1. Although steels are metastable at room temp it takes dry ice temperatures to induce phase change. "Tin pest" disease proceeds slowly at first at 10 centigrade. Im never saw any reference in military journals that Napoleon or his officers attributed loss of buttons as significant.
2. Group 5 and group 6 alloys: bismuth, antimony, arsenic, tellurium, selenium, etc.; accelerate tin pest. Copper, Silver, Cadmium, Indium etc. alloys reduce tin phase change. Copper and Silver as intermetallic compounds Cu3Sn and Ag3Sn. Lead does not not form a solution in solid Tin, so it does not prevent tin phase change. However 50:50 Pb:Sn will leave a Lead matrix behind.
4. Pure Tin was unlikely a button material as it was too soft to hold the coats closed. More likely a Pewter Tin-Antimony-Lead alloy.
4. Russians had a Tin coin few years back. They wrestled with the Tin pest problem.
5. Tin whisker formation is proportional to Tin content of solder. Tin-Lead Solder is usually reflowed which reduces surface stress but Bright Tin plating is not reflowed.
6. Although Lead causes neural issues in young mammals in infinitessimal amounts, Silver is much more toxic to adult humans.
Karen, beta tin (white tin) is stable at room temperature, not metastable. Alpha tin (gray tin) becomes the stable phase below about 56°F. So below 56°F, beta tin is metastable - it is no longer the lowest energy state, but there is still an activation energy associated with the phase transformation.
I think the term you wanted to use is "allotropic," not "metastable."
It's interesting how non-committal Le Couteur is about the historical veracity of the "Napoleon's buttons" story, given that it's the title of her book (although the story itself is actually only mentioned briefly in the introduction).
When people talk about "pure tin" in electronics applications, they mean that it is 97% pure or more. However, I believe this still leaves enough room for alloying additions (antimony, bismuth, or lead) to suppress alpha tin formation.
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