Role of segregated P and S in intergranular stress corrosion cracking of Ni

Abstract

The effect of P and S on the intergranular corrosion and stress corrosion of Ni has been evaluated with corrosion tests, crack tip corrosion modeling and stress corrosion tests, and modeling. Tests were conducted in I N H2SO4 at 25 °C over anodic potentials of +0.3 V to 2.0 V (SCE). Corrosion tests were conducted on Ni + P and Ni + S alloys and Ni samples ion implanted with P and S. The corrosion results showed that both P and S reduce passivity in Ni and enhance the corrosion rate but that P is oxidized and dissolves in the electrolyte while S remains on the surface. This difference in surface behavior of P and S was used to explain the difference in intergranular corrosion (IGC) and inter-granular stress corrosion cracking (IGSCC) behavior of Ni + P and Ni + S. The active crevice and crack walls with Ni + S is thought to reduce the concentration gradient and hence the transport of Ni+2 from tip to mouth which in turn reduces the crack tip corrosion rate. By comparison of the geometry, potential dependence and rates of IGC and IGSCC of Ni + P it was concluded that IGSCC was not controlled solely by anodic dissolution but that a mechanical contribution was involved. Strain assisted corrosion was ruled out while a corrosion assisted intergranular fracture process was shown to be feasible.