Passive film under anodic polarisation

It is convenient to consider three stages of anode polarisation with regard to temperature effects, (a) under film-free conditions, (b) under film-forming conditions and (c) at the active-passive transition. 

The technique may be understood in terms of metallic passivity, i.e. the loss of chemical activity experienced by certain metals and alloys under particular environmental conditions as a result of surface film formation. Equations 15.2 and 15.3 suggest that the application of an anodic current to a metal should tend to increase metal dissolution and decrease hydrogen production. Metals that display passivity, such as iron, nickel chromium, titanium and their alloys respond to an anodic current by shifting their polarisation potential into the passive regon. Current densities required to initiate passivity are relatively high [Uhlig and Revie 1985] but the current density to maintain passivity are low, with a consequent reduction in power costs [Scully 1990]. 

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