Corrosion

 Corrosion attacks all engineering materials, especially metals.Corrosion is any chemical action which harms the properties of a material.It reduces the life of a material and increases the cost of a structure.For example, a steel bridge must be repainted regularly to protect it from rust.Various metals have therefore been developed to resist corrosion.Among them are the stainless steels.These metals contain from 12 to 35% chromium which forms a very thin layer or film of chromium oxide on the surface of the metal.This film protects the metal from corrosion.Alloys made from copper and nickel are also corrosion-resistant.For example Monel metal, which contains roughly 60% nickel and 30% copper, is resistant to both fresh and salt water corrosion.It is therefore used for marine engine parts, and for surface like ship' propellers which are in contact with sea water.Cupronickels, which contain a smaller proportion of nickel, have a similar resistance to fresh  and sea water.They are mainly used to make tubes.

 When two different metals touch other in the presence of moisture, corrosion occurs.This type of corrosion is known as galvanic or electrolytic corrosion because it has an electrical cause.The metals and the moisture act like a weak battery and the chemical action which results corrodes one of the metals.If, for example, aluminium  sheets are riveted with copper rivets, the aluminium near the rivets will corrode in damp conditions.

 No material can be completely corrosion-resistant.Even stainless steels will corrode.Engineers can, however, fight corrosion.For example, they can use high-purity metals because these metals are more resistant than alloys.They can also make sure that two dissimilar metals are not allowed to touch each other.Finally engineers can protect the surfaces of metals in many different way.One of the most common methods is to paint them.