Zinc Pest was Re: Times have changed!

Craig Zeni

On Sep 16, 2018, at 5:15 PM, main@RealSTMFC.groups.io wrote:

6b. Re: Times have changed!
From: Daniel A. Mitchell <danmitch@tir.com>
Date: Sun, 16 Sep 2018 17:15:19 EDT

The one you show are in exceptional condition. From my experience, perhaps 75% of of such old castings have turned to dust due to “Metal Rot” (intercrystaline corrosion). It’s due to impurities in the diecast metal, and varied from “pot to pot” in the casting process, even from the same manufacturer.

Such rot is like a bad cancer, there’s really no cure. The corrosion occurs between the metal crystals, forcing them apart. The part will first swell, distort, warp, and become covered in white powder … eventually the powder is all that’s left. If the part is not yet in awful shape you can try to get a mold off it, and thus “save” in for future reproduction.

Dan Mitchell

Back in my college days when I was studying metallurgy, one of my professors who happened to be a tinplate train collector and restorer told me his theory was that poison Zamac aka zinc pest isn't an electrolytic problem or a corrosion problem or a temperature problem. It's an atomic problem, a metallurgical problem with impurities - typically lead, antimony and bismuth. Zamac, like almost all metal, has a 'grain' structure. When liquid metal solidifies, the metal doesn't start freezing uniformly - it begins at nucleation points in many places more or less at the same time. A cubic atomic structure of atoms grows out of each of those nucleation points forming a grain. As the grains continue to grow they bump into each other, forming grain boundaries. When the contaminated Zamac alloy is poured and solidifies, the contaminant atoms are trapped inside the grains...but the contaminants don't like being inside the grains. Through a process known as diffusion the impurity atoms move thru the grain structure heading for the boundaries. As the impurities gather at the boundaries they push the grains apart...which is what you see with the 'poisoned Zamac' that we see in old models that are swelling and crumbling. The long time it takes for the failure to happen is consistent with diffusion theory as well. Is my professor right? Beats me...

Trying to save those parts with zinc pest will effectively be rearranging the deck chairs on the Titanic...how long it takes your particular ship to sink is the great unknown but it will eventually sink.

Craig Zeni
Cary NC

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