Zinc Coatings

Abstract

Preventing ferrous materials from corrosion by applying a coating of zinc has been practised for over two hundred years. In the past numerous tests and studies have been carried out so that the life of a zinc coating, in a particular environment, can now be predicted with reasonable accuracy. Zinc prevents corrosion of the underlying metal in three ways:

• (i) In the first instance by providing a tough metallic coating, which seals the surface of the basis metal.

• (ii) Zinc exposed to moist air forms mainly basic zinc carbonate which acts as a barrier between the zinc and the atmosphere. In clean atmospheres this protective layer is self-maintained and corrosion of the zinc takes place only very slowly. In polluted atmospheres, particularly those contaminated by sulphur dioxide, the basic zinc carbonate is continually dissolved and replenished. This process continues until all the coating is gradually corroded away.

• (iii) Finally, if any mechanical damage or gradual corrosion exposes the steel, the surrounding zinc becomes sacrificial and cathodic protection of the steel occurs.