EFFECT OF POROSITY ON RESISTANCE OF EPOXY COATINGS TO COLD-WALL BLISTERING

Six epoxy-based coating systems were tested for their resistance to de gradation in the form of ''cold-wall'' blistering. The coatings were a pplied to type 1010 steel (UNS G10100) substrates and exposed to heate d water for up to 10 months in Atlas test cells as a modified version of NACE Standard TMO174. The performance of the coatings was measured by the exposure time for the coatings to start blistering, the time fo r the corrosion potential (E(corr)) of the coating substrates to shift toward active values, and the delamination (or blistering) rate of th e coatings. Good cold-wall blistering resistance was observed for two polyamine-cured epoxy coating systems with porosity levels < 1 vol%. T hese two coating systems exhibited longer lifetimes than the other coa ting systems under four different test conditions that varied coating thicknesses and exposure temperatures, Intermediate performance was ob served for a polyamine-cured epoxy with an average porosity level > 1 vol%. Poor cold-wall blistering resistance was shown by a polyamide ep oxy system, an amine adduct epoxy system, and an amido-amine epoxy sys tem, all of which had porosity levels > 1 vol%. Most of the coating sa mples exhibited linear blistering rates, which indicated that the kine tics of cold-wall blistering were diffusion controlled. The two coatin g systems that showed the best resistance to cold-wall blistering also showed the lowest blistering rates, indicating that these coatings ma y have had lower permeabilities and/or better adherences than the poor er performing coatings, probably as a result of their lower porosity l evels and similar compositions.