Micro-electrochemical polarization study on 25% Cr duplex stainless steel

Micro-electrochemical characteristics of 25% Cr duplex stainless steel (DSS) were investigated by a new electrochemical technique, a micro-droplet cell. Anodic polarization tests were carried out in pH 5.6 acetate buffer solution for annealed DSS solution, for the alloy aged at 850 °C, and for the alloy aged at 475 °C. The anodic peak current density for the ferrite (α) phase was measured to be higher at slightly lower potential compared with that for austenite (α) phase in solution annealed DSS, suggesting that α was electrochemically more active than γ, which has a lower Ni content. The polarization curve for the region where the sigma (σ) phase coexisted with secondary austenite (γ2) phase, (σ+γ2), in the alloy aged at 850 °C showed two anodic current peaks, which possibly corresponded to those for γ2 and σ, respectively. The anodic peak current density and passive current density for the region of (σ+γ2) in the alloy aged at 850 °C were higher, and the current scattering corresponding to the initiation of metastable pitting corrosion was more evident than those for α and γ in the solution annealed alloy. This indicates that the (σ+γ2) region was probably electrochemically active, and thus sensitive to active dissolution. The γ2 phase containing lower contents of N, Cr, and Mo compared with the primary austenite (γ) phase as well as the Cr-depleted zone around the σ phase appeared to be the main cause of the degradation of the stability of the passive film. An anodic current peak was also observed in the polarization curve for the region that had experienced spinodal decomposition of ferrite phase to Fe-rich α phase and Cr-rich α′ phase (α+α′). The anodic current peak for the region of (α+α′) in the alloy aged at 475 °C was found at lower potential and the peak current density was higher than those for primary α and γ in a solution annealed alloy. The precipitation of α′ phase induced the formation of a microscale Cr-depleted region, which is active and hence degrades the stability of passive film.