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", 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 sped up, and seemed to feed on itself.

With the adoption of RoHS regulations in Europe, and similar regulations elsewhere, tin pest is an increasing problem, with some manufacturers using pure tin to comply with RoHS regulations by removing the lead from formerly tin/lead alloys.

The allotropic transformation

At 13.2 degrees Celsius (about 56 degrees Fahrenheit) and below, pure tin transforms from the (silvery, ductile) allotrope of β-modification "white tin" to brittle, α-modification "grey tin". Eventually it decomposes into powder, hence the name "tin pest".

The decomposition will catalyze itself, which is why the reaction seems to speed up once it starts; the mere presence of tin pest leads to "more" tin pest. Tin objects at low temperatures will simply disintegrate.

Explanation by Dr. Ron Lasky

WHAT IS TIN PEST?Tin is a metal that is allotropic, meaning that it has different crystal structures under varying conditions of temperature and pressure. Tin has two allotropic forms. "Normal" or white beta tin has a stable tetragonal crystal structure with a density of 7.31g/cm3. Upon cooling below about 13.2 C, beta tin turns extremely slowly into alpha tin. "Grey" or alpha tin has a cubic structure and a density of only 5.77g/cm3 . Alpha tin is also a semiconductor, not a metal. The expansion of tin from white to grey causes most tin objects to crumble.The macro conversion of white to grey tin takes on the order of 18 months . The photo, which is likely the most famous modern photograph of tin pest, shows the phenomenon quite clearly.This photo is titled "The Formation of Beta-Tin into Alpha-Tin in Sn-0.5Cu at T <10 C" and is referenced from a paper by Y. Karlya, C. Gagg, and W.J. Plumbridge, "Tin pest in lead-free solders", in Soldering and Surface Mount Technology, 13/1 [2000] 39-40.This phenomenon has been known for centuries and there are many interesting, probably apocryphal, stories about tin pest. Perhaps the most famous is of the tin buttons on Napoleon's soldiers' coats disintegrating while on their retreat from Moscow. Since tin pest looks like the tin has become diseased, many in the middle-ages attributed it to Satan as many tin organ pipes in Northern European churches fell victim to the effect.Initially, tin pest was called "tin disease" or "tin plague". I believe that the name "tin pest" came from the German translation for the word "plague" (i.e. in German plague is "pest").To most people with a little knowledge of materials, the conversion of beta to alpha tin at colder temperatures seems counter intuitive. Usually materials shrink at colder temperatures, not expand. Although it appears that the mechanism is not completely understood, it is likely due to grey alpha tin having lower entropy than white beta tin. With the removal of heat at the lower temperatures a lower entropy state would likely be more stable.Since the conversion to grey tin requires expansion, the tin pest will usually nucleate at an edge, corner, or surface. The nucleation can take 10s of months, but once it starts, the conversion can be rapid, causing structural failure within months.Although tin pest can form at <13.2C, most researchers believe that the kinetics are very sluggish at this temperature. There seems to be general agreement in the literature that the maximum rate of tin pest formation occurs at -30 C to -40 C.

Possible historical examples

cott 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, making for caches of food and kerosene deposited on the way in. In early 1912, at the first cache, there was no kerosene; the cans &mdash; soldered with tin &mdash; were empty. Members of the expedition later died in the cold and blizzards, only eleven miles from a massive depot of supplies.

The cause of the empty tins is unknown. Some observers blame poor quality soldering, although tin cans over eighty years old have been discovered in Antarctic buildings with the soldering in good condition. In any case, the lack of kerosene was just one factor in the deaths. One source observes,

The likely cause of death for Scott's polar party was some combination of scurvy, gangrene, starvation, dehydration, and hypothermia. [ [http://www.wilderness.net/library/documents/glover.pdf On Robert Scott's mission to the South Pole] ]

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 cold 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. [http://www.straightdope.com/columns/080509.html]

Modern tin pest since adoption of RoHS

With the adoption of Restriction of Hazardous Substances Directive (RoHS) regulations in Europe banning most uses of lead, and similar regulations elsewhere, the problem of tin pest has come back with some manufacturers using pure tin, who previously used tin/lead 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 fall off the leads. After heating back up, 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 particles of tin move around. Tin pest can be avoided by alloying with small amounts of antimony or bismuth, which prevent the decomposition. Silver, indium, and lead have also been used, but lead has been mostly banned by RoHS.

References

http://www.indium.com/blogs/Dr-Lasky-Blog/entry.php?id=310