Zamac rot

Published 06/08/20

Zamac (or Zamak) is a metal alloy with a base of zinc and alloying elements of aluminum, magnesium and copper. The name zamak is an acronym of the German names for the metals: Zink, Aluminium, Magnesium and Kupfer. Being cheap and easy to melt, after WWI it became widespreadly used. Nowadays there are basically five types of zamac (2, 3, 4, 5 and 7), with different proportions of the four base elements. However, the most commonly used type is zamac 3.

On this model the metal is cracking, to the point that with time pieces will fall off.

In terms of our hobby, when we refer to a “diecast model” we’re talking about zamac. Zamac is a very good alloy for models, being cheap, readily available and very easy to cast. A manufacturer with good molds can get very crisp details with zamac. Nevertheless, the alloy is not free of problems. And a possible big issue is the so-called zamac rot (or zinc pest). It happens when the alloy contains lead impurities. With that, if the alloy used in the model has lead, the model may develop zamac rot.

Bubbles under the paint is a sure sign of zamac rot.

Because of the lead, the zinc alloy suffers an intercrystalline corrosion process. The process will happen in varying degrees, but if the ambient humidity is high (say above 65%), it will be worse. What you will see is irregularities under the paint, such as small cracks and fractures, blisters or pitting. The process is irreversible, and with time the model will literally crumble to pieces.

My friend’s two Alfas are basically goners…

Back in my 1:18 days it was something very common. In fact, other than lack of space, the fear of zamac rot was a deciding factor for me to sell my collection. So far, however, I hadn’t seen this happen with 1:43 models. But to my surprise this morning an old friend and collector sent me these pictures. These two Altaya Alfas are from his collection, and as you can see, they are rotting. Unfortunately, there’s nothing he can do and eventually they will crumble 😲.

And how can you avoid it? You can’t 😣. A dry environment will only delay and slow the process, but it will eventually happen. I don’t expect to see this occurring with better diecast brands, but you never know. That being so, this is the most important reason why I prefer resin models.

Food for thought.

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