conditions, in aqueous ammonia and ammoniacal sea water environments. These studies revealed that the presence of ammonia in sea water impairs the load bearing capacity of the alloy, and the aqueous ammonia environments tend to cause brittle stress corrosion cracking failures, which are often premature and/or catastrophic. However, in addition to induced residual stresses from manufacturing/processing, the operating conditions in marine environments subject these pipes to external forces and widely varying pressures and fluid flow rates, which can cause both static and cyclic stresses. Experiments conducted under SSRT conditions, however, could not predict the operative failure modes and mechanisms for the alloy when exposed to ammoniacal environments under the combined action of static and cyclic stresses. In the present investigation, the stress corrosion failure characteristics of the alloy studied previously in SSRT, were subject to testing under static load with superimposed fixed interval, low amplitude loading cycles. These tests were conducted in air and in 5 and 10 wt-% aqueous solutions of ammonia. The results and observations help to elucidate the role of superimposed cyclic stresses on the stress corrosion failure behaviour of the alloy in the ammoniacal environment.