Update on Corrosion Performance of CBPC Coatings in Aggressive Bridge Environment

摘要

Coatings are widely used to mitigate corrosion of structural steel in aggressive humid environments. However, the service life is often diminished in aggressive environments. Repair of coatings can be costly due to materials, labor and environmental controls. Novel coating systems are commercially available for steel bridge application. As a part of research to assess novel coatings, Chemically Bonded Phosphate Ceramic (CBPC) coating was investigated. Research was done on CBPC coating considered various exposure environments such as inland, beach and salt-fog exposure. Preliminary results after 8-month outdoor testing and 5800 hours salt-fog exposure revealed early degradation of outer CBPC layers but also indication of corrosion mitigation. Thus long term testing including up to 24 months in outdoor environments and up to 14600 hours in salt-fog exposure was conducted to evaluate coating and corrosion performance after extended exposures. To assess the corrosion damage due to exposure, the coating was evaluated by visual inspection, coating thickness, adhesion measurement and X-ray diffraction. The formation of iron hydrogen phosphate hydrate[(Fe(H_2PO_4)32H_2O] and iron phosphate hydrate [Fe_3(PO_4)_8H_2O] was thought to provide apparent protection from enhanced corrosion but there is a probability of steel substrate corrosion in extended exposure in humid environment.