Characterization of the oxide film formed on alumina-forming austenitic stainless steel in deaerated, DH, DO, and OT supercritical water

Abstract

The effects of water chemistry (deaerated (DO < 10 ppb), dissolved hydrogen (DH 1.6 ppm), dissolved oxygen (DO 2 ppm), and oxygen treatment (OT 300 ppb O2 and 100 ppb NH3) on the corrosion behavior of alumina-forming austenitic stainless steel (AFAs) were systematically investigated. A dual-layer oxide scale composed of an inner Fe/Cr/Al oxide layer and an outer Fe-rich oxide layer was observed on the AFAs. Water chemistry was found to have a negligible effect on the inner Fe/Cr/Al oxide layer; however, the morphology of the outer Fe-rich oxide layer was found to depend significantly on the chemistry. No facetted and dense hematite, which effectively protected the alloy and led to lower weight gain (less oxidation) over prolonged exposure, was formed in the outer oxide layer in oxidizing environments (DO and OT). Conversely, columnar magnetite with a porous structure was formed in outer oxide layer in reducing environments (deaerated and DH) in the absence of oxygen, with a higher weight gain observed. The addition of NH3 increased the pH of the supercritical water (SCW), thereby promoting alumina dissolution and rendering the OT conditions undesirable for use in a supercritical water-cooled reactor (SCWR). This study provides valuable insight into the behavior of AFAs in SCW and highlights the importance of carefully controlling SCW chemistry to optimize material performance.