Synergistic corrosion mechanisms of supercritical water and Na2SO4 deposition layer: Austenitic stainless steels 316 L and 304

Abstract

This study investigated the corrosion behavior of 316 L (316LSS) and 304 (304SS) stainless steels under a Na₂SO₄ deposition layer in supercritical water (SCW). The salt layer reduced oxide film densification versus pure SCW, with 304SS exhibiting more severe corrosion (e.g., pitting) due to the absence of Mo which resists pitting corrosion. Without the salt layer, oxide film thicknesses were 6.50 µm (316LSS) and 1.60 µm (304SS); in contrast, 6.52 µm and 9.33 µm, respectively, indicating thicker in SCW with the salt layer. At 723 K and 25 MPa, higher water density increased Ni/Fe ion solubilities, while the salt layer trapped these ions in the microenvironment between the layer and oxide film, generating a locally acidic and oxidizing environment via hydrolysis of Ni/Fe ions. This process accelerated metal ion leaching and oxide dissolution, further enhanced in the presence of SO₄^2-. Na₂SO₄ deposition layer and SCW synergistically intensified corrosion in both alloys.