Abstract
Three laboratory based accelerated corrosion tests have been investigated with the aim of developing improved procedures to assess the corrosion protection properties of organic paint films. These methods were; i) standard salt-spray (ASTM B117-85), ii) wet/dry cycle corrosion test using a low concentration (NH4)2SO4/NaCl electrolyte and iii) a corrosion/weathering test combining wet/dry corrosion cycles with UV-condensation cycles described in ASTM G53-84. Results are reported for an epoxy-polyamide/polyurethane paint system on phosphated steel and also for an alkyd/alkyd system on cold rolled steel. Corrosion and paint degradation were assessed visually and by SEM. Corrosion product composition was analysed briefly by XPS. Salt-spray testing resulted in corrosion, blistering and paint degradation which appeared unrealistic if compared with 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 towards filiform corrosion. In contrast, the combined corrosion/weathering cycle test resulted in corrosion, blistering and overall paint degradation, including loss of gloss, which appeared, qualitatively at least, to be more realistic. Possible reasons for the observed differences are addressed. It is concluded that, for paints, if an accelerated corrosion test is to be successful, then the concomitant effects of weathering and corrosion should be considered together.