The Effects of the Slow Adsorption of Anions and Some Organics on Iron Dissolution Kinetics

Abstract

The mechanism of anodic dissolution of iron in acid solutions has been the subject of experimental investigations and theoretical discussions by many authors during the last 30 years. Many of the proposed reaction mechanisms can be divided into three groups: (i) those which, following Heusler’s “catalytic mechanism,”(1) postulate a single, two-electron charge transfer rate-determining step; (ii) mechanisms based on that originally proposed by Bockris, Dražic, and Despic(2) (BDD mechanism), with two consecutive, single-electron charge transfer steps, including adsorbed OH species in the reaction mechanism, and (iii) the anion participation mechanism, a modification of the BDD mechanism, advocated by Kolotyrkin and co-workers,(3) Bech-Nielsen and co-workers,(4,5) and others. A detailed analysis of the relevant literature was presented in a recent review.(6) The purpose of this chapter is to show that most common inorganic anions (SO 4 2− , ClO 4 − , Cl−, Br−, I−), similarly to many organic molecules, adsorb on the iron surface and inhibit the anodic reaction. Therefore, there is no justification for incorporating the anions into the formation of the activated complex of the main anodic dissolution reaction, except perhaps in very high anion concentration solutions, when the main dissolution reaction is totally inhibited.