Discoloration Failure of Topcoated Intumescent-Coated Steel

Abstract Exterior steel elements were coated with a three-coat system to provide functional and aesthetic characteristics. The lower portion of the columns were coated with a primer, intumescent, and topcoat, while the intumescent was omitted from the application to the upper portion of the columns. Within months after application, a tacky substance of contrasting coloration was observed on the topcoat, and, in these areas, the topcoat was poorly bonded to the intumescent. An on-site evaluation was conducted which included evaluation of the areas of tacky coating and collection of samples for laboratory study. During on-site evaluations, the tacky substance was observed to exist only where the intumescent coating was present. Laboratory evaluations included chemical analysis of the tacky substance and coating materials. Additionally, mock-ups of the coating systems, including products similar in composition to the applied topcoat as well as the specified topcoat with two variations of intumescent coating application, were performed to understand conditions leading to the development of the tacky substance.