GB985024A - Improvements in or relating to electrolytic etching - Google Patents

Abstract

<PICT:0985024/C6-C7/1> <PICT:0985024/C6-C7/2> <PICT:0985024/C6-C7/3> Metal such as Al, e.g. for electrolytic capacitors or printing plates, is electrolytically etched to increase its surface area by a fluctuating unidirectional current produced by a repeated sequence comprising charging a capacitor from a D.C. source and then temporarily short circuiting the capacitor across an etching bath. Fig. 1 shows Al foil being etched in a bath 7, washed in a bath 20, e.g. of distilled water, and dried by lamps 21. Charging and discharging of a capacitor 4 is controlled by switches 2 and 5 or, in a modification, by switches 15 and 5. The switching sequence at 2 and 5 is controlled at 23. Fig. 2 shows the voltage Vc of capacitor 4 and etching current Ie during operation. From t1 to t2 switches 2 and 5 are closed, from t3 to t4 both switches are open, and from t4 to t5 switch 2 is closed and 5 open. The etching bath contains 2% NaCl solution at 74 DEG C. and is stirred by air or gas from a fan 6. Fig. 3 shows a further embodiment wherein individual electrodes 38 are etched while connected to a current rail 35. The time interval t1 t2 may be lengthened so that after the current has fallen to the value of the rectifier current, the rectifier current flows for a short while as a steady D.C. before it is switcher off. Capacitor discharge etching may be followed by D.C. etching at 5-20 Amps./dm.2. After etching, dielectric oxide may be formed on the metal in a forming bath at a voltage not exceeding 100 volts, e.g. 17 or 64 volts.