Aluminum Does Rust, Just Not the Way You're Thinking

To begin, it must be said that while the term "rust" is defined as iron oxide and therefore rusting is something that can only happen to iron and iron alloys, asking whether or not aluminum "rusts" gets to an important question. Really the question is about corrosion but because aluminum is an element and not an alloy of iron, the question is more properly posed as "Does aluminum corrode?" Let's find out.

From soda cans to car doors to street signs to airplanes, aluminum (or aluminium in the UK) is truly ubiquitous in our world. In fact it is the most used nonferrous metal and the most abundant metal in the earth's crust. The cost of obtaining refined aluminum can be quite high, as it is energy-intensive to get it from its most common ore, bauxite, and the waste from the process of attaining it, known as “red mud”, is one of the most pressing toxic waste issues facing modern industry. Only a few years ago a red mud containment facility in Hungary overflowed, endangering the lives of hundreds and nearly polluting one of Europe’s most important rivers. But society currently allows this risk because aluminum is so versatile.

Aluminum can be used for so many different applications because it is light (i.e. it has a low density) and durable. Certainly durability is the result of various factors, but for aluminum there is a most important factor. This is that aluminum is resistant to corrosion or rust - or so it seems. The aluminum that we see in everyday life is actually already corroded. Aluminum reacts very quickly with oxidizers in the air and immediately develops what is called a passive layer of aluminum oxide surrounding the pure metal. This passive layer protects the metal from further surface corrosion.

So, now you might be thinking "Great! All our street signs and airplanes are resistant to corrosion because they already corroded a bit. They'll be on the road and in the sky forever. No need to worry at all." Unfortunately it's not quite that simple. The aluminum oxide layer protects the pure aluminum inside from the familiar surface corrosion, but there is a much more pernicious type of corrosion which can affect aluminum: pitting corrosion.

Pitting corrosion is a type of galvanic corrosion which happens to metals that are protected by a passive layer, like aluminum and stainless steel. Pitting corrosion happens in a very small area of the metal which by some means has its passive layer removed. This can happen chemically or simply from a scratch. Once the protective oxides are removed, oxidizers (like chlorine and sulfides) in the air or water in the surrounding environment begin to attack the underlying metal and corrode it locally. The rate of pitting corrosion will increase if there are excess oxidizers in the air, as is the case for many developing markets, including certain parts of China. Because this type of corrosion tends to dig into the metal, pitting corrosion is notoriously hard to detect. It also causes serious structural issues which can lead to total failure of the metal. When it comes to the world’s airplanes and other places where aluminum protects lives, this kind of degradation is simply unacceptable.

While in storage and shipping, there are packaging materials which can save aluminum from the perils of pitting for extended periods; in situe, and in the very long-term, there are no complete solutions. One of man's few techniques for corrosion control, and arguably the most used, is passivation, i.e. the forced addition of a passive layer of oxidized compound to the exposed surfaces of a metal. But aluminum by nature exhibits the property of passivation thus eliminating it as a choice for further protection (it must be said that adding more or thicker passive layers will add that much more time for the pitting corrosion to reach the material being protected, but the point here is that it is only a matter of time; corrosion is inescapable). In this way, pitting corrosion, along with its close cousins, proves itself to be one of the most dangerous forces facing a modern society which relies so heavily on natural and artificial passivation for protection of the equipment on which so many lives depend.

More from our series on pitting corrosion, and the related topic of corrosion of electronics.

Austin A40 image care of Charles01 via Wikipedia