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Copper nickel alloys 90/10 and 70/30 are used extensively in saltwater systems operating on marine platforms and offer high corrosion and biofouling resistance. However, systems using these materials were reported to fail prematurely. Preliminary investigations revealed pollution of harbor waters with tangible levels of ammonia and sulphides which was reported to be the primary cause of failure. The present author(s) have performed laboratory and field investigations to identify the stress-material-environment interaction of the copper nickel alloys extensively in use on seawater system of marine platforms. Results obtained point to the fact that polluted seawaters can affect the material properties of the alloy leading to premature failure(s) of systems at stresses well below the designed stress. The paper presents the correlation of the results of the laboratory and field tests of 90/10 and 70/30 copper–nickel alloys in seawaters polluted with ammonia and sulphide pollutants.

The authors carried out failure analysis of bent and branched copper-nickel alloy pipelines that had failed in marine environments. These failures were almost always dominated by a brittle stress-corrosion cracking (SCC) mode and could often be attributed to the presence of ammoniacal byproducts in the operating environment. Attempts were made to reproduce the marine-type field failures in the laboratory by testing a Cu-5.37%Ni alloy, similar to the material used in failed pipelines. The tests were performed under slow strain rate test (SSRT) conditions in aqueous ammonia and ammoniacal seawater. Results revealed that the ammonia-induced brittle SCC failures were predominant and reduced the load-bearing capacity of the alloy. The real-life failures are not simple SSRT-type failures. The operating conditions, in addition to the induced residual stresses from manufacturing/processing, subject the system pipes to external forces and widely varying pressures and fluid flow rates. This combination of stresses can produce both static and cyclic stress conditions, similar to a static load coupled with a low-amplitude cyclic load. Tests conducted under superimposed cyclic stresses on prestressed specimens were found to accelerate the stress-corrosion failures in the present copper-nickel alloy in an ammoniacal environment.