Corrosion and Weathering of Paints for Atmospheric Corrosion Control

Abstract

Three laboratory-based accelerated corrosion tests were investigated to develop improved procedures for assessing corrosion protection properties of organic paint films. The methods investigated were: standard salt-spray (ASTM Standard B-117), a wet/dry cycle corrosion test using a low-concentration ammonium sulfate/sodium chloride [(NH4)2SO4/NaCl] electrolyte, and a corrosion/weathering test combining wet/dry corrosion cycles with ultraviolet (UV) condensation cycles. Results were reported for an epoxy-polyamide/polyurethane paint system on phosphated steel and for an alkyd/alkyd system on cold-rolled steel. Corrosion and paint degradation were assessed visually and by scanning electron microscopy and x-ray photoelectron spectroscopy. Salt-spray testing resulted in corrosion, blistering, and paint degradation that appeared unrealistic compared to results after 12-month exterior exposures to natural industrial atmospheres. Wet/dry cycle corrosion testing produced paint delamination from scribe lines caused apparently by the relative volume of underlying corrosion products, as well as a clear tendency toward filiform corrosion. In contrast, the combined corrosion/weathering test resulted in corrosion, blistering, and overall paint degradation, including loss of gloss, which appeared qualitatively more realistic. It was concluded that the concomitant effects of weathering and corrosion should be considered together if an accelerated corrosion test for paints used in atmospheric corrosion control is to be successful.