What Does Activation Controlled Corrosion Mean?
Activation controlled corrosion is a type of corrosion which arises from activation polarization. Activation is the changing of a passive surface of a metal to a chemically active state.
Activation polarization is usually a controlling factor during corrosion in media containing high concentrations of active species or strong acids. Mixed potential theory can be used to explain activation controlled corrosion.
Corrosionpedia Explains Activation Controlled Corrosion
Corrosion is an activation-controlled chemical reaction, the rate of which is greatly affected by temperature. Typically, corrosion rate increases significantly as temperature increases. Activation polarization is usually the controlling factor during corrosion in strong acids.
The increased corrosion rate results from increased activation energy for chemical and electrochemical reactions, increased diffusion rates in the electrolyte, and increased transport through the electrolyte or environment and across films that may be formed on the metal surface. If a corrosion reaction is activation controlled, then stirring or increasing agitation has no effect on the corrosion rate.
Activation overpotential controls the electrode reaction at a low reaction rate. The cathodic reaction 2H+ + 2e- = H2 is, in the acid solution, one of the processes controlled by the activation overpotential. The activation overpotential varies with the kind of metal and the electrolytic condition. In most cases, metal dissolution and metal-ion deposition are controlled by the activation overpotential.
For purely activation controlled processes, each reaction can be described by a straight line on a potential, E versus log current, Log i plot, with positive Tafel slopes for anodic processes and negative Tafel slopes for cathodic processes. Mixed potential theory can be used to explain cases where corrosion processes are purely activation controlled or cases where concentration controls at least one of the corrosion processes.