To improve price stability, stainless steel manufacturers have focused on developing new stainless steel reduced Ni and Mo content, known as rare metal. SUS821L1 and SUS323L, lean duplex stainless steels, were registered in the list of the JIS （Japanese Industrial Standards） in 2015. Corrosion resistance of SUS821L1 and SUS323L are almost equal to that of SUS304 and SUS316L respectively. SUS304 and SUS316L are known as major austenitic stainless steels. In addition, these lean duplex steels have strength equal to that of SUS329J3L or SUS329J4L, known as traditional duplex stainless steels. In this report, characteristic properties of these lean duplex stainless steels with that of SUS304 or SUS316L are described, and some suitable application examples are introduced.

In the previous studies, adsorption and corrosion inhibition of various anions and an organic cation inhibitor, tetrabutylammonium ion （C₄H₉）₄N^＋（ TBA^＋） on the Fe surface in deaerated 1 M HClO₄ have been discussed based on the hard and soft acids and bases principle. The present investigation deals with adsorption behaviors of adsorptive anions, Cl^-, I^- and SH^- and also the anions plus TBA^＋ on the Fe surface in 1 M HClO₄. The values of the surface coverage, θ for the anions and/or TBA^＋ were obtained from the electric double layer capacitances determined by impedance measurements. TBA^＋, which has no unshared electron pair, was adsorbed on the surface by electrostatic interaction. The anions which hold unshared electrons rather chemisorbed than electrostatically adsorbed on the surface by the formation of a stable soft acid and soft base interaction. The highest values of θ for I^- and SH^-, classified into soft bases, were observed at the potentials near the zero-charge potential, E_pzc because the Fe surface at E_pzc behaves the softest acid. TBA^＋ was adsorbed on the anion-chemisorbed Fe surface by electrostatic interaction, resulting in the high θ values.

It is well known that precipitates on the surface of stainless steels would be initiation points of pitting corrosion. In this study, it was investigated that an effect of Cu precipitates with tens of nm diameter on early stage corrosion and pit initiation of stainless steel. The diameters of Cu precipitates in 18％Cr-3％Cu ferritic stainless steel were controlled by aging condition which are 500-700℃, 1-100h, and the diameters were 0-51nm in grain and 104-125nm at grain boundary. In pitting potential measurement, the Cu precipitates on the surface of specimens were dissolved below pitting potential. It was suggested that the holes formed by the dissolution of the Cu precipitates initiated pits at pitting potential. Pit density of the pitting potential measurement had correlation with number of corrosion point in cyclic corrosion test. The pit densities of specimens with intragranular Cu precipitates whose average diameters were more than 35 nm were larger than that of specimens with less than 35 nm diameter precipitates. Therefore, it was suggested that there is a critical diameter of Cu precipitates which can affect pit initiation.