The What, Why and How of Aluminum Corrosion

Aluminum is one of the more versatile metals that exists. According to government statistics, it is among the top three most abundant elements in the earth's crust, and is used in just about every walk of life, frequently as a surface metal. From automobiles to airplanes, marine vessels, roofing and consumer products, aluminum is truly ubiquitous. In fact, according to the Aluminum Association, approximately 75% of all the aluminum that's ever been produced in America remains in use, frequently in an alternative form.

"Aluminum can withstand weight-bearing forces yet is extremely light."

It's omnipresence and robustness is largely attributable to its superior chemical makeup; it can withstand weight-bearing forces yet is extremely light, only a fraction of what steel and copper weigh. It's also resistant to corrosion, which is part of the reason aluminum is leveraged so frequently as a surface metal in the automotive industry. Cars are constantly in moisture-rich environments, but thanks to aluminum, vehicle exteriors hold up exceedingly well in wetness, which has helped to lengthen the lifespan of motor vehicles several times over in recent decades. Studies from organizations like R.L. Polk and the Department of Transportation say the typical lifespan among automobiles today exceeds 11 years.

While aluminum may be resistant to corrosion, it's not immune from it. Fortunately, learning more about what causes the surface material to corrode, the type of corrosion it encounters and what solutions offer corrosion resistance can help you get the most out of this multifaceted material.

What causes corrosion on aluminum?
Similar to other metals, aluminum corrosion occurs when it is exposed to moist environments for a prolonged period. The combination of oxygen from the air and water triggers a chemical reaction called oxidation. Here, the aluminum gains more oxygenated molecules.

What makes aluminum vulnerable to corrosion is its anodic nature. By definition, anodes are more reactive to corrosive environments, which is any place that is wet or humid. This is part of the reason why aluminum is often used as a sacrificial anode so it can protect other metals from corrosion. This is also true of zinc in many settings.

While aluminum may be resistant to corrosion — in that it takes longer for it to erode compared to other surface metals — it does eventually succumb to it, depending upon how long aluminum is exposed to environmental factors. Naturally, its hardness and durability diminishes the longer it is exposed.

What does aluminum corrosion look like?
One of the unique aspects of metal is the way it reacts to its natural surroundings. For instance, unlike iron, which will flake and resemble the classic orange-brown appearance that comes with rust, aluminum corrosion is much lighter in appearance, usually in some shade of white. And instead of flaking, aluminum oxide is hard and forms a surface surrounding the impacted area. This is another reason aluminum is often used as a sacrificial anode, because it forms a protective layer even after oxidizing rather than chipping away like iron does when it rusts.

How long does it take for aluminum to oxidize?
It's very difficult to say with precision how long it will take for aluminum to oxidize, given so many variables are involved. What can be said is it typically happens faster than what occurs with steel due to its anodized nature and the way it reacts in the presence of oxygen. It's the manner in which aluminum oxidizes — forming a hard outer skin — that allows it remain as resilient as it does than it would if the resulting effects were the opposite, by peeling or getting softer.

What are the different types of corrosion and how are they related to aluminum corrosion?
Just as aluminum is highly versatile, it can also be paired with other surface metals, such as copper, silicon, manganese, magnesium and zinc and other alloy varieties. Which ones it is paired with can influence the type of corrosion it experiences, and as a result, how aluminum corrosion appears.

Galvanic corrosion
When two or more dissimilar materials are combined — and it's exposed to a corrosive environment, such as water — galvanic corrosion is possible. Otherwise known as dissimilar corrosion, galvanic corrosion can only happen when metals with opposing chemical properties are coupled.  For example, since aluminum and titanium have very different chemical structures, galvanic corrosion is a possibility.

Typically, galvanic corrosion happens faster than other types of corrosion."

Typically, galvanic corrosion happens faster than other varieties, which is why coating is a key element to improving corrosion resistance, particularly when materials are used for surfaces that are placed under stress or must withstand heavy loads.

Filiform corrosion
Filiform corrosion is similar to galvanic corrosion, in that it can occur when aluminum is combined with other metals. The way it differs is that it's exclusive to environments where coatings are used. In this way, filiform corrosion is less damaging to metal because it's usually present and observable only on surfaces, thus is mostly superficial. Scratches, swirl marks and beards are types of filiform corrosion.

Poultice corrosion
Another type of corrosion that can affect aluminum is called poultice corrosion. Poultice corrosion is most common in environments where there is a lot of dirt and debris. A classic example for aluminum is when it's used on the body of light vehicles and trucks. During bad weather, the roads are lined with salt and sand to increase traction. This inevitably gets kicked up and attaches to a car's body, wheel frames and undercarriage.

Fuel tanks, windows frames and deck boxes on sport boats are also common breeding grounds for poultice corrosion.

How can you mitigate aluminum corrosion?
Again, while aluminum may be resistant to corrosion, it's not impervious to it; without a proper anti-corrosive solution, it will break down eventually. The best way to overcome this is by applying the coatings that serve as a shield. This coating can counteract the corrosion by slowing the process down.

No matter how you're using aluminum, you can obtain the best coating solution by testing its stability and longevity. For that, there's Auto Technology. We have the environmental testing laboratories and proficiencies that can inform your corrosion resistance strategy. Please contact us to learn more.