Corrosion resistance and cracking susceptibility of 316L stainless steel in sulfuric acid-containing supercritical water

Cracking susceptibility and characteristics were studied on non-sensitized UNS S31603 stainless steel by slow strain rate tests in supercritical and sub-critical water containing 0.01 mole/kg-H₂SO₄ at 400°C/25 MPa, 400°C/60 MPa, and 360°C/25 MPa with two levels of oxygen concentration, 8 and 800 ppm. The stainless steel had excellent corrosion resistance in 0.01 mole/kg-H₂SO₄ at 400°C when oxygen level was as high as 800 ppm and this was attributed to a thin and compact scale of Fe-Cr spinel and γ-Fe₂O₃ formed on the alloy. Cracking severity was higher for the supercritical conditions at 400°C than in the sub-critical water at 360°C. Although intergranular mode was dominant, a few traces of transgranular cracks were found in the less oxidizing 8 ppm O₂ condition.