Electrolytic colouring (EC colouring) (Topic: 14069)

Electrolytic colouring is also a two step colour anodising process. It is carried out by means of dipping the profiles in a colouring tank after anodising and rinsing. The difference from dye colouring is in the use of electric current. The colouring agent is also different. It is possible to use direct current (DC) or alternating current (AC), but most electrolytic colouring processes use AC. DC current can cause a defect called spalling where the anodic film flakes off the substrate. This type of current the colouring processes are also more sensitive to impurities.

In the colouring tank the electrodes (e.g. stainless steel) are placed at the tank wall. The colouring electrolyte solution is usually composed of sulphates of nickel, tin, cobalt, or copper in addition to an acid. Under the action of electrical energy, metal particles are deposited at the bottom of the pores in the anodic oxide coating. The pores are then sealed. The pH in the solution is between 1 and 5,5 (dependent on the metal ion species used), the current density between 0,1 and 0,5 A/dm2 and the voltage used is between 5 and 20. The temperature is between 15 and 35°C. Air agitation of the bath is not recommended because of oxidation of the metal salts, and also that air pockets would create areas of different (lighter) colouring.

The finishes produced are light brown, brown, pink, maroon (red-brown), and black. This EC-colouring processes anodic oxide layers which have a good wear and outdoor resistance.