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.
본 연구에서는 Fe-20Cr 스테인리스 강의 두 단계 포텐셜 디케이 거동이 나타나는 원인을 규명하고, 이를 응용하여 산성 환경에서 Fe-20Cr 기 페라이트 계 스테인리스 강의 부동태 피막의 용해 저항성을 비교할 수 있는 피막용해저항지수를 개발하고자 하였다. 동전위 분극 시험, Mott-Schottky 분석 및 X-선 광전자 분광분석기를 이용한 부동태 피막의 깊이 프로파일 분석 결과 부동태 피막이 바깥쪽의 Fe 가 농축된 피막 층과 안쪽의 Cr이 농축된 피막 층의 이중 층으로 이루어져 있으며, 이로인해 두 단계의 포텐셜 디케이 거동이 나타남을 밝혔다. 이어서 합금 원소인 Cr, Mo, W, Cu, Ni가 스테인리스강의 부동태 피막의 용해 저항성에 미치는 영향을 수치화 하였으며, 각 합금 원소의 영향은 피막용해저항지수 = %Cr + 1.6 % Mo + 11.7 % Cu + 49 % W - 0.53 % Ni 로써 나타낼 수 있었다. 특히 W의 경우 부동태 피막 내에 안정한 W 산화물 층을 형성하여 부동태 피막의 용해 저항성을 크게 증가시키는 것으로 밝혀졌다.
