Effect of Porosity on Resistance of Epoxy Coatings to Cold-Wall Blistering Available to Purchase

Six epoxy-based coating systems were tested for their resistance to degradation in the form of “cold-wall” blistering. The coatings were applied to type 1010 steel (UNS G10100) substrates and exposed to heated water for up to 10 months in Atlas test cells as a modified version of NACE Standard TM0174. The performance of the coatings was measured by the exposure time for the coatings to start blistering, the time for the corrosion potential (E_corr) of the coating substrates to shift toward active values, and the delamination (or blistering) rate of the coatings. Good cold-wall blistering resistance was observed for two polyamine-cured epoxy coating systems with porosity levels < 1 vol%. These two coating systems exhibited longer lifetimes than the other coating systems under four different test conditions that varied coating thicknesses and exposure temperatures. Intermediate performance was observed for a polyamine-cured epoxy with an average porosity level > 1 vol%. Poor cold-wall blistering resistance was shown by a polyamide epoxy system, an amine adduct epoxy system, and an amido-amine epoxy system, all of which had porosity levels > 1 vol%. Most of the coating samples exhibited linear blistering rates, which indicated that the kinetics of cold-wall blistering were diffusion controlled. The two coating systems that showed the best resistance to cold-wall blistering also showed the lowest blistering rates, indicating that these coatings may have had lower permeabilities and/or better adherences than the poorer performing coatings, probably as a result of their lower porosity levels and similar compositions.