ADVANCED MATERIALS & PROCESSES | JANUARY/FEBRUARY 2025 23 corrosive soils with very low concentrations of soluble chloride ions, and is due to selective dissolution of copper and internal oxidation of tin[6]. SASANIAN SILVER COIN A silver coin is part of a large collection of coins excavated between 1932 and 1935 from the Qasr-e Abu Nasr archaeological site in south- central Iran[7]. The coin has been dated to 689-90 AD and is shown in Fig. 3 together with a small metallographic cross section. The coin was also non- invasively analyzed using micro-x-ray fluorescence spectroscopy (micro-XRF). This determined the coin to be a silver alloy (Ag: 96.8, Au: 1.5, Cu: 0.7 wt%). The low copper content (0.7 wt%) signifies that no copper had been added for coin debasement. The coin has obviously undergone significant damage in the form of broken-off pieces, relatively large cracks, and deformation. Metallo- graphy revealed a highly deformed microstructure with internal corrosion along copper-rich layers that are the remnants of coring occurring when the metal was cast. In this respect, the coin’s internal condition is similar to that of the Egyptian vase[8]. ROMAN BRIDGE PILE-SHOE Many wooden piles sheathed with iron pile-shoes, and dated to between 340-400 AD, were recovered from the Maas riverbed in the Netherlands in the 1990s. One combination accidentally fell onto a concrete floor during storage. Three of the four bars comprising the pile-shoe were broken: two were old fractures, but the third was fresh, see cracked, and corroded. It was investigated by x-ray radiography, inductively coupled plasma mass spectroscopy (ICP-MS) for metal analysis, metallo- graphy (optical and SEM), x-ray diffraction (XRD), micro-Raman spectroscopy, and SEM + EDS for corrosion product analyses. The vessel proved to be made from two components: a single, very thin sheet of tin bronze (Cu: 89.3, Sn: 10.1 wt%) and a massive disc of arsenical copper (Cu: 95.1, As: 2.2, Pb: 2 wt%) inside the base[3,4]. Metallography showed that the tin bronze had undergone considerable thermomechanical working and annealing to obtain the thin sheet, including fine repoussé decorations. The corrosion was extensive, consisting of intergranular and transgranular attack, microcracking, and large through- cracks, leading to fragmentation. These damages most probably occurred owing to SCC caused by stresses from retained deformation (mainly coldwork) and the burial environment. Note: SCC in ancient tin bronzes has been recently demonstrated[1,5]. The corrosion analyses revealed a multi-layered corrosion crust including an internal tin-rich corrosion layer with some cuprite (Cu2O) and an external copper carbonate layer (mainly malachite, Cu2CO3(OH)2). This is termed Type I corrosion[6]. It is characteristic of longterm corrosion in low-to-moderately Fig. 1 — Visual and x-radiography views of the Egyptian vase, showing extensive restoration, missing pieces and hairline cracks. The cracks labelled A follow external chasing (decoration) grooves. Fig. 2 — View of the fragmented tin bronze vessel and an SEM-BSE metallograph of the internal corrosion and cracking damage. Fig. 3 — Visual images of the Sasanian silver coin (Access No. 36.30.298, courtesy of The Metropolitan Museum of Art) and an SEM-BSE metallograph showing the highly deformed microstructure and internal corrosion layers.
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