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imatfaal

Corrosion (?) on an Carbon/Alu Fork

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I was servicing my commuter bike and noticed a little (about 2mmx3mm) blister on the once smooth surface of my front fork. I have photographed it and added it to the post here

post-32514-0-84738200-1396017437_thumb.jpg

 

The fork was sold as carbon-alu which normally means a carbon blades attached to a aluminium crown and steerer tube (ie other end from photo). The dropouts (where the wheel goes) and the disc brake connection seem also to be metallic.

 

The little blister broke as soon as I investigated and was full of white powder (some still remains around edges of photo) - it looks as if others may be forming higher up the photo.

 

What is going on? Is this aluminium corrosion of some sort? Any ideas how to stop it?

 

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Paint?

 

I don't think so - there is a distinct crater with rough edges/surface. The blister was raised - but underneath the white gunk is a pitted dip

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Did you see this bit from the Boeing link

About carbonfibre plus aluminium?

 

Fiber-reinforced plastics are corrosion resistant, but plastics reinforced with carbon fibers can induce galvanic corrosion in attached aluminum structure.

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Most aluminium alloys are weak against corrosion, including under simple rain - and road conditions can be worse, salt above all. White powder is a known result of aluminium corrosion.

 

Especially the harder aluminium alloys in the 7000 and 2000 series corrode quickly and fail earlier than banal steel. I had put a AA7049A rod in the soil; one day later, it lost wide plates from its surface ("exfoliating corrosion"), totalling half a millimeter thickness. Steel hulls in contrast last for years in sea water.

 

A fews alloys resist corrosion: the 1000, 3000 and 5000 series, but are very soft, and some alloys among the 6000 series, which aren't famously strong.

 

In very limited tests, I saw no effect of galvanic couples hence am not convinced up to now. I expect carbon to do very little to aluminium - but the polymer matrix contains acids, phenols, amines and more, including chlorine from epoxy polymerization, which are better candidates to promote corrosion.

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Did you see this bit from the Boeing link

About carbonfibre plus aluminium?

 

Yes thanks - I must admit I was unconvinced, not on the engineering but whether it was what had happened to mine as it was so reminiscent of iron rust blisters

Most aluminium alloys are weak against corrosion, including under simple rain - and road conditions can be worse, salt above all. White powder is a known result of aluminium corrosion.

 

Especially the harder aluminium alloys in the 7000 and 2000 series corrode quickly and fail earlier than banal steel. I had put a AA7049A rod in the soil; one day later, it lost wide plates from its surface ("exfoliating corrosion"), totalling half a millimeter thickness. Steel hulls in contrast last for years in sea water.

 

A fews alloys resist corrosion: the 1000, 3000 and 5000 series, but are very soft, and some alloys among the 6000 series, which aren't famously strong.

 

In very limited tests, I saw no effect of galvanic couples hence am not convinced up to now. I expect carbon to do very little to aluminium - but the polymer matrix contains acids, phenols, amines and more, including chlorine from epoxy polymerization, which are better candidates to promote corrosion.

 

My frame is 6015 aluminium but I am not sure about the Al of the Carbon-Al forks. The fact that you mention the white powder - which was so obvious to me on inspection - being a known result of Al corrosion bolsters my feeling that salty-water got beneath the enamel/paint and caused corrosion.

 

Any ideas on prevention - good clean out to remove any salty residue and coat of automobile touch up paint?

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AA6015 should be excellent against corrosion, with only soft aluminium alloys being better. The other... it all depends on which one it is. At AA2024 ou AA7075, you've lost.

 

Corrosion protection: remove salt is a good idea, because chlorine ions are the one enemy of aluminium. Paint isn't very efficient on aluminium, and may promote corrosion if the composition doesn't fit. The proper help is anodization; again, it's excellent on AA6015, fair but useful on AA7xxx series, not so efficient on AA2xxx.

 

Anodization is a cheap operation, but usually done in an industrial context, so it all depends on your diplomatic skills. Normally, the part is fully stripped of any coating before. On AA6015 the result will be clear if desired, on AA7xxx it would be dark grey.

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The little blister broke as soon as I investigated and was full of white powder (some still remains around edges of photo) - it looks as if others may be forming higher up the photo.

 

What is going on? Is this aluminium corrosion of some sort? Any ideas how to stop it?

 

Does white powder is similar to this?

 

post-100882-0-53309400-1396857203_thumb.jpg

 

These are mine aluminum electrodes used in electrolysis of water with high voltage, high current, for a few minutes. After ~1 hour they would be totally wiped out.

 

Before experiment they looked like this plate and were rectangles (not round edges, it's result of experiment):

post-100882-0-91053600-1396857445_thumb.jpg

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