ACCELERATED TEST METHOD FOR ASSESSING CORROSION AND WEATHERING OF PAINTS FOR ATMOSPHERIC CORROSION CONTROL

Three laboratory-based accelerated corrosion tests were investigated t o develop improved procedures for assessing corrosion protection prope rties 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 corros ion cycles with ultraviolet (UV) condensation cycles. Results were rep orted for an epoxy-polyamide/polyurethane paint system on phosphated s teel and for an alkyd/alkyd system on cold-rolled steel. Corrosion and paint degradation were assessed visually and by scanning electron mic roscopy and x-ray photoelectron spectroscopy. Salt-spray testing resul ted in corrosion, blistering, and paint degradation that appeared unre alistic compared to results after 12-month exterior exposures to natur al industrial atmospheres. Wet/dry cycle corrosion testing produced pa int delamination from scribe lines caused apparently by the relative v olume of underlying corrosion products, as well as a clear tendency to ward filiform corrosion. In contrast, the combined corrosion/weatherin g test resulted in corrosion, blistering, and overall paint degradatio n, including loss of gloss, which appeared qualitatively more realisti c. It was concluded that the concomitant effects of weathering and cor rosion should be considered together if an accelerated corrosion test for paints used in atmospheric corrosion control is to be successful.