TABLE OF CONTENTS The term paint is normally 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 millimetres. The term coating is not confined to thick films.
Metal coatings (e.g. zinc on steel) can also provide a useful and economic form of protection for steelwork and plant in the appropriate circumstances. Some metallic coatings (e.g. hot-dip galvanizing) 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 disbondment or corrosion and eventual breakdown.
The thickness of the coating also affects permeability, and in general, there is a correlation between thickness and life. However, with relatively thick coatings as obtained with some modern materials such as epoxies the relationship is not so well established. There may be a limiting thickness above which either little additional protection is obtained or the increase in cohesive strength reduces its...
