DEGRADATION O ORGANIC COATINGS ON STEEL - MATHEMATICAL-MODELS AND PREDICTIONS

Conceptual and mathematical models are developed for blistering proces ses induced by the corrosion of defect-containing coatings on steel su bstrate ''posed to electrolytes. Models were developed based on cation transport along the coating/steel interface from the anodic sites at the defect to the cathodic sites under the coatings. The models accoun ted for both an initial induction period before cations have broken th rough into the blister and a subsequent propagation period when ions a ccumulate within the blister. The mathematical models are solved analy tically to predict cation fluxes and concentrations along the interfac e and within the blisters. Solutions of the models are expressed in te rms of dimensionless parameters. Model variables include blister size, distance between the blister and defect, ion diffusivity and potentia l gradients. The conditions under which ion transport is likely to be controlled by either potential or concentration gradients are establis hed. Model results indicate that large blisters are more likely to gro w than small ones because higher cation concentrations can build up wi thin them. Implications of this conclusion for maintaining the integri ty of organic coatings are discussed.