Why Does Zinc Protect Steel?

Zinc, by its very nature, is a “reactive” metal. That is, it tends to corrode quite readily when exposed to moisture.

Why then does it protect steel when the coated sheet is exposed to rainfall?

When zinc corrodes in the presence of moisture, it undergoes a chemical reaction. In this reaction, the zinc

actually changes chemical form; the metallic zinc on the surface changes to other chemical compounds,

primarily zinc hydroxide. As the surface dries, the hydroxide film on the surface is converted to either a form of

zinc oxide or zinc carbonate (formed by reaction between the zinc hydroxide and carbon dioxide in the air).

These compounds form a relatively continuous protective film on the surface of the zinc. Thus, when the surface

is again exposed to rain or condensation, the protective film serves as a barrier between the moisture and the

zinc coating, protecting the zinc underneath. These types of chemical films are called passive films. It is the

presence of this passive film that slows down the reactivity of the zinc, thereby dramatically reducing the rate of

corrosion of the zinc coating.