Corrosion fatigue crack growth behavior of nickel base alloys in sodium thiosulfate solution

The effects of heat treatment, loading frequency and applied potential on corrosion fatigue crack growth (FCG) rates in either alloy 690 or alloy 600 in sodium thiosulfate solution were investigated. Corrosion fatigue tests were performed in 0.1 M sodium thiosulfate solution at 25°C at an R ratio of 0.1, with a loading frequency of 0.1 and 1 Hz, and under various applied potentials. The experimental results indicated that the FCG rate, at 1 Hz, in alloy 690 was nearly independent of heat treatment and anodic applied potential; meanwhile, that in alloy 600 varied with the heat treatment and applied potential. Under cathodic applied potential and at a low ΔK regime, the FCG rates in alloy 690 were slightly higher than those at open circuit potential. The environmental contribution to the FCG rate in alloy 690 was observed at 0.1 Hz, while a much more pronounced effect was observed for sensitized alloy 600 at the same loading frequency. Fractographical examinations of all the fractured surfaces showed that the fatigue cracks propagated in the transgranular mode in alloy 690 regardless of whether solution annealed or thermally treated, in air and in the solution at different applied potentials and frequencies. However, an intergranular mode of cracking was observed for the sensitized alloy 600 in 0.1 M thiosulfate solution at +500 mV and at 0.1 Hz, which had a markedly higher crack propagation rate.