We all know that rust is that reddish-orange coloured layer that forms on the surface of some metals due to oxidation caused by moisture or water. In addition, metals also react to strong acids and industrial chemicals. If nothing stops the corrosion, the rust continues to attack and can even disintegrate the metal.
Not all metals contain iron or are prone to corrosion or rust. To prevent these problems at an industrial level, the correct choice of rust and corrosion resistant metal is essential.
Metals that do not rust
Rust compromises the mechanical characteristics of the metal, alters the chemical properties and is usually not aesthetically ideal. For these reasons, attempts are made to avoid having to deal with rust and the negative effects it brings by using metals that do not rust. These are the most common:
Stainless steel is a mixture of several elements that, in most of its variants, contain the same amount of iron, which can react and form an oxide layer. However, most stainless steel alloys also contain a high percentage of chromium oxide, at least 18%, which reacts before the iron. The chromium oxidises rapidly to form a protective chromium oxide layer on the surface of the iron. This layer both resists corrosion and prevents oxygen from reaching the steel beneath it. Molybdenum and nickel, also present in stainless steel, increase rust resistance.
Aluminium is another metal that is resistant to rust and corrosion. This is because its alloys contain very little iron, and, as mentioned above, without iron the metal cannot rust, although it does generate a reaction. When the aluminium alloy is exposed to water, a layer of aluminium oxide quickly forms on the surface. This layer is quite resistant to corrosion and protects the metal.
Galvanised steel is the most rust and corrosion resistant metal. This metal is nothing more than carbon steel coated with a layer of zinc. The zinc acts as a barrier that prevents oxygen and water from reaching the steel, thus protecting it from corrosion. Even if the zinc layer is peeled off, it will still protect the surrounding areas of the inner steel through cathodic protection and a protective layer of zinc oxide.
Red metals: copper, bronze and brass
These three metals contain almost no iron and therefore do not rust, but they do react with oxygen. Copper oxidises over time and forms a greenish layer that protects the metal from further corrosion. Bronze, on the other hand, is a mixture of copper, tin and smaller amounts of other elements, which makes it a more corrosion-resistant alloy than copper. Finally, brass, an alloy of copper and zinc, will also resist corrosion.
As we have made clear for each of the materials, it is true that these metals do not rust, but that does not mean that they do not corrode. They have their own forms of corrosion, such as the green layer that forms on oxidised copper or the pitting that can appear on stainless steel. However, these forms of corrosion do protect the underlying metal from rusting.
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