α-tin

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'', ''tin plague'', or ''tin leprosy''. It is an autocatalytic process, accelerating once it begins. It was first documented in the scientific literature in 1851, having been observed in the pipes of pipe organs in medieval churches that had experienced cool climates.

With the adoption of the Restriction of Hazardous Substances Directive (RoHS) regulations in Europe, and similar regulations elsewhere, traditional lead/tin solder alloys in electronic devices have been replaced by nearly pure tin, introducing tin pest and related problems such as tin whiskers.

Allotropic transformation

At and below, pure tin transforms from the silvery, ductile metallic allotrope of β-form ''white tin'' to the brittle, nonmetallic, α-form ''grey tin'' with a diamond cubic 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 Â°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''. The decomposition will catalyze itself, which is why the reaction accelerates once it starts. The mere presence of tin pest leads to 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 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. The tin cans were recovered and no tin pest was found when analyzed by the Tin Research Institute. Some observers blame poor quality soldering, as tin cans over 80 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. This appears to be an urban legend, as there is no evidence of any failing buttons, and thus they cannot have been a contributing factor in the failure of the invasion. Uniform buttons of that era were generally bone for enlisted, and brass for officers. Critics of the theory point out that any tin that might have been used would have been quite impure, and thus more tolerant of low temperatures. Laboratory tests of 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 the invasion. Nevertheless, some of the regiments in the campaign did have tin buttons and the temperature reached sufficiently low values (below −40 Â°C or °F). In the event, none of the many survivors' tales mention problems with buttons and it has been suggested that the legend is an amalgamation of reports of blocks of Banca tin completely disintegrated in a customs warehouse in St. Petersburg in 1868, and earlier Russian reports that cast-in buttons for military uniforms also disintegrated, and the desperate state of Napoleon's army, having turned soldiers into ragged beggars.

Modern tin pest since adoption of RoHS

With the 2006 adoption of the Restriction of Hazardous Substances Directive (RoHS) regulations in the European Union, California banning most uses of lead, and similar regulations elsewhere, the problem of tin pest has returned, since some manufacturers which previously used tin/lead alloys now use predominantly tin-based alloys. 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. This cycle 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 phase change.

See also

* Bronze disease – destruction of bronze artifacts by corrosion
* 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

External links

{{DEFAULTSORT:Tin Pest
Metallurgy
Tin
Allotropes
Corrosion