Corrosion on aluminium frame

Underscore

I replaced the middle chain ring on my commuter at the weekend (a Spesh Sirrus). Rather than putting it on the workstand, I did the work while it was hung by its front wheel on the garage wall and I noticed that an area of paint toward the bottom of the downtube is bumpy/blistered, like you would get if the frame were steel and had started to rust. It would seem that the frame has started to corrode - perhaps due to a flaw in the paint allowing salt water ingress during the winter? Anyway, is there anything I should do? I assume that I should wire-brush down the area and repaint with something like Smootherite...

TIA,

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craker

I've run a bare aluminium frame for ~5 years now as an on and off commuter (very unloved bike :twisted: ) and I'm not aware of any reaction it's had in that time to the elements. It's got scratched and there might be some unexplained surface discolouration but there's certainly no corrosion.

If you want to sand prime and paint it's up to you but I can't see it making any practical difference,

Gussio

What you are seeing is aluminium oxide which (I believe) once it forms on unprotected aluminium creates a protective barrier over the metal beneath. In short, not something to worry about.

kurako

Alu won't react with salt water due to the aforementioned aluminium oxide. You'd need some concentrated acid before you get any fireworks. I have an aluminum frame which has been in all weathers for over 10 years now. The paintwork is a mess but the frame is solid so don't worry about it.

Drysuitdiver

as above, its not like iron oxide which continues to eat away at the iron. aluminium is fine once the alu oxide has formed. it looks awful but its not a problem

kurako

FYI. Don't worry. Alu is very reactive with oxygen and the oxide layer forms instantly. The bubbling is more likely from impurities in/ under the paint.. You need to submerge it in acid to dissolve the protective layer.

http://www.youtube.com/watch?v=wYS_KK_kmNY
http://www.youtube.com/watch?v=63wZWCnyD6A

Underscore

Well, thanks all. It looks so much like the bubbling you get when steel is rusting under paint that I assumed that it was/is a continuing reaction. I'll probably brush it down and re-paint at some point but at least I know there's no rush now...

Cheers,

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dilemna

Not so much frames but aluminium handlebars can suffer terribly after a few seasons as sweat from hands can seep through bar tape which then reacts with the surface of the aluminium bars below eating into it. Check your bars as failure doesn't bare thinking about ......... I stripped off the bar tape of some old Super Italia Pro 2 ITM handle bars and was shocked so have changed these.

Underscore

I've recently changed from flat-bars to butterflies on this bike so I'm OK on that point. However, if this oxide layer will stop salt water reacting with Aluminium, I wonder why it doesn't do the same thing with sweat?