The impact of pH and temperature on copper corrosion products in subcritical conditions: delivering insight into the treatment of archaeological copper alloys

Corrosion profiles extant on archaeological copper alloys from aerated marine contexts often comprise CuCl beneath or intermixed with a Cu2O layer, which is normally overlaid by a range of green blue Cu2OH3Cl polymorphs. Depending on prevailing conditions, other chloride bearing compounds and basic copper carbonates, may also be present. Aesthetically pleasing patinas can be disrupted by post excavation hydrolysis of CuCl producing voluminous Cu2OH3Cl polymorphs. This can be controlled by using the inhibitor benzotriazole, except in the presence of low pH caused by hydrolysis. Alternatively, the removal of soluble chlorides (Cl-), using various aqueous alkaline wash solutions, offers an approach to fulfilling the aim of controlling post-excavation corrosion. Enhancing washing effectiveness by using subcritical conditions, that are claimed to aid desalination, has been experimentally studied for marine archaeological iron but has received limited attention for copper alloy desalination. This research investigates the potential of subcritical fluid technology to release Cl- and transform compounds that are commonly present on marine archaeological copper alloys. The impact of the treatment variables pH, temperature and time on analogue pressed pellets of the commonly occurring corrosion products, atacamite (CuCl2OH3Cl), clinoatacamite (CuCl2OH3Cl), malachite Cu3(CO3)2(OH)2, nantokite (CuCl) and cuprite (Cu2O) was studied. Sets of analogues comprising 3 pellets were treated in subcritical conditions at pH8 (NaHCO3) and pH10 (NaCO3) for up to 480 minutes, with analysis of the concentration of Cl- and pH values in the eluate at time intervals of 15, 30, 45, 60, 120 and 480 minutes. The surface and cross section of samples was analysed with SEM-EDS and ground pellet samples with XRD to detect whether transformation had occurred. To translate this into contexts for treating naturally forming corrosion product profiles, analogues were produced by corroding copper alloy coup