Corrosion of Type 304 Stainless Steel in Mixed Anhydrous Nitric and Sulfuric Acids^★

The corrosion behavior of Type 304 stainless steel was investigated in anhydrous HNO₃-H₂SO₄ solutions at temperatures from 50 C (122 F) to 100 C (212 F). The HNO₃ concentration ranged from 0 to 100 percent by weight. Type 304 stainless steel corrosion rates were determined by gravimetric analysis and by measuring the relative loss of the constituent materials, such as iron, chromium, nickel and manganese, using atomic absorption spectrometry. The corrosion rate was found to increase with HNO₃ concentration until a composition of 20 weight percent HNO₃ is reached. After this point, the corrosion rate decreases with the addition of HNO₃ to a minimum at 90 weight percent HNO₃ - 10 weight percent H₂SO₄. Increasing the HNO₃ concentration beyond this point rapidly increases the corrosion rate. The corrosion rates measured gravimetrically were substantiated by analysis of the elemental constituents in solution.