General informationsUntil now, systematic investigations on the analysis of the different modes of inhibition in electrode reactions are rather rare. Specially, interface-inhibition is investigated more frequently than electrolyte-layer inhibition.Hence, the following study only should be evaluated as an attempt to sketch the methodology and, if possible, to incite research especially on limits and reliableness of the methods. It is one of the aims of the Working Party "Inhibitors" of the European Federation of Corrosion to promote the development of this methodology.Generally, investigation in this field has to visualize three objects: 1. to detect inhibition 2. to determine the effects of inhibition 3. to investigate the causes of inhibition and its modes These objects will treturn in each of the following sections.Without differentiation, inhibition of an electrode reation can be detected by the occurring polarization or overvoltage*). As it is well known, inhibitors of whatever kind may increase the overvoltage of the electrode reaction. This can be indicated by the shiR of steady state current-density-potential curves which may be recorded statically or by low polarization sweep, (12, 38) of polarograms, (38) by cyclic voltammetry (7) or by correspondent measurements varying the Faraday-impedance with the frequency (37,38,138,139).A hysteresis occurring by recording backward the steady-state current density-potential curve may indicate slowly progressing and time dependent processes such as secondary inhibition of whatever kind (47, 139) or adsorption phenomena.But in some cases, inhibition even may occur though polarization or overvoltage are not observed. This may arise wben polarization of the inhibitor is hidden by compensating or even over-compensating depolarization. In such a case, the inhibitor is electrochemically very reactive. By its electrochemical reaction it may produce a remarkable depolarization.Hence, another general indicator of inhibition should be used which indicates inhibition even in cases of overlapping polarization and depolarization. It is the behaviour of potential-capacitance curves during the electrode reaction (11, 38, 153). Inhibitors are diminishing the double-layer capacity. Its change by inhibition may be ") See definition of Vetter (2): overvoltage -q = E -E,, polarization 11 = E -cr, where E the potential during electrolysis, E , the equilibrium-potential and sr the rest-potential (f.i. corrosion-potential).
Jabrgung Heft 611972calculated from the initial slope of the potential-time curve at constant polarization current (transient galvanostatic processes) (1). The double-layer differential capacity may be determined too by alternating current measurements 2, 3). On mercury, as a supplement, both the comparison of potential-capacity (4,5, 38) and electrocapillary curves (4,5, 36, 38 153) recorded in absence and presence of additives can announce inhibition.Besides this, all the mentioned methods can inform on the potential range of the inhibition. The comparison of results...
