Aluminium corrosion - protection

[penster]

My road bike has carbon front forks with an aluminium crown. The underside of the crown, where the front wheel splashes it with water, has become corroded. I don't want it to get any worse. Is there anything I can coat it with to protect it?
Mick

 

[domtyler]

Yes, coat it with a thin layer of Al2O3.

 

[Chris James]

As Dom has extremeley helpfully pointed out, aluminium forms a protective 'passive layer' of oxide on it's surface, in the same way as stainless steels do. This layer forms pretty much instantaneously on bare aluminium as it is so reactive.

This passive layer is actually very stable and good at preventing further general corrosion. Unfortunately this layer is very thin and is attacked by salt, specifically chloride ions at high concentrations (salty water dries out and becomes salt deposits / strong saline solution). They puncture the layer and lead to pitting. You can also get white corrosion deposits on the aluminium.

What you need to do is wash off the desposits and clean that area regularly. This will prevent salts building up in that area and allows the passive layer to protect from further corrosion. Mudguards also help.

 

[fossyant]

Ah but like lots of folk, how do we keep it looking pretty - as it causes the paint to flake and bubble.......it's not as easy to treat as steel....

I'd be interested.... or is it a case of catching a chip early and painting it......

 

[LLB]

The only answer is to use autosol to buff it back up and then laquer it.

In the mean time, Scott Oilers FS365 works very well at protecting from road salt. I've used it for years on my bikes & motorcycles, and it does work.

http://www.mandp.co.uk/productInfo.aspx?catRef=529278

 

[Chris James]

Ah but like lots of folk, how do we keep it looking pretty - as it causes the paint to flake and bubble.......it's not as easy to treat as steel....

I'd be interested.... or is it a case of catching a chip early and painting it......

I can't see why an aluminium alloy frame would be any more difficult to paint than a steel one. I wouldn't know though as I have only had steel bikes!

With all paint systems it is vital that all the corrosion products are removed before painting. This is because paint is both oxygen and water permeable and so existing corrosion can continue beneath the paint, whioch causes the paint to spall off.

So if you catch the chip early and clean it right back (sanding or grit blasting) then it be okay I woudl have thought. Maybe the fact that aluminium oxide is strongly adherent (and the base metal so reactive) makes it more difficult to clean up?

 

[02GF74]

I can't see why an aluminium alloy frame would be any more difficult to paint than a steel one. I wouldn't know though as I have only had steel bikes!

For aluminium you need to use an etch primer otherwise the paint won't stick. Sorry but you'll have to google to find out its properties.

 

[Chris James]

For aluminium you need to use an etch primer otherwise the paint won't stick. Sorry but you'll have to google to find out its properties.

I guess it is an acid etch to puncture the passive layer.

I worked for a number of years in the electroplating industry and you had to use to pretty unpleasant etches to get plating to stick to aluminium. Steel was easy!