Pulse anodising (Topic: 14065)

In order to obtain an anodic oxide coating of acceptable surface quality in a faster way, pulse anodising is increasingly used. Pulse anodising needs special rectifiers which can cycle the current density from a base level up to a maximum level and back again, over and over again. For instance the current density can be increased from the base level and up to the max. level, and kept at this level for 0,25 seconds, and then dropped down again to the base level for 0,1 seconds before the next increase of the current density. The pattern of these changes is called square pulse. See figure below.

Pulsation of the current density.

If the current density is increased just for a very short time and then lowered back to a base level again, the temperature does not have time to increase. When anodising with use of a pulse rectifier the result is an anodic oxide layer with high wear and corrosion resistance due to the small diameter of the oxide pores. The oxide layer is obtained in 20ñ25% faster time. To produce an oxide layer with a thickness of 25 µm in a normal anodising process the time will approximately be 50 minutes. Using pulse anodising the same oxide thickness will be obtained in approximately 38ñ40 minutes. That means a saving of 10ñ12 minutes in addition to a raise of the quality of the anodic oxide coating.

In normal anodising the current density is kept at a level of 1,4ñ1,8 A/dm2 to obtain optimum wear