Stability of Reference Masses II: The Effect of Environment and Cleaning Methods on the Surfaces of Stainless Steel and Allied Materials

A study has been made of the surfaces of Immaculate 5, En58AM and Nimonic 105 materials by X-ray photoelectron spectroscopy, (a) after cleaning using simple washing methods and (b) after contamination in the laboratory environment for periods of up to 156 days, using filtered and unfiltered laboratory air. All surfaces are covered by a thin oxide and this is further covered by a thin layer of carbonaceous contamination of atmospheric origin. Some of the contamination is removed by surfactants contained in various proprietary cleaners or by boiling water (used to mimic steam-cleaning methods). The first exposure to boiling water increases the chromium content of the oxide. Exposure to the environment causes the carbonaceous contamination thickness to grow at a rate described by either simple diffusion theory or by a "logarithmic" theory such that the increase in mass of a kilogram artefact is around 15 μg/year after five years. If the laboratory air is filtered and moves at a low rate the carbonaceous contamination growth is low, but if the filter is removed and the flow rate raised, the carbonaceous contamination rate increases. It is recommended that masses made from these materials are initially subject to a wash in boiling water for 5 minutes to develop a stable oxide thickness, stored in static filtered air to reduce the carbonaceous contamination and are cleaned using a dilute aqueous solution of gentle surfactant cleaners or possibly by a steam-cleaning method. Some further work confirming this by weighing is required.