I bought a '99 Trek 5200 new in January 2000. I have about 50,000 miles on it and it's in fine shape, except for one very frustrating thing. My frame has two aluminum cable stops for the rear brake, epoxied to the top tube. No fastenings. These cable stops corrode to powder in 3-4 years. This is not good, to say the least. I understand the mechanism: corrosion of dissimilar materials (carbon and aluminum), far apart in the galvanic series. For them to corrode, they need to be in electrical contact, and subjected to an electrolyte (sweat, dirty water, or just high humidity).

There is another possible mechanism for the corrosion: the cable housings and cable ends interacting with the aluminum. I suppose that's possible, but I don't use anything unusual for cable housings or ends. Right now, my cable housings need replacing and they're corroded into the stops so I can't remove them. I can see that if I pull hard on them at all, the rotten aluminum will crumble.

Anyway, they corrode, and take the surrounding paint with them, now much of the top tube paint is gone. Trek has warrantied the frame twice and it needs it again. I'm without the bike for 2-3 months, and they make me take it to a dealer for disassembly and reassembly or they won't warranty it. And this costs money. They send the frame away, Trek replaces the cable stops, repaints, and returns it. In less than a year's time, I can see the stops begin to corrode again. Trek claims that my frame is the only frame out there that this is happening to. I find that hard to believe. I'm not a particularly heavy sweater, and usually ride fast enough that the sweat does not drip vertically off my nose onto the top tube. My area does not salt the roads, and I only ride this bike in the rain during occasional brevets or if caught out.

So I'd like a little advice before I send the thing off again for another failed "repair."

I have a friend who bought a 5200 back in '92 and has had two new ones under the warantee. On the last one we stripped off the components and the bike shop sent it off for the exchange. It does take a while, but it is a new frame and fork.
A bike shop doesn't need to take it apart for you.

For some reason, Trek won't replace, just repair and repaint, at least so far. Gregg's at Greenlake in Seattle told me they had to R&R the components. Only replacement now would be a Madone.

I think Gregg's sees you as a source of repeat income. You should be able to take the stripped frame to them for shipment to Trek and retrieve it in that form for rebuilding if you can do it yourself.

When you get it back again, I'd put a heavy coat of paint over the cable guides as soon as you get it. You need to protect them better than the factory paint job. And, yes, there are people who's sweat is particularly corrosive. I don't know what it is or why but they do exist.

Good idea. The factory doesn't paint them at all. I can dab on some epoxy paint, and should bed the cable guides in grease, too.

I misspoke myself in my previous answer to the comment, as we used to say back in the day. I remember the stops as being bare aluminum because the paint falls off them after only a couple of months. They corrode from the inside because it's electrochemical corrosion, not surface corrosion. Thus adding more paint won't make any difference. The Trek paint film is in fine shape when it falls off.

Electrochemical corrosion needs a complete conducting path. Obviously the factory paint has a missing area somewhere since, if the cable guides were completely encapsulated in paint, they would never corrode.

Does that explain why the paint on my 00 5200 is flaking off in big pieces? It started at the ends of the chain stays and cable and bottle bosses. Now it's so you can just chip big flakes off with your thumbnail. I need to take this thing in to the shop and see if they can get Trek to repaint it.

Any recent experiences with this exact issue? I've got an '02 5200 with a front cable stay that appears to be on it's last leg. I'm just wondering what Trek will do...

If you bought the bike new and have proof Trek should replace when it breaks. I like in trek country and have many friends get replacement frames from them over the last 40 years. If it is used you are SOL. Roger

do you actually have to wait for the brake cable to let go before they will replace/repair it?