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Corrosion

Corrosion

Corrosion is a complicated chemical process. In extremely simple terms, it could be described as a reaction between a substance and its surroundings. In the case of the sheet metal used in cars, it is iron which reacts with water and oxygen to form ferric hydroxide or what we usually call rust.

One of the first steps in manufacturing iron is to remove the oxygen from the iron ore. This is necessary to make the iron sufficiently strong for further processing. Pure iron is, however, chemically unstable. The iron always strives to "take back" the oxygen and form ferric hydroxide (rust), which is stable.

If iron is to be able to form ferric hydroxide to any greater extent, then water is needed as well as oxygen. The water acts as an electrolyte and starts the electromagnetic process that helps the unstable iron to absorb oxygen from the atmosphere.

Ferric hydroxide has a larger volume that iron, which makes the sheet metal crack when it corrodes. More oxygen and water forces its way in and the corrosion worsens.

The corrosive process will be appreciably accelerated if the electrolyte (water) contains salt. Other factors that facilitate corrosion are heat and impurities in the surface of the iron.

The metal sheet is protected against corrosion by eliminating the factors that lead to corrosion. The most common way is to prevent water and oxygen from coming in contact with the metal. On Saab 9-3, this is achieved by painting the metal in several coats. In addition, most of the body is zinc coated, which makes the metal less susceptible to corrosion.