Technical Note: The 100-mV Potential Decay Cathodic Protection Criterion

Mixed-potential theory was used to evaluate the 100-mV potential decay criterion for cathodic protection (CP). It was shown that the criterion may be enhanced if it is linked to the current that induces the negative potential shift. Thus, when the kinetics of the cathodic reaction exhibit activation-controlled behavior, the open-circuit corrosion rate (corrosion rate in the absence of protection) can be estimated from the negative potential shift achieved and the local applied current density. This can be used to monitor development of steel passivity induced by a cathodic current. When the cathodic reaction is under mass-transfer control, the anodic reaction may well be under activation control. The anodic reaction rate, while the current is applied, then may be determined. For activation- and mass transfer-controlled cathodic kinetics, the achievement of 100 mV of negative potential shift by the low current densities normally applied when cathodically protecting steel in concrete indicates that, in theory, the steel corrosion rate is negligible.