Performance Evaluation of Field Exposed Painted Constructional Steels

The major protective coatings applied to structural steelwork are paints, metal coatings and combinations of both. The choice is partly governed by the actual enviro-nmental conditions and partly by economic considerations. Organic coatings used on steel provide an effective barrier protection by isolating steel from the attacking species. The barrier properties of the coating are impro-ved by increased thickness, by the presence of pigments and fillers that increase the diffusion path for water and oxygen, and by the ability to resist degradation. Degradation allows access of reactants to the coating / substrate interface without the necessity for diffusion through the coating and it is an electrochemical process which follows the same principles as corrosion of uncoated steel. As coatings age in a corrosive environment, the interconnecting network of pores within the coating event-ually become saturated with water, salts, etc., exposing the metal substrate to a corrosive environment. An impor-tant property of a coating is its resistance to water penetration and there is a direct correlation between resistances and the ability of the coating to protect the underlying steel from corrosion. A generic three coat paint system was taken from a leading paint company for carrying out exposure tests