The essential role of the microstructure and composition in the corrosion resistance of laser-decontaminated surfaces

Abstract

Metal equipment in nuclear facilities produces a wide range of radioactive wastes during the long-term service of reactors. Simulated specimens with non-radioactive isotopes Co were used instead of extremely radioactive 60Co to improve the analysis feasibility. Herein, laser decontamination was performed on 304 stainless steel with a stable isotope Co contamination layer. The microstructure and composition were characterized by transmission electron microscopy to investigate the effect of laser decontamination on the outermost surface. The results showed that laser decontamination altered the surface morphology and microstructure. The high-energy laser instantly melted and vaporized the surface, thereby removing the contaminants. Molten pool craters and corrugated protuberances were formed on the surface through the ejection of metal vapors and droplets, and the dual-phase microstructure containing γ-Fe and strain-induced martensite transformed into δ-Fe and γ-Fe phases in the recast layer. Corrosion resistance measurements showed that laser decontamination decreased the corrosion current density and the corrosion rate, whereas increased the passivation film resistance and charge transfer resistance. This indicates a remarkable improvement in comparison with the as-received specimens, mainly due to the formation of a chromium-rich oxide layer (Fe2CrO4), which acted as a separator, thereby isolating the solution from the substrate.