Creation of alloy design concept for anti air-born salinity weathering steel

Alloy designing methods, such as Ca or enhanced Ni addition together with elimination of Cr, for anti air-born salinity weathering steel were established, utilizing various findings in terms of colloidal function of rusts. Addition of Ca to steel is a measure aiming at increasing pH at corroding interfaces, resulting in stabilization of iron oxy-hydroxides, alteration of ion exchanging properties of the colloidal corrosion products, and maintaining passive state of locally bare steel surfaces at defects of rust layers. Enhanced nickel prescription decreases active dissolution rates of the steel, to reduce acidification of the corrosion interface caused by hydrolysis of metallic ions. So formed rust may contain Ni(II) ions to stabilize Fe(II,III) oxides, which may be beneficial to alter its ion-exchanging properties to even facilitate alkalization at steel/rust interfaces. Elimination of chromium is essential to improve corrosion resistance of low alloy steels applied to the coastal atmospheres because of its acidifying properties. Practicality of the established concept was confirmed through 9 year exposure tests of so-prescribed steels to the severe coastal environment, resulting in the first commercialization of anti air-born salinity weathering steel. This paper mainly describes authors' seed investigating activities carried out from 1984 to 1989 together with the recently obtained penetration curves.