Effects of alloying elements (Mn, Si, Cu) on the passivity and repassivation kinetics of stainless steel alloys

Abstract

High Mn-N stainless steels (SSs) are being developed to replace the expensive austenitic stainless steels $(\gamma-SSs)$, containing high Ni content (8~12 wt. % Ni). Ni in these SSs is replaced with a combination of Mn and N. Mn (a γ-stabilizer) is not only 7 to 8 times cheaper per ton than Ni but it also increases the nitrogen solubility, and these two together compensate for the Ni deficiency in these new high Mn-N SSs. Nitrogen in addition of functioning as an austenite stabilizer also improves corrosion properties and has several other benefits such as increased strengthening and retardation of sensitization. The addition of Si (a ferrite former) and copper (an austenite former) in SSs can also have a substantial effect on the passivity and electrochemical properties. So therefore, effects of these alloying elements (Mn, Si, Cu) on the passivity, repassivation kinetics and stress corrosion cracking (SCC) behaviour of stainless steels are of considerable importance. The effects of these alloying elements on the repassivation kinetics and SCC behaviour of SSs were investigated by rapid scratch electrode technique (RSET) and slow strain rate test (SSRT). The structure and defect density of the passive films was investigated by photoelectrochemical technique and Mott-Schottky analysis. Polarization experiments and electrochemical noise analysis (ENA) of the high Mn-N SSs, showed that as Mn content of the alloys was increased, localized corrosion resistance was decreased as confirmed by decrease in pitting potential $(E_{pit})$ of the alloys. The repassivation kinetics, measured in terms of the current density i(t) flowing from the scratch as a function of the charge density q(t) that has flowed from the scratch, showed that, the addition of Mn decreased the repassivation rate of the alloys in 0.5M NaCl solution at $80^\circ C$. During the initial stages of repassivation, passive film nucleated and grew according to place exchange model in which log i(t) is l...