INTERMEDIATE PRODUCTS OF ATMOSPHERIC OXYGEN REDUCTION AND THE INTEGRITY OF METAL ORGANIC COATING INTERFACE

The mechanisms of delamination and undercoating corrosion in systems protected by organic material(s) remain unclear. The observed types of failure have been interpreted in various mechanistic terms having a common assumption that the hydroxyl ions generated at the metal surface as products of the reduction of atmospheric oxygen are responsible for the loss of protective properties. The possibility of formation of superoxide and/or peroxide species and the effects of their chemical interaction with the coating seem to have been neither considered nor studied. They are presently discussed in terms of the concentration and stability of peroxide species and of a possible mechanism of their attack on the organic coating. Results of the recent rotating ring-disc electrode (RRDE) studies of the mechanism of oxygen reduction at steel and zinc surfaces in alkaline media are summarized and compared. Although the actual mechanism differs somewhat, peroxyl ions are present as stable products of oxygen reduction in the entire interfacial potential region corresponding to the oxide-covered surfaces in both the passive and prepassive potential regions. Only at bare and/or adsorbed-hydrogen-covered metal surfaces are hydroxyl ions the only final products. The mechanistic effects of the presence of a number of foreign anions at the zinc and steel interfacial regions were studied using the RRDE method. A possible connection between the presence of certain anions and corrosion protective properties are discussed. Further mechanistic considerations of delamination and undercoating corrosion should take into account the presence of the highly aggressive peroxide and nascent superoxide ions whose decomposition may involve destructive chemical interaction with organic material and/or other components of the coated system.