Aluminum oxide layer

[SEBASTIANOFA]

Hello, I would like to ask a few things about aluminum: the protective oxide layer that forms spontaneously in contact with air, in the absence of corrosive agents,in normal room temperature, no water or mechanical actions, remains unchanged over time, continuing to protect the underlying aluminum for an indefinite time?
I mean, if nothing breaks the barrier, can aluminum hypothetically be protected “forever”?

 

[IRstuff]

Ostensibly, yes; the caveat is that the layer is quite thin, and isn't necessarily contiguous, i.e., there may be voids or pits that can allow corrosion to get under the layer. The Lock Picking Lawyer has a video of a Gallium attack on aluminum, but he did have to damage the surface first.

https://www.youtube.com/watch?v=jeghGhVdt9s

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[TugboatEng]

It's unpopular to paint aluminum boats because the paint protects the aluminum from forming this oxide layer and leads to crevice corrosion. Aluminum, left to form it's own passive layer, aluminum can be quite tolerant of of some harsh environments including marine.

 

[EdStainless]

Or if the surface wasn't clean to start with there may be places that an oxide film didn't form.
A little oil or grease, even finger prints is all that it takes.
If it is the correct Al alloy as Tug says it is better left bare.
The wrong alloy and it never stands a chance.

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[SEBASTIANOFA]

ok thanks, so if I had to polish an aluminum motorbike frame to a mirror effect like it did when it left the factory, I would just go and remove the protective barrier that has formed over the years, right? Will this reform in a short time? are there any contraindications or aspect to focus on for this work? some say they use fine sandpaper, such as 1000-3000 and then polish, a bit like paint. Would it be advisable to wash the frame thoroughly after polishing to remove polish residues or other trapped dirt that could lead to crevice corrosion?

 

[TugboatEng]

The protective layer gives the aluminum a dull look. If you polish it you're going to need to maintain it as it will dull over time. Some polishes leave a wax film. If you do not maintain the polish you will see pitting type corrosion occur, likely due to then waxes in the polish causing crevice corrosion.

If you keep the parts in a climate controlled environment three effects will be minimized.

Do note that many cars come with polished aluminum wheels. They have a clear coat finish applied to them. It's quite durable (lasts 10-20 years) but if it's ever damaged crevice corrosion will cause pitting. It can always be removed and re-applied as with most coatings.

 

[SEBASTIANOFA]

what a complicated material that aluminum is ... painting it is complicated, keeping it greasy is not good ... are there no treatments similar to cataphoresis for steel? is anodizing also possible on large parts? are there any contraindications?

 

[EdStainless]

Al is easy, just don't expect it to be shiny.
I see bicycle components all of the time that people have polished because they think that looks neat.
If it is a part that you can keep touching up then fine.
But parts with lots of hidden areas and high probability of getting dirty and wet it isn't a good idea.

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[TugboatEng]

The aircraft industry uses chromate conversion coatings to make aluminum more tolerant of painting.

A user here mentioned hydration as a cause of adhesive failure recently. They mentioned a treatment to prevent this (proprietary). I speculate that silanes could be used to treat the surface between abrasive blasting and painting for a more durable coating.

 

[IRstuff]

Typically, aluminum mirrors have a deposited oxide coating to protect it; the analogous treatment for your case might just be clear coat

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[SEBASTIANOFA]

I don't understand one thing, if the paint does not allow the oxide layer to form, isn't it a bit the same thing that should happen when you screw bolts into the threaded frame? The screw scrapes off the oxide protection and then tightens with the frame, yet usually the threads are healthier than the rest of the frame, perhaps because they still seal from oxygen? Also, if a part of the frame is subject to rubbing, such as moving cables or adjusting bolts, is it a good idea to cover these parts with silicone grease or wrap coat?

 

[TugboatEng]

The threads are protected from moisture/sealed provided no lock washers are used. But, you will find a ring of corrosion around the outside of the head of the bolt.

Crevice corrosion doesn't occur under the paint, it occurs at the paint/aluminum/air interface. It starts at a hole or break the coating and then propagates from there.

 

[EdStainless]

And there are holes in every paint job

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[SEBASTIANOFA]

ok guys thanks for the information, so to recap, wanting to keep "forever" an aluminum frame in non-use conditions, like a museum, the best thing to do is wash it well with neutral detergent, dry the interstices and the points where the water could collect and leave it naked as it is. No cavity wax or grease. With these conditions, without water , uv lights or too humid air, I shouldn't see corrosion triggers even in the long term, right?