What is corrosion?
Most materials and compounds that we use are not – in the longer term – permanent, at least in the forms that we normally utilise them. They are broken down and change into more stable compounds with the elements of which they are composed. Where metals and metallic materials are concerned, we say that they corrode; in everyday language we say they rust.
Iron and other metals are extracted from naturally occurring minerals which are more stable compounds that the metals themselves. It is, therefore, natural that corrosion processes lead to the metals reverting to compounds that are very like the mineral.
Protective oxide layer
Steel has a certain resistance against corrosion. This so-called passivity is due to a thin and invisible layer of oxides being formed on the surface. This process takes place through a reaction between the metal and the oxygen in the surrounding environment. The oxide layer reduces the corrosion rate dramatically; it is then said that the material has become passivated i.e. less affected by environmental factors such as air and water.
Different alloying elements offer different kinds of rust protection
Certain steels are especially adapted to acquire better corrosion resistance. One example of this is stainless steel, where added alloying elements – above all chromium (Cr) – lend to the material a very good corrosion protection.
Stainless steel with self-repairing for lasting value, a film from Team Stainless
Another example is so-called weathering steel. The admixture of a little extra copper (Cu) in the steel gives rise to the rapid formation of a protective rust patina over the surface of the entire product. The naturally formed oxides subsequently protect the underlying steel against continued corrosion. This process is applied, for example, in steel chimneys, bridges and plants in corrosive environments. Weathering steel is also encountered in different types of embellishment on building facades or in artworks such as steel sculptures.
Other types of protective layer
To reduce or prevent corrosion of those steels that are classed neither as stainless nor weathering, some kind of protection is needed. This can be accomplished in several different ways with many different types of corrosion protection products.
One alternative is to create a dense layer e.g. paint that prevents the steel reacting with the surroundings.
Another way is to apply one or several substances onto – or in contact with – the steel less noble than the underlying steel and that thereby corrode instead. Many steel products are given a long-term protection through zinc or mixtures of zinc and aluminium, or zinc and titanium, being applied to the steel. The zinc coating oxidises and thereby prevents corrosion of the underlying material. A certain runoff of zinc from galvanised materials exposed to e.g. rain does indeed occur. This runoff is dependent on the rain intensity, the water quality etc and cannot be equated with the corrosion rate. On top of the zinc layer, it is then possible to apply a coat of paint to further protect the surface.
100 years of stainless steel
The Stainless Steel Centenary was celebrated in 2013. For this centenary, a film was made of the history behind stainless steel and what it is used for today.