Critical pitting and repassivation temperatures for duplex stainless steel in chloride solutions

Both the critical pitting temperature (CPT) and critical repassivation temperature (T_r) for two kinds of duplex stainless steels (DSS, namely UNS S31803 and UNS S32750) were investigated in 1 mol/L NaCl solution using the cyclic thermammetry method. Potentiodynamic anodic polarization technique was employed to validate the cyclic thermammetry technique. In addition, the site of pitting nucleated preferentially on the DSS had been confirmed by scanning electron microscopy (SEM). The results demonstrated that there exhibits a hysteresis loop in cyclic thermammetry curve, revealing that the propagating pits could repassivate during the cooling half-cycle. The CPT and T_r for UNS S31803 were 59.6 °C and 36.5 °C, whilst the CPT and T_r for UNS S32750 were 87.5 °C and 70.5 °C, respectively. Pitting was always observed preferentially in the austenite phase. The results can be partially explained based on the changes in chemical composition of ferrite and austenite phases. Moreover, a semi-quantitative model is proposed to explain the existence of T_r.