A review on the passivation ability of nanostructured steel surfaces created by nanocrystallization treatment

Passivation is a process through which metals are protected from corrosion due to the formation of a nanometer-thick oxide layer on their surfaces. The great practical significance of passivation is that it protects metals and alloys from corrosion that occurs in the atmosphere, water and aggressive chemical environments. Studies on passive film formation and surface passivity of metals and alloys are not new. Substantial efforts have been made in recent decades to understand the thermodynamics and kinetics of the formation, growth and also breakdown of the passive films under various environmental conditions. It has been reported that the electrochemical stability of the passive film is mainly dependent on its chemical composition and structure (e.g. thickness, compactness, etc.) which are in turn influenced by the chemical composition and microstructure of the substrate as well as the conditions of the surrounding environment (i.e. pH, temperature, etc.). So far, various methodologies including surface treatments, surface coatings, inhibitors, etc. have been used to enhance the passivation ability and passive film stability upon the metals and alloys. The aim of this review paper is to investigate works that have been undertaken to improve the electrochemical corrosion behavior of the passive film formed on a steel surface, with particular focus on surface nanocrystallization treatments to understand the effect of grain size on the passivity behavior. It has been shown that currently there is no general agreement in the literature on the effects of grain refinement on the passivation ability and corrosion performance of stainless steels. The main reason for this disagreement is the complex contribution of various factors resulted from different manufacturing processes and surface treatments which could pose complex effects on corrosion resistance of a material. Each one of these factors has its own effect on the passivity and corrosion behavior and they are also highly interdependent on each other. One of the significant gaps in corrosion-related literature pointed out in this paper is that when studying the passivity and corrosion resistance of nanocrystalline stainless steels, all the possible determining factors like grain size, grain boundary character, texture, chemical composition, phase composition, surface roughness, residual stresses and inclusions should be considered.