Common Causes of Aluminium Corrosion

Unlike iron rust, aluminium corrosion doesn’t spread as an orange-red stain. However, a dull aluminium oxide coating does form on the surface of the lightweight metal when it’s exposed to the atmosphere. Not to worry, this oxide acts as a protective barrier. That’s a helpful material feature, but what if the corrosive element isn’t removed? Deal with that issue and others by knowing the common causes of aluminium corrosion.

Atmospheric Reactance 

Corrosive attacks take place on aluminium parts when they’re exposed to the atmosphere, which is why it’s hard to find the metal in its naturally shiny state. It tends to grey and lose its glossy finish. Cleaned of that oxide coating, it regains that polish, but it’s only a matter of time before the oxygen in the air wipes away that mirror-like finish. Worse still, humid environments and salty locales accelerate the reaction. Hence, marine-grade aluminium sheets are designed to resist corrosion.

Galvanic Corrosion 

Still located at the seafront, the aluminium is protected from the sea air, yet it’s still acquiring a nasty white stain. It’s spreading like some kind of infection. This time around, the alloy is touching another type of metal. Due to the conductive environment and their dissimilar atomic structures, the two metals function as a battery, with electrons flowing between one alloy and the other. Unfortunately, the lightweight aluminium takes on the role of the anode, so it’s sending this charge to the second metal. In plain terms, that anodic material corrodes because its atomic structure is disintegrating. The solution here is clear: keep dissimilar metals apart.

Crevice and Intergranular Fatigue 

A weld defect or machine error has left a small crevice in the component. The weak spots referred to here include boreholes, cracks, fractures, seams, fastener contact areas, and more. Look for white powdery deposits and heavy pitting under parts overlaps and weld heat-affected-zones. As for intergranular fatigue, this occurs when the actual grain of the part forms a number of distinct edge boundaries. Manufacturing impurities and anodic pathways are known to cause this issue.

Even the formation of a protective aluminium oxide layer can backfire if that coating doesn’t develop evenly. The white powder and pitting undermine the thin but strong alloy, which leads to an unfavourable maintenance report and the chore of having to scrub the surface clean. Then, if the pitting has really sunk deep, even this effort won’t be enough to save the part. Finally, when looking at structural parts, beware of stress corrosion cracking, a condition that occurs when a susceptible aluminium alloy is heavily loaded and exposed to humid air.

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