|Galvanic corrosion on "Frankenbikes"||brider|
Oct 23, 2002 7:35 AM
|There's been a discussion brewing on the Slowtwitch site in regards to the potential of galvanic corrosion in the carbon rear stays bikes that seem to be the latest marketing rage. Any one remember the old Treks and Specialized Allez Epics? Carbon tubes bonded to aluminum lugs. Failures abounded. Carbon reacts with aluminum to set up a low-grade battery (of sorts), which erodes the bond between them. How many of you think the bike comanies have a handle on this? Are we going to see another rash of bond failures due to the carbon rear ends? Comments?|
|That's actually a good point||joekm|
Oct 23, 2002 8:11 AM
|To get the galvanic action you speak of, you need an electrolyte solution. As it happens, sweat, rainwater, Gatorade, or just about any other common moisture source will do just fine. Therefore, such construction must isolate moisture from the joints - and do it in a permanent way. There was a monster truck of an airplane made back in the 60's called the Helio Courier. It's structure was a cromoly space frame mated to an aluminum semi-monoquot shell. The designers solved the galvanic corrosion problem by spraying molten aluminum on the cromoly space frame. This allowed them to mate the two structures while isolating them from moisture. |
Anyway, I think you bring up a good point. I'd be curious to know how they are addressing this myself.
|The sins of their fathers-||filtersweep|
Oct 23, 2002 8:33 AM
|Don't Treks's already have internal aluminum lugs on their ubiquitous OCLV frames? Doesn't Look STILL make externally lugged (AL) carbon frames?
I would like to think that this issue has been addressed. How many carbon forks have aluminum dropouts? Or an aluminum steerer. This issue is already present on a bunch of non-Frankenstein bikes- I'm "assuming" that manufacturers have learned how to deal with the issue.
Oct 23, 2002 8:41 AM
|Gerard of Cervelo is pretty active on the Slowtwitch board, and he's made some statements regarding the isolation of carbon from aluminum through the use of fibergalss. Problem is, the rear triangles require some EXTREMELY tight tolerances, and he doubts that many of these comanies are well versed in isolating carbon from aluminum. |
And to address the OCLV issue -- actually, the lugs that Trek uses are carbon as well.
|I've been following the same discussion....||sn69|
Oct 23, 2002 10:51 AM
|I don't know if it's still posted to his site, but Craig Calfee used to discuss the issue at length. Likewise, Carl Strong has added some comments recently at the Strong Forum (www.hydromedia.com), and he's promised to explain more about the issues after he gets his factory moved. I would be curious to hear what Cyfac and Colnago do to offset the potential problem.
One of the tubesets that Gerard has been talking about is Columbus' new XLR8R Carbon, which bonds aluminum lugs with carbon tubes. Check out the Cyfac site for a picture of their prototype. I wonder what Columbus might have to say about the issue.
|Columbus Tubing's US Rep:||sn69|
Oct 23, 2002 11:01 AM
|KOEHN PROJECT ENGINEERING
Mr. Norbert Koehn
Perhaps Mr. Koehn can shed some light on the issue.
Oct 23, 2002 8:45 AM
|It was a problem on the early trek bikes but I think that it has been taken care of. One method I know some companies use is to insulate the carbon from the aluminum with a layer of fiberglass. There are other methods out there too, like using titanium in the joints. Basically I think that the problem was recognized on those early bikes and has been dealt with. It does make you wonder about all those cheap forks with aluminum steerers that are pouring out of taiwan though.|
|Titanium and Carbon....||joekm|
Oct 23, 2002 9:12 AM
|I believe that both of these are in the same category wrt galvanic corrosion and both are highly resistant to it anyways. In other words, there should be no problems mating carbon to titanium. However, a metal such as 6061 aluminum is classified a couple of categories away from both titanium and carbon. Therefore, you still would have to deal with the possibility of galvanic action between the titanium and the aluminum. End result is that you still have to isolate moisture (or gas) from the joint.|
|Can't really mix materials without long term problems||LC|
Oct 23, 2002 10:12 AM
|It seems fashionable to mix a variety of materials lately, but I just keep thinking this is a bad idea. These aluminum/carbon stay frames are pretty new, so we will just have to wait a little and see the results but I suspect there will be problems. There certainly has been a rash of carbon seatposts in aluminum frame problems lately. There are many little things like BB shells, headsets and drop outs of differing materials that should be concidered. How many little batteries have you got on your bike?
