Mitigation of ammonia-induced SCC in a cupronickel alloy by additions of MgCl2
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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.
During the testing process, it was established that chlorides of sodium and magnesium also had a role to play on the ammonia-induced SCC. Further tests were therefore designed, and this paper summarizes test results, which point to the possible mitigation of ammonia-induced SCC in cupronickels by the addition of MgCl2.
Keywordsaccelerated corrosion testing cracking behavior environmental failures environmentally assisted cracking failure mechanism magnesium mechanical property estimation
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- 1.P.T. Gilbert: “Corrosion Resisting Properties of 90/10 Copper-Nickel-Iron Alloys with Particular Reference to Offshore Oil and Gas Applications,” Brit. Corros. J., 1972, 14(1), pp. 20–25.Google Scholar
- 5.D.C. Agarwal and S. Seshadari: “Low-Nickel Copper Alloy—Role of Ammonia and Corrosion Retarding Media,” Proceedings of the 11th National Congress on Corrosion, Vadodara, India, July 17–19, 2003, e-Publication by NCCI, India.Google Scholar
- 6.Corrosion in Metals and Alloys—Stress Corrosion Testing—Part 7: Slow Strain Rate Testing, International Standard ISO 7539–7: 1989(E), pp. 1–4.Google Scholar
- 8.ASTM Standard E-8, ASTM, Philadelphia, PA, 1992. Originally published as E-8-24T, last previous edition E8-90 under American Association State, Highway and Transportation Officials Standard, AASHTO No: T 68, pp. 130–45.Google Scholar