How & Why steel corrodes

Steel sheet is a very versatile product. It comes in many sizes and types, and is applied to many end uses including steel buildings, automotive panels, signs, and appliances. The low cost, strength and formability of steel sheet are some reasons for its widespread use. Unfortunately, as are most steel products, it is prone to rusting, a phenomenon that causes the surface to become unsightly and, over time, may contribute to product failure. For this reason, steel is protected by a variety of methods ranging from internal alloying (stainless steel, for example), to coating with paints or metallic coatings.

Corrosion is an electrochemical process that, in the case of steel sheet, oxidizes the iron in the steel and causes it to become thinner over time. Oxidation, or rusting, occurs as a result of the chemical reaction between steel and oxygen. Oxygen is always present in the air, or can be dissolved in moisture on the surface of the steel sheet. During the rusting process, steel is actually consumed during the corrosion reaction, converting iron to corrosion products. In the case of most low-carbon steel sheet products, iron oxide (rust) develops on the surface and is not protective because it does not form as a continuous, adherent film. Instead, it spalls, exposing fresh iron to the atmosphere which, in turn, allows more corrosion to occur. This aspect of steel sheet behaviour is very undesirable, both aesthetically and from the aspect of service life. Eventually, often sooner than desired, the steel sheet is corroded sufficiently to cause degradation in the service life, i.e., loss of structural strength, perforation and intrusion of water, etc.

Fortunately there are many coatings that can be applied to steel in a very cost-effective manner to confer sufficient corrosion protection to steel so that it can be used for a multitude of demanding applications