Crack Tip Chemistry Modeling of Intergranular Stress Corrosion Cracks of Nickel Containing Segregated Phosphorus and Sulphur

摘要

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. Tests were conducted in 1N H sub 2 SO sub 4 at 25 degrees C over anodic potentials of +- 0.3V to 2.OV (SEC). A one-dimensional mass transport model (considering diffusion, migration, and nickel sulphate precipitation) was successfully developed to predict the corrosion rates at the tip and crack walls. Corrosion tests were conducted on Ni + P and Ni + S alloys and Ni samples ion implanted with P and S to develop a data base for the model. 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. The model predicts that the active tip and crack walls with Ni + S will result in precipitation of NiSO sub 4 at very shallow crack depths and extremely low crack tip corrosion rates. Both IGC and IDCC are essentially absent from the Ni + S system at 900 mV (SEC). The model predicts much larger crack tip corrosion rates for the Ni + P system with an active tip and passive walls. (ERA citation 10:024030)