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Old 01-09-12, 05:16 PM   #1
Berylbite
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Mixing metals

A question for all you metallurgists.
What are the big faux pas of the types of metal bolts and components that you mount to your bike? don't you develop little electric charges that cause rust when two different types of metal touch?
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Old 01-09-12, 05:47 PM   #2
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it's really hard to avoid this, and so I always use some kind of grease. If you are going with aluminum into steel or aluminum and carbon, the fastener should be coated with something proven to be an anti-seize grease. Over the summer, I had a stainless steel bolt get stuck in a steel fitting in less than a week. My fender paid the price.
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Old 01-09-12, 06:51 PM   #3
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Yes, galvanic corrosion can be a problem. I use copper based anti-seize on my seat posts and stems.
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Old 01-09-12, 08:03 PM   #4
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There are several ways of reducing and preventing this form of corrosion.

One way is to electrically insulate the two metals from each other. Unless they are in electrical contact, there can be no galvanic couple set up. This can be done using plastic or another insulator to separate steel water pipes from copper-based fittings or by using a coat of grease to separate aluminium and steel parts. Use of absorbent washers that may retain fluid is often counter-productive. Piping can be isolated with a spool of pipe made of plastic materials or made of metal material internally coated or lined. It is important that the spool has a minimum length of approx 500 mm to be effective.

Another way is to keep the metals dry and/or shielded from ionic compounds (salts, acids, bases), for example by painting or encasing the protected metal in plastic or epoxy, and allowing them to dry.

Coating the two materials or if it is not possible to coat both, the coating shall be applied to the more noble, the material with higher potential. This is necessary because if the coating is applied only on the more active material, in case of damage of the coating there will be a large cathode area and a very small anode area, and for the area effect the corrosion rate will be very high.

It is also possible to choose metals that have similar potentials. The more closely matched the individual potentials, the lesser the potential difference and hence the lesser the galvanic current. Using the same metal for all construction is the most precise way of matching potentials.

Electroplating or other plating can also help. This tends to use more noble metals that resist corrosion better. Chrome, nickel, silver and gold can all be used.

Cathodic protection uses one or more sacrificial anodes made of a metal which is more active than the protected metal. Metals commonly used for sacrificial anodes include zinc, magnesium, and aluminium. This is commonplace in water heaters. Failure to regularly replace sacrificial anodes in water heaters severely diminishes the lifetime of the tank.
http://en.wikipedia.org/wiki/Galvanic_corrosion
The metals in anti seize compounds work as the sacrificial anodes.
Depending on application, you can use different ones.

Bikes in most applications are pretty forgiving of this, but like
others, I use anti seize on pedal threads, freewheel threads,
alloy seat posts, and spoke nipples.

In plumbing, you use special connectors to join copper refits
to older galvanized steel piping, to extend the life of your
repairs.
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Old 01-10-12, 03:11 AM   #5
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Originally Posted by unterhausen View Post
it's really hard to avoid this, and so I always use some kind of grease. If you are going with aluminum into steel or aluminum and carbon, the fastener should be coated with something proven to be an anti-seize grease. Over the summer, I had a stainless steel bolt get stuck in a steel fitting in less than a week. My fender paid the price.
Just as a matter of interest, where was the fitting? It's unusual for stainless to bind to alloy steels in such a short time, in fact, of all the couples, they're the closest in common use. Stainless to stainless is by far and away worse, but that's a cold-welding phenomenon rather than just gross corrosion and seizure.

The worst by far and away is carbon against a zinc alloy or against aluminium. I wouldn't even use an antiseize in those conditions. I'd have a jointing paste like aerograde PL32 or JC5A in there, but they arent all that easy to get hold of outside of amateur aerospec places.
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Old 01-10-12, 07:41 AM   #6
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Just as a matter of interest, where was the fitting? It's unusual for stainless to bind to alloy steels in such a short time, in fact, of all the couples, they're the closest in common use. Stainless to stainless is by far and away worse, but that's a cold-welding phenomenon rather than just gross corrosion and seizure.
It was the fitting in the chainstay bridge for the fender. I took the bike to France, but before I left I removed the bolt that was in there. I had to cut it out, should have been a warning. Rode PBP, and 4 days later when I was packing to return to the U.S., I couldn't get the new bolt out. I am not sure I would say it was a typical case of galvanic corrosion, but the bolt was coated in rust and it was a real struggle to get it out. In fact, I have been riding around without a rear fender since August, and I finally got it out this past weekend. There was much cursing and whining.
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Old 01-10-12, 04:55 PM   #7
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This is was sparked the inquiry.
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Old 01-10-12, 07:09 PM   #8
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stuck seatposts are a rite of passage for all bike owners. Otherwise, everything I see is steel, no problems there
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Old 01-12-12, 07:02 AM   #9
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One of the reason forward-facing seat clamps became so popular :-)
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