Micro-mechanic model for cathodic blister growth in painted steel

Abstract

A micro-mechanic blister growth model is proposed for a coating system consisting of a polymer film applied to a steel substrate exposed to salt solutions. The mechanism of the blister formation is based on corrosion-induced disbondment of the coating at the defect periphery coupled with the stress driven diffusive transport of liquid along the coating/substrate interface. By considering the coating as a semi-double cantilever beam loaded by a moment at the periphery and a distributed load along the beam length due to mass transport, a fifth order ordinary differential equation is derived for the beam deflection. The solution is obtained, which yields the functional relationship between the blister growth rate and applied bending moment. The predicted blister growth velocity compared favorably with experimental observations on a paint coated steel panel immersed in a five percent salt water solution. Model predictions allow the construction of a blistering zone/nonblistering zone map, which should help to design better protective coatings for steel.