Many bridges in the United States require rehabilitation to prevent corrosion. This paper discusses overcoating as a less costly alternative to the traditional practice of enclosing sections of a bridge for abrasive blasting, disposing of the removed paint, and repainting. To evaluate how various overcoat materials perform when they are applied to different types of aged steel substrates, Federal Highway Administration researchers selected six lead-free and low-volatile organic compound materials to apply over coated, aged, and rusted surfaces. Using a cyclic, accelerated testing method, they studied the overcoat systems in the laboratory and through field exposures, evaluating performance by assessing surface failures and rust creepage developed at scribes (scratches made through the overcoat surface down to the steel substrate). Comparing the results yielded a number of insights into overcoat performance when applied to the three types of substrates. Although rust creepage developed at the scribe and grew linearly over time for each sample, the overcoat materials performed differently though applied to the same primer substrates. The difference in performance depended upon the wetting or penetrating properties of the individual overcoat material, which contains different types and amounts of solvent and resins with varying penetrating power. The longer the overcoat material takes to cure, the more solvent is available to soften the coating substrate and thereby increase the primer's adhesion to steel. As a result, the researchers concluded that the difference in the amount of rust creepage at the scribe was due to the variability in primer. Overall, calcium sulfonate (CSA) performed the best on all three substrates. However, it is a soft material that picks up dirt easily. Given these strengths and weaknesses, bridge owners are advised to use their best judgment in deciding whether to use CSA as an overcoat material.
Abstract