Aluminum/Carbon = bad
Aluminum/Titanium = bad
Aluminum/steel = bad
Titanium/carbon = ok
Titanium/steel = bad
Magnesium/anything = really bad
|Titanium and Stainless Steel are an OK to mix....||joekm|
Oct 23, 2002 11:11 AM
|Chromium and nickle content steels, while not as corrosion resistant as titanium, are in the same category wrt galvanic corrosion.|
|oh heck with it, here's how the metals rank.....||joekm|
Oct 23, 2002 11:22 AM
|From the standpoint of repairs to aircraft grade materials with respect galvanic corrosion, metals are typically put into four categories arranged from most likely to corrode to least likely. They are: |
I: Magnesium and it's alloys
II: Aluminum alloys (often further broken into two sub-groups, but I won't get into that here), along with Cadmium and Zinc.
III: Iron, Lead, Tin and their alloys (except stainless steel)
IV: Stainless Steels, Titanium, Chromium, Nickle, Copper,and their alloys along with graphite (i.e. carbon)
The further apart the categories of any two metals, the greater the potential for galvanic corrosion between them.
|re: Galvanic corrosion on "Frankenbikes"||terry b|
Oct 23, 2002 12:44 PM
|I think people are confusing "carbon fiber" with "carbon." There is a pretty big difference, since carbon fiber bike parts are (usually) a passivated material (elemental forms sitting in an inert medium) and are also usually coated with some sort of scratch protection. The carbon stays that Columbus produces are not raw - they have a pretty thick clear coat and should therefore be resistant to forming a chemical bond with aluminum. This is not to mention that you'd have to either sweat enough to force sodium into the joint via capillary action, or perhaps soak your bike in the ocean since the reaction cannot take place spontaneously. All that assumes that aluminum can react with whatever material is holding the CF in matrix.
I guess if we really want to worry about it we should pull our carbon steerers out of our King headsets and see if the top ring is permanently bonded. And we'd better violate the MFGRs warning about greasing carbon seatposts.
|Bing-bing-bing! We have a correct answer! -Aplause-||Ahimsa|
Oct 23, 2002 3:04 PM
|Excellent work Terry B.!
Yep, last time I checked, my carbon fibre seatpost was a carbon weave buried in a relatively thick layer of non-reactive plastic resin.
I suppose I could sand the resin off until I get to the bare carbon threads...maybe the resulting "battery" would power my headlamp, no?
Or perhaps I could just buy a glass frame and rub my wool jersey on it?
Heh heh heh!
|You are correct, however......||joekm|
Oct 24, 2002 4:45 AM
|You are absolutely correct in that, if the carbon itself is isolated from a moisture or gas electolyte then you eliminate the potential for galvanic corrosion. However, it does not take much electolyte for the action to begin and I think the issue here is wear of the resin layer over time. Additionally, there are structural issues. For example, say you've got carbon seat stays going into an aluminum monostay. Now, I don't know the answer here, that would require a frame building expert but, would the additional resin required to protect the interface (and there will be a step-change in stiffness at that point creating a stress concentration) comprimise the intended behavior of the structure? |
I'm not saying that it can't be done. I'm just saying that, even considering the resin matrix of the carbon fibre material, it's still a valid point because of the potential for wear at the interface over time. Personally, I'd be more concerned about that cosmetic "course weave" layer that shows on a lot of carbon fibre components then the actual structural layup.
While were on the subject, I've also wondered if adding vertical compliance to an aluminum frame by using carbon seat stays creates a fatigue issue at the chainstays and bottom bracket.
...any frame building experts care to comment?
|late but pertinent||lonefrontranger|
Oct 24, 2002 4:56 AM
|Trek and Specialized had issues with their old carbon/al lugged frames true. I believe this was reported to be issues with the bonding agent however, NOT the materials used.
For what it's worth, Giant got it right even back in the early 90's. I had a Cadex lugged CF / AL frame, and the guy I sold it to still races it. I also know of 3 or 4 Giant Cadex frames in the Front Range area that are still being raced or used for ultra events. Each of these frames is over 10 years old.
Boy the Cadex tubes sure did break when used for MTBs though. This, however, was tube failure (from being bashed into trees and rocks), not joint failure.
I have seen an early ('96 or 97) Trek 5200 OCLV that pulled apart from its CF lugs at the BB shell. Does this mean Trek doesn't know what they are doing? Nah, probably just a bad QC day.