Hot-dip galvanizing of steel products and components offers a long-lasting protection against a wide range of mild-to-severe corrosive media. In this section, several types of such media in which a hot-dip galvanized product can be used are introduced:
(a) Highly corrosive industrial media
Industrial media typically contain various gases, sulphur vapour, and other particles. The presence of moisture, normally as rain, can cause severe corrosion on galvanized steel due to the reaction between sulphur compounds and rain drops. The products of such reactions are zinc sulphates and sulphides which can readily dissolve in the rain drops at the steel surface and destroy the protective zinc coating. A bare steel surface is easily corroded and the lifetime of the steel component is drastically reduced as a result.
(b) Moderately corrosive industrial media
Such industrial media have lower levels of corrosive particles and hence, corrosion in such environments is less severe than the highly corrosive media. The atmosphere of industrial estates is a typical example of a moderately corrosive medium.
(c) Suburban areas
This type of environment may include the urban and suburban areas, containing significantly lower corrosive agents.
(d) Mild coastal atmospheres
The corrosion severity of such media is lower than the suburban areas. However, the coastal winds contain chloride ions, which can react with the zinc coating and produce zinc chloride. This compound readily dissolves in water and destroys the protective layer promoting the corrosion of bare steel surface.
(e) Tropical coastal atmospheres
The temperature of such coastal media is mild and the corrosion severity is typically lower than the mild coastal atmospheres.
(f) Rural areas
The corrosion in the rural areas is lowest due to a low level of sulphur and other particles in air.
The following diagram illustrates the ratio of corrosion for galvanized steel in various types of media.