stoo
Likes Dirt
A bees dick under 31.6 ( so the sides are a good tight fit ), then hit it with the steel wool in the lathe for a nice finish.
Galvanic corrosion gurus, another stuck dropper post in carbon frame thread!
- Thread starter ctguru
- Start date
African or European? Or Australian? Have you got Henry Hokes micrometer?
pink poodle
気が狂っている男
So...google sent me to Wikipedia, and Wikipedia says:
"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte. This same galvanic reaction is exploited in primary batteries to generate an electrical voltage."
I'm fairly science illiterate. So in have 2 simpleton questions:
1) Is this scenario actually galvanic corrosion? Or something else?
2) if so, what triggers it on a bike? How do you prevent it happening? Which materials are more/less at risk?
"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte. This same galvanic reaction is exploited in primary batteries to generate an electrical voltage."
I'm fairly science illiterate. So in have 2 simpleton questions:
1) Is this scenario actually galvanic corrosion? Or something else?
2) if so, what triggers it on a bike? How do you prevent it happening? Which materials are more/less at risk?
Knuckles
Lives under a bridge
1) Yes.
B) The anode/cathode ratio (larger CF composite area to a smaller aluminimum area). ie an anodised post with a tiny breach in the anodising will corrode expotentionally faster than a raw alloy post, and a alloy nipple in a carbon rim, will corrode much faster than an alloy seat tube with a carbon seat collar.
It is the carbon reinforcing fibres in the composite material that are conductive, and air is a sufficient enough electrolite to stimulate a current.
Ti droppers is the answer. *edit* Unless you have an ally frame, then the galvanic process will be pretty much the same, but with the frame corroding instead.
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Flow-Rider
Burner
With the aid of washing your bike all the time and water pocketing in certain areas plus minerals and ph level in the dirt you ride around getting washed in with the mix of water. It may cause problems over a long time.
ctguru
Likes Bikes and Dirt
This thread made me go and take a look at my frame/seatpost and im assuming this is the same sort of corrosion going on?
Bike is 12months old, seatpost has been moved around a few times for height adjustment and regreased once. Its been 5-6 months at least since it was last touched and at that time i had just purchased the Motorex Carbon Grease so a very light coating was applied. I dont recal noticing any corrosion at that point in time.
Is it possible the carbon grease could speed up the corrosion?
Travis.
Bike is 12months old, seatpost has been moved around a few times for height adjustment and regreased once. Its been 5-6 months at least since it was last touched and at that time i had just purchased the Motorex Carbon Grease so a very light coating was applied. I dont recal noticing any corrosion at that point in time.
Is it possible the carbon grease could speed up the corrosion?
Travis.
Attachments
Knuckles
Lives under a bridge
Nope.
The grease will create an insulating barrier. However, if there are patches that lose the grease during insertion, washing, etc, these areas will corrode at an accelerated rate. The smaller the contact patch of alloy against the carbon, the more rapidly the galvanic reaction in these localised areas. If you have a QR seat collar, whip the post out after each ride, or at least if the bike is going to sit for a while, and give it a quick wipe with the grease before reinserting prior to riding.
Or you could try and oxidise the surface of the alloy, as oxygen anions are a galvanic inhibitor. Hence why Ti is classed as noble rather than active. Just below carbon on the galvanic scale.
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Nambra
Definitely should have gone to specsavers
Not speaking from experience, but reading this thread got me curious as I have an alloy post in a carbon frame and may run into this trouble as well.
Reading item number 13 here make sense to me, noting too that item 14 suggests that caustic is only suitable for steel frames. The galvanic action will probably result in an alkaline corrosion compound (salt), and a weak acid like lemon juice or vinegar (or Coke?) applied for a period might be a safer route to try in the first instance to try to get the post to loosen on its own without resorting to mechanical force.
