Stress Corrosion Cracking of Sensitized 304 Austenitic Stainless Steel in Sulfurous Acid

Stress corrosion cracking investigations on U-bend samples of sensitized 304 austenitic stainless steel have been conducted in sulfurous acid solutions in the concentration range, 0.40 to 10% at ambient temperatures. Samples were found to fracture in solutions of more than 3% sulfurous acid. Chemical analysis of solutions after tests showed appreciable reduction in concentration of H₂SO₃, formation of H₂SO₄, and also the presence of Fe, Cr, and S ions. Some model experiments were carried out to ascertain the chemical species causing stress corrosion cracking and the role of oxygen and FeS in generating them. Bubbling of oxygen gas through (1) solution of H₂SO₃ with sample, (2) distilled water with FeS chips, and (3) solution of H₂SO₃ with FeS chips and sample, resulted in the formation of tetrathionic acid. Results indicate that the sulfurous acid does not directly cause cracking but it is the tetrathionic acid formed by either the interaction of sulfurous acid and austenitic stainless steel or the interaction of FeS and oxygen, that is responsible for cracking. Metallorgraphic examinations of the fracture confirmed intergranular cracking.