Galvanic corrosion

[myveonix]

Last week replaced these alu nipples with proper brass (taper grip) nipples on a carbon rim. Became near impossible to obtain and retain good spoke tension.

[seven]

Anodised coatings on alu helps to delay the process

[V12man]

Its a galvanic reaction, not catalytic.

 

Caused by dissimilar metals simultaneously in contact with each other and with a body electrolyte. One metal will usually be severely corroded while nothing happens to the other.

Since it is conductive and can react, carbon fibre can act as a 'metal' for purposes of a galvanic couple. Batteries are probably the most ubiquitous examples of a galvanic couple in action.

 

The term electrolyte means basically any liquid that can transport an electric current - so, water, sea water, vinegar, blood, battery acid etc are all potential electrolytes. The stronger the concentration of salts in the liquid, the more effective it is as an electrolyte. This is why sea water is more corrosive than freash water.

 

There is a thing called the 'galvanic series' which is a list of galvanically active materials (nearly all metals) ranked in order of half-cell potential (reactivity). The further apart two materials are in this series, the more likely that the more reactive one will be corroded when it is used in simultaneous contact with the other and an electrolyte.

 

So.....how to prevent or reduce galvanic corrosion?

 

1) break the electrical circuit - by insulating the two metals from one another with a non conductive material like plastic or by insulating one or both from contact with the electrolyte (e.g. paint).

2) remove electrolyte - keep the metals dry....always

3) if two metals will contact each other in a wet environment, choose metals that are close to each other in the galvanic series where possible ( eg copper and brass will do better than a combination of alu and stainless steel.)

4) Dilute the electrolyte with fresh water

5) coat parts in moisture repellent coatings such as grease, oil, paint.

6) use a sacrificial anode ( attach a body of much more reactive metal - it will corrode preferentially - common on ships and submerged metal structures)

7) use impressed current to hold the part needing protection at a lower voltage relative to others. ( pipelines and railways are often protected this way)

 

Ok that's the lecture....now what can you do with your wheels.....?

 

A) keep them away from sea water and sea spray

B) dry immediately after washing and wash less frequently

C) use brass nipples rather than alu. It's closer to carbon in the galvanic series.

D) store indoors away from salty/ humid air

 

Sent from my SM-G935F using Tapatalk

And keep them far away from where pool chemicals are stored.

[JXV]

And keep them far away from where pool chemicals are stored.

Yup....not strictly galvanic but pool acid turns alu into hydrogen and a big sticky mess. Chlorine pretty much wrecks most metals except Ti

 

Sent from my SM-G935F using Tapatalk

[droo]

Corrosion is between the aluminium rim and brass nipple not the spoke, unless the spoke is stainless steel.

 

Most spokes are stainless.