A Percolation Model for Passivation in Stainless Steels

Abstract

It is well-known that Fe-Cr, Ni-Cr, and Fe-Ni-Cr alloys require about 12 at. % chro-mium to become "stainless. " The transition around 12 % Cr is quite abrupt (i), especial-ly when expressed as the change in passiva-tion potential (Ep) (Fig. I). The occur-rence of this transitional behavior is insensitive to alloy composition (apart from Cr) and is probably almost independent of temperature. This suggests that the geomet-rical arrangement of Cr atoms in the lattice is important for passivation, independent of other factors. The most important geometri-cal consideration in this problem is the degree of connectivity of Cr atoms through the three-dimensional (3D) lattice, which is described by percolation theory (2). Perco-lation processes characteristically show a sharp threshold (pc) which represents the concentration of occupied sites (or bonds) at which a connected cluster of "infinite" extent suddenly appears (Fig. 2).