Rust Never Sleeps is the name of a famous record album produced years ago by Neil Young and his band Crazy Horse. The title makes sense in a metallurgical context because rust refers to the oxidation of ferrous metals that continues without stopping unless preventive steps are taken regularly. One-time fixes will not do the trick.
The decay is quiet, persistent and inevitable. Because rust never sleeps.
What is rust and why will aluminum never rust
Rusting happens when iron or steel reacts with oxygen and moisture. It is a type of corrosion that occurs exclusively on iron-based metals, and causes the iron surface to weaken, flake and disintegrate. It is an iron oxide that is reddish-orange in color, which you probably know, since rust is also the name of a color.
Aluminum is never going to rust. Never. That is a fact. It is chemically impossible for aluminum to rust, because rust is a chemical reaction that only occurs in iron and iron-based alloys like steel, and aluminum contains no iron.
That said, aluminum can corrode. Is this making sense to you? If not, please let me explain.
All rust is corrosion, but not all corrosion is rust
I have an idea that semantics may be the issue here, because I have often seen and heard corrosion and rust, as words, used interchangeably, and they are not the same thing. Yes, corrosion and rust are both the result of oxidation reactions – where an element combines with oxygen. But I repeat: Rust is a type of corrosion that refers specifically to metals containing iron. And aluminum contains no iron.
Corrosion is a process where metals deteriorate as the result of environmental chemical reactions such as oxidation, where an element combines with oxygen. It can affect metals like steel and aluminum, and it can lead to structural failing or to the loss of material strength, among other things. But on the flip side, some forms of corrosion actually protect the metal. Aluminum oxide is an example.
Aluminum oxide is formed from a chemical reaction when aluminum is exposed to oxygen. It is a protective coating – a layer, and a form of corrosion that protects the metal rather than destroying it, like rust.
The four most common types of aluminum corrosion
Even though pure aluminum is highly resistant to the negative effects of corrosion, it is too soft for many applications. By adding small amounts of elements like magnesium, silicon, copper or zinc, etc., we get an aluminum alloy with improved properties. Due to the presence of salt or industrial chemicals, aluminum alloy can corrode when its naturally formed oxide film is damaged. Using aluminum in areas with very low or very high pH can also lead to corrosion.
The four most common types of corrosion are galvanic, pitting, crevice and inter granular corrosion.
- Galvanic corrosion can occur when two different metals are in electrical (metallic) contact and are connected by an electrolytic bridge (such as water containing salts).
- Pitting corrosion occurs in the presence of an electrolyte and is most commonly associated with chloride-containing environments, although other aggressive species can also initiate pitting.
- Crevice corrosion occurs in narrow, confined spaces where a stagnant electrolyte is trapped, leading to local breakdown of the protective oxide layer.
- Intergranular corrosion in aluminum alloys is the preferential attack along grain boundaries caused by compositional differences or precipitates, while the grain interiors remain relatively unaffected.
When aluminum corrodes, it commonly develops a dull gray appearance or a white, powdery oxide layer on the surface.
Select specific aluminum alloys to prevent corrosion
Protective coatings such as anodization can help you prevent corrosion. Another way to prevent corrosion is to select specific alloys. Examples of these are the marine-grade aluminum alloys in the 5xxx and 6xxx series, which have been engineered to resist degradation caused by salt spray and humidity. 5xxx alloys are typically superior to 6xxx in aggressive marine exposure.
Alloys in the 2xxx and 7xxx series should be avoided unless heavily protected or specially heat‑treated in coastal areas or salty environments, due to their high amounts of copper or zinc.
In preventing corrosion, these are the most sound and traditional methods available. Recent patent activity also shows that further innovation is coming in areas such as:
- Replacing toxic chromium treatments with non-chromate coatings
- Developing "self-healing" coatings
- Modifying alloy compositions
One example is the addition of trace elements to prevent intergranular corrosion, where the metal decays along its internal grain boundaries. Another is a new suite of alloys in which nanotechnology shows promise for improving hardness and durability while maintaining corrosion resistance, though most applications remain under development.
A third refers to an additive manufacturing alloy designed for 3D printing, with a highly resistant passive film that outperforms traditional alloys in seawater environments.
Patents related to coatings that can "repair" themselves include one where layers of 8-hydroxyquinoline (8HQ) or lithium salts are applied. When the coating is damaged, the inhibitors are released to passivate the exposed aluminum, creating a new insoluble layer that stops corrosion before it spreads. Another “self-healing” concept is an ultrathin film containing cross-linked nanoparticles. It provides a shield with no use of toxic chromium.
In fact, many of the recent patents I’ve seen focus on non-chromate conversion coatings, due to environmental regulations. These describe processes for preparing anodized coatings that use low-temperature, nickel-free, and chromium-free solutions to fill the pores of the aluminum oxide layer, thereby extending the life of the metal in harsh environments.
A lot of good things are continuing to happen for aluminum, the metal of the future.