If you still need to apply force, then that item 13 essentially says to create a method to apply pressure between the seat mount on the post, and the top of the seat tube on the bike, rather than try to twist the seat post out. This makes sense because the frame would have to be designed for an axial compression load down the seat tube, considering it has to bear the weight of a rider through all the rough stuff, rather than huge twisting forces. By using some sort of expanding jig arrangement, you'll be putting a axial compression force down the seat tube equal to the pulling force on the seat post. Before doing that, I'd still apply mild heat then cold to the seat post to create mild expansion then rapid contraction of the post inside the seat tube to hopefully break or at least weaken the corrosion bonding. As the OP says, carbon fibre has a much lower coefficient of thermal expansion to aluminium, so there is opportunity to expand and contract the alloy seat post inside the seat tube and create gentle mechanical force on the galvanic bonding to try to weaken it without brute force.
Obviously whatever you do is at your own risk and I'm not saying that any of the above is correct or appropriate, although I would try it on my bike if I had to.
Be keen to hear how you go, regardless of what you try; I think patience will be a virtue in this instance too - don't reach for the hammer (or other similarly violent tool) first!
Edit: Sorry I missed ctguru's previous link to the sheldon brown seatpost page - does make sense though...
Reading item number 13 here make sense to me, noting too that item 14 suggests that caustic is only suitable for steel frames. The galvanic action will probably result in an alkaline corrosion compound (salt), and a weak acid like lemon juice or vinegar (or Coke?) applied for a period might be a safer route to try in the first instance to try to get the post to loosen on its own without resorting to mechanical force.
If you still need to apply force, then that item 13 essentially says to create a method to apply pressure between the seat mount on the post, and the top of the seat tube on the bike, rather than try to twist the seat post out. This makes sense because the frame would have to be designed for an axial compression load down the seat tube, considering it has to bear the weight of a rider through all the rough stuff, rather than huge twisting forces. By using some sort of expanding jig arrangement, you'll be putting a axial compression force down the seat tube equal to the pulling force on the seat post. Before doing that, I'd still apply mild heat then cold to the seat post to create mild expansion then rapid contraction of the post inside the seat tube to hopefully break or at least weaken the corrosion bonding. As the OP says, carbon fibre has a much lower coefficient of thermal expansion to aluminium, so there is opportunity to expand and contract the alloy seat post inside the seat tube and create gentle mechanical force on the galvanic bonding to try to weaken it without brute force.
Obviously whatever you do is at your own risk and I'm not saying that any of the above is correct or appropriate, although I would try it on my bike if I had to.
Be keen to hear how you go, regardless of what you try; I think patience will be a virtue in this instance too - don't reach for the hammer (or other similarly violent tool) first!
Edit: Sorry I missed ctguru's previous link to the sheldon brown seatpost page - does make sense though...
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Knuckles
Lives under a bridge
You're obviously not an engineer then.
pink poodle
気が狂っている男
Thanks. I think I get it. A little.
So...Does the smaller exposed area concentrate the galvanic forces? Magnifying their power, thus the faster corrosion?
Thanks Knuckles.
I guess im my case a more regular cleaning and regreasing would be best. Least it came out with zero effort despite the corrosion.
Does this require any special cleaning, any tips for cleaning the corrosion? (Guess the best solution is a carbon seatpost?)
Travis.
I guess im my case a more regular cleaning and regreasing would be best. Least it came out with zero effort despite the corrosion.
Does this require any special cleaning, any tips for cleaning the corrosion? (Guess the best solution is a carbon seatpost?)
Travis.
Nambra
Definitely should have gone to specsavers
I am actually... I've just broken a lot of shit over the last 20 years and I'm careful now.
Knuckles
Lives under a bridge
Nope. It's the similar reaction as in a battery. An increase in area differential, increases the electrochemical potential, displacing electrons from the alloy with a stronger force. Think of it as a ramp, the smaller the metal contact the steeper the ramp, the electrons slide down to the bottom (CF) faster and easier.
Knuckles
Lives under a bridge
Ahhhhh......a retired engineer. :boink:
Flow-Rider
Burner
I can just see it already, cheap MTBs for sale and come with free seat posts. :llama: