Scanning Auger Microscopy and X Ray Photoemission Spectroscopy Studies of Metallic Archaeomaterials

ABSTRACTWe report on the surface microchemistry of a Roman iron object and of a Roman lead pipe ‘fistula’. The original patina of the iron sample consists mainly of oxidic phases whose lateral distribution is imaged with a submicron spatial resolution. Metallic iron in the bare surface is covered with a Fe(lll)-Fe(ll) oxidic layer which is several times thicker than in a modern ferritic steel. We find that the presence of a flux minimizes lead oxidation in the joint of the fistula as well as in comparison, ‘modern’ lead soldered samples. In one of these comparison samples the lateral distribution of the chemical species present is imaged with a spatial resolution of about 1000 Å.

The initial stages of lead oxidation in pH 9 to 14 aqueous solutions

The initial stages of lead oxidation in a range of alkaline solutions of varying pH and ion content have been investigated with the use of cyclic voltammetry and potential step techniques. In both carbonate and borate buffered solutions (pH > 9), and in more alkaline solutions, Pb oxidation commences at a potential of ca. 140 mV vs. RHE. The initial product is considered to be Pb(OH)2, due to the proximity of this potential with the theoretically predicted one, and due to its invariance (vs. RHE) with pH. In pH 14 solutions, Pb(OH)2 is soluble, producing Pb(OH)3− and only a small amount of a stable surface film. At lower pH's, Pb(OH)2 is not predicted to be soluble, and in borate-buffered solutions, a stable, reducible film appears to form. In carbonate-buffered solutions, the surface film is very difficult to remove electrochemically. Although XPS analyses show the film to be essentially free of carbonate, it is clear that carbonate must be involved in some way in inhibiting film reduction. Keywords: lead, lead hydroxide, lead oxide, surface films, carbonate solutions, borate solutions, alkaline solutions, cyclic voltammetry.

The influence of oxygen pressure on lead oxidation kinetics

The dependence of the oxidation rate of lead films on the pressure of oxygen was investigated by ellipsometric and resistance techniques for oxygen pressures between 200 Pa and 100 kPa and in the temperature range 323-423 K. Measurements of the pressure dependence were obtained by studying the change in oxidation rate following an abrupt change in pressure. The results of all experiments indicated a small positive dependence, though significant variations in the values were observed both with extent of oxidation, and between experiments conducted at different temperatures. During the first few minutes of the reaction an unusual trend was noted in the observed kinetics which could be interpreted as being due to absorption of oxygen, by the metal close to the metal-oxide interface, resulting in changes to the metal conductivity and the refractive index, determined by ellipsometry, of the lead substrate.