Rust compromises the mechanical characteristics of metal, alters the chemical properties, and is (usually) not considered aesthetically pleasing. It is for these reasons that many people in charge of design and material selection try to avoid having to deal with rust and the negative effects that come with it. One way to eliminate the threat of rust is to use metals that don’t rust.
Metals That Don’t Rust
Common metals that don’t rust include:
- Stainless Steel (some grades)
- Galvanised Steel
- Red Metals (Copper, Brass & Bronze)
Aluminium cannot rust. This is because of the fact that rust is iron oxide, and most aluminium has virtually no iron in its composition. However, aluminium does oxidize, but it actually protects the underlying unoxidized aluminium. This is because the aluminium oxide is more corrosion resistant than the aluminium alloy it rests on. This is different than rust, because rust will flake away from an iron alloy thus allowing the rusting process to continue until material failure.
Stainless steel is another example of a metal that does not rust. It is important to note the some grades are more likely not to rust than others. Austenitic stainless steels such as 304 or 316 have high amounts of nickel and chromium. The chromium combines with the oxygen before the iron is able to do so, preventing rust formation. This forms a chromium oxide layer that is works in a very similar way to the aluminium oxide layer previously discussed. It protects the underlying metal because it is very corrosion resistant. On the other hand, ferritic or martensitic stainless steels may be susceptible to rust because they contain less chromium.
Galvanised steel is technically a coated material, but it is worth mentioning here. Galvanised steel is a carbon steel that would most likely rust if it did not have one or more layers of zinc applied to it. The zinc layer acts as a sacrificial metal for the steel. This means that the zinc layer on a galvanised steel sheet will combine with the oxygen more readily than the iron in the steel will. This creates a zinc oxide layer that prevents the formation of iron oxide, thus eliminating the possibility of rust forming. However, if the zinc coating is damaged, or if the galvanised steel is placed in extreme environments, the zinc coating may be rendered ineffective and the galvanised steel will rust. Galvanised steel is also susceptible to “white rust” which is a type of corrosion that forms under specific conditions, typically involving water or condensation. The formation of white rust does not necessarily damage the material or the zinc coating.
Copper, Brass, and Bronze
Copper, brass, and bronze do not rust for the same reason as aluminium. All three have a negligible amount of iron in them. Therefore no iron oxide, or rust, can form. However, copper can form a blue-green patina on its surfaces when exposed to oxygen over time.
These Metals Don’t Rust But…
These metals don’t rust, but that does not mean that they do not corrode. They have their own forms of corrosion, such as pitting that can occur in stainless steel or the blue-green tarnish found on oxidized copper. Furthermore, if they are brought into contact with a carbon steel or other type of steel that does rust, iron deposits can be made on the surface of these materials that will oxidize and create rust.
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