Black Alloy
Posted on Wednesday, May 31st, 2006 at 11:39 am
Tin, lead and lead alloy corrosion
Articles of tin are seldom encountered in archaeological sites. This metal is found more often in various alloys, particularly in combination with copper for bronze and/or tin pewter. Gettens notes that tin seldom survives in archaeological sites because of the transformation of tin to a mix of stannous and stannic oxide by direct intercrystalline oxidation (SnO and SnO2) or to a loose powdery gray tin, commonly referred to as ‘tin pest,’ by allotropic modification The alteration compounds of tin in a marine environment have not been adequately studied; it is known, however, that sodium chloride stimulates the corrosion of tin. Ingots of tin that were completely oxidized to tin oxide were recovered from a Bronze Age shipwreck off the coast of Turkey (Bass 1961). Although not often mentioned in literature, tin sulfide can also be expected to be found where sulfate-reducing bacteria are active in anaerobic environments.
Lead is commonly found in shipwrecks; it was used on ships for weights, cannonballs, sheeting, and stripping. Lead is a stable metal in neutral or alkaline solutions that are free from oxidizing agents, especially if carbonates are present in the water. Basic lead carbonate (2PbCO3 • Pb[OH]2) and lead oxides (PbO and PbO2) are formed under most archaeological conditions where there is prolonged atmospheric exposure. The gray lead carbonate and lead oxide generally form a protective layer on the artifact that prevents further oxidation. Both these corrosion compounds are found on lead from a marine environment, but lead chloride (PbCl2), and especially lead sulfide (PbS) and lead sulfate (PbSO4), are also common.
Gettens noted that few occurrences of lead sulfide have been reported on archaeological objects, but more recent research shows that the primary lead corrosion product in anaerobic marine environments is lead sulfide, while lead sulfate is commonly found on objects recovered from aerobic marine environments. It is not unusual in shipwreck excavations to find the remains of lead straps that have been completely converted to a black slush. The bulk of this corrosion is most likely lead sulfide which results from the action of sulphate-reducing bacteria. Some intermediate forms of lead oxides (PbO and PbO2) may be formed, and oxysulfides are also present. Lead often exhibits extensive corrosion attack when it is in contact with wood. Lead strips that were nailed onto a ship’s keel have been observed in a state of severe deterioration. The oxygen-consuming, decaying wood and the marine encrustation that forms over the lead apparently creates the anaerobic conditions conducive for the metabolism of the sulfate-reducing bacteria; in addition, the decaying wood provides nourishment for the bacteria.
Lead alloys, such as old pewter, which is an alloy of tin and lead, oxidize to the same compounds as the two parent metals. The condition of different pewter pieces varies widely both between and within archaeological sites, primarily because of different local conditions and varying percentages of tin to lead in each individual object. In general, leaded pewter always survives in better condition in marine environments than does lead-free pewter; this is most likely due to the formation of lead sulfate (PbSO4) that protects the surface of the artifact. Lead-free pewter suffers extensive corrosive attack in aerobic sea water and is often completely mineralized as stannic oxide (SnO2) and lead sulfide (PbS), and various very brittle, mineralized antimony and tin (SbSn) compounds are formed. In contrast, in anaerobic environments, both leaded and lead-free pewter survive in good condition through the protective formation of lead and tin sulfide films. In fact, the only corrosion present on pewter recovered from anaerobic marine environments may be a thin sulfide film on the surface of well-preserved metal. Various combinations of lead carbonate, lead oxide, lead sulfide, lead chloride, and tin oxide are possible. Pewter objects often have wart-like blisters on the surface of the metal, which possibly result from localized contaminations of salts. These should not be removed, for under most of them there are either holes or pits in the metal.
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