3.4.1 Processing – Surfaces
The topics “material selection” and “welding” were already covered in the previous chapters. The inner surfaces of vacuum chambers and components are a significant factor for achieving the operating pressure in the high vacuum and UHV. The processing must be carried out under the condition, to minimize the effective surface and to produce surfaces with small desorption rates.
The surfaces of vacuum chambers and components are often fine glass bead blasted after welding and mechanical processing. High-pressure glass beads with a defined diameter are blown onto the surface. Sealing surfaces cannot be blasted and so they are covered during blasting. The process seals the surface, it levels it microscopically, removes near-surface layers, such as discolorations and creates a decorative appearance. Surfaces to be blasted must be clean and free of grease, the grit media must be replaced regularly, especially when changing groups of materials, such as ferritic and austenitic stainless steels.
Brushing is used for post-treatment of weld seams. The brushes used must be made of stainless steel and must not be contaminated by other materials, so that no foreign matter is introduced into the surface. The same goes for polishing. Polish or abrasion may not be added to the surface and/or must then be removed completely. The effective surface area must not be increased by microscopic roughening.
Pickling is an effective method for cleaning the surface. Impurities and about a 1 to 2 µm thick layer are dissolved. Relevant pickling parameters such as the concentration of the stain, the temperature or the pickling time must be strictly observed to avoid overpickling. After pickling, it must be rinsed intensively to remove all remnants of the pickling liquid. The surface roughness is changed only insignificantly by the pickling process.
Electro-polishing is the selective anodic dissolution of the metal into an electrolyte by using a DC power supply. In this case, typically 12 to 15 µm are removed from the surface to produce a crystalline pure surface. In order for the surface to be evenly removed, an electrode suited for the component must be manufactured frequently. This makes the process complex. In addition, the CF sealing surfaces must be covered, as the electric field strength is locally increased as the edges, resulting in an increased removal of material. Among UHV users, the process is disputed. The inclusion of hydrogen into the surface or through the electrolyte remnants on the surfaces are discussed. As with pickling, after the electro-polishing the components must be thoroughly rinsed. In addition, a leak test should be performed afterwards, as material is removed in the area of the weld seams. Depending on the previous condition, electro-polishing can cut the surface roughness by half.