CORROSION PROTECTIVE PERFORMANCES OF COMMERCIAL LOW-VOC EPOXY URETHANE COATINGS AN HOT-ROLLED 1010-MILD-STEEL/

The protective properties of low-VOC epoxy/urethane paint systems of c ommercial grade have been investigated using a variety of techniques s uch as electrochemical impedance spectroscopy (EIS). One epoxy-polyami de mastic/urethane, three high-solid epoxy-amine/urethane coatings, on e solvent-free epoxy-amine/urethane, one water-based epoxy-amine and o ne high-VOC alkyd paint system (used as paint reference system) were a pplied on hot-rolled 1010 mild steel panels and exposed for up to 2000 h in the salt spray cabinet (SSC) or for 1 year at an outdoor marine test site. These paints were tested for their barrier properties, corr osion-induced adhesion loss and visual defects, as well as for their f lexibility and resistance to direct impact. The barrier properties inc reased in the following order: alkyd < water-borne epoxy < high-solid epoxy-amine/urethane < high-solid epoxy-polyamide mastic/urethane and solvent-free epoxy amine/urethane. The alkyd and the water-borne epoxy systems are characterized by a very large increase in water uptake an d film pore area during the exposure time, leading to an appreciable c orrosion rate at the interface such that important visual defects are noticed on the plate surface as the exposure time is long enough. The three high-solid epoxy-amine/urethane paints had a low increase on the ir pore area over time to a degree typical of moderately degraded coat ings. After 2000 h of exposure in the SSC, they presented slight corro sion at the interface and little water uptake except for one paint sys tem where water penetration and corrosion were more considerable. The three paint systems had slight blistering defects detectable by the na ked eye. Among all the epoxy systems, the epoxy-polyamide mastic/ureth ane and the solvent-free epoxyamine/urethane had the best corrosion pr otection barrier properties, with a low water penetration, a limited a nd constant corrosion rate over time and a low film pore area. The low -VOC epoxy/urethane coatings are characterized by a drastic low elonga tion-at-break except for the epoxy-polyamide mastic/urethane and one o ne-coat epoxy-amine/urethane having a low resistance to water penetrat ion. All the paint systems under consideration were unable to sustain the corrosion-induced adhesion loss in the SSC and at the outdoor mari ne test site. (C) 1997 Elsevier Science S.A.