Long-term corrosion of copper alloys in the soil: new aspects of corrosion morphology in archaeological vessels from south-western Iran

Abstract

A group of copper-based objects excavated at Deh Dumen cemetery, in south-western Iran, was studied and analysed to examine the long-term corrosion morphology and mechanism in the soil burial environment. For this purpose, twenty-two samples from twenty-one copper-based vessels were studied and analysed using X-ray diffraction, scanning electron microscopy—energy dispersive X-ray spectroscopy, micro-Raman spectroscopy and metallography techniques. The results of the analyses showed that the majority of vessels are made of tin bronze, along with two arsenical copper samples. The extent of corrosion observed ranges from very thin corrosion crusts to thick crusts and entirely corroded structures. These three identified corrosion morphologies display a multi-layered corrosion stratigraphy as well as the preserved limit of the original surface. The corrosion crusts include internal tin-rich and external copper-rich layers, and the main corrosion mechanism for the formation of multi-layered corrosion crusts is decuprification or selective dissolution of copper during the long-term burial time in a moderately Cl-contaminated soil. The three identified corrosion morphologies are similar to the previously published morphologies, but some clear deviations are apparent and are discussed here.

A Bronze Lebes from the Coastal Cemetery of the Phaleron Delta, Greece: Excavation and Conservation of the Find

“…Through the XRD analysis, it was attested that the corrosion layers are composed of cuprite, malachite and atacamite, while there was also recognised the presence of quartz as a result of sand intrusion from the burial environment. As far as cuprite goes, apart from the characteristic red colour, which is its principal hue, can also appear in hues of different intensity, which vary from orange to yellow [24,28], a fact that was observed, as well, in the microscopic observation of the examined lebes fragments (Figure 14a). Colour gradations in the cuprite layers occur depending on the alteration of the Cu/Sn ratio and, more precisely, the lighter red colour indicates corrosion areas or bands rich in tin, while, as the colour gets darker, the content in copper increases [23,24,26].…”

“…The quartz (SiO2), which was also recorded, derived from remains of soil material that had penetrated in the porous structure of the corrosion in the course of the long period of time the artefact remained buried. The absence of tin compounds, such as oxide of tin -cassiterite (SnO2), from the diffractograms, is related to the difficulty in the detection of these compounds by the method of XRD, because of their very low crystallinity or amorphous structure in the bronze patinas [23,26,28]. As a result, it was attested that the main corrosion products that were documented in the examined samples were copper (I) oxide (cuprite), basic copper carbonate hydroxide salt (malachite) and copper trihydroxychloride (atacamite).…”

“…The microscopic observation, especially in fracture areas, ascertained, as well, the existence of unstable layers between the corrosion products, which show an off-white/whitish hue and which cause disruption of the material, surface detachments and segregation of the artefact (Figure 15). These off-white phases are tin-rich and their formation is due to the process of decuprification or selective leaching of Cu, in which the copper ions are dissolved and removed from the metallic structure, while, at the same time, the tin in the inner part of the alloy is oxidized [23][24][25][26]28,42]. The quantity of copper that will leach from the metal's structure, will subsequently react with the anions contained in the soil (in our case chloride and carbonate ones) and will be deposited on the external surface of the artefact [3].…”