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Galvanic action anyone?


Johan Bornman
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A sick Cervelo was brought into my workshop last week.

 

The threaded insert for screwing in the BB was coming out. These things are make of aluminium and glued in place. This one's glue came undone and the BB was now moving around inside the frame. It is the left side.

 

Easy-peasy. Pull the crank, pull the loose shell, mix some resin, glue it back, fit the BB and go ride.

 

 

But not so fast. Look what I found inside once I removed everything.

 

20100601_110357_Cervelo_BB.JPG

 

This view is through the left BB aperture. The carbon tube visible by my finger is the downtube.

 

I discovered that the insert that came loose, is partially inserted inside another alu tube and glued in place too. Imagine a short carbon tube (the BB shell), and inside that is an alu tube as wide as the BB shell. Two threaded, flanged cups are then inserted from each end and also glued in place.

 

The jagged edge in the photo is the corroded outer tube.

 

The cup itself doesn't look as bad but is pretty pitted too.

 

20100601_110749_Cervelo_Cup.JPG

 

Here's the cup. It is threaded on the inside and the BB screws directly into that. Note the deep erosion on the right hand side. The corroded ridge is about 1mm deep - halfway through the cup.

 

Like all modern bikes' BBs, there was water inside. I wish they would just drill drainage holes in the shells at the factory, but they don't.

 

Question: What caused this?

 

 

 

 
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Arent carbon frames made by winding fiber over a mould (polystyrene)? What they use to remove the ineer mould could be what caused this...

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There had to be some heavy chemical reaction in there to cause

 

a) The glue coming loose in the first place

b) The corossion never seen anything like that before.

 

Let me do some homework and speak to my friends studying chemistry maybe they have a theory
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Nope, there is actually a chemical reaction between the carbon fiber and the Al.

 

I remember wanting to some carbon work about two years ago. And in my quest of becoming a expert on the matter (a few google searches later) I found numerous articles - even some nasa ones if I can recall where they speak about the problems of having carbon fiber up against a metal surface.

 

Plenty of info on the web :

 

http://www.google.co.za/#hl=en&source=hp&q=carbon+fibre+aluminium+corrosion&aq=f&aqi=&aql=&oq=&gs_rfai=&fp=f242465d82a80e9c

 

 

 

I think you are going to have to coat the metal surface with something first - I remember some people used polyester resin to coat the metal. Then you use the normal epoxy for the fiber sheets.

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Ja, carbon fibre and aluminium (and many other alloys) react and cause corrosion. To prevent this, the aluminium has to be electrically insulted from the carbon fibre, for example by using a layer of fibre glass or a gel coat. This corrosion is exacerbated by the presence of moisture, which as JB said is present in most BBs.

 

 

 

However, the bonding layer used to glue the aluminium tube should have prevented the contact in the first place. Haven't a clue as to what happened to the glue. Maybe it wasn't uniform, and the galvanic corrosion started at a blemish in the bond? Turbobok2010-06-03 05:16:14

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However' date=' the bonding layer used to glue the aluminium tube should have prevented the contact in the first place. Haven't a clue as to what happened to the glue. Maybe it wasn't uniform, and the galvanic corrosion started at a blemish in the bond? [/quote']

How big is the electrode potential of the aluminium-carbon system? Presumably, if there's enough water with enough dissolved salts in it, the aluminium and carbon don't actually need to be in direct contact. It's possible they only put the bonding material on the areas where carbon contacts aluminium and none on the other parts of the tube, such as the middle part.

 

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Aluminium is actually quite a thermodynamically unstable metal, and has quite a high potential for corrosion (for a metal, second only to Lithium, but I speak under correction there). Therefore any moisture would provide the necessary means for the corrosion to start.

 

 

 

Good point about the possible lack of glue in the middle! I wonder though, Cervelo would surely know this? Was this just a once off mistake - and badly bonded? I wonder what their protection against this is - just the resin?

 

 

 

I read somewhere a while back that if the aluminium surface area is larger than that of the carbon, corrosion won't take place. How much larger is has to be and why this is - I don't know, just an interesting aside.

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