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Thursday, March 22, 2018

Scale-Craft Zinc Casting Breakdown?

Some of the casting materials used in OO models (particularly Famoco and Graceline) have a strong tendency to break down over years, depending on the batch of metal used to make them. A few years ago I posted an article on this “rot,” which is really more formally known as zinc pest. 

I have rarely noted any symptoms of this with Scale-Craft castings. My thinking is they used a better die casting material. I did, not too long ago, run into several hopper cars with some issues on the bodies – blistering -- which I was able to clean up and paint over after a brief soaking in vinegar. The flaws are not too visible.

But then, these castings just came in. WOW! The zinc has broken down badly in several of these truck sideframes, and also the body of an S-C baggage car (seen at the bottom of the photo) is in rough shape.

The question is, what causes this? Turning back to the Wikipedia article also linked from my earlier article, zinc pest

…was first discovered to be a problem in 1923, and primarily affects die-cast zinc articles that were manufactured during the 1930s, 1940s, and 1950s. The New Jersey Zinc Company developed zamak alloys in 1929 using 99.99% pure zinc metal to avoid the problem, and articles made after 1960 are usually considered free of the risk of zinc pest since the use of purer materials and more controlled manufacturing conditions make zinc pest degradation unlikely.
Affected objects may show surface irregularities such as small cracks and fractures, blisters or pitting. Over time, the material slowly expands, cracking, buckling and warping in an irreversible process that makes the object exceedingly brittle and prone to fracture, and can eventually shatter the object, destroying it altogether. Due to the expansion process, attached normal material may also be damaged. The occurrence and severity of zinc pest in articles made of susceptible zinc alloys depends both on the concentration of lead impurities in the metal and on the storage conditions of the article in the ensuing decades.

They don’t expand on that point about storage conditions, but I suspect that is part of the issue of these specific castings. Maybe S-C used a bad batch of zinc, but in my real-life professional world of the French horn, another thing is known to attack finishes of instruments; breakdown of the materials inside the case (foam, etc.), creating an aggressive chemical environment if the fumes are not vented out (the problem being seen when a case is closed for months at a time).

As to these trucks, there was likely some trigger based on storage conditions, a condition that perhaps also led to rust on the steel parts. Fortunately, some of the truck castings will clean up OK and the wheels are all good and will be put to use. And the aluminum frames of the S-C cars are both perfect, unaffected as they contain no zinc.

I should note that the Wikipedia article adds that “Zinc pest is different from a superficial white oxidation process ("Weissrost") that may affect some zinc articles.” Some of what you see in old castings is this oxidation process (it looks somewhat like mold on the surface of the model). Truth be known, on these parts in the photo we may be seeing both processes.

As to prevention today, so long as you keep your models in a situation where they can breathe a bit (not in a tightly sealed container) there should be no problems. As I said in my original article, if a casting is going to go bad it probably already has by now.

UPDATE: On taking apart the S-C steel side cars I found this surprise. Look at the roof at the bottom of the photo in particular, it is shrunken in the middle and damaged. Both roofs are damaged. Based on the loose materials inside my working theory is that the windows were a nitrate film stock that broke down. Lucky that it did not start a house fire! Maybe the storage location was also very humid. The fumes from this chemical breakdown probably also influenced the development of the bad zinc pest. I am able to save all the brass parts at the least.

UPDATE II: I did work over the four least damaged 6-wheel truck sideframes seen in the cars in this article, and have them solid. Method involves soaking in vinegar for short periods of time and then scraping off the material impacted by the zinc pest, which is softened by the vinegar. They are now assembled into nice working trucks (using the 3D printed bolsters), will look fine when painted black.

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