Does aluminum rust?

We often use the term "rust" to mean corrosion, but these two terms do not mean the same thing.

Distinguishing between Rust and Corrosion

Rustis a coating that occurs on the surface of iron or steel as itoxidizes, which is the result of anelectrochemical reactionbetween iron and oxygen due to oxygen and moisture in the atmosphere as well as any moisture directly deposited on the surface of the metal. If moisture is absent, the rusting process will depend solely on atmospheric oxygen.

Rust is reddish brown or brownish yellow in color, flaky andbrittle. It can be of different types, such as hydrated ferric oxide or ferric oxide-hydroxide, which is FeO(OH) or Fe(OH)₃.

However, rusting of any type does not provide reliable protection against any furtherdeterioration.Green rustcan be formed onrebars, which are used in cement concrete pillars for marine underwater applications. In this case iron reacts withchlorideswithout the presence of atmospheric oxygen.

As rust forms, the iron and steel metal parts lose their integrity as the layers of rust gradually form, flake and fall off likescalefrom the parent metal, due to its brittleness. Rusting weakens the metal part. It also enables the growth of organisms such as bacteria and leads to aesthetic issues. (SeeTesting For Microbiologically Influenced Corrosion in Pipelinesfor more on bacterial growth.) In due course, the parent metal will be fully converted to flakes of rust, leaving no trace of the parent metal.

Figure 1. Flakes of rust.(Source:john_pittman,CC BY-NC-SA 2.0)

Aluminum Oxidation

Theoxidation of aluminum, on the other hand, as an electrochemical reaction due to the presence of atmospheric oxygen, is called corrosion. This is different from rusting because the flaking and scaling off process is totally absent in this case. Generally, a thin aluminum oxide (Al₂O₃) film of 4 nm thickness forms on the surface due to oxidation, whichprotects the substrate from further corrosionbecause the film is not brittle. Thealuminum oxide filmis fairly stable in the presence of water.

However, the water's pH may affect the stability of the film. When the Al₂O₃film is in contact with chlorides, formation of dialuminum hexachloride begins. When moisture is present it can result increvice corrosionandpitting corrosion被动的,由于刺穿₂O₃film.

Alloys such as aluminum bronzes take advantage of the aluminum'spassive filmformation during oxidation. (Learn more about aluminum and aluminum alloy corrosion inThe Corrosion Properties of Aluminum and Its Alloys.) The oxide film can be recognized by its appearance as a dull gray layer. During the oxidation itself the film hardens to provide a stable protective coating on the exposed surface of the affected metal. Further electrochemical deterioration is fully halted by the protective Al₂O₃layer produced during the initial oxidation.

In certain applications where the aluminum oxide film is repeatedly rubbed off and rebuilding of the oxide film is impeded,alkalineandacidicenvironments can readily promote localized pitting corrosion of aluminum and its alloys. Localized pitting exposes more parent metal to the environment, so any newly formed protective film is destroyed and thus the pitting continues to progress. Pitting results in unsightly holes all over the surface, accompanied by a reduction in strength and the initiation offatigue cracks. However certain alloys of aluminum have greater corrosion resistance.

A hard aluminum oxide film can also be purposely formed in the factory through a process calledanodizing. A plasma electrolytic oxidation process causes the formation of crystalline Al₂O₃with greater hardness on the surface.
Because Al₂O₃isamphoteric, it reacts with acids as well as bases. While reacting with NaOH, it produces sodium aluminate, which hashardnessand a high melting point. It is also an insulator of electricity.

Aluminum cannot rust as it contains no iron particles. However, itis susceptible to corrosion in the presence of oxygenthat is readily available in the surrounding air. The electrochemical reaction between aluminum and oxygen causes the formation of a thin, hard protective film of aluminum oxide.