THE SUSCEPTIBILITY OF 90CU-10NI ALLOY TO STRESS-CORROSION CRACKING INSEAWATER POLLUTED BY SULFIDE IONS

Electrochemical polarization measurements and slow strain rate tests ( SSRT) of a 9OCu-10Ni alloy in highly sulfide polluted seawater were co nducted to investigate stress-corrosion cracking (SCC) behavior. The s everity of the SCC depends on the sulfide concentration in the seawate r. The severity increases as the concentration increases. Because the major time in SCC is spent in the initiation process of the propagatin g crack, the fracture toughness has only a minor effect in the compone nt life failed by SCC. The SCC behavior of CDA706 is strictly linked t o sulfide concentration in the range of 100 to 1000 ppm. The general c orrosion of Cu-Ni alloys in low (<100 ppm) and high (>100 ppm) sulfide polluted seawater increases due to the selective copper dissolution. Cyclic polarization measurements confirmed that the corrosion rate dec reases slightly as the sulfide concentration increases. Fitting tenden cy was high in the low concentration range of sulfide and low in the h igh concentration range. The presence of stresses in SCC removes the p rotective layer as it increases during testing of the specimen or duri ng the actual service of a component. The authors propose that film ru pture occurred, and two proposed SCC mechanisms were operational, name ly sulfide stress cracking associated with the anodic dissolution in t he low sulfide concentration range and hydrogen embrittlement, which w as dominant in the high sulfide concentration range. It was found that a synergism exists between sulfide and stress that enhances the effec t of the latter.