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Corrosion and Rusting



Introduction

Some people may be annoyed by their car “wearing out”. Kids may have trouble with rust forming on their bicycles. One may think how to prevent rusting, but do one knows what is happening when a metal corrode?

“Corrosion is defined as the involuntary destruction of substances such as metals and mineral building material by surrounding media, which are usually liquid (i.e. corrosive agents).” Most metals corrode. During corrosion, they change into metallic ions. In some cases, the product of corrosion itself forms a protective coating. “For example, aluminium forms a thin protective oxide layer which is impervious to air and water. In other cases (e.g. iron), however, the coating either flakes off or is pervious to both air and water. So the whole piece of metal can corrode right through.”

The most common forms of metallic corrosion are caused by electrochemical reactions, wherein two metallic phases (e.g., iron oxide and iron) react in the presence of electrolytic solution. Another mechanism of metallic corrosion is caused by chemical reaction, which explains how the protective layer of the metal is formed.

Rusting is the corrosion of iron which is the most widely used structural metal. Most of it is used in making steel. The wide range of products made from steel includes all types of vehicles, machinery, pipelines, bridges, and reinforcing rods and girders for construction purposes. Therefore, rusting causes enormous economic problem and is the reason why extensive measures of corrosion protection have had to be developed.

The economic importance of corrosion and corrosion protection can be shown by the following example: “It is estimated that roughly 3% of the annual production of steel is lost by corrosion. In 1974, 140 millions tons of steel were produced in the United States at a cost of...