Establishment of film dissolution resistance equiva-lent(FDRE) based on the potential decay behavior of the stainless steels

Abstract

In this thesis, potential decay behavior of Fe-20Cr based stainless steels and establishment of the film dissolution resistance equivalent of ferritic stainless steels under acidic condition were explored. Potential decay behavior of Fe-20Cr alloy was examined by potential decay test, X-ray photoelectron spectroscopy, and Mott-Schottky analysis. The potential decay behaviors charac-terized by a short plateau followed by $1^{st}$ decay and a long plateau followed by $2^{nd}$ decay to an active state were closely associated with the duplex layer structure of passive film. We demon-strate that the short plateau followed by $1^st$ decay is due to rapid dissolution of outer non pro-tective Fe-rich $oxide/Cr(OH)_3$ layer, whereas the long plateau followed by $2^{nd}$ decay is attribut-ed to slow dissolution of inner protective Cr-rich oxide layer. Next, the effects of alloying elements (Cr, Ni, Cu, W, Mo) on the resistance to uniform dissolution of Fe-20Cr based SSs was examined. Passive film formed on each alloy was inves-tigated by XPS depth profile. Fe-20Cr SS showed the $τ^{2nd}$ of 9460 s, and slope of $τ^2nd$ vs. wt. % Cr was about 2200 s / wt. % Cr. The increase in the Ni content adversely affected the resistance to uniform dissolution due to the formation of the ferrite-austenite duplex phase. The increase in Cu content enhanced the resistance to uniform dissolution. The increase in W content also improved the resistance to uniform dissolution by forming stable W oxide in the barrier layer of the passive film. The presence of Mo increase the resistance to uniform dissolution. According to the results, the film dissolution resistance equivalent (FDRE) was established as FDRE = % Cr + 1.6 % Mo + 11.7 % Cu + 49 % W - 0.53 % Ni.