Is this corrosion?

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rmerka
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Joined: Wed Mar 06, 2013 4:23 pm
Location: Austin, TX

by rmerka

Primer and paint? And open up that drain hole?

11.4
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Joined: Tue May 23, 2006 4:33 am

by 11.4

Hard to say what was happening. You don't have deep surface corrosion. Some of the problem might have simply been flux left on the metal before the frame was painted. Sooner or later, it accumulates moisture and bubbles up. But I would have expected more pitting if that were the source of it all. The color doesn't necessarily mean anything -- it could be the color of chain lube or god knows what else that has tinted it over the years.

The question might be whether you have corrosion internally. There's nothing to point specifically at that, except that one would like an explanation for the external corrosion. When there's a galvanic potential created in a situation like this, if the insulation is breached, the corrosion can occur elsewhere on the part because everything is charged and susceptible. The weld between the bottom bracket shell itself and the socket for the chain stay unit seems wonky -- that little ridge that erupted makes me wonder whether the right welding wire was used. Someone who builds bikes for a living needs to opine on that one.

I wish I could say something that ruled out any significant damage. If you want to paint over it, I'd get some anticorrosive paint specifically made for aluminum, I'd go back again and scrub off every last little bit of paint or flux or whatever until you have uninterrupted bright metal, and paint it well. I'd frankly leave just the aluminum paint on there for now -- it's not like everyone will be looking at the underside of your bottom bracket. Then you can see more clearly if anything develops again. I've seen lots of frames, both steel and aluminum, with lots of corrosion that needed to be treated but was basically harmless in its current state if treated. Yours might be the same. I was concerned about a carbon effect because that cable guide was a bit funky and clearly some post-assembly engineering, which is always an invitation to carbon-based corrosion. I'm still on the fence about it -- I sanded down one frame with a lot of it, and it wasn't like one could see corrosion growing out from between the carbon and the alloy, but even the factory agreed that the joint was failing. It might be worth it to send photos to their warranty service -- if the problem here was a manufacturing defect, then even if the warranty is officially expired they still have to consider legal liability in an accident. They ought to be helpful with a diagnosis.

by Weenie


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youngs_modulus
Posts: 668
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

Oh dear.

There is no flux on your frame because no flux is used to TIG-weld aluminum. Flus is used to braze steel frames.

Corrosion can occur elsewhere because everything is charged and susceptible? Thats not right. That's not even wrong.

To the OP: if the stays are secure in the bottom bracket lug, your frame is probably fine. Mask off the rest of the frame and then rattle-can it (spray paint it) with a couple of coats of black. There's no need to fret about finding "aluminum-specific anticorrosive paint," whatever that is. Anything that will keep water out will do.

There's no need to overthink this. Repaint the affected area and go enjoy your bike.

Cheers,

Jason

11.4
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Joined: Tue May 23, 2006 4:33 am

by 11.4

youngs_modulus wrote:Oh dear.

There is no flux on your frame because no flux is used to TIG-weld aluminum. Flus is used to braze steel frames.

Corrosion can occur elsewhere because everything is charged and susceptible? Thats not right. That's not even wrong.

To the OP: if the stays are secure in the bottom bracket lug, your frame is probably fine. Mask off the rest of the frame and then rattle-can it (spray paint it) with a couple of coats of black. There's no need to fret about finding "aluminum-specific anticorrosive paint," whatever that is. Anything that will keep water out will do.

There's no need to overthink this. Repaint the affected area and go enjoy your bike.

Cheers,

Jason



I'm always delighted to learn more on any subject. I learned what I knew from extended conversations with BMC technical support, and always appreciate corrections to what I had previously heard.

My father was a much wiser man than I, and brilliant. He once told me that knowledge made some people wise, made other people a$$holes. Sometimes, in how posters communicate on this forum, this might be some advice to be heeded.

youngs_modulus
Posts: 668
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

Cool story. I take it your dad never discussed name-calling or smug sanctimony with you.

