Steam treating steel to obtain uniform oxide coating - esp. for protecting pipes from corrosion in nuclear power plant

摘要

Uniform and adherent oxide layers with a prescribed thickness are obtd. on workpieces made of unalloyed or alloy steels by treatment with superheated steam at 200-700 degrees C. The novelty is that the selected treatment temp. is held constant to within plus-or-minus 3 degrees C., and a defined uniform steam turbulence with Reynolds number 800-10,000 is created. The workpieces are pref. heated at max. 10 degrees C/minute in air or inert gas to max. 250 degrees C. before the steam treatment, then cooled at max 20 degrees C/minute in air or inert gas after steam treatment. The steam is pref. generated using water contg. 50 mg/l salts and as free from O2 as possible. Used e.g for protecting high tensile steel from corrosive media, esp. uranium halides, halogens, and hydrogen halides in nuclear energy technology, by forming a hard layer of Fe3O4 on pipes etc. carrying steam or vapours, and rotating at high speeds.