Mitigation of ammonia-induced SCC in a cupronickel alloy by additions of MgCl2
- 66 Downloads
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
Unable to display preview. Download preview PDF.
- 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