Crevice corrosion and microthrowing power

Abstract

The two concepts of crevice corrosion and electrolytic microthrowing power seem to have been connected very seldom, in spite of the fact that, on a closer look, microthrowing power would be expected to be a decisive factor in crevice corrosion of stainless steels. Whereas the electrolyte resistance is the predominating factor for the macrothrowing power, i.e. an even current distribution on macro-scale electrodes, the microthrowing power, i.e. an even current distribution on small-scale electrodes, like crevices, is governed by the product, |κ·dη/d i| of the electrolyte conductivity κ and of the slope dη/d i of the polarization curve. In the passive stage, preceding initiation of crevice corrosion dη/d i is exceptionally high, whereas κ is quite high in the concentrated and acid crevice solution formed in the propagation stage. Calculations are presented for the distribution of current and electrode potential on the crevice walls and of total crevice current and ohmic potential drop from the bottom to the mouth of a thin ( h = 0.01 cm) and deep ( L = 10 cm) metal/metal crevice of type 316 stainless steel in the preinitiation, initiation and propagation stages of crevice corrosion. The calculations are mainly based on equations derived by Wagner (Plating, 48(1961) 997) for the microthrowing power in the electodeposition of metals into thin and deep crevices on cathodes. As a basis for the calculations, we have determined the passive current density, i p, which has been found to be about two powers of ten lower than has commonly been assumed. The results of the calculations agree well with existing experimental data on current and potential distribution in crevices. The importance of differentiating between creviced anodes under potentiostatic control and naturally working crevice corrosion cells is emphasized.