Tech Aluminium/Carbon Corrosion

jdd

Maximum Pace
Hardest Crash
Jul 26, 2008
2,512
639
133
Kanazawa
#1
I read this w/regard to airplane manufacturing:

"Unlike aluminum, which corrodes when it comes into contact with carbon fiber, titanium works well with carbon fiber..."

...and while it goes on to say that there are special aluminum coatings that prevent that, I was just wondering if CF frames that use a few alu parts here and there have similar issues?
 

microcord

Maximum Pace
Aug 28, 2012
914
294
83
Tokyo
#2
"Unlike aluminum, which corrodes when it comes into contact with carbon fiber, titanium works well with carbon fiber..."
Umm? Aluminium corrodes near-instantaneously whether or not it contacts CF. Or an extremely thin (and usually insignificant) layer of it does. I think that this layer becomes deeper with time but still remains thin; does CF somehow stimulate the process?
 

jdd

Maximum Pace
Hardest Crash
Jul 26, 2008
2,512
639
133
Kanazawa
#3
This is way beyond my technical comprehension, but after a google or few:

Aluminum alloys are extremely vulnerable when they are coupled to a carbon composite. Figure 3 shows the anodic and cathodic polarization curves of aluminum alloys and carbon composites, respectively. It is clear that the rate of galvanic corrosion in seawater is controlled by the oxygen reduction reaction. What this means is that any condition that leads to an increase in the rate of oxygen reduction will cause an increase in the rate of galvanic corrosion. During the galvanic corrosion, a white, jelly corrosion product will be formed on the surface of the aluminum.

proxy.php?image=http%3A%2F%2Fwww.corrosionpedia.com%2Fimages%2Fuploads%2F6518ffc77e1843759e7e0eeec894976f.png%3Fwidth%3D400&hash=8f51259e7c7788f96ae0943fcc72730a
I'm not sure what this means for bikes, which is what I was wondering.
 

TCC

Tokyo Cycling Club
Jun 30, 2013
2,362
1,291
133
#4
Not sure what this is doing in the Helpful thread (I can move it to its own thread if you want...)

But; corrosion of aluminium connected to carbon fibre depends on a few things.

-It needs to be electrically connected (ie, wet) to really kick in the corrosion. If it is not wet for continued amounts of time, the corrosion will most likely be unnoticeable (don't quote me on that; I have no actual physical evidence of my own to back this up).
-If the aluminium has an anodized surface, that prevents corrosion to a degree.
-If the carbon is painted, then again, no issue. Any isolation of the surfaces with a non-conductive material (non-metallic paint, glue, chewing gum, etc.) will do it.

But yeah, Carbon + Aluminium = Galvanic Corrosion.
 

TCC

Tokyo Cycling Club
Jun 30, 2013
2,362
1,291
133
#5
Actually, there might be an overlooked safety issue here...

If you have an aluminium steerer tube, with a carbon stem which you are sweating down onto (salt water hugely accelerates things, through surface reactions, apparently, which is why seawater is so harsh to stuff like this)...

The aluminium corrosion increases the diametre of the steerer, with everything else remaining equal, this would possibly crack the stem where it connects to the steerer. You don't want that.

Same with Aluminium seat posts with a carbon sleeve around them, or just going straight into a carbon frame. That could end in the seat tube splitting...
 

TCC

Tokyo Cycling Club
Jun 30, 2013
2,362
1,291
133
#6
I am publicly blaming you, @jdd, for piquing my interest in this enough that am now searching for Ohmmeters on Amazon, so I can measure the conductivity of all elements on my bike, to found out potential risk areas, and isolate these using titanium or kevlar sleeving.
 

jdd

Maximum Pace
Hardest Crash
Jul 26, 2008
2,512
639
133
Kanazawa
#7
No problem with having this moved. I've got time on my hands and so have been bumbling around the web a lot.

And yeah, steerer and posts would seem to be it.
 

TCC

Tokyo Cycling Club
Jun 30, 2013
2,362
1,291
133
#8
Any two conductive non-similar materials will initiate galvanic corrosion when touching though, so this is not an issue isolated to carbon and aluminium...

Anything that has 'seized' on a bike has undergone galvanic corrosion.
 

TCC

Tokyo Cycling Club
Jun 30, 2013
2,362
1,291
133
#11
So, in regard to what's been said, would a CF stem be the best choice 4 a CF neck?


'Neck' is probably called a 'steerer tube' nowadays, I have no idea on that one.
1. Stop being from the past.

2. An aluminium stem would be OK for a carbon steerer, as the nature of the way it fits, means there is room for a little corrosion development on the inside. A small amount of corrosion on the surface of the aluminium stem would essentially reduce the diameter of the hole, and with everything being equal, this would slightly up the torque of the stem bolts by a tiny bit. I can't see this causing any real usage problem. A carbon stem on a carbon steerer would not have any problems. An aluminium stem on an aluminium steerer, would not have any problems. It is only a carbon stem on an aluminium steerer which might get issues. But who is sticking a mega bucks carbon stem on a cheapo aluminum steerered fork, anyway?
 
Aug 27, 2012
581
234
73
London, UK
www.macrophotofly.com
#13
BTW Same corrosion magnification issue with (i) steel and Carbon and (ii) with Steel and Aluminium together. Stainless steel is okay.
Two things to add to the conversation as a man who used to work in the car industry with reinforced plastics,
  • A lot of what is called carbon, is actually glass reinforced plastics (Carbon sounds so much more sexy than glass). Mid-price (and possible some expensive) "carbon" seat posts where they are trying to engineer a degree of flex are likely candidates for GRP. I understand the FSA SLK range is mainly GRP with some carbon reinforcing, where as the K-force range has a far higher degree of carbon usage. A certain famous UK sports car manufacturer used by Mr J Bond has entirely GRP panels, not real "carbon" panels. Glass doesn't conduct and hence GRP doesn't cause the same level of corrosion when water is present
  • As mentioned above, coatings can make a big difference to corrosion rates. Providing the coating is not compromised (e.g. scratching the anodised aluminium part through to the bare metal) then corrosion rates drop considerably.
 

TCC

Tokyo Cycling Club
Jun 30, 2013
2,362
1,291
133
#14
As mentioned above, coatings can make a big difference to corrosion rates. Providing the coating is not compromised (e.g. scratching the anodised aluminium part through to the bare metal) then corrosion rates drop considerably.
Yep, or if by some genius stroke of design, a bare steel or aluminium bolt goes through both materials to fasten them together, and completing the circuit.