Hiroki Ota

Corrosion Behavior of 21%CR Ferritic Stainless Steel at Atmospheric Environment

21%Cr ferritic stainless steel (SS) is prevailing as a substitution of TYPE304 because it does not contain Ni, expensive ingredient. In this study, 21%Cr SS and TYPE304 were exposed for 5 years at seashore area in Okinawa, and corrosion resistance of these steels was evaluated from appearances and the maximum pit depth expected by Gumbel distribution. As a result, the estimated maximum pit depth value of 21%Cr SS was less than that in TYPE304 with #800 and 2B surface, while it was equivalent with HL surface. Growth rate of maximum pit depth could be expressed by the equation of x=aT n , where x is maximum pit depth, T is time, a and n are constants, and n was estimated to 0.2 with any surface in both steels. Therefore, it could be predicted that 21%Cr SS could exhibit superior or even equivalent corrosion resistance to that in TYPE304 when exposed longer period.

High Corrosion Resistance 21%Cr-0.4%Cu Ferritic Stainless Steel Contributing to Resource Conservation

With prices for metal resources such as nickel and molybdenum soaring, there is a heightened sense of crisis concerning resource scarcity. While Type304, the most common stainless steel, offers excellent corrosion resistance, its price is affected significantly by the cost of nickel because of its 8% nickel content. The stainless steel that has the same corrosion resistance as that of Type304 and does not contain nickel and molybdenum has been required. JFE Steel Corporation has developed a new 21%Cr-0.4%Cu stainless steel, the world’s first ferritic stainless steel, which offers equivalent corrosion resistance to Type304 while containing absolutely no nickel or molybdenum, two rare metals. The newly developed steel contains 21% chromium with the addition of 0.4% copper. The development of the steel is based on a new discovery that the passive films of stainless steels could be strengthened by the synergy effect of high chromium content and copper addition. Copper addition enriches the chromium content in passive films after field exposure. Newly developed 21%Cr-0.4%Cu stainless steel is adopted for many applications as a substitution for Type304, including commercial kitchenware, building materials and industrial machinery. The steel is expected to be a new standard of a ferritic stainless steel as a substitution for Type304.