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 Nimo nic 105 materials by X-ray photoelectron spectroscopy, (a) after clean ing using simple washing methods and (b) after contamination in the la boratory environment for periods of up to 156 days, using filtered and unfiltered laboratory air. All surfaces are covered by a thin oxide a nd this is further covered by a thin layer of carbonaceous contaminati on of atmospheric origin. Some of the contamination is removed by surf actants 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 env ironment 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 aro und 15 mu g/year after five years. If the laboratory air is filtered a nd moves at a low rate the carbonaceous contamination growth is low, b ut 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 subjected to a wash in bailing water fo r 5 minutes to develop a stable oxide thickness, stored in static filt ered air to reduce the carbonaceous contamination and are cleaned usin g a dilute aqueous solution of gentle surfactant cleaners or possibly by a steam-cleaning method. Some further work confirming this by weigh ing is required.