When you think of metals that are reactive, you may think of something that rusts, tarnishes or even burns with contact with air! Aluminum, a metal that will bond with oxygen almost faster than you can give it an opportunity, would be at the top of that list.
However, look to your left and right: from aircraft to soda cans, architectural facades to today’s models of vehicles, aluminum is always an example of perseverance, often defying the laws of chemistry that technically define it.
How could one of the most reactive metals with oxygen be so very durable and resistant to corrosion? The answer is not just a freak of nature; it is a brilliant act of chemical self-preservation, that has led to aluminum becoming one of the most useful and driven materials in the world today.
What Does “High Affinity for Oxygen” Actually Mean for Aluminum?
At a basic level, a metal’s “affinity for oxygen” refers to how easy it is for its atoms to lose their electrons to oxygen atoms to make an oxide. In the scale of the periodic table, aluminum (Al) is very electropositive, meaning that it is willing to lose its three valence electrons.
In contrast, oxygen is a very strong electron acceptor, and together this intrinsic chemical drive makes an oxide of aluminum very exothermic (energy-liberating).
You can think of it like a magnet: aluminum atoms are highly attracted to oxygen atoms. This is why pure aluminum reacts nearly immediately with oxygen in the air every time it is exposed to a fresh surface.
That is, the reaction does not creep along slowly like rust on iron; instead, the reaction is much more rapid, occurring often in milliseconds.
How Does This Rapid Oxidation Prevent Corrosion?
This is where the magic – or science – of passivation starts to take place. When aluminum’s surface comes into contact with oxygen (even the oxygen in the air) it doesn’t just form any oxide, it forms a dense thin layer of aluminum oxide (Al2O3) that is very stable.
This coating is referred to as a “passive film” because it maintains the remainder of the aluminum “passive” or unreactive to oxidation.
Unlike the case of iron rust where the oxide flakes off and leaves new fresh metal underneath, aluminum oxide adheres permanently to the surface of the aluminum so there is no more reactive surface left in contact with the environment.
As an analogy, imagine building a wall around a city that is consistently under siege from an invading army.
The wall you built represents the aluminum oxide, protecting the bulk aluminum metal from surroundings and isolating the reactive metal from any corrosive agents like water, salts, or pollutants from reaching the underlying metal and continuing to react.
Is the Aluminum Oxide Layer Truly Impermeable?
While “impermeable” is an intense word, the aluminum oxide layer is very effective. It is generally incredibly thin, on the order of a few nanometers (50 – 100 Angstroms) in thickness, which is roughly 1/50,000th of the width of a human hair! The aluminum oxide layer, its atomic size and thickness notwithstanding, is dense and closely packed in its atomic structure.
This density is what is important. Unlike porous rust, which will allow oxygen and moisture to penetrate the layer of rust and subsequently allow the corrosion process to continue, the aluminum oxide lattice acts as an effective and highly selective diffusion barrier. Oxygen and moisture have a difficult time passing through this dense structure to diffuse to the pure aluminum.
The beauty of the Al2O3 layer is derived from the strong ionic bonds within its structure. These ionic bonds yield a very stable and inert compound. The protective film is also an electrical insulator, which prevents the electrochemical corrosion process that requires the flow of electrons.
What Happens If the Protective Oxide Layer Gets Damaged?
This is where aluminum’s positive affinity for oxygen works to its advantage. The protective layer of aluminum oxide forms almost instantly.
If the surface is scratched, abraded, or otherwise damaged, exposing fresh aluminum metal to the environment, the aluminum underneath will react with any available oxygen and immediately form the protective oxide layer.
In an oxygen-rich environment, such as ambient air or oxygen-saturated water, that self-repair mechanism is nearly instantaneous.
In this case, as soon as that protective wall is breached, it instantaneously rebuilds, providing constant protection. The ability to self-repair is a significant reason so much aluminum is used in demanding applications.
Are There Any Environments Where Aluminum’s Passivity Fails?
The aluminum oxide layer is effective, but it has its limitations, which depend on the pH level of the environment.
