Intercrystalline corrosion in die cast zinc alloys
The earliest mass production of zinc alloy castings was developed soon after 1920 when the addition of aluminium was found to produce a strong alloy that did not attack the iron and steel parts of the die casting machine. Unfortunately this particular mix was found to be very susceptible to intergranular corrosion due to the presence of small amounts of impurities such as tin, lead and cadmium. At that time the purest zinc available still contained 0.05% lead and 0,02% cadmium, but adding 0.1% magnesium was found to reduce the corrosion sufficiently for commercial purposes.
Meanwhile in an experiment using laboratory grade 99.993% pure zinc the intercrystalline corrosion did not develop. It was known then that purity was a necessity. Around 1930 zinc of 99.99% purity became available and the modern range of alloys were developed, known as Zamak.
Frank Hornby originally produced electric train sets. The first Dinky toys were produced under the name Modelled Miniatures in 1933 as accessories to his train sets. The large scale production of Dinky Toys took off in 1934.
In theory intercrystalline corrosion in zinc alloys made after 1931 should be a rare occurrence. But the cost of the high purity zinc was an unnecessary extravagance for an ephemeral product like a child’s toy and so a lower grade could be used. Also the diecasters could easily degrade the molten alloy by accidental introduction of lead and tin from solder, incorrect recycling which reduced the magnesium, or even by casually throwing in tinfoil from cigarette packets.
There is no treatment that can stop this corrosion developing in off-grade alloys. However, for corrosion to occur it requires an electrolyte (a liquid containing a salt) and oxygen. The rate at which it proceeds will be increased with heat.
The conservation approach for long term storage is therefore to attempt to inhibit the activity of the electrolytic cell by sealing the object in sachets that remove the oxygen and moisture, and keeping them in a cool temperature.