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I found an aluminum bike in the trash and it seems like it has been exposed to the rain, especially because some of the metal is slightly rusty.

The frame is a different thing, because altough it is made in aluminum, it has worm-like bright shapes, with a small relief (when I pass my fingers it isn't as smooth as the rest of the frame).

This:

is this aluminum rust?

So, my questions:

  1. Is this "rust"? (I know aluminum doesn't have corrosion like steel does, but this is IMO somekind of corrosion).

  2. Is this a damaged in a point that it is dangerous to use it again?

  3. Can I solve it? (I tried sanding a small bit and it takes care of it, of course the shine is gone in those bits).

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Looks like ancient Indian rock art to me. ;-> –  Moab Jul 7 '11 at 21:50
    
Thanks everyone for their answers! –  jackJoe Jul 8 '11 at 7:50
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3 Answers

up vote 4 down vote accepted

Yep, "raw" aluminum develops a layer of corrosion fairly rapidly. Thankfully, unlike with iron/steel, the corrosion is "tight" and eventually develops to a thickness where additional corrosion is prevented (in normal circumstances).

The corrosion can be prevented by either coating the aluminum with a lacquer-like coating or "anodizing" it. Anodizing is a process where the aluminum is dipped in an electrolyte solution and a current is run through it, to develop a layer of "super corrosion" that is smooth and fairly hard/scratch resistant.

I'm guessing the bike was lacquered and the coating is breaking down. Once one little spot of breakdown appears, the corrosion below will push up the adjacent areas, creating a growing spot that turns into the "worms" you see. There is no structural problem as a result of this -- it's purely cosmetic.

The thing to be concerned about would be if there were any spots (particularly around the joints) where a salt-like crust was forming. This would indicate that the aluminum is "crystalizing" in the weld, and fractures, if not already present, are imminent.

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+1 for mentioning anodizing so eloquently (I forgot about that). –  ʍǝɥʇɐɯ Jul 7 '11 at 12:03
    
Not that anodizing is really practical for a bike, but it's what you see on, eg, aluminum storm windows (and, conveniently, it takes stain well so it can be used to color the aluminum), and it's the finish most people assume is "bare" aluminum. –  Daniel R Hicks Jul 7 '11 at 15:51
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This is surface oxidation.

Freshly polished aluminium loses its sheen almost instantly, most shiny aluminium products have a layer of lacquer to mitigate against that.

Scratches can appear worse than they are. You can leave it and the oxidised surface will not 'get any worse' or any deeper. There are no safety concerns here.

As for tidying up the appearance, you might want to look at what motorists do with their wheels. Aluminium wheels on cars oxidise really badly if they are neat 'alloy', hence all of them are painted to look like aluminium. Many concoctions are provided for the motorist wanting to tidy up the appearance of the wheels after they have scraped a kerb too many.

Remember to park so your frame does not get scratched, put end-caps on your cables and patches of tape where they rub the frame. In that way you should have a tidy frame.

For now another option is to put a sticker on the frame over the offending area(s).

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It appears to be minor surface corrosion. From that picture, it could also simply be scratches, but I assume not if you can sand it away.

It isn't (usually) structurally damaging or unsafe for use, unless it also gets much worse than you see there.

Polish it away with an aluminum polishing compound and a rag, if you want the best appearance after.

Otherwise light sanding is fine.

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