- Very Acidic Environments (pH < 4): In strong acids, the aluminum oxide film can dissolve, especially for acids with a pH < approximately 4. After the film dissolves, the reactive aluminum underneath is exposed and will corrode rapidly unless the oxide films are able to reform quickly enough.
- Very Alkaline (Basic) Environments (pH > 9): In strong bases (pH > approximately 9), the film can also dissolve, exposing the aluminum and leading to rapid corrosion. Hence, it is generally recommended you do not use strong alkaline cleaners on aluminum.
- Chlorides: The aluminum oxide passive film can be locally attacked by chlorides (Cl−), which are abundant in salt water and marine environments. Chlorides can locally attack the oxide layer and lead to a type of localized corrosion known as pitting corrosion. Once a pit has formed, it can become an anodic site, which will further corrode in that confined area.
- Galvanic Corrosion: Aluminum can also suffer from galvanic corrosion when it is electrically connected to a less “noble” (less reactive) metal (such as copper, steel, or brass) in the presence of an electrolyte (moisture or saltwater). In those galvanic couples, since aluminum is more electrochemically active, it will act as the anode and corrode preferentially, protecting the more noble metal.
Knowing these limitations is important for alloy selection and design in particular applications.
How Do Industries Enhance Aluminum’s Natural Corrosion Resistance?
Because of the remarkable natural corrosion resistance of aluminum, other industries have created processes to allow human control of this oxide layer:
- Anodizing: This is an electrochemical process that purposely thickens the natural aluminum oxide layer. The aluminum is immersed in an electrolyte and an electrical current passed through it to grow a thick, hard, and durable oxide (this action produces what is known as an anodic coating.) Anodizing can also connect the porous structures which may be both dyed for aesthetics or sealed for added corrosion protection.
- Chemical Conversion Coatings (Chromate and Non-Chromate): These conversions use different chemical reactions to apply a very thin protective coating over the aluminum surface but rather than electrochemical reactions, they are done by chemical reactions. Rather than using hexavalent chromium to accomplish this goal (now banned due to environmental requirement), today they are done with zirconium and titanium compounds achieving very similar corrosion protection and providing a very solid surface for paint and other coatings.
- Painting (including powder coating): Another barrier from corrosion is to use a paint or powder coat, in these cases a coating is put over the top of metals to provide protection. This is usually done in conjunction with anodizing or conversion coatings. Thus painting will provide better protection and for a much longer time.
- Alloying: The alloying elements associated with aluminum have a major impression on its corrosion resistance. So for example high purity aluminum is very resistant to corrosion. However, as certain alloying additions are added, including copper, the lifetime of that resistance decreases. The 5xxx series alloys (with magnesium) introduces the best corrosion resistance particularly for salt water environments.
Why Does This Matter for Everyday Life and Industry?
The relationship of aluminum to oxygen is not only a curiosity, but also drives both engineering, and many everyday products. For example:
- Sustainability: Aluminum products are truly long-cycle products as they essentially cannot corrode, so they need to be replaced far less frequently, which contributes to a sustainable material cycle.
- Light-weighting: Aluminum can lead to the lightest design in aerospace, automotive and transport industries since it usually does not require heavy or complicated protection systems against corrosion (this is especially true for steel), which can add unnecessary weight to these transport industries, thus providing an opportunity to improve fuel efficiency and reduce harmful emissions.
- Low-maintenance: Aluminum, when in normal environmental conditions, typically does not need maintenance any more than any other metal and often less, thus providing lower long term maintenance costs.
- Unmatched versatility: Aluminum is used in an almost endless number of applications and products due to it weight, strength (especially in alloys), and resistance to corrosion, from food packaging to boats and marine vessels.
conclussion:
Aluminum and oxygen are a wonderful example of how a deceptively reactive trait can ironically provide unparalleled stability and usefulness. Aluminum’s high reactivity with oxygen is not a weakness, it is actually its strength.
The quickly-solidified and self-healing oxide barrier aluminum creates converts it from a very reactive metal into a corrosion-resistant power player; quietly supporting us in innumerable ways, from the tiniest electronic files to the largest aircraft, because of the unique power of oxygen and aluminum.