SCC Failure Prediction of Austenitic Stainless Steels in Acid Solutions: Effect of pH, Anion Species, and Concentration

Stress corrosion cracking (SCC) of type 304 (UNS⁽¹⁾ S30400) and type 316 (UNS S31600) austenitic stainless steels in acidic chloride and sulfate solutions has been investigated as functions of pH and concentration by using the constant load method. It is found under a constant stress condition of 437 MPa that the logarithmic relationship between steady-state elongation rate (i_ss [m/s]) and time-to-failure (t_f), which were obtained from the corrosion elongation curve (elongation-time), shows a good linear function with a slope of unity in these solutions, irrespective of material (type 304 and type 316), pH, and anion concentration. This result supports that i_ss becomes the parameter for prediction of time-to-failure as previously reported, it is also found that the ratio of transition time to time-to-failure holds constant with a value of 0.57 ± 0.02 independent of pH, anion species, concentration and material. Furthermore, i_ss below which no fracture occurs within the laboratory time scale (<10⁷ seconds) is estimated to be of the order of 10^-10 m/s and less, which means that i_ss becomes the parameter for criterion on the evaluation of SCC susceptibility. On the basis of the results obtained, the critical values of pH and concentration for both type 304 and type 316, below which no SCC occurs, are speculated.