Aluminium corrosion-resistance

[Yellow Fang]

Is aluminium corrosion-resistant? I've just read that it's not, but I've never noticed any corrosion on my aluminium bike.

 

[spandex]

It is it is called aluminium oxide and looks white

 

[tyred]

Bare aluminium will always have a coating of corrosion. It actually forms a barrier to protect from further corrosion. It's a bad idea to keep polishing aluminium to make it shiny as you strip away the protective layer. This type of corrosion is harmless. It's the dull powdery type of corrosion which you need to look out for on ally.

 

[brodie]

Bare alu does react with oxygen to form an oxide layer on its surface which protects it from further oxidation, and it makes it look dull. Anodising is just alu oxide applied by the manufacturer.

But that oxide layer, whether it is anodising or formed naturally on bare alu, doesn't protect against damaging corrosion, usually from salty water, either road salt or sweat. It turns alu into a white powder.

 

[simon_brooke]

If you put it in salt water in contact with a bit of ferrous metal, it will corrode extremely quickly. Under normal atmospheric conditions, however, the oxide forms a protective skin and stops it oxidising further.

Obviously painted is better, but bare aluminium is OK so long as you keep it away from salt water!

 

[Piemaster]

+1 on salt water.
Aluminium on the deck areas (of the supply boat I work on) can best be described as dissolving rather than corroding once any paint protection comes off or is damaged.
Aluminium anodes are used in a sacrificial manner in underwater areas (around propellers) rather than lose material from the steel hull.

 

[Davidc]

Not just salt water, but a number of industrial atmospheric pollutants as well. Mercury is spectacular as the oxide layer dissolves in it quickly - before mercury use got banned it was a school science demo, little drop on an aluminium sample, which just disintegrated into a white powder.

A bike frame in Al should have a paint layer which helps, and components made of it are often treated (e.g. anodised), but where any of these are scratched keep an eye on them.

If you see lots of white powder worry.

 

[Yellow Fang]

I've never ever seen any white powder on my bikes, even on the replaceable gear hanger which doesn't appear to be anodised or painted.

 

[Davidc]

I've never ever seen any white powder on my bikes, even on the replaceable gear hanger which doesn't appear to be anodised or painted.

That's good, usually good quality ali lasts a long time.

As has been said aluminium is generally self protecting. Anodising doesn't really show, it just looks the slightly dulll matallic finish we're all used to. The gear hanger should be one of the strong alloys, and many of them seem to resist abrasion well, which helps keep the oxide layer intact. The surface oxide layer is extremely thin (a few molecules), so if it's working the surface still looks like metal, just dull.

The only time I've seen aluminium corrosion was in an alloy called dural, used for scaffolding poles and similar. The pole held a light and a loudspeaker and had been frequently sprayed by seawater. When it cracked it was clear that there was a fracture, probably from impact, which had trapped water. You'd probably notice damage like that on your bike.

 

[fossyant]

Easy, just look after the stuff, just like you would anyway. Touch up scratches/chips -stops corrosion getting under the paint.

 

[andrew_s]

Exposed aluminium is generally OK for corrosion. It tends to be a problem in more hidden away areas that don't dry out so easily. E.g. seatpins or quill handlebar stems inside the frame, unanodised handlebars under the bar tape etc
You can get it anywhere if you store the bike somewhere damp though.

 

[rog 1974]

dont know about bikes but on my old LR 110 the main prob with the allu body panel corrosion was where it came into contact with other kinds of metals.

 

[StirlingCrispin]

Where you have to watch is if you get bubbling under the paintwork.

This happened to me round the bottom bracket area on my Cannondale touring bike. Guess some paint had chipped and then water got underneath the paint work.

The surrounding paint could be flaked off to reveal this:

Sanded off the excess oxide and then covered the area with undercoat and protective paint from B&Q. That was two years ago it's fine now.

 

[jimboalee]

Used to do anodising at school.

Chainrings and brake stirrups in pretty colours.