Nanoparticles are being considered in the development of durable coating systems for structural steel highway bridge structures due to their beneficial electrical and mechanical properties. Since the 1980s, three-coat zinc-rich primer coatings have become widely used to protect steel bridges against corrosion. Still, around $500 million is spent annually for maintenance painting of steel bridges, triggering a trend to enhance coating system durability and reduce maintenance requirements.
In CORROSION 2017 paper no. 9539, “Corrosion Performance of Nano-Particle Enriched Epoxy Primer for Marine Highway Bridge Application,” by S.F. Fancy, M.A. Sabbir, K. Lau, and D. DeFord, the authors report on a study that investigated the corrosion performance of a nanoparticle-enriched zinc-rich primer (NPE-ZRP) for structural steel under aggressive marine exposures. Corrosion protection by a zinc-rich primer is provided by galvanic coupling of the zinc pigments with the steel substrate; therefore, an important parameter for the primer is electrical conductivity between zinc pigments and the steel. The authors note that the quantity of zinc pigments alone, even with content as high as 80 to 90% by weight, cannot ensure effective electrical continuity to provide long-term galvanic corrosion protection. Other factors can affect electrical continuity, including the shape of the zinc pigment and formation of zinc oxidation products. Studies have found, for example that efficient electrical contact can’t be provided by spherical pigments.
Improving electrical conductivity by adding materials such as di-iron phosphide, carbon black, extender pigments, and aluminum pigments to the epoxy resin with zinc particles has been studied. Reports indicate that adding carbon nanoparticles improves the mechanical and electrical performance of a zinc-rich epoxy primer, and with a lower zinc concentration than typical zinc-rich primer formulations.
For an initial evaluation of the coating and corrosion behavior of a NPE-ZRP, the authors implemented short-term testing to identify important coating characteristics and electrochemical parameters that could affect the coating system’s overall durability. The NPE-ZRP was applied over A36 (UNS K02600) carbon steel coupons with coating thicknesses ranging between 150 and 300 µm. Testing was comprised of four-months of outdoor exposure to a harsh marine environment and 60 days of water immersion in a sodium chloride (NaCl) solution.
The outdoor exposure tests subjected the NPE-ZRP coated samples to environments at a beach test site in the Florida Keys and an inland test site ~16 km (10 miles) from the South Florida coast. Before testing, a 25.4-mm long and 0.5-mm wide scratch was made on one set of samples so the effect of local coating damage could be determined. A crevice environment was simulated on another set of samples by placing an acrylic plate over the coupon surface to create a crevice that was exposed to air and moisture. The coated samples were then mounted on a south-facing exposure rack and oriented at a 45-degree angle toward the horizon, and exposed from April 22 to August 22, 2016. During exposure in the summer months, the temperature highs were typically between 80 to 95 °F (27 to 35 °C) with relative humidity sometimes >90%. Cumulative rainfall was ~250 mm for the beach site and ~500 mm for the inland site. Exposure to ultraviolet light was relatively high at both test sites during the exposure period.
The solution immersion tests, performed to simulate runoff and pooled drainage water, immersed the NPE-ZRP coated steel coupons in a 3.5 wt% (0.6 M) NaCl aqueous solution for 60 days. Corrosion testing included open-circuit potential and linear polarization resistance measurements. Activated titanium was used for the counter and reference electrodes for each test cell.
After testing, the authors observed surface discoloration for the samples in all exposures—from a dark grey color to a whitish appearance that was thought to be related to zinc corrosion products caused by exposure to the high relative humidity and rainfall exposure outdoors, and solution immersion in the laboratory. Significant coating deterioration, such as rusting, blistering, and cracking, however, was not evident for samples in either the outdoor or immersion tests. Comparatively higher coating bond strength was identified for all exposures, and barrier protection was observed. No significant changes in coating thickness were measured.
Although the intact coating samples showed good corrosion protection following exposure, moderate surface rust was observed on the scribed samples. After an initial period of high corrosion activity, consumption of the exposed zinc at the scribed region continued at a slow rate without incremental rust formation, which indicated zinc corrosion activity and some level of beneficial cathodic polarization. Even though rust was observed at defect sites, the authors note that the extent of corrosion probably would have been greater if zinc pigment was not present in the primer matrix near the defect sites.
They conclude that the benefits of including nanoparticles in zinc-rich primer coated steel could not be easily ascertained in the chloride solution immersion tests; however, they suggest that some level of enhanced conductivity was present in the NPE-ZRP coated samples since steel corrosion was generally limited at the defect sites even though anodic currents were high and consumption of zinc pigments adjacent to the defects was evident. The authors suggest further testing to determine the role of the nanoparticles in the polarization behavior of the steel, and to assess the influence of nanoparticles on the galvanic coupling of dispersed zinc pigments and the long-term durability of NPE-ZRP.