Comparison of under-film corrosion behaviors under unsheltered and sheltered exposure conditions

Abstract

Heavy-duty paints are applied to steel structures built in coastal areas. Some structures, such as bridges, are composed of parts that are directly exposed to the rain and other parts that are not exposed to the rain. In the latter parts, corrosion environments are sometimes severer than the former parts. To compare the under-film corrosion behaviors under such different exposure conditions, painted steel sheets were examined under unsheltered and sheltered exposure conditions. After the exposure tests, they were evaluated and analyzed by Electron Probe Micro Analyzer (EPMA), Raman imaging and microscopic FT-IR on cross-sections. In Raman imaging, the distributions of chemical states of corrosion products were obtained and compared with the elemental mapping results by EPMA. These detailed analysis data made it possible for us to consider corrosion behaviors. As a result, uniform corrosion of substrates was found under the unsheltered exposure condition, and localized corrosion of substrates was found under the sheltered exposure condition. In addition, not only distributions of sea salt particles but also chemical states of corrosion products under coating films were different between unsheltered and sheltered exposure. Based on these results, it is considered that corrosion products and sea salt particles were partially washed away by rain under the unsheltered exposure condition but remained under coating films under the sheltered exposure condition. In case of the latter condition, it is considered that sea salt particles in high humidity produced liquid films by deliquescence and caused anodic blisters and localized corrosion.