Stress Corrosion Cracking of 60/40 Cupro-Nickel Alloy: Fractography and Chemical Analysis^⋆

The copper-nickel alloys have found extensive application in steam generation systems. In general, these materials have excellent resistance to corrosion-erosion and stress corrosion cracking (SCO. However, service failures, characterized by intergranular cracking and believed to be the consequence of stress corrosion, have occurred. The use of harder tempers to meet the demands of higher operating temperatures and pressures has increased susceptibility to SCC. Further, the treatment of feedwater with organic amines or hydrazine, which produce ammonia upon breakdown, enhances corrosion and stress corrosion of these materials.^1,2  Accelerated stress corrosion cracking (SCC) was induced in copper alloy 720 (40-43 Ni, 1.5-2.5 Fe, 0.5-1.7 M_n, balance Cu) by exposing stressed c-ring specimens to the vapor over ammoniacal solutions in autoclaves at 500 F (260 C). The scanning electron microscope (SEM) was employed for fractographic analysis of the resultant intergranular cracking and for micro-chemical analysis of the corrosion products and corrosively attacked metal at the stress corrosion crack surface. The chemical attack is characterized by a process of copper depletion operative at the crack tip. Following sufficient exposure, the stress corrosion crack walls develop a two-layer corrosion product. A nickel-rich phase, adjacent to the alloy, is covered by a copper-rich phase outside.