Surface Analytical Study of Cleaning Effects and the Progress of Contamination on Prototypes of the Kilogram

To examine the stability of prototypes of the kilogram after cleaning, surface analysis techniques, including X-ray photoelectron spectroscopy (XPS), were applied to specimens cut from prototype material of Pt-10% Ir alloy. Traditional steam-jet cleaning (SJC) was compared with ultrasonic cleaning with solvents (UCS) in terms of cleaning effects and the progress of contamination. The results show that mass changes due to metallic factors such as oxidation, reduction and dissolution were negligibly small. In terms of carbon decrease and adsorption of water, UCS was superior to SJC. From analyses of peak area ratio by XPS, it was shown that the cleaner the surfaces, the higher the rate of contamination. But the contamination levels converged to a common value after a six-month exposure to air, irrespective of the cleaning method used. This finding is of great importance for the preservation of prototypes. The contaminating substances were found to be hydrocarbons from the ambient air. The mass gain for the surfaces of a prototype due to hydrocarbons after a six-month exposure to ambient air is estimated to be 4,3 μg for SJC and 16 μg for UCS.