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Oxidation on Aluminium Boats: What is it?

Blog | 12 January 2018

Oxidation is a condition that most of us associate with iron. Ferrous products just corrode unless they’re specially treated. As for oxidation on aluminium boats, is this a big problem? Unfortunately, marine aluminium does oxidize, although it doesn’t rust like steel. Remember, aluminium oxide coatings protect the underlying metal. They provide an invisible barrier, a dull film that stops corrosion. With that said, what’s the problem?

Dealing with Aluminium Oxide 

Years of exposure to the air cause this problem, although the issue isn’t terribly corrosive. No, aluminium alloys naturally develop an oxide film. That coating impairs the metal’s lustrous good looks, but it doesn’t really harm the structural characteristics of the alloy. In fact, the oxidized metal membrane acts as an atmospheric barrier. That all sounds well and good, but what if this layering effect isn’t properly administered? Aluminium oxide coatings, at least when they’re applied within a controlled environment, perform as a shell-like finish. However, should that coating form at any other time, there’s trouble in store for the craft. Painted metal fittings end up peeling because of the oxidation effect. As the paint lifts away, water migrates into the open wound and weakens the alloy.

Identifying Pitting Depassivation 

Look at the tiny holes on the hull of an aging tinny. The natural oxidation is failing because of the chlorides in the salt water. On a freshwater lake, the issue is less likely. No, it’s out on the briny seawater that this pitting eats into the hull of an aluminium tinny. The ‘depassivation’ term enters this scenario when the outer oxidized film breaks down. Specially applied post-process finish or aluminium oxide, the barrier layer collapses. As it disintegrates, tiny pinholes develop on the metal in and around the boat. Left to its own devices, this effect will eat away a formerly durable aluminium frame.

Uncovering Galvanic Corrosion 

A metal tinny becomes a giant battery when it sits on a salty fluid. The briny water assumes the role of an electrolytic compound. As for the electrodes in this weak circuit, they’re the two dissimilar metals that are in contact with each other. Unfortunately, aluminium loses its electrons during this battery-like action. The result is metal fatigue, a process where the aluminium part corrodes. Keep those dissimilar metals apart, use a sacrificial anode, and stop galvanic corrosion in its tracks.

Marine oxidation is often a desirable prospect, even though it dulls the metal. The protective film acts as an oxygen barrier, after all. If that finish or another special coating disintegrates, surface pitting will form because of depassivation action. Finally, galvanic corrosion exacts a high price, but it is an entirely preventable process.

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