Microstructure and phase composition of bronze Montefortino helmets discovered Mediterranean seabed to explain an unusual corrosion

Abstract

Two Monterfortino helmets, recovered in the Mediterranean seabed, show unusual features with respect to the more common helmets of the same period and found in underwater environments. Hence, they were investigated by a multi-analytical approach, which allowed us to identify the compounds constituting the helmets and to make some considerations about their metallurgy, although all the metal was converted to degradation products. The helmets, originally made in bronze, have maintained their original shape because of copper sulphides formation. The observed differences in composition between the two helmets were attributed to the position modification, of one of them, into the seabed along centuries. For the first time, a microstructural investigation permits to reconstruct the history of the aging processes involved in the total oxidation of roman bronze helmet metal.

New Insight on Archaeological Metal Finds, Nails and Lead Sheathings of the Punic Ship from Battle of the Egadi Islands

“…The biofilm developed by D. desulphuricans at the metal surface accumulates with exposure time forming a poor protection patina [ 11 ]. Some examples are the sulphides are present in corroded bronze that came from two quite recent wrecks, Wasa (1628 in Stockholm Habour) [ 41 ] and Mary Rose (1545 near Portsmouth) [ 42 ], and from two Montefortino helmets [ 43 ].…”

The Fontanamare Discovery (Sardinia Coast, Italy), a Case of Underwater Corrosion of Bronze Coins

“…It should be noted that the content of Cu is low until 50 µm depth due to selective leaching. The alloy does not show visible corrosion at depths greater than 50 µm, below the boundary between the externally corroded layer and up to the inner core metal [34][35][36].…”

Early Stages of Metal Corrosion in Coastal Archaeological Sites: Effects of Chemical Composition in Silver and Copper Alloys

“…The thin sulfide layer lays on other copper corrosion compounds, specifically copper hydroxychlorides, as evidenced by the presence of relatively small amounts of Cl and O. In the literature, there are many occurrences of copper sulfide naturally grown on copper-based artifacts which have been buried for a long time in soil [38,39] or aqueous sediments [40,41]. It is known that they form under anoxic conditions in the presence of decomposing organic matter and sulfate-reducing bacteria (SRBs).…”