Corrosion-protective effect of organic coatings at the defect - influence of surface preparation and anti-corrosive pigments of base coating

In practice of corrosion protection a clear influence of surface preparation and pigments of base coating has been widely observed on defect-induced delamination of organic coatings. In the presented paper this mechanism was studied using the Scanning Kelvin-Probe. This instrument enables determination of the corrosion potential by high resolution contactless measurements even with presence of insulating organic coatings. During these investigations the following results were obtained: Cathodic delamination takes place in case of coatings with zinc phosphate primers on blast cleaned steel substrates. Using zinc dust primers, especially the edges of the defect are cathodically protected by anodic dissolution of zinc as part of the zinc dust primer. Because of the formation of zinc oxides, increasing load duration can lead to a deactivation of zinc dust and a progression of the corrosion process below the coating. On hand-cleaned steel surfaces rust reduction takes place at the edge of the defect in form of a cathodic partial reaction which is independent from pigments of the base coating. This reduction is increased by the galvanic cell. In case of cyclic loading reduced rust is reoxidized during dry phases and can re-participate in the process during wetting. In the result the thickness of the rust-layer is growing and phase transformations in the rust lead to the loss of adhesion of the coating starting from the edge of the defect.