Passivation and Surface Film Formation

Abstract

Passivation is a process in which the metal surface transforms from an active state to an inactive state owing to the formation of a barrier layer. The passivation of zinc has been the subject of numerous studies as shown in Table 3.1. It should be noted that there is a clear difference between the studies made in strong alkaline solutions and those made in other solutions. In general, the studies made in strong alkaline solutions are related to battery applications, and the focus is on the maximum current prior to passivation and the time to passivation. This focus arises because passivation is a problem in alkaline batteries under a high discharge rate. On the other hand, studies made in neutral and slightly alkaline solutions are generally related to corrosion, and the focus is on the conditions and processes of passivation as well as on the stability of the passive films. Accordingly, the material presented in this chapter is organized in two main sections; dealing with passivation in alkaline solutions and passivation in other solutions. Prior to these two sections, a description of the conditions and characteristics of passivation is provided. A later section is devoted to anodization, an anodic process used to produce a solid surface film which generally passivates the surface. The last section discusses the stability of passivation and passivation breakdown.