ON ELECTROCHEMICAL TECHNIQUES FOR INTERFACE INHIBITOR RESEARCH

The electrochemical behavior related to different modes of inhibition effect is discussed for interface inhibitors. The action coefficients of an inhibitor on the anodic and cathodic reactions (f(a) and f(c), r espectively) of a corrosion process are defined. An equation is deduce d which denotes that if the inhibition effect is caused by geometrical ly blocking the surface of metal electrode by adsorbed inhibitive spec ies, the corrosion potential in the inhibitor-containing solution will be nearly equal to that in the solution without the inhibitor. In thi s case the inhibition efficiency (eta) equals the coverage (Theta) of the adsorbed species and can be estimated by polarization resistance ( R(p)) measurements. In addition, the plot of eta vs the ''relative cov erage'' (mu) estimated from interfacial capacitance measurements will be a straight line through the original point of the coordinate. If th e inhibition is not due to the geometric blocking effect, which can be judged by the noticeable shift of corrosion potential as the inhibito r is added into the solution, then eta cannot be estimated by R(p) and does not equal Theta. Instead, transfer resistance (R(t)) can always be employed to estimate inhibition efficiency whatever the mode of the inhibition effect. It is argued that theoretically no Tafelian straig ht lines can be measured in solutions with interface inhibitors unless Theta is independent of the electrode potential E or f(a), and f(c), of the inhibitor change linearly with E. Equations of faradaic admitta nce at the corrosion potential in inhibitor-containing solution for di fferent modes of inhibition effect are deduced. In addition to the est imation of R(t), R(p) and interfacial capacitance, information on the adsorption of the inhibitive species can also be gathered by EIS measu rement. In the case of the geometric blocking effect, the EIS display in the inhibitor-containing solution will be similar to that in the bl ack solution when eta is low, while a single capacitive loop will disp lay when eta is high. If the inhibition is not due to the geometric bl ocking effect, two time constants will be involved in the EIS display measured in the inhibitive-containing solution no matter what the disp lays of EIS in the blank solution are, and in this case the effect of E on Theta can be predicted based on the EIS and polarization curve me asurements. Copyright (C) 1996 Elsevier Science Ltd