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Corrosion

Corrosion

Corrosion is a complicated chemical process but put simply, it can be described as a reaction between a substance and its surroundings. In the case of cars, it is the iron in the panels that reacts with water and oxygen to form ferric hydroxide, what we usually call rust.

One of the first steps in the manufacture of iron is to remove oxygen from the iron ore. This is necessary so that the iron will be strong enough to withstand further refinement. Pure iron, however, is a chemically instable state. Iron continuously strives to regain its oxygen and form ferric hydroxide (rust), which is a stable state.

In order for iron to form ferric hydroxide to any greater degree, moisture is also required in addition to oxygen. The moisture acts as a so-called electrolyte and starts an electrochemical process that assists the instable iron in absorbing oxygen from the atmosphere.

Ferric hydroxide has a greater volume than iron, which means the panel will crack when it is corroded. More oxygen and moisture can then penetrate and aggravate the corrosion.

The corrosion process is considerably aggravated if the electrolyte (the moisture) contains salt. Other factors that can aggravate corrosion are heat and impurities on the surface of the iron.

The panels are protected against corrosion by eliminating the factors that lead to corrosion. The most common method prevents moisture and oxygen from coming in contact with the metal. On the Saab 9-3, this is achieved by painting the panels in several coats. Much of the body is also zinc-coated, which helps to make the metal less susceptible to corrosion.