Passivation of Nickel Monocrystal Surfaces

Abstract

Abstract Steady state potentiostatic polarization measurements on low index nickel monocrystal surfaces in 1N H2SO4 indicate a strong anisotropy in passive current densities. The nature of this orientation dependence appears to be related to such properties of the solid state as the crystal growth axis. For example, in the case of crystal growth parallel to the 〈110〉 axis, the degree of protection provided by the passive film on a {111} surface is greater than that on either {110} or {100} faces, i.e., the passive current densities are in the following order: {111}<{110}<{100}. It is interesting to note than an identical anisotropy has been reported in the rates of gaseous oxidation on nickel surfaces. On the other hand, precisely the reverse dependence is observed in passivation studies when the growth direction of the monocrystal is parallel to the 〈100〉 axis. The above results are examined in terms of the crystallography and defect structure of the film, the metal/film interface, and the metal substrate and their relevance to current theories of passivation is considered.