Chemical stripping of bulk saturated NEG coatings

In the recent years the installation of non-evaporable getter (NEG) coated dipole and quadrupole chambers at the GSI heavy ion synchrotron (SIS 18) have been carried out and ware completed in January 2010. The NEG coated chambers was part of different upgrade activities of the existing UHV system [1]. Those non-evaporable thin film getters, consisting of Ti–Zr–V, are deposited onto the vacuum chambers using three magnetron sputtering facilities built at GSI, each fulfilling different geometric shapes and dimensions. A detailed description of the production and the functional principle is given in Ref [1]. In brief, after coating and activation a highly chemical reactive surface is present in the vacuum system getting dissociated residual gas components by means of oxidation, nitration and carbonisation of the surfaces metallic constituents. The capacity of the NEG surface is approximately one monolayer for CO and CO2 (10 15 particles per cm). In the environment of the SIS18 the operation time of an activated NEG surface is typically longer compared to the venting cycle time due to maintenance. The live time capacity of a 1 μm NEG thin film is limited to 30 – 50 reactivations [2]. After the bulk of the NEG film is completely saturated, it has to be stripped of the vacuum chamber before applying new NEG coating. Industrial stainless steel pickling utilizing solutions containing nitric and hydrofluoric acid similar to buffered chemical polishing (BCP) [3] has proven to work on thick walled chambers (>2 mm) [4] but is not feasible for delicate dipole and quadrupole chambers of only 0.3 mm wall thickness because of nonuniformities especially at welded joints. Therefore, an alternative method was proven and will be described in the following.