Abstract
Various mechanisms by which organic coatings protect against corrosion have been discussed in the literature. It has long been established that protective coatings cannot be regarded as barriers against oxygen and water, as sufficient reactants will penetrate the coating to sustain a corrosion reaction [1]. Mayne instead suggested that the coating functions as a barrier between anodic and cathodic sites on the metal surface [2]. Stratmann and coworkers proposed that the diffuse double layer, as described in the Gouy-Chapman theory of charged surfaces, is extended by the coating due to its low concentration of ions, reducing the driving force for electrochemical reactions [3]. More recently, Mills and Jamali proposed that the corrosion protection is actually provided by the surface oxide, while the coating serves to protect the oxide layer [4].
In principle there are three mechanisms for corrosion protection: Thermodynamically (cathodic protection), kinetically (passivity), and by removing the corrosive reactants with a perfect barrier. If these same principles are to be applied to protective organic coatings, the barrier mechanism can be ruled out since the coating is not a perfect barrier to oxygen and water. A zinc-rich primer may provide cathodic protection, while a coating without sacrificial pigments must rely on passivity, i.e. the mechanism proposed by Mills and Jamali. This paper presents a study of the oxidation of steel under a protective coating, in support of the oxide protection and passivity theory. By electrochemically reducing the surface oxide before applying the paint, oxide growth and passivation of the steel under the paint was investigated using Scanning Kelvin Probe and XPS.