Mechanism of Improvement on Corrosion Resistance by Copper Addition to Ferritic Stainless Steels

Abstract

The mechanism of improvement on corrosion resistance by copper addition to ferritic stainless steels was studied electrochemically. Cathodic reaction of copper added ferritic stainless steels is accelerated with progress of corrosion, presuming as the effect of fine metallic copper precipitated on surface. Anodic reaction of ferritic stainless steels is decelerated by copper addition because of not only the hinderance from deposits on surface but also the decreasing effective anodic reaction area by precipitated metallic copper on the active anodic point on surface. Especially, the passivation potential of 19%Cr steel shifted to nobler side into the vicinity of its natural potential by copper addition. It was considered from the above properties, that even if the copper added and Nb stabilized 19%Cr steel gets rusty to start the anodic dissolution only a little degree, cathodic reaction at rusting point is immediately accelerated to exceed the passivation current (exceed the anodic peak value) resulting in the passive state virtually. Namely the copper added Nb stabilized 19%Cr steel shows the excellent corrosion resistance since an auto-passivation function of a negative feed back type.