440A 스테인리스강의 내식성에 미치는 첨가 원소와 열처리의 영향

Abstract

440A martensitic stainless steels which were modified with reduced carbon content(~0.5%) and addition of small amount of nickel, vanadium, tungsten and molybdenum were manufactured. Effects of alloying elements and tempering temperatures on the uniform and pitting corrosion were investigated through the electrochemical polarization tests. 1. Characteristics of uniform corrosion in 1N H2SO4 ①When tempering temperature is constant, corrosion current density in active-passive transition point, Icorr, decreased a little with an increase of austenitizing temperature. ②When austenitizing temperature is constant, longer holding time showed a little lower Icorr and Ipass, passive current density. ③When austenitized at 1050℃ and tempered at 350~750℃, best anti-corrosion properties were obtained at 350℃ tempering temperature while worst at 450℃ and 550℃. ④When tempered at below 450℃ and above 550℃, similar and good anti-corrosion characteristics were obtained regardless of alloying elements added, showing anti-corrosion characteristics are influenced more by tempering temperature than by alloying elements. 2. Pitting corrosion in 3.5% NaCl ①When tempering temperature is constant, the lowest pitting potential, Ep, was obtained when austenitizing temperature was 1250℃ and this is because of the grain coarsening. ②When austenitizing temperature is constant, longer holding time showed a little higher value of Ep. ③When austenitized at 1050℃ and tempered at 350~750℃, the highest Ep was obtained at 350℃, while the lowest at 450℃ and 550℃ ④When tempered at 450℃ and 550℃, Ep was increased a little when 0.4wt.% of tungsten was added. ⑤When tempered at 450℃, more pitting was observed. And pitting was formed at regions where Cr concentration is low or grain boundaries are intersecting and showed irregular shape.