Tin Pest. A tin corrosion or discoloration or powdery surface texture caused by prolonged exposure to temperatures below 18° centigrade. Tin pest is a chemical reaction changing metallic tin (betta-tin) to powdery form (alpha- tin). When it is first noticed tin pest should be removed immediately (by scraping off if necessary – use a soft pencil eraser on coins and medals) as it will spread over the entire object's surface by exposure to cold environment. Tin does not corrode from moist air as does iron (to form rust), but only from exposure to prolonged cold temperature. Also called tin plaque.