Sunday, March 28

Carbon Corrosion

Corroded Statue of Liberty - from Wikipedia
Y'all know what corrosion is. In regular steel, it's referred to as "rust." Aluminum tends to turn to a whitish powder. Zinc is used as an anode to keep boat components from corroding.

Corrosion is not necessarily a BAD thing. It is galvanic corrosion that makes a battery work. Still, you do not want to try to use your expensive bike components to power your headlight.

Enough of that, what about corrosion and composites, you might ask? I'd respond it is a reasonable question, given the title of this post. Well, there's good news and bad news. The first, is that your new carbon bike will NOT corrode away any time soon. In point of fact, it is THE most "noble" material used in bicycle construction. It is even more stable than gold. You see, materials are susceptible to galvanic corrosion in the presence of a conductor and a more noble material. If you look at the figure below, you'll see that carbon is at the very top (I did not include materials such as gold which are only slightly worse than carbon). The "steel is real" crowd will be relieved to see that their bike material of choice IS, in fact, better than aluminium. Bringing up the bottom are things like zinc and magnesium. As I recall, a couple of years ago, Colnago built a bike with a Magnesium frame. Just because you CAN do something does NOT mean it is a good idea.

The Further Down the Scale, the Easier Carbon Will Make it Rot!
The BAD news is that your carbon won't rot away in front of your eyes, but those expensive aluminum and magnesium components ARE at risk. If, for example, you have an aluminum frame and a carbon seat post, for example, the potential exists for corrosion of the frame due to that contact. You have risk for corrosion to those cool dropouts that hold your wheel to your carbon fork.

Lest you think this is overdramatic, back when the F/A-18 fighter went into service, one went into the drink. They fished it out a couple of weeks later and the aluminum was basically GONE.

There are a couple of approaches to avoid trouble. First, avoid "intimate" contact between the carbon and metal that is more reactive than stainless steel. A rubber pad would suffice, or a swatch of fiberglass. Undamaged (no scratches or rock chips) paint and primer. Even WAX. Yay, WAX! Just keep the two apart so they can't pretend to be a battery cell. Hopefully, your bike manufacturer took this precaution in things like gluing the fork ends into the carbon. If not, you're still left with the second strategy.

The second strategy is to KEEP THINGS CLEAN AND DRY. Dirt and moisture in combination really get the galvanic action going. If it's dry, there is no medium for the electrons to zip around in. On the other hand, if you leave it out in a damp garage next to the pool chlorine tablets, you will have interesting things happening before you know it. Especially, if you have a cyclocross bike such as I do, do not wait a week before cleaning after a race.

Anyway, I don't think I have to tell you how to look for corrosion in the metal. You'll detect IT pretty easily. Clean it away until you get down to sound metal. Reprime and repaint. When you are out of sound metal, it's time for a visit to your LBS. Remember, keep that puppy clean and dry - and ESPECIALLY watch that carbon seatpost in that aluminum frame! Ham should be happy with THIS post. His carbon frame may outlast his grandchildren's grandchildren if he's lucky. They'll just have to replace the metal bits as they rot away.

I don't know if y'all can take a log of comfort in it, but I've now had Buddy for over a year and there is still sound aluminum in the frame, though I haven't attempted to remove the seatpost recently. Hmm...

A Carbon Seatpost In an Aluminum Frame is Not the Best Galvanic Combination
The Ti Rails are Not a Likely Problem


Ham said...

So let me get this right - carbon frame + carbon seatpost = good. But there is a clamp on top of the seatpost and, the dropouts at the end of the fork and seat stays..... [gulp]

Still, I have to say I've not heard of that being a problem, so I won't start to worry too much.

Learning that carbon/alu had galvanic action amazed me - I would have thought the resin acted as an insulator.

Steve A said...

Just keep it clean and dry. You weren't really planning to leave that expensive carbon bike out in the rain for the winter anyway. It would not have time for the metal bits to corrode because the thieves would have it in THEIR clean and dry locale.

As for the fork ends, titanium or good stainless would be pretty resistant to rot. Aluminum would also work fine if galvanically isolated by the manufacturer as part of the bonding process. In truth, if the adhesive had a carrier, THAT would probably be enough to isolate the two materials unless you decided to dunk the fork in a tank of salt water for a current generation test. I do NOT think that Trek would warranty such a test.

You can actually try this as an experiment and generate your own carbon blog post that I will read with interest. Fill a small bowl with salt water. Drop an aluminum foil piece in and watch it over the next couple of weeks. Repeat the process with a hunk of carbon in there in close proximity to the aluminum. Report the results.

Big Oak said...

Huh. I was feeling pretty good about my Tricross until now. Guess I better clean it off more regularly.

Steve A said...

Big Oak, the photo at the link below might have been a corrosion-related aluminum failure. It's tough to really tell from the picture, but it does look sorta powdery white in it. Maybe he stored his Tricross out in the garage...

My own Tricross is more than a year old and I have not used it gently. While I only did one cross race, I have put well over 4000 miles on it, so it doesn't seem to be an aluminum fatigue thing. Hey, Ham got nervous from a previous carbon post. Looks like it's moving around the circle.

Really, if corrosion was a serious problem in well maintained Tricrosses, there'd be serious recall action going on. Still, it WILL last longer if you keep it looking purty.

PInarovo said...

Good to see the material for my bike frame isn't on your list: wood. Although it does have aluminium inserts for the headset and seat tube. It does look stunning when waxed!

Steve A said...

PInarovo, Wood is lovely, but you have not yet heard about composites in the sun. I will be sure not to omit wood! Actually, wood has it's own corrosion problem in that it can oxidize very rapidly at relatively low temperatures. Often referred to as "burning."

Pinarovo said...

True, but at least that rapid oxidation can be stopped. Get a magnesium frame burning and it isn't so easy.

Anonymous said...

My mountain bike is totally done for (don't think I've ever washed the mud off). But apparently I'm set, carbon seatpost in the carbon bike, aluminum seatposts in the aluminum bikes.

Big Oak said...

Thanks Steve for that link. That's the exact fork I have.

Pinarovo, watch out for powder post beetles!

Steve A said...

When this series is all over, I'll take your bikes in trade in exchange for ancient Schwinn Varsities if you wish. Seriously, remember that bikes of every possible material have been failing since long before our grandparents grew up. As for this chapter, clean and dry maximizes life. Wet and dirty and salty next to the pool chlorine tablet bucket will NOT.

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