Folks-
The subject of electronic anti-corrosion, galvanizing, and whatnot came
up on the Volvo-Net mailing list a while back; here's part of my
contribution to the discussion:
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Folks-
eris@tc.fluke.COM (Chris Beckmeyer), in his musings about "electronic
anti-corrosion" asks (in part) ...
>>> why wouldn't sacrificial zincs work on a car like they do on my
boat? <<<
Because your boat is immersed in water, and your car isn't.
The boat's sacrificial zincs work on the galvanic corrosion that occurs
because the dissimilar metals on your boat are immersed in a weak
electrolyte (seawater) and the least noble metal in the whole melange is
the one that will eventually be disolved. If you don't have the zincs,
the least noble piece of underwater metal is probably something like a
bronze through-hull fitting for the head, and you don't want that to rot
away, do you?
On a car, the galvanic corrosion is localized because the car isn't
sitting in an electrolyte bath like the boat is. Most rust on a car is
the result of a local electrolyte, like a couple of drops of road-salt
contaminated water seeping into a body seam. A sacrificial zinc three
inches away wouldn't even be in the "circuit" made up of the two pieces
of body metal and the drop of salty water.
However, there is a way to spread the sacrificial zinc all over the
car. You can galvanize it, covering it all with a thin layer of zinc.
Volvo says they do that, though I don't know the method they use.
The best way is to hot-dip the entire assembled body structure in molten
zinc, since that seals all the welds and seams.
The least effective way is to use pre-galvanized body sheet metal. The
problem there is that each spot weld results in a small area where the
zinc has been vaporized and the steel left bare.
Now, why they don't just build the cars out of passivated stainless
steel is beyond me. The incremental cost difference would be trivial.
>>> there are a zillion old volvos out here in seattle, most of them in
the part of town settled and for the most part, still populated by
scandinavian fishing folk. do they know something they're not tellin'? <<<
Well, fish oil is the anti-rust ingredient in Rustoleum brand paint ....
-Shel
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