All galvanic corrosion needs to occur is electrical contact between dissimilar metals. Meaning they have to touch. I've had aluminum that touched a carbon frame corrode to powder. Doesn't need to be sweat. Just damp sea air can do it.

I've had a welded quill stem break in half at the weld. I happened to be crossing a bad RR track just after an all-out descent down a windy road, doing 40 or so. The stem hung together by a tiny bit of weld, so I didn't go down and even managed to finish the ride. I sent to the stem back to the manufacturer. They offered to replace it. Uh, no.

Aluminum doesn't fracture in layers like this. See the red circles. The break should be more clean if it was overtightening. The green circles show signs of corrosion on the unbroken areas of the stem.

I should mention that dissimilar metal corrosion usually occurs on bikes which have been ridden on rollers or trainer a good bit.

Galvanic corrosion between alloy and carbon is a common problem for seatposts.It's one of the reasons carbon assembly paste exists. It also happens on bars (as shown above) and I've seen it on stems, too.

OP, what is the stack limit on that fork. You have a lot. Regardless of whether this contributed to the issue, I would strongly suggest going with the higher rise stem and fewer spacers also as shown above.

The stem failed along where I would expect to see a riser from welding on the tabs. Without the stem in my hands I would guess that it it fatigue from over torquing the stem bolts plus the added stresses of supporting more of the rider's weight due to the use of arrow bars with the large number of spacers being a slight but additional factor.

Since you need to replace the stem regardless, it would be prudent to discuss addressing this with your mechanic.

It can if it was only cracked at the top and the final part failed with a downward force (rider). The hard anno on the surface of the stem can make the last section peel like that.

There does look like corrosion as well. There could be more than one factor involved in the failure.

I don't think its annodized, they are carbon wrapped aluminum stems

So what you're saying is...the carbon asploded, like carbon is wont to do, and took the aluminum out with it?

I'm with Bob Dopolina- bottom line, more than one substantial factor leading to the failure. While sweat/sea air/etc make sense chemically and on paper, looking at what's going on inside the stem in the second photo makes me wonder exactly how much of either would need to find its way into the stem to cause such a problem. I'd also be curious to hear what Profile has to say about the failure, because if anyone knows about failure models for the stem...it's them.

There are many valid points made by both sides though this one strikes me as most accurate. To support this point, force onto the stem would be much heavier at the top compared to the bottom where the only single flake occurs (read: Carbonfiberboy's account), which also could be little more than paint layers with an ultra thin layer of AL attached.

Simple question for the corrosion support, has it been documented what life to expect before said corrosion? Be it in the saltiest, most humid seaside climates with the rider who rides/trains hard as often as possible? Is two years realistic expectancy?

I can't see that in the photos.

That would explain the peeling then. It is a piece where the bond failed before the carbon and it tore away from the stem.

Don't forget this is a tri setup. The force from rider weight is different than with road bars. Combine that with the large number of spacers and high setup and that might be all to cause it to snap.

I had wondered about this. Heavy rider, forwardish position, some corrosion, hitting a pothole due to lack of handling skills (tri guy after all), and crack it goes. Good thing this won't affect how poorly he climbs or how ridiculous his sleeveless jersey looks.

Which tri jokes did I miss?

Thanks all for your replies. I did wrote to Profile Design to hear their thoughts, but haven't received an answer yet.

The sweat issue was brought because we did saw some corrosion on the stem, as you can seen on the pictures below, and the paint did peel off very easily. I do sweat a lot and I do use the trainer once every two weeks, for a 2 hour session, so it is exposed to a lot of sweat. However, I'm sure this should not have happened, the stem should handle sweat without issues, that's why this is so puzzling to me and think that maybe the stem was defective.

Regarding weight, yes, I am a bit heavy, as "therhodeo" pointed out (tri guy lol), I weight 185 pounds. The amount of stacks is because that's how the bike was setup during the bike fitting.

I guess as some of you say, this could have been the result of various factors, but just the thought of this happening downhill scares the #$&% out of me. Don't know if the mix of the stem being AL with a Carbon wrap caused this, so I guess I'll stay with an AL only stem.

I got a new Bontrager AL stem, and I guess from now on I will check the stem more regularly and change it once every year, since I wouldn't want this to happen again.

Here are the additional photos:

First off...let me assure you that unless you're 5' 3"...185 isn't "heavy". Don't be so hard on yourself.

Second...that indeed does look like serious corrosion and to counter what you said...massive amounts of sweat will corrode aluminum.

Is this the stem here?:

https://www.amazon.com/Profile-Design.../dp/B005FEP11Y

If so the "carbon wrapped aluminum" things seems gimmicky to me...I wouldn't run one. Second...the stem looks like it corroded from the inside out...sweat got between the stem and fork and literally ate it form the inside out.

Like the others said...you're luck you're not seriously injured and I'm glad to hear you're ok. I'd thank my lucky stars, get a new stem and periodically check it for corrosion if you sweat a ton like that.

What is the advantage of a carbon wrapped stem over an aluminum stem or even just a carbon stem?

pulls chicks.

I am 5'9

And yes, the link that you posted was the stem that I was using.

As you said, I am really thankful that this happened at a very low speed, just testing the bike before the triathlon. So yes, I will test more often the stem, just to be sure that no signs of corrosion arise. TY !

In my case, no advantage at all !! In fact, maybe the carbon wrapping was the one responsible of hiding the AL corrosion, grrrrrr .

Do you (anyone) really think that carbon-wrapped aluminum stems are dangerous, or just speculating? I've had this one JRFOTO Carbon Fiber Stem Carbon Fiber Handle Bar Stem (a cheaper variety) for 8-9 months and now you're making me nervous.

ps, re: the advantage, it looks sharp! Same as carbon anything, more or less.

The description says that it can be used for school and work, so how bad can it possibly be?

In all seriousness, the pictures look like a pot metal stem covered with poorly adhered simulated carbon. I personally wouldn't trust it.

No. The takeaway here is to limit sweat from accumulating on any parts, i don't think anodizing/painting would have saved this part either.

None that I can see...it's gimmicky. If you want a carbon stem...pony up the $$ and get one. I really do not so I went with Alu Ritchey.

I agree 100%...the thing just seems like a poor design. Again, I'm glad all this cost you was a stem.

I can believe the sweat theory. I have had Trek aluminum frame replaced due to sweat damage, and that was without any carbon contact. If sweat got between the stem and carbon steerer tube, I have no doubt it could result in a failure.

After what just happened to me, I wouldn't trust it. I would go either full AL or full Carbon. After reading some posts, think that AL is safer for such an important part on the bike, but I could be wrong. I'm just too psyched right now

Well that's absolutely terrifying. Glad the OP wasn't hurt too badly. Good luck finding a suitable and hopefully sturdier stem replacement.

Yes, that's totally galvanic corrosion. Looks exactly like the brake stops on my carbon frame before replacement. And thinking about it, I think the welded AL stem that failed me was also Profile Design. Forged AL for me. I got rid of the quill stem on that bike with an adapter.

Another thing to keep in mind is the rider's preride test. If I'm running aerobars, I sit on the top tube and reef on those suckers. I've popped stem bolts doing that. You can lock the front brake and work the drops, too.