Zinc plague
Zinc pest , also known as zinc corrosion , is the slang term for intergranular corrosion that can affect parts containing zinc .
This affects all types of zinc die castings that have been made with unsuitable alloy mixtures.
General problem
Problems of this kind occur particularly frequently with products that were manufactured in times of material shortages. In Germany, for example, shortly before the Second World War, during the war and shortly afterwards, only inferior or contaminated alloy mixtures were available for toys and other non-war products. Higher quality material was used for armaments production or was not available again after the war.
Affected are z. B. old coins, but also die-cast toys such as model trains or model cars . However, machines and engine parts, e.g. B. historical vehicles. Since the affected parts expand (i.e. become larger), attached parts that are in themselves are often damaged as well.
From the mid-1950s onwards, the problem was pushed back through improved starting materials and more precise adherence to the material mixes in die casting. Parts from this time and after are considered safe.
However, problems arose for well-known manufacturers after outsourcing their production lines to China, as unsuitable alloys were used there for cost reasons. Thus Marklin have to admit that products were affected again by zinc pest after the relocation of production.
causes
In order to use zinc as a casting alloy, mainly aluminum and copper are added. Aluminum reduces the solution attack of the iron-containing casting mold by the zinc, while copper increases the strength and hardness of the component, albeit at the expense of dimensional accuracy. A small amount of magnesium can also be added to compensate for the harmful effects of components such as lead, tin or cadmium. However, this only works up to certain limit values.
Said limit values are standardized today in DIN EN 1774.
Corrosion processes
In the case of selective corrosion, (air) moisture attacks the eutectic and dissolves it out. This form of corrosion occurs with alloys that form a solid solution. Aluminum, iron, cobalt and chromium are particularly affected.
Intergranular corrosion, as a form of selective corrosion, dissolves the structure along the grain boundaries and increases in volume. Due to the increase in volume and the resulting stresses inside the component, bubbles can form on the surface of the component. Temperature fluctuations also have a negative effect on the stresses in the component.
The phenomenon is considered unstoppable.
The process is not related to the tin plague .
Individual evidence
- ^ Unrest in Swabia: zinc plague eats Märklin's nerves. In: Handelsblatt. March 24, 2008, accessed March 12, 2018 .
- ^ Stephan Hasse: Foundry Lexicon . 19th edition. Schiele & Schön, Berlin 2007, p. 375 f .
- ↑ Standard DIN EN 1774 zinc and zinc alloys, cast alloys, in block form and in liquid form . Table 1: Chemical composition of zinc alloys in block form and in liquid form. Beuth, 1997, p. 5 .
- ↑ Arthur Burkhardt: Technology of zinc alloys . 2nd Edition. Springer, Berlin 1940, p. 11 .
- ↑ William D. Jr. Callister, David G. Rethwisch: Materials science and materials technology: an introduction . 1st edition. Wiley-VCH, Weinheim 2013, p. 642 .
- ↑ Boris Nogowizin: Theory and Practice of Die Casting . Schiele & Schön, Berlin 2011, p. 13 f .
- ↑ Holger Hoche: Examples of damage: fractures and cracking on components made of die-cast zinc . In: Practical Metallography . tape 51 , no. 12 , 2014, p. 869 .
