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Galvanic corrosion and spoke twist/windup: cause, prevention and cure.

Wednesday, August 20, 2014 by  
Filed under Daily Blog

August
20
2014

This morning I want to follow up on Monday’s blog, which was focused on a broken spoke and the issue of spoke windup/spoke twist. You may want to give that blog a quick read if you missed it.

My spoke broke inside the rim, so it was actually a spoke nipple failure (the spoke itself did not break).

As I mentioned in Monday’s blog, I’ve had problems with spoke windup when truing my wheels (Zipp 404 Firecrest full carbon clinchers). Even though I’d use a drop of oil at the spoke nipple and the Park Bladed Spoke Holder tool (BSH-4), the spoke would often turn with the spoke nipple, causing the spoke to twist (see photo, click to enlarge):

Spoke windup.

Spoke windup.

 

Spoke twist is a serious problem, as it weakens the spoke, makes the wheel less aerodynamic, makes truing very difficult and wheels are less likely to remain in true because the twisted spokes tend to “pop” and slightly unwind while riding.

Because a number of spokes were twisted on my rear wheel, I decided to take the wheel to my Zipp dealer for a complete spoke replacement. When I got to the shop, I talked about the problem with the wheel builder, Steve, who I already know (Steve is the mechanic who did the initial build on my 2013 Trek Madone 5.9, and he did a extremely impressive job).

The cause of my windup woes? Galvanic corrosion.

Zipp wheels use Sapim Aero (bladed) spokes, which are made of steel, along with aluminum spoke nipples. Steel + aluminum + high humidity + my sweat = corrosion. This would also explain why my front wheel has had no issues with spoke windup (my sweat falls on the rear wheel).

I had three major questions, and Steve was kind enough to take the time to address them all.

First, I wanted to know if brass nipples could be used when Steve re-laced my wheel. I’d done some research before taking my wheel in, and learned that brass nipples would not corrode like aluminum nipples do. Steve said absolutely. There’s a very small weight penalty (almost not even worth mentioning), but apart from that there’s no downside to brass. I went with brass nipples for the re-build, obviously.

Second, I wanted to know how to prevent the issue (I wanted to know this for my own edification, and also because my front wheel still has aluminum nipples). Steve suggested oiling the nipples not just when I’m truing, but as a regular maintenance item. He also suggested Liquid Wrench as the lubricant, as it’s thinner than chain lube (which is what I’d been using), and penetrates better.

Finally, I wanted to know the best procedure for freeing a spoke from a corroded spoke nipple. Steve had a fantastic solution to this problem: Liquid Wrench, and heat. What Steve does is put a drop of Liquid Wrench at the spoke nipple, then apply heat to the spoke about an inch above the nipple using a small butane torch (I bought the Blazer GB4001 Stingray). The oil will actually start to boil down in the nipple, freeing the spoke. This procedure, Steve said, will not harm the carbon wheel. Steve said he was going to have to perform this process on every single spoke to disassemble my wheel.

I’ve learned a lot over the past couple days, and it feels great to know that the troublesome issue of spoke windup is something I can completely prevent. I hope this information helps some of you out, too.

John Stone Fitness Comments

8 Responses to “Galvanic corrosion and spoke twist/windup: cause, prevention and cure.”
  1. Galvanic corrosion is a big problem in industry. The telephone companies had to deal with it wit their copper lines, Also gas lines and water lines have the same problems depending on the soil but they all suffer some sort of it. One of the fixes is a small charge is applied to the metal fo keep it from happening. Think about it.

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  2. I’ll preface this by saying that I’m no expert in galvanic corrosion and I’m mainly posting this for your input.

    My understanding is that brass and aluminum are fairly far apart on the galvanic chart with steel alloys falling between them. So by switching the the nipple from aluminum to brass, you’ve changed the steel spoke from the cathode to the anode in the reaction. So the steel would now be the material subject to corrosion.

    We can’t determine the exact potential difference between the metals without knowing which alloys are being used. But it’s possible that there is now more galvanic potential difference between steel and brass than there was with steel and aluminum. But it will present differently now that you’ve switched anode and cathode.

    I’m curious if it will present at all or if your additional protection and scrutiny will keep it in check.

    Thoughts?

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    • Clearly I’m far from a galvanic corrosion expert, too, so take the below with that in mind. My comments are based on a few days of research, and from talking with people more knowledgeable that I am.

      The electrode potential difference between steel and brass is much smaller than the difference between steel and aluminum and, as you correctly pointed out, the larger the electropotential gap between two alloys or metals, the greater likelihood galvanic corrosion will occur.

      Looking at the Anodic Index, I can see that the relative electrochemical voltage of brass and steel is just .1 volts (-0.40 and -0.50, respectively), while it’s .45 volts between aluminum and steel (-.95 and -0.50, respectively).

      An electropotential difference of 0.15 or less is generally recommended in applications where corrosion might occur.

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