Tin pest
Tin pest is an autocatalytic, allotropic transformation of the element tin, which causes deterioration of tin objects at low temperatures. Tin pest has also been called tin disease, tin blight or tin leprosy (Lèpre d'étain).
It was observed in medieval Europe that the pipes of church pipe organs were affected in cool climates. As soon as the tin began decomposing, the process accelerated.
With the adoption of the Restriction of Hazardous Substances Directive (RoHS) regulations in Europe, and similar regulations elsewhere traditional lead/tin solders have been replaced by solders containing primarily tin, making prevention of tin pest and related problems such as tin whiskers a modern technological challenge.[1][2]
Contents
Allotropic transformation
At 13.2 °C (about 56 °F) and below, pure tin transforms from the silvery, ductile metallic allotrope of β-form white tin to brittle, nonmetallic, α-form grey tin with a diamond structure. The transformation is slow to initiate due to a high activation energy but the presence of germanium (or crystal structures of similar form and size) or very low temperatures of roughly −30 °C aids the initiation. There is also a large volume increase of about 27% associated with the phase change to the nonmetallic low temperature allotrope. This frequently makes tin objects (like buttons) decompose into powder during the transformation, hence the name tin pest.[3]
The decomposition will catalyze itself, which is why the reaction speeds up once it starts; the mere presence of tin pest leads to more tin pest. Tin objects at low temperatures will simply disintegrate.
Possible historical examples
Scott expedition to Antarctica
In 1910 British polar explorer Robert Scott hoped to be the first to reach the South Pole, but was beaten by Norwegian explorer Roald Amundsen. On foot, the expedition trudged through the frozen deserts of the Antarctic, marching for caches of food and kerosene deposited on the way in. In early 1912, at the first cache, there was no kerosene; the cans — soldered with tin — were empty. The cause of the empty tins could have been related to tin pest.[4] Some observers blame poor quality soldering though, as tin cans over eighty years old have been discovered in Antarctic buildings with the soldering in good condition.
Napoleon's buttons
The story is often told of Napoleon's men freezing in the bitter Russian Winter, their clothes falling apart as tin pest ate the buttons. Whether failing buttons were indeed a contributing factor in the failure of the invasion remains disputed; critics of the theory point out that the tin used would have been quite impure and thus more tolerant of low temperatures. Laboratory tests provide evidence that the time required for unalloyed tin to develop significant tin pest damage at lowered temperatures is about 18 months, which is more than twice the length of Napoleon's Russian campaign.[4] It is clear though that some of the regiments employed in the campaign had tin buttons and that the temperature reached sufficiently low values (at least -40 °C).[5] However, none of the many survivor's tales mentions problems with buttons and it has been suggested that the legend is an amalgamation of a case of disintegrating Russian tin buttons in an army warehouse in the 1860s[6] and the utterly desperate state of Napoleon's army turning soldiers into ragged beggars.[5][7]
Modern tin pest since adoption of RoHS
With the adoption of the Restriction of Hazardous Substances Directive (RoHS) regulations in Europe and California banning most uses of lead, and similar regulations elsewhere, the problem of tin pest has returned, since some manufacturers who previously used tin/lead alloys, now use pure tin. For example, the leads of some electrical and electronic components are plated with pure tin. In cold environments, this can change to α-modification grey tin, which is not electrically conductive, and falls off the leads. After reheating, it changes back to β-modification white tin, which is electrically conductive, and can cause electrical short circuits and failure of equipment. Such problems can be intermittent as the powdered particles of tin move around. Tin pest can be avoided by alloying with small amounts of electropositive metals or semimetals soluble in tin's solid phase e.g. antimony or bismuth, which prevent the decomposition. Silver, indium, and lead have also been used, but lead is not soluble in tin's solid phase.
See also
- Gold-aluminium intermetallic - giving rise to Purple plague or White plague, another failure mode for electronic components due to the formation of a crystalline substance.
- Zinc pest - decay of zinc by an unrelated intercrystalline corrosion process.
References
- ↑ A Tin Pest Failure, Journal Journal of Failure Analysis and Prevention, ISSN 1547-7029 (Print) ISSN 1864-1245 (Online), Volume 9, Number 5, 10/2009, pp.461-465, Neil Douglas Burns, online abstract via www.springerlink.com
- ↑ Tin Pest Control National Physical Laboratory, 'www.npl.co.uk
- ↑ Janey Levy Tin, The Rosen Publishing Group, 2009, ISBN 1-4358-5073-4, page 20
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External links
- tin pest, transformation of beta tin into alpha modification (grey tin) Time lapse video of tin pest on ingot via www.youtube.com
