There is an ever increasing use of the term ‘galvanizing’ to describe the coating of steel with zinc. The term is derived from the name of Italian scientist Luigi Galvani. Due to no legally binding definition the term is now used to describe a whole host of systems. We try to clarify some important misconceptions. Steel can be coated with zinc using various processes, however, not all are the same even though they may attach the term ‘galvanizing’ within their description. The processes will vary with respect to how they are applied, the thickness of the zinc coating and its characteristics. All of these have an important influence on vital issues such as performance and physical properties.
The immersion of steel in a bath of molten zinc is referred to as hot dip galvanizing. This term however can be used to cover two slightly different processes:
Hot dip galvanizing – post fabrication;
individual components that make up a staircase, for example, are chemically cleaned and immersed in a bath of molten zinc. This creates a metallurgically bonded coating that provides complete coverage of the steel component with a coating thickness of 50 to 200 micrometers, both internally and externally for hollow sections, including edge protection.
Continuously galvanized steel strip – pre fabrication;
large coils of steel strip are automatically fed via a system of rollers through a cleaning phase and quick immersion (2/3 seconds) into a zinc bath. This provides a hot dip galvanized coating that is however much thinner (5 and 40 micrometers) than that achieved via post hot dip galvanizng. Also the galvanized strip steel is a preliminary product which is further processed following galvanizing, i.e shaped, punched and cut to size. This destroys the protective zinc layer at the cut and punched edges. Due to the difference in coating thickness and hence performance, hot dip galvanizing and continuously galvanized sheet are used in different ways. Pre galvanized sheet is mainly used in internal environments such as air conditioning systems whereas post hot dip galvanized steel is mainly used for outdoor applications, as a rule, due to the requirement for longer periods of protection.
Grit blasted steel surfaces are coated with droplets of semi-molten zinc sprayed from a special gun. The coatings are always slightly porous, sensitive to shortcomings in the preparation of the steel, and need skill in application. They cannot be applied to the inside of tubes and hollow sections.
Zinc is deposited electrolytically from a solution of zinc salts. Only very thin coatings are economically feasible and coatings are not uniform except on the simplest shapes. A zinc plated coating may also be formed – with variable thicknesses – by mechanical deposition. This is sometimes referred to as mechanical galvanizing but unlike galvanizing there is no metallurgical bond with the base steel.
Steel parts are tumbled in zinc dust at temperatures just below the melting point of zinc. Coatings are thin but closely follow the contour of the steel parts.
Zinc dust paints
These are formulated with a high proportion of metallic zinc dust pigment to give electrical conductivity, and are applied to clean – generally blast-cleaned – steel. The formulation and application of the paint needs careful control for satisfactory performance.