34 - Paint coatings for the plant engineer

Abstract

The term “paint” is used to describe the liquid material before application and “coating” after it has been applied, dried, and cured. Organic-based coatings form the largest use for protection and decoration, and these can vary widely in properties and characteristics, ranging in thickness from a few microns to several millimeters. Metal coatings can also provide a useful and economic form of protection for steelwork and plant in the appropriate circumstances. Some metallic coatings act as a complete barrier to the environment, but most corrosion-protection metal coatings and all organic coatings are permeable to moisture and gases to some extent. The rate of permeability varies with different types of coating, but in all cases protection of the substrate is mainly afforded by adhesion. Good adhesion, which is largely a product of adequate surface preparation, prevents the lateral spread of moisture and contaminants which would undermine the coating and lead to disbandment or corrosion and eventual breakdown. The thickness of the coating also affects permeability and there is a correlation between thickness and life. There may be a limiting thickness above which either little additional protection is obtained or the increase in cohesive strength reduces its adhesive strength. Coatings should always be applied as closely as possible to the manufacturer's recommended thickness.