An investigation into the corrosion of Ag coins from the Greek colonies of Southern Italy. Part I: An in situ FT-IR and ERS investigation of the behaviour of Ag in contact with aqueous solutions containing 4-cyanopyridine

The electrochemical behaviour of polycrystalline Ag and Ag(111) electrodes in contact with neutral chloride and perchlorate solutions containing 4-cyanopyridine (4CP) was studied. The understanding of the interaction of organic adsorbates with precious metals and alloys in given environments can provide guidelines for the design of suitable corrosion inhibition systems meeting the peculiar requirements of archaeological standards. 4CP adsorbs onto Ag both in the presence and in the absence of chlorides and acts as a reversible film former, suitable for use with historical items. 4CP has been shown to alter markedly the anodic kinetics of Ag: active corrosion in chloride environment is changed to a pseudo-passivating behaviour and potential oscillations, typical for the formation of thick AgCl layers, are suppressed. In situ spectroelectrochemical FT-IR measurements were carried out using both p- and s-polarised radiation in order to clarify the potential-dependent behaviour of 4CP. FT-IR data were complemented by in situ electroreflectance experiments, which highlighted changes in the electronic structure of the Ag surface in the presence of 4CP. Cyclic voltammetry and differential capacitance measurements were employed to asses the electroactivity and adsorption ability of 4CP. The results obtained disclosed that 4CP is adsorbed intact in a neighbourhood of the corrosion potential in perchlorate and chloride solutions, it desorbs at high anodic potentials and it reacts cathodically.