[Closed] can this be prevented? what's the cause? why one and not the other? (FM)

That is pretty much what I learned years ago with 1:24 high end FM's mostly

DM's not so much. Bad quality control in China and then there are the cheaters that don't follow the recipe mixture. White metal cars don't do this as far as I know. Only die cast

Absolutely correct

I've lost several prized models to this decast-rot. Very disheartening.

So it is quite similar to stone worm. Makes perfect sense now.

Yes, Chris is right. Impurities in the mix are the problem.

It's not caused by slight variations in the metal mix (which would probably just reduce the strength of the metal slightly but probably wouldn't have any great structural effect on a model which is not subject to any real mechanical stresses). The problem is caused by particles of impurities which have got into the metal. (Some of the old pre-war Dinkies which were badly affected were made from scrap metal which contained impurities that weren't removed during the melting and casting process. Some relatively recent Russian diecasts were made from crappy impure recycled metal and suffered very badly from so-called "metal fatigue". Mind you, I've also had a couple of nice Tekno models which swelled up and fell apart).

Instead of just staying put, these impurities somehow migrate over time to the spaces between the metal grains, where they clump together and push the metal grains apart. Imagine a brick wall where something gets into the mortar between the bricks and makes it swell up till the bricks get pushed apart and the wall develops cracks. Same kind of thing. The model doesn't just crack, it swells up. Sometimes you can see big gaps appearing between the metal body and the plastic glazing unit, for example. Or the body buckles because it is expanding while the floor (made of a more stable material) to which it is riveted stays the same length so it distorts the body. I've also seen the opposite effect in some early Japanese models, where the body casting is stable but the floor is trying to expand, so it bows.

Although poor storage conditions are not the direct cause, the regular expansion and contraction of models exposed to heat or sunlight over the years can encourage the migration of these impurities and cause this problem, whereas models from the same maker which are protected from extremes of temperature may show little or no sign of trouble.

Nothing to be done about it, unfortunately, although I know owners of pre-war Dinkies have desperately painted the inside of the body shell with epoxy resin to try and keep things together a while longer, but the disintegration marches on regardless. Heartbreaking to see in a well-loved model.

thanks so much Chris...

Glad mine isn't there (yet).

photo no longer available

Maz and Chris are correct - heat and paint have nothing to do with it - it's the impurities.

More specifically, it comes down to electron valances at an elemental level. For example, tin and copper can compatibly be alloyed to form bronze. But throw some lead into a zamak pour (for example) and bad things happen.

I personally have seen this with the afore-mentioned Russian diecasts, Japanese Micro Pet (a shame because they were beautifully made), Crescent racers, and the Ertl 1:43 '64 Chevy. But my Crescent Ferrari hasn't had any acceleration of the its slight cracking in over 30 years.

You are welcome Galen but not sure it helps in this situation.

Thanks, Graeme.

I've heard similar stories over the years about impurities in the molten alloy. Tailing or ...

trimmings woud lbe swept up from the floor and recycled intot eh next alloy batch being melted down. Including lead dust, dirt, the occasional cig wrapper, etc. Waste not / want not back then!

also see: https://www.maronline.org.uk/intergranular-corrosion-again/