Should I worry?

ajoten

So I'm getting this blistering thing going on where my carbon seat stays go into the aluminium frame.



Is this cosmetic or a cause for concern?

nevica

Hello,

On the metal side blistering is usually a sign of rust. On the carbon side it looks like the lacquer coating of the carbon fibre is peeling. If you are really concerned why don't you try and take it into a bike shop which knows about carbon fibre?

ajoten

Why? Because it's Sunday so the shops aren't open, and in this on-demand age the forum might be able to provide an answer immediately.

g00se

The blistering is Aluminium corrosion, not rust (iron/steel oxide). Normally, alu oxide corrosion is nothing to worry about as it's air-tight and only forms on the surface (think grey scaffolding poles). However, bubbling like that is a sign of salt corrosion, where road-salt water soaks through the powdery alu corrosion and takes the corrosion deeper. I guess you've had some salty crud stuck around that ledge between the two and it's started off there.

I've had some on the alu crown of a carbon fork and all the bike shops said it was risky to ride and couldn't recommend it. The corrosion was near the bonding between the carbon and the crown and there was a risk it would have compromised it. I ended up replacing the fork to be safe.

I guess there is a similar risk to yours. However, the bonding surface between the two bits will be large and there's only a tiny bit of corrosion.

See what the shop says - but I would think there's an argument to scrape the paint off of the metal, dig into the powdery oxide with a compass point and see how far it goes. Then clean out all the corrosion to shiny metal and paint/seal it up.

Monty Dog

Whilst there appears to be some corrosion under the paint on the alloy, I don't think is relate to the problem with the lacquer. There is probably some micro-movement of the joint between the alloy and carbon, lacquer is very brittle, it may have been applied a little too thick and over time it's simply cracked and peeled. If there was sign of a white powder residue (aluminium oxide) and there was creaking, then there would be cause for concern.
I would rub off the flaking lacquer (wet and dry) and apply some lacquer or even clear nail varnish to seal the surface and keep an eye on it.

maringirl

If cf is going to go it is always around where it bonds to metal where the water and salt can get in. This is why a lot of folk don't ride cf bikes in winter! It needs an expert eye cast over it - its your safety that matters.

ajoten

Hmm. Thanks everyone. There has been creaking (I thought bottom bracket), will take it in today.

ajoten

@MontyDog... white powder residue? See pic:

ajoten

I'm sure noone is particularly worried about my mither, but fyi...

LBS #1 = dunno, we'd have to send it up north to our carbon fibre specialist.
LBS #2 = there's no movement and you could risk it, we wouldn't, get a new frame or bike. Oh and your wheels need a service.
LBS #3 = there's no give in the CF, it's purely cosmetic, get some nail varnish on it.

racingcondor

I've known of couple of Alu/Carbon bikes that seperated at the join between the materials (usually at the bottom bracket) and the first sign is usually creaking.

I wouldn't worry about the laquer coming off the carbon but that oxidisation could be a sign that the frame is on its way out so I'd take it to a good shop and get it checked out. It's not a problem it the oxidsation is just external but as you can imagine if its internal it can seperate the bond between the tubes.

Personally I'm with LBS #2. There are a few things on a bike that you don't want to fail. Anything that holds the wheels or handlebars on tops the list.

ajoten

LBS #4 = nah, bond still good, just flaky lacquer.
LBS #5 = looks like water's got in, replace frame.

[i.e. it's all very well saying "a good shop"... I no longer know what that means, given the huge variation in opinion!]

Monty Dog

If there is white, powdery residue, have you tried cleaning it off? As above, I would simply clean it off and apply a suitable sealant - there's little you can do otherwise, but you'll likely extend the life of the frame.
There doesn't appear to be any evidence of any compromise to the structural integrity of the joint so plenty of life left in the frame - the failure mode due to galvanic corrosion is a build-up of the aluminium axide which results in cracking / splitting of the tubes. Keep an eye on the joint, listen for any creaking and provided it stays that way don't worry about it.

desweller

As it's on the seat stay it's not a massive problem even if it does fail; it will just push up towards the seatpost. Unlike a chainstay failure, where the rear triangle will come properly adrift...