CORROSION MECHANISMS FOR COPPER AND SILVER OBJECTS IN NEAR-SURFACE ENVIRONMENTS
MICHAEL B. McNEIL, & BRENDA J. LITTLE
4 SILVER CORROSION
An example of silver corrosion is conversion of silver to cerargyrite, as indicated above line (c) in figure 1. The region between the diagonal lines that bound the upper hatched region approximates the effective oxidizing behavior of near-surface, fully aerated seawater (Garrels and Christ 1965). Most shallow sea chemistries fall into this region. Conditions are near this region in shallow land burials where the major source of groundwater is rain or surface water percolating through soils. Deep groundwaters and waters from coal mines may not fit this particular model (Garrels and Christ 1965). Cerargyrite is stable in seawater and chloride-rich shallow land burial.
Silver and its alloys are subject to corrosion by reduced sulfur species. In air (e.g., in a museum) H2S can result from biodegradation of polymeric materials that contain sulfur, producing monoclinic acanthite (Banister 1952; Bauer 1988). In marine environments silver objects corrode under the influence of SRB to produce acanthite and cerargyrite. The formation of body-centered cubic argentite (Ag2S) has been reported (North and MacLeod 1986). Abiotic corrosion of pure silver in water that contains dissolved sulfides produces acanthite with traces of argentite (Campbell et al. 1982).
The formation of argentite as a corrosion product is noteworthy because naturally occurring mineral argentite has not been reported in the biosphere and artificial argentite made with pure silver cannot be quenched to room temperature (Roy et al. 1959). Reports of mineral argentite appear to be the result of misidentification of acanthite crystals transformed from argentite without a change in shape. Argentite formation in quantity is limited to silver objects buried in sediments for archaeological periods (Gettens 1963a; North and MacLeod 1986). If Cl− is present, argentite or acanthite combined with cerargyrite is formed.
There are three polymorphs of Ag2S. Monoclinic acanthite is stable up to 176°C (Kracek 1946). Body-centered cubic argentite is stable from 176°C to a temperature between 586°C and 622°C (Kracek 1946), above which the stable form is a face-centered cubic polymorph (Djurle 1958; Barton 1980). The high-temperature polymorph has never been observed in corrosion. Reactions interconverting Ag2S polymorphs are very fast. When acanthite forms in abiotic experiments, it does so in extremely small quantities as a surface phase (Campbell et al. 1982).
The laboratory database on sulfides and their reactions with silver can be summarized as follows: (1) corrosion of silver by reduced sulfides, whether H2S (Sinclair 1982; Volpe and Peterson 1989) or organic sulfides (Sinclair 1982), produces acanthite; (2) carbon disulfide does not produce corrosion; (3) the corrosivity of organic sulfides appears to be controlled by transport mechanisms and thus by vapor pressures; and (4) the rate of sulfidation is strongly affected by NH3 and by alloying silver with iron (Biestek and Drys 1987). Abiotic aqueous corrosion of silver in the presence of reduced sulfur species produces acanthite in bulk (Birss and Wright 1981; Campbell et al. 1982) with possible trace argentite (Campbell et al. 1982).
The conversion of macroscopic quantities of silver to any sulfide in the biosphere requires the action of SRB, since the total reduced sulfide content of seawater and most groundwaters with pH below 9 is small. Except for the action of SRB, the conversion of sulfate to reduced sulfide requires geological time. Mor and Beccaria (1975) estimate zero sulfide concentration in Mediterranean seawaters having a typical sulfate concentration of 0.2 g/l and an SRB population of 10–102 per ml.
These observations support the hypothesis that formation of argentite is limited to precipitation of silver in the presence of copper ions by a reduced sulfur species. This theory is consistent with the observation that argentite corrosion products are sometimes accompanied by jalpaite (Ag3CuS2) (North and MacLeod 1986). Argentite formed from pure silver is unstable at room temperature, yet there are two reasons why argentite should precipitate during the corrosion of archaeological objects. Argentite containing several percent copper is usually associated with jewelry and coinage. Unlike acanthite, it can accommodate nearly 30% copper in its lattice (Shcherbina 1978). The phenomenon is parallel to the production of akageneite (βFeOOH) rather than goethite (αFeOOH) in the corrosion of meteorites. The formation of akageneite is attributed to its ability to accommodate significant Cl−, while the goethite lattice can accommodate little or none (Buchwald 1977; Buchwald and Clarke 1989). Precipitation of a mineral from an impure environment favors a high-entropy, low-packing fraction crystal structure capable of accommodating impurity atoms (Goldschmidt 1953).
Argentite formation occurs when an object made of a silver-copper alloy is in a water-saturated deposit that contains SRB capable of maintaining reducing conditions and bacteria (perhaps ammonia producers) capable of oxidizing and thereby solubilizing both silver and copper atoms. A layer of sand or soil restricts the ability of the metal ions to escape the biofilm, so that concentrations of copper and silver ions within it rise to levels that cause precipitation. The precipitation of argentite is favored for the reasons given above. Jalpaite forms in regions where the copper concentration is high enough to favor this mineral. In the absence of thermodynamic data the specific concentration cannot be quantified.
Argentite and jalpaite could, in principle, be stabilized when formed by corrosion of pure silver in a copper-rich environment (such as that associated with a mixture of silver and copper coins), but for practical purposes the presence of either argentite or jalpaite implies that the silver artifact originally contained significant copper.