Improvements in or relating to electrolytic treatment of metals more particularly aluminium for increasing the effective surface

907,264. Electrolytically increasing the surface area of metals. PLESSEY CO. Ltd. Sept. 19, 1960 [Sept. 21, 1959], No. 32012/59. Class 41. In electrolytically etching aluminium or other metal foils to increase their surface area a bi-directional current is used having a voltage wave cycle by far the greater portion of which is positive-going but which is such that successive positive-going portions are separated by short negative-going portions of considerably less amplitude than the positive-going, e.g. a cycle as shown in Fig. 1. At a given frequency there is an optimum value for the minimum voltage in a cycle. The etching may be carried out in a single step by passing the aluminium foil through an electrolytic cell in which it forms the anode, the cathode or cathodes of which is a plate or plates of aluminium. Possible electrolytes are HCl with dissolved A1 at pH 3.5; AlCl 3 solution with water added until the density is 1.005 g/ml.; the AlCl 3 solution with added HNO 3 . The aluminium, after being etched, is formed and aged at 500 volts D.C. in a solution of boric acid. It is stated that the method of etching is believed to be applicable to tantalum subject to the choice of a suitable electrolyte.