How Does Aluminum Form an Oxide Film That Resists Corrosion?

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How does aluminum form an oxide film that resists corrosion? Aluminum reacts with the environment around it, and the reaction produces a film that separates the metal from further attack. This explainer answers the question directly, explains how the film forms and how it can fail, and lists what the sources do not settle. It draws on a 2021 review of aqueous corrosion in aluminum alloys and a 2020 review of corrosion in additively manufactured aluminum alloys, both hosted on the NIH PubMed Central site.

Quick answer

• The film is “spontaneous aluminum oxide/hydroxide film formation at the interface,” according to the 2021 review.
• Once formed, the film “protects the material and impedes further reaction with the environment.”
• The film is described as “generally self-renewing and self-healing.”
• The review names a threat from halides: “The passive film breakdown may be initiated by aggressive halide anions and lead to localized corrosion (pitting).”

How the film forms

The film forms on its own. The 2021 review states that “The passivation is related to spontaneous aluminum oxide/hydroxide film formation at the interface.” The word “spontaneous” is the source’s own description of how the film appears.

The 2020 review refers to the surface layer as the “native oxide film,” and it describes how the film’s properties depend on surface state. It says that “These methods greatly affect the surface state and therefore, the surface energy, which influences the nucleation and growth of the native oxide film.” Surface finish therefore has a direct effect on how the film starts.

Why the film protects the metal

The protective role is the central point. The 2021 review says that “The passive film protects the material and impedes further reaction with the environment.”

The film can also repair itself. The review notes that “The passive film is generally self-renewing and self-healing.”

The alloy composition changes the picture. The review explains that “The alloys tend to passivate because of high Al concentration.” It also notes that “The Al–Cr alloys, for example, can form protective passive layers of considerable thickness in different electrolytes.”

What the film is made of

The film is a mix of oxide and hydroxide. The 2021 review uses the phrase “aluminum oxide/hydroxide film,” and it describes the passive layer as the solid that forms on the metal surface. The review also mentions a “passivity region” where “the solid hydroxide exists,” and it states that “In this region, Al is protected by a passive layer.”

The review describes the passive region as the place where “passive layer formation on the metal surface” happens. It notes that “The passivation is reflected by rapid current density increase or stabilization at potentials higher than E p (passivation potential) on the polarization curve.”

How the film breaks down

The main weak point is chloride. The 2021 review states that “In halide-containing solutions, however, the alloys are prone to pitting corrosion.” It then explains the mechanism: “The passive film breakdown may be initiated by aggressive halide anions and lead to localized corrosion (pitting).”

The review also says that the film’s stability can decline in some conditions. It describes “a decreasing stability of the spontaneously formed passive film” and notes that “ten times higher Al dissolution rates were found at the OCP” in chloride-containing hydrochloric acid.

The breakdown also shows up in electrical tests. The review says that “The increase is a result of passive film breakdown and happens at potential higher than transpassive potential,” which means the current rises sharply once the film fails.

Why manufacturing changes the film

The 2020 review shows that the way a part is made changes its film. It states that “close-to-the-surface pores can compromise the passivation properties of the native surface oxides, and therefore the pitting resistance of the material.” Pores near the surface give the film weak spots.

Surface defects matter too. The review explains that “Unmolten powder on the surface of the printed part can compromise the passive behaviour of the material by introducing defects in the native oxide film and greatly increases the surface roughness.” Roughness and defects both make the film less uniform.

Microstructure can make the film thicker or thinner. The review reports that a finer microstructure “resulted in a thicker and more stable native oxide film” in one case. In another, the review says that “Larger separate Si precipitates restrict the formation of a compact oxide layer.” The composition and the structure of the alloy both shape the result.

Surface treatment can also change the result. The review notes “an improvement of the corrosion resistance of as-produced parts by bright dipping in a phosphoric/nitric acid bath,” which it attributes to “the removal of the oxide film formed during printing.”

Residual stress is another factor. The review states that “surface residual stresses could, in general, influence the surface energy and therefore, the nucleation and growth of the native oxide film, affecting as a consequence the passivity and pitting resistance of the material.”

For related reading, see why rain and salt corrode outdoor steel and why acidic soil stops root growth in plants.

How we chose these sources

The two sources cover different sides of the question. The 2021 review covers aqueous corrosion and passivation in aluminum alloys in general. The 2020 review covers corrosion and protection in additively manufactured aluminum alloys, which shows how processing changes the film. Both pages returned HTTP 200 on direct requests, and both are hosted on the NIH PubMed Central site.

What we could not verify

The 2021 review states that “The passive film is generally self-renewing and self-healing,” but the sources do not give one repair rate for every environment. The review’s own account of stability shows that the film can decline under some chloride conditions, so the self-healing claim has limits.

The 2020 review focuses on additively manufactured alloys. Its findings on pores and surface defects describe those parts, and the sources do not show how the same results apply to cast or wrought aluminum sold for decor.

What the sources do not say

The sources do not give the thickness of the film on a typical aluminum fixture. They also do not give a time to failure for an outdoor piece, and they do not describe a coating or cleaning product for home use. The 2020 review states that “Nevertheless, the AM material presented a much higher value of pitting potential” in its tests, but that is a comparison of two test materials, not a forecast for any product.

Frequently asked questions

Does aluminum rust?

The sources do not use the word rust for aluminum. They describe its surface chemistry in terms of an “aluminum oxide/hydroxide film,” which protects the metal from further reaction.

Why does the film form without a coating?

The 2021 review says that the passivation “is related to spontaneous aluminum oxide/hydroxide film formation at the interface,” so the film is not described as a separate coating.

What breaks the film down?

Halide anions are the named example. The 2021 review states that “The passive film breakdown may be initiated by aggressive halide anions and lead to localized corrosion (pitting).”

Can the film repair itself?

According to the 2021 review, the passive film is “generally self-renewing and self-healing.” The sources do not give a repair rate for every condition.

Does the way aluminum is made change the film?

Yes, according to the 2020 review. It says that “close-to-the-surface pores can compromise the passivation properties of the native surface oxides, and therefore the pitting resistance of the material.”

Sources

• Aqueous Corrosion of Aluminum-Transition Metal Alloys Composed of Structurally Complex Phases: A Review. Materials, 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8467597/ (HTTP 200, checked 2026-10-28)
• Corrosion and Corrosion Protection of Additively Manufactured Aluminium Alloys: A Critical Review. Materials, 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC7663725/ (HTTP 200, checked 2026-10-28)