Corrosion protective performances of commercial low-VOC epoxy/urethane coatings on hot-rolled 1010 mild steel

Abstract

The protective properties of low-VOC epoxy/urethane paint systems of commercial grade have been investigated using a variety of techniques such as electrochemical impedance spectroscopy (EIS). One epoxy-polyamide mastic/urethane, three high-solid epoxy-amine/urethane coatings, one solvent-free epoxy-amine/urethane, one water-based epoxy-amine and one high-VOC alkyd paint system (used as paint reference system) were applied 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, corrosion-induced adhesion loss and visual defects, as well as for their flexibility and resistance to direct impact. The barrier properties increased 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 and film pore area during the exposure time, leading to an appreciable corrosion 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 their pore area over time to a degree typical of moderately degraded coatings. After 2000 h of exposure in the SSC, they presented slight corrosion at the interface and little water uptake except for one paint system where water penetration and corrosion were more considerable. The three paint systems had slight blistering defects detectable by the naked eye. Among all the epoxy systems, the epoxy-polyamide mastic/urethane and the solvent-free epoxyamine/urethane had the best corrosion protection barrier properties, with a low water penetration, a limited and constant corrosion rate over time and a low film pore area. The low-VOC epoxy/urethane coatings are characterized by a drastic low elongation-at-break except for the epoxy-polyamide mastic/urethane and one one-coat epoxy-amine/urethane having a low resistance to water penetration. All the paint systems under consideration were unable to sustain the corrosion-induced adhesion loss in the SSC and at the outdoor marine test site.