On electrochemical techniques for interface inhibitor research

Abstract

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 , respectively) of a corrosion process are defined. An equation is deduced which denotes that if the inhibition effect is caused by geometrically blocking the surface of metal electrode by adsorbed inhibitive species, the corrosion potential in the inhibitor-containing solution will be nearly equal to that in the solution without the inhibitor. In this case the inhibition efficiency (η) equals the coverage (Θ) of the adsorbed species and can be estimated by polarization resistance ( R p ) measurements. In addition, the plot of η vs the “relative coverage” (μ) estimated from interfacial capacitance measurements will be a straight line through the original point of the coordinate. If the inhibition is not due to the geometric blocking effect, which can be judged by the noticeable shift of corrosion potential as the inhibitor is added into the solution, then η cannot be estimated by R p and does not equal Θ. 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 straight lines can be measured in solutions with interface inhibitors unless Θ is independent of the electrode potential E or f a and f c of the inhibitor change linearly with E. Equations of faradaic admittance at the corrosion potential in inhibitor-containing solution for different modes of inhibition effect are deduced. In addition to the estimation of R t , R p and interfacial capacitance, information on the adsorption of the inhibitive species can also be gathered by EIS measurement. In the case of the geometric blocking effect, the EIS display in the inhibitor-containing solution will be similar to that in the black solution when η is low, while a single capacitive loop will display when η is high. If the inhibition is not due to the geometric blocking effect, two time constants will be involved in the EIS display measured in the inhibitive-containing solution no matter what the displays of EIS in the blank solution are, and in this case the effect of E on Θ can be predicted based on the EIS and polarization curve measurements.