THE ELECTROCHEMISTRY OF CORROSION BENEATH CORROSION DEPOSITS

Based on the uniform corrosion mechanisms beneath corrosion deposits described in a preceding theoretical study, the present paper shows that certain deposits attain a steady state only at the free corrosion potential. Except for the natural corrosion potential, electrochemical investigation techniques can therefore only be used to study quasi-stationary states, where the electrochemical reactions and transport phenomena are in dynamic equilibrium with the instantaneous thickness of the deposit. The electrochemistry of a metal covered by soluble or anionic insoluble deposits is very close to that on bare metal (deposits ''transparent'' to the imposed polarization). Conversely, deposits of the insoluble cationic type compensate nearly integrally the effects of polarization, thus behaving as veritable passive layers. It is also shown that irreversibility effects are present in the growth regime control of deposits under imposed polarization. This may lead to multiple quasi-stationary states. For example, anodic or cathodic pulses can cause an insoluble deposit to change from cationic to anionic, or vice versa. A particular consequence is the existence of a pitting or general anodic depassivation potential for insoluble cationic deposits. Similarly, there is a protection or cathodic passivation potential for insoluble anionic deposits. Altogether, electrochemical methods shall be used, not only to measure corrosion rates, but also to study the intrinsic stability of the feature of observed deposits. This should enable us really to predict long-term corrosion rates.