Romarchite and associated phases as common corrosion products on pewter artifacts from marine archaeological sites

Corrosion products were examined from typical pewter artifacts originating from six different submerged archaeological sites, dating to between ca. A.D. 1550 and 1733, along the eastern seaboard of North America and in the Caribbean Sea. The artifacts were viewed as 270-450-year long experiments revealing the phases and mechanisms of tin corrosion in seawater. All of the samples analyzed exhibit abhurite (Sn3O(OH)(2)Cl-2), romarchite (SnO), and hydroromarchite (Sn3O2(OH)(2)) forming at the expense of the underlying artifact. Textural analysis suggests that abhurite is the first alteration product to form at. the expense of the pewter; romarchite subsequently develops and then hydroromarchite. The outermost corrosion layers on several of the most corroded artifacts also exhibit cassiterite (SnO2) as a significant and apparently final phase to form during alteration. The absence of this mineral on many samples demonstrates that, while samples appeared to be stable under the conditions that were present, cassiterite had not yet had time to form. The very limited stability field for romarchite, based on data presented by Seby et al. (Geochimica et Cosmochimica Acta, 65, 3041-3053, 2001), suggests that its presence on these artifacts may be the result of a kinetic effect. The universal appearance of this mineral on corroding tin suggests that. it is a required step in the oxidation of pure tin to the final most stable phase of cassiterite. The stability of romarchite and its effectiveness as an agent of passivation can provide insight into not only the formation of tin oxides but, the rate of tin corrosion. This can have significant implications in the field of artifact preservation as well as more widespread industrial applications. (C) 2004 Wiley Periodicals, Inc.