The mechanism of the action of inorganic inhibitors

Abstract

The studies on the inhibition of the corrosion of iron and steel made in the Oak Ridge National Laboratory originated in the discovery of the inhibitory action of the pertechnetate ion, TcO₄ ^-. The properties of technetium compounds relevant to inhibition are presented, together with an outline of the hypothesis of electrostatic polarisation by the XO^4n- ions which led to the discovery.

Results of several studies related to mechanism are given. These demonstrated the extreme effectiveness of the pertechnetate ion, in comparison with other inhibiting anions, even in the absence of oxygen or other oxidising agents. It was shown that TcO₄ ^-, CrO₄ ^2- and MnO₄ ^- all exert an effect on the metal that does not depend upon reduction of the inhibitor. The result of this action is that the metal may be passivated anodically by much lower current densities than are requisite in non-inhibiting electrolytes, and with passage of no more charge than is necessary to form the passive film.

The nature of the interface between metal and electrolyte in both active and passive conditions is briefly discussed in relation to the alternative reaction paths which lead to active dissolution or to passivation, respectively. The various ways in which the inhibitor may enter into these processes are presented from the point of view of electrode kinetics, as modified by the specific interactions in which the structure of the inhibitor may playa part. In effect, the inhibitor reduces the ‘activity’ of the metal, as represented by its exchange current, so that it is then capable of being passivated as easily as a stainless alloy.