THE EFFECT OF CHLORIDE-IONS ON STRESS-CORROSION CRACKING OF TYPE-304 AND TYPE-316 AUSTENITIC STAINLESS-STEELS IN SULFURIC-ACID-SOLUTION

Stress corrosion cracking of type 304 and type 316 austenitic stainless steels has been investigated as a function of chloride ion concentration in 0.82 kmol m-3 pure sulfuric acid solution at 353 K under a constant stress of 392 MPa by using a constant load method. At a concentration of up to approximately 2 x 10(-2) kmol m-3 type 316 has no SCC susceptibility as well as the result in the pure sulfuric acid solution, while type 304 has SCC susceptibility the same as that in the pure sulfuric acid solution except for a narrow concentration range of around 1 X 10(-3) kmol m-3, at which concentration type 304 has no or little SCC susceptibility. The behavior of type 304 at the narrow concentration range means that chloride ions serve as inhibitor for SCC. At higher concentrations of more than 2 x 10(-2) kmol m-3 the SCC susceptibility of both steels increases with increasing chloride concentration, which implies that chloride ions act as promotor for SCC. In addition, the logarithm relationship between time to failure and steady state elongation rate is found to become a good straight line with a slope of unity independent of the material and chloride concentration. The steady state elongation rate becomes a parameter for the prediction time to failure even in the mixed solution of sulfuric acid and the chloride ions. The present results obtained are qualitatively explained in terms of adsorption of chloride ions at crack tips through a SCC process.