In all seriousness, it wouldn't have killed me to be a bit more gentle.

The poor OP, though, was worried about whether his frame was broken. Because you had a few long talks with BMC, you felt equipped to not only tell him that it was probably corroded into oblivion but also that somehow it was welded with the wrong filler rod(?!). You asked for professional input, so I gave it.

I came down on you so heavily because I wanted to make it clear to the OP that he could rest easy and ignore your speculations. But I'd hate to think that my sarcasm dissuaded others from asking questions, regardless of their technical sophistication. Good questions are generally much more interesting than the right answer.

I'll try to be sweeter in the future.

OP: How's that frame? Are you feeling any better about repainting it and riding it?


Cheers,

Jason

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kbbpll
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by kbbpll

Somehow missed in this discussion is the part where after riding in the rain, Lightweenie has to take his seatpost out and turn the bike upside down. Really? And that FD cable guide looks like it's rusted from top to bottom. Still shifts ok?

youngs_modulus
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Location: Portland, OR USA

by youngs_modulus

That's a fair point. Lightweenie, what's the scoop?

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Lightweenie
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Joined: Sun Aug 03, 2014 1:00 pm

by Lightweenie

Thanks both 11.4 and young's_modulus for your answers!

The frame seems fine, the connection between chain stays and BB seems rock solid. I reassembled the frame without paint for now, and rode it two times on the indoor trainer and everything seems fine.

I am considering to try getting access to a drill and add a small drain hole before repainting that spot at some point soon.

@kbbpll: surprisingly it shifts perfectly fine. I tried to remove this piece of cable in order to replace it with a new one but I couldn't get it out so I reassembled everything as is. If there is some trick for removing/replacing this I'd be interested to know. Also, since I have not seen any other frame with such a piece of cable in the BB, I was wondering wether it was not meant to be this way, and the hole was thought of as both cable guide and drain hole somehow?

About this water issue, when I removed the BB there was a little bit of mud inside the shell. I suspect this is also related to the fact that my left BB piece was not very smooth in contrast to the right one. So I also replaced the BB when reassembling. Perhaps apart from opening a small drain hole I should try to locate where the water gets in, there don't appear to be any holes or anything like that (the frame has external cable routing).

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jekyll man
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Location: Pack filler

by jekyll man

it might be that your frame was originally meant to have a guide with a tube moulded in ala Cannondale, and was rebuilt after this got broken with a bit of homemade "engineering" to stop the cable wearing through the frame.

As for water ingress, it can get in anywhere. Just drill a 2mm hole in the bottom of the bb shell where its not covered by the cable guide. So long as it can get out again.
(or just take the seat post out occasionally, and tip the bike up for 10 mins)...
Official cafe stop tester

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Lightweenie
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Joined: Sun Aug 03, 2014 1:00 pm

by Lightweenie

I just wanted to follow up on this. So I rode the frame for almost two more years, until a couple of months when I bought myself a new carbon frame and moved all components to the new frame.

My plan was to get some 105 groupset or something similar on the older frame and use it as a winter bike and commuter. So today I tried to work a bit on it, managed to remove the outer cable, and noticed that there was a sharp edge bordering the weld as if some material was missing :
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So I took a small screwdriver and it was indeed quite soft, I was able to make a small hole in the metal very easily:

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My guess is that the frame is not really ride-able right now, and thank god nothing happened while riding. It is a pity that I will probably have to reconsider my winter bike/commuter plans, because I really liked this frame. Any ideas if I can still do something with it? It is quite nice, reasonably light at 1200gr and has a 315gr (cut) fork. Otherwise I guess I'll have to just dispose of it.

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Frankie - B
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by Frankie - B

this was a good story to read and learn from. I think that the frame you have there is sadly KIA. I'm sorry.
'Tape was made to wrap your GF's gifts, NOT hold a freakin tire on.'
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