Effects of zinc addition on the corrosion behavior of 316L stainless steel and Stellite 6 cobalt-based alloy in PWR primary water

The effects of zinc addition on the general corrosion of 316L austenitic stainless steel and Stellite 6 cobalt-based alloy were studied in simulated pressurized water reactor primary water with 0, 10, and 40 μg·L^–1 zinc additions. The corrosion rate and metal release rate of the two materials were determined by using the weight loss method. The surface and cross-sectional morphology, thickness, and element distribution of the oxide film were examined by scanning electron microscopy and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS). The crystallographic structures of the outer and inner oxides were identified by atomic-resolution TEM imaging with fast Fourier transform. The results reveal that the addition of 10 μg·L^–1 Zn did not have a statistically significant effect on the corrosion rate and corrosion product release rate of 316L stainless steel within the initial 1000 h of exposure. Increasing the concentration of Zn to 40 μg·L^–1 significantly reduced the corrosion rate, corrosion product release rate, and oxide film thickness of 316L stainless steel. Specifically, the oxide film thickness decreased from 250 to 95 nm. For the Stellite 6 cobalt-based alloy, composed of γ-Co matrix and Cr₂₃C₆ carbides, the galvanic corrosion effect between γ-Co matrix and Cr-rich carbides was observed under all zinc addition conditions. The oxide film thickness of Cr₂₃C₆ carbide in the Stellite 6 cobalt-based alloy was comparatively thinner than that of the γ-Co matrix, which is mainly ascribed to the preferential oxidation of the γ-Co matrix. Extensive interfacial oxidation was observed between γ-Co matrix and Cr₂₃C₆ carbide, which can be attributed to the occurrence of electrochemical galvanic corrosion between these two phases. The carbides have a higher nobility compared to the γ-Co matrix. The corrosion rate, corrosion product release rate, and oxide film thickness of Co-based alloy all decreased with increasing zinc concentrations. The average oxide film thickness of the preferentially oxidized γ-Co substrate decreased from 200 to 60 nm, and the average oxide film thickness of Cr₂₃C₆ carbide decreased from 100 to 40 nm. Microstructural analysis of the oxide film demonstrated that Zn²⁺ promotes the denser outer oxide film formation by forming a zinc-incorporated spinel, which retards the outward diffusion of metal ions and the inward diffusion of oxygen ions, reduces the oxygen partial pressure at the oxide/metal interface, and promotes the formation of a continuous protective Cr₂O₃ inner oxide film, thus significantly suppressing the corrosion and metal release of 316L austenitic stainless steel and Stellite 6 cobalt-based alloy in high-temperature water.