Effect of Ni²⁺ in occluded volume on the oxidation behavior of 304 stainless steel in high temperature water

In the occluded volume of high temperature pressurized water loop in nuclear power plants, poor-controlled water chemistry due to sluggish flowage could induce accelerated degradation of structural materials. In this paper, an occluded volume shaped nuclear-grade 304 stainless steel sample was immersed in oxygenated high temperature water containing a certain concentration of Ni²⁺. It was found that from the outer to the inner side of the sample, the proportion of spinel in the oxide film decreased while that of hematite increased and Ni content in the outer oxide film decreased. It was thought that the concentration of Ni²⁺ could influence the oxidation behavior of material significantly. The occluded volume could impede the inward diffusion of Ni²⁺, causing a concentration gradient of Ni²⁺ along depth of the volume and gradual change of oxide film characteristic.