ELECTROLYTE PENETRATION THROUGH EPOXY COATINGS ELECTRODEPOSITED ON STEEL

A.C. impedance measurements carried out on epoxy-resin electrocoated steels were used to determine the pore resistance of the organic film as a function of time of exposure to 3% sodium chloride solution. The thermogravimetric analysis of coatings after various exposure times enabled determination of the quantity of electrolyte inside the pores, while gravimetric liquid sorption experiments were used to evaluate water uptake inside the coating. The results were interpreted in terms of a model in which water rapidly penetrates macrocapillaries (macropores) present as defects in the external portion of the coating and extra fine capillaries formed by the polymer net, followed by water and ions penetration through the macropores. The number, dimensions and shape of the pores through the film, estimated by optical microscopy, additionally confirmed the proposed model of electrolyte penetration. It was shown that conduction through the coating depends only on conduction through the macropores, although the quantity of electrolyte inside the macropores is only one tenth of that inside the polymer net.