Although protective coatings are widely used to mitigate corrosion of structural steel bridges in aggressive humid environments, their service life is often diminished. Repairs can be costly due to materials, labor, and environmental controls.
As a part of an investigation to assess novel coatings that are commercially available for steel bridge application, a chemically bonded phosphate ceramic (CBPC) coating was examined by researchers with the Florida International University (Miami, Florida, USA) and the Florida Department of Transportation (Gainesville, Florida, USA). The study considered various exposure environments such as inland, beach, and salt-fog exposure.
The CBPC coating consists of an acid phosphate and a water-based slurry that contains base minerals and metal oxides (e.g. magnesium oxide/magnesium hydroxide [MgO/Mg(OH)2]). These two components are mixed together and sprayed on a metal surface with a dual component spray gun. According to the commercial literature, the acid phosphate and oxides in the slurry interact with the metal substrate to form an insoluble passivation layer of stable oxides (2 to 20-μm thick) that contains ~60% iron with phosphate, potassium, magnesium, silicon, hydrogen, and oxygen. Additionally, the commercial literature says the coating can be used in environments with temperatures ranging from 35 to 200 °F (1.7 to 93 °C) and 0 to 99% humidity.
The researchers assessed the performance of the CBPC coating in outdoor exposures by placing steel coupons with a CBPC coating at a beach test site in the Florida Keys as well as an inland test site in South Florida ~10 mi (16 km) from the coast. CBPC-coated coupons for testing were provided by the manufacturer. The coated samples were exposed at 45 degrees facing south. Exposure times of four, eight, and 24 months were proposed to identify coating degradation and corrosion development over time. Temperature and relative humidity at the two outdoor test sites were comparable, but the precipitation for inland site was higher than for the beach site.
Samples also were exposed to salt-fog conditions with 5% sodium chloride (NaCl) solution for 2,200, 5,800 or 14,600 h (according to ASTM B117, “Standard Practice for Operating Salt Spray (Fog) Apparatus”) to evaluate the effects of the aggressive exposure conditions on the integrity of the coating and corrosion durability of the steel coupons. The salt-fog chamber temperature was ~32 °C. The samples were placed at a ~40-degree inclination with support along the bottom edge of the coupon.
To assess the corrosion damage incurred, the coating samples were evaluated by visual inspection, coating thickness, adhesion measurement, and x-ray diffraction. Assessment of coating thickness and coating adhesive strength before and after exposure were also made.
The researchers concluded that the CBPC coating surface degraded in moist environmental conditions, with degradation in the form of scouring and chalking on coating surface. The bulk coating degraded by forming enhanced pores during exposure in presence of moisture. An intermediate reaction product, Fe(H2PO4)32H2O, formed during exposure and is thought to be protective. However, there is the possibility of steel substrate corrosion during long exposure times in moist environments.
More information can be found in CORROSION 2017 paper no. 9593, “Update on Corrosion Performance of CBPC Coatings in Aggressive Bridge Environment,” by M.A. Sabbir, S.F. Fancy, K. Lau, and D. DeFord.