Corrosion rate measurements under conditions of mixed charge transfer plus diffusion control including the cathodic metal ion deposition partial reaction

A modified polarization equation is presented, based on mixed charge-transfer and diffusion control of anodic and cathodic partial reactions, and including the cathodic metal ion deposition partial reaction (back reaction). This equation can be used as a basis for curve fitting current/potential data to obtain corrosion rate parameters and would be particularly useful for zinc and steel corrosion in near neutral stagnant solutions. Errors due to neglect of the back reaction, under mixed kinetics, can be significantly larger than the corresponding values under charge-transfer controlled kinetics, even for values of E k well removed from E 0, M (e.g. at E k, M =64 , 90 mV, errors are 5%, 1% as compared to −65%, −53%, respectively). These errors are always negative, indicating that calculated corrosion rates underestimate actual rates. Errors increase as the ratio i k /i L, H increases, as E k approaches E 0, M and as either b a, M or b c, H increases (holding the other constant). As i k approaches i L, H , the buildup of metal ions at the metal surface increases further. Because of slow mass transport of metal ions away from the metal surface, the back reaction has a more significant effect; hence, errors due to neglect of the back reaction increase. If both the mass transport and the back reaction are neglected, these errors can be as large as −90%.