Aluminium frame corrosion...

[pedalhead]

Hi all. I've got a 1996 Giant ATX 990 that's seen fairly light use & been sat in various sheds & garages for the past 10 years or so. I'm planning on stripping the parts for another build, but noticed some weird looking damage on the BB. I'm pretty sure it's not caused by any impact, but rather it just looks like it's crumbling away. I've also noticed some bubbling underneath the paint on a part of the downtube, akin to steel corrosion, but of course this is an aluminium (CU92) frame. I've not seen corrosion like this on aluminium before, but I wonder if it's a characteristic of the CU92 (aluminium/copper alloy) that Giant used (perhaps still do?). Any thoughts?

 

[Anthony]

That would be a bit odd as Alcoa claim that 6013 alloy is more resistant to corrosion than 6061 and promote its use in the aerospace industry for applications where corrosion resistance is particularly important.

I think any paint bubble creates suspicion that some chemical reaction is taking place underneath. If moisture is trapped between metal and paint, it can happen - particularly with powder coating where there is no primer. I would take away the bubble, clean and prime the exposed metal and touch in the paint.

 

[Skinwhistle]

Looks like the corrosion is on the weld seam?!? Best pick away the paint, wire brush loose corrosion then examine around the area! Hopefully it's just surface stuff and only cosmetic? I'd strip down the B/B too just to make sure moisture isn't trapped??

 

[pedalhead]

yup it's right at the weld. Cheers guys, I'll strip it down & have a closer look.

 

[Chute55uk]

my trek is alcoa and being 21 years old has no corrosion like yours has, another alcoa trek i stripped of paint also showed none of what yours is displaying, the difference of course is both the treks are bonded not welded so it might have something to do with the welds rather than the frame itself

 

[Anthony]

The filler rods used for the weld are a similar material to the tubes, or at least compatible, and you also have a bubble on the down tube, so this must be a reaction between the powder coat and the aluminium when moisture has been trapped for a long time. I feel sure it must be very localised though, not in any way structural.

 

[pedalhead]

Yikes, I think this frame is going up on the wall. A small amount of brushing, not even with a wire brush just a Park gear clean brush, resulted in this... :shock:

 

[Raging_Bulls]

I'd get in touch with a welder first. Chances are it's salvageable, but it'll probably need to be stripped and painted again.
Depending on the sentimental value, you might opt to repair it instead of replacing it.