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Hot dip galvanizing versus zinc-rich paint

Engineering and technical staff often get confused over the various terms bandied about in South African industry such as ‘galvanized’ or ‘cold galvanizing’ or ‘zinc-rich paints’. So, what’s the difference?

It is metallic zinc in hot dip galvanizing that affords cathodic protection and barrier protection to galvanized steel. The extent of protection offered is directly proportional to the zinc coating thickness.

This is not so for zinc-rich paint, which consists of fine zinc powder dispersed in a dry film of paint resins. A further factor to be considered is the environment to which these coatings would be exposed. Paint coatings are notorious for having pinholes that allow the external environment to penetrate the coating, something hot dip galvanizing avoids.

The key difference is that hot dip galvanizing results in the zinc coating forming a metallurgical bond with the underlying steel, whereas zinc-rich paints (cold galvanizing) merely adhere to the steel surface. Each has its place in corrosion-protection applications.

Care should therefore be taken when selecting zinc-based coating systems for chemically-aggressive environments. Zinc, being an amphoteric metal, is attacked by both acids and alkalis. Zinc coatings should only be used in the pH range of 6 to 12. Zinc phosphate and zinc chromate containing paints do not provide cathodic protection as they are corrosion inhibitors rather than sacrificial coatings and provide protection by a totally different mechanism.

When considering zinc-rich paints, only those that contain sufficient quantities of metallic zinc dust provide cathodic protection. There must obviously be sufficient zinc particles present to ensure that they are in electrical contact with each other in order to provide a common anode. Individual isolated zinc particles dispersed in the paint binder or resin will not provide protection, as they would essentially be insulated from the steel substrate and each other.

On the other hand, if too much zinc dust is added to the paint, there may be insufficient binder available to glue these particles together, giving a weak coating with poor adhesion and cohesion. In accordance with ISO 12944, all zinc-rich paints should contain a minimum of 80% zinc in the dry film in order to function as sacrificial primers. From the point of view of zinc content, hot dip galvanizing is the ultimate zinc-rich primer. However, there are occasions where a zinc-rich paint is the answer.

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