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 the production of iron is to remove oxygen from the iron ore. This is necessary to ensure that the iron will be sufficiently strong for further processing. Pure iron is a chemically unstable substance, however. It endeavours the whole time to "get back" the oxygen and form ferric hydroxide (rust), which is a stable substance. For iron to form ferric hydroxide to any greater extent, however, water is also necessary in addition to oxygen. The water acts as an electrolyte. It starts an electrochemical process which makes it easier for the unstable iron to absorb oxygen from the atmosphere. Since ferric oxide has a larger volume than iron, the sheet metal cracks when attacked by corrosion. Additional oxygen and water can penetrate the metal and worsen the corrosive attack.
The corrosive process will be appreciably accelerated if the electrolyte (water) contains salt. Other factors which facilitate corrosion are heat and impurities in the surface of the iron.
Sheet metal is protected from corrosion by eliminating the factors which give rise to a corrosive attack. The most common method is to prevent water and oxygen from coming into contact with the metal. On the Saab 900 this is done by applying several coats of paint to the sheet metal. In addition, large parts of the body are galvanized to make the sheet metal less prone to corrosion.