Baths for anodising aluminium and its alloys - contg chromium trioxide and chromium chromate to obtain translucent oxide film

摘要

Bath for the anodic oxidn. of unalloyed or alloyed aluminium, consisting of an aq. soln. contg. chromium with valencies VI and III, also contains 20-100 g/l CrO3 and 0.5-15 g/l chromium chromate (CrO4)3Cr2. The bath pref. contains 20-100 g/l CrO3 and an amt. of (CrO4)3Cr2 between 0.5 g/l and the limit of its solubility at the working temp. of the bath, esp. to obtain 0.076-2.3 g/l CrIII; The pref. process uses a bath with max. temp. 65 degrees C at 5-90 volts, esp. to obtain a translucent coating assuring corrosion protection and also revealing the macrographic structure of the workpiece. Alternatively, the pH may be maintained above 5.5 to obtain only a barrier layer. The bath can be used to obtain a translucent coating which reveals structural defects inthe surface of the Al substrate. Cf conventional baths, which need about 7 days to reach equilibrium between the different Cr ions, this bath can be used immediately.