Soon after the introduction of wound foils for Al electrolytic capacitors, concepts emerged for increasing the specific capacitance(F/m2) of the anode. Initially, this was achieved through mechanical roughening of the anode surface prior to forming.
The use of electro-chemical etching from the mid-1930s onward increased the effective surface of the aluminum anode greatly. During such etching, the foil runs through sequentially arranged electro-chemical etching baths in the presence of several
thousand amps from the foil to the bath. After cleaning, the roughened anode foil is oxidized (formed) and cleaned again.