The Stern-Geary and related methods for determining corrosion rates

Abstract

Calculation of the current-potential relations for corroding electrodes as a function of the cathodic and anodic transfer coefficients and as a function of the difference between the corrosion and reversible potentials demonstrates the deficiencies of the Stern-Geary equation for determining corrosion rates. The calculations also demonstrate that a linear current-potential relation is not predicted except over very small ranges of potential, e.g. 2 mV. As a consequence, it is essential in using the Stern- Geary and related methods to determine the slope dE/di at the corrosion potential. The Stern-Geary method is not applicable for systems where the corrosion potential is close to the reversible potential for either of the participating reactions and fortuitous linearity of currentvoltage plots may be erroneously interpreted as indicating that the Stern-Geary method is applicable. Methods of determining the corrosion rate which depend upon measuring the difference ΔE in corrosion and reversible potentials are not likely to be successful because of their sensitivity to the value of ΔE. A new derivation of the slope dE/di at the corrosion potential is presented and it is shown that the Stern-Geary and Mansfeld-Oldham equations may be derived as special cases. This new derivation is not immediately useful for determining the corrosion rate but it is shown that by measuring the dependence of the corrosion potential on concentration of cathodic reactant then the new derivation can be useful for rapid and accurate corrosion rate measurements even for small values of ΔE.