Cathodic cleaning results in the liberation of hydrogen; at a given current density twice as much hydrogen is evolved as is oxygen at the anode. The cathodic cleaning is assisted by the repulsion of the negatively charged particles of dirt, in these strongly alkaline solutions most colloidal particles carry a negative charge. The negatively charged work will also attract positively charged ions such as copper, zinc, or other metal ions which are discharged at the surface, forming a metal smut. For this reason different solution tanks should be used when nonferrous metals are being cleaned by both cathodic and anodic cleaning. The principle reaction taking place at the cathode is the liberation of hydrogen, which may penetrate the metal and become occluded or absorbed by it. Under these conditions steel may become very brittle. Nickel surfaces and nickel base alloys must be electrocleaned cathodically to avoid passivation of the surface. The passivated surfaces will not plate satisfactorily.
Anodic cleaning results in only half the gas liberation that is obtained with cathodic cleaning at the same current density. While it charged anodically, the work will rapel metallic ions. In fact, smuts formed during cathodic cleaning are dissolved or dislodged during an anodic cycle. This action contributes to the efficiency of anodic electrocleaning. Copper and copper base alloy should be cleaned anodically for only short periods unless specially inhibited cleaners are used; otherwise corrosion or an oxidized or tarnished surface will result.
The use of periodic reverse current with alkaline derusting compounds for the removal of rust and scale is widely practiced. It eliminates the use of acid descalers which is not derirable for machined surface or where bleedout after acid descaling can cause problems. Oxides can be removed without etching or smut formation. Another cleaning process are Electroless Plating.
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