Activation control, anodic partial process

Although most corrosion systems can be described by the limiting models presented above, there are instances where control of the corrosion system is a combination of both types, viz., activation controlled anodic partial process with two cathodic partial processes - one under activation control and another under transport control. Examples are iron corrosion in acid solution with inorganic contaminants (, 18) and oxygen ( ). The corrosion current density in such systems is... 

Mixed potential systems with the cathodic partial process under transport control and the anodic partial process under activation control is typical of many corrosion systems. For the cathodic partial process to be under transport control. Equation 44 must be unity or larger. This occurs when the absolute value of the difference between the equilibrium electrode potential of the cathodic partial process and the corrosion is on the order of one volt. This condition prevails for most metals of interest in corrosion studies if oxygen... 

When there are two partial process in a mixed potential system and both are under activation control, the most probable forms of the current densities of the anodic and cathodic partial processes are Equations 33 and 35, respectively. For an isolated metal, the overpotential (since the corrosion potential represents the perturbed electrode potential in this case) is... 

Another source of error involves cases in which both the anodic and cathodic reactions are not charge transfer controlled processes, as required for the derivation of Eq 25. Modifications to Eq 25 exist for cases in which pure activation control is not maintained, such as in the case of partial diffusion control or passivation [35]. Other researchers have attempted to calibrate the polarization resist2ince method with gravimetrically determined mass loss [36], In fact, polarization resistance data for a number of alloy-electroljrte systems have been compared to the observed average corrosion currents determined from meiss loss via Faraday s law [28], A linear correspondence was obtained over six orders of magnitude in corrosion rates. 

---