Further electrochemical deterioration is fully halted by the protective Al 2O 3 layer produced during the initial oxidation. During the oxidation itself the film hardens to provide a stable protective coating on the exposed surface of the affected metal. (Learn more about aluminum and aluminum alloy corrosion in The Corrosion Properties of Aluminum and Its Alloys.) The oxide film can be recognized by its appearance as a dull gray layer. When moisture is present it can result in crevice corrosion and pitting corrosion, due to puncturing of the passive Al 2O 3 film.Īlloys such as aluminum bronzes take advantage of the aluminum's passive film formation during oxidation. When the Al 2O 3 film is in contact with chlorides, formation of dialuminum hexachloride begins. However, the water's pH may affect the stability of the film. The aluminum oxide film is fairly stable in the presence of water. Generally, a thin aluminum oxide (Al 2O 3) film of 4 nm thickness forms on the surface due to oxidation, which protects the substrate from further corrosion because the film is not brittle. This is different from rusting because the flaking and scaling off process is totally absent in this case. The oxidation of aluminum, on the other hand, as an electrochemical reaction due to the presence of atmospheric oxygen, is called corrosion. (Source: john_pittman, CC BY-NC-SA 2.0) Aluminum Oxidation (See Testing For Microbiologically Influenced Corrosion in Pipelines for 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.įigure 1. It also enables the growth of organisms such as bacteria and leads to aesthetic issues.
In this case iron reacts with chlorides without the presence of atmospheric oxygen.Īs rust forms, the iron and steel metal parts lose their integrity as the layers of rust gradually form, flake and fall off like scale from the parent metal, due to its brittleness. Green rust can be formed on rebars, which are used in cement concrete pillars for marine underwater applications. However, rusting of any type does not provide reliable protection against any further deterioration. It can be of different types, such as hydrated ferric oxide or ferric oxide-hydroxide, which is FeO(OH) or Fe(OH) 3. Rust is reddish brown or brownish yellow in color, flaky and brittle. If moisture is absent, the rusting process will depend solely on atmospheric oxygen. Rust is a coating that occurs on the surface of iron or steel as it oxidizes, which is the result of an electrochemical reaction between iron and oxygen due to oxygen and moisture in the atmosphere as well as any moisture directly deposited on the surface of the metal. Distinguishing between Rust and Corrosion
We often use the term "rust" to mean corrosion, but these two terms do not mean the same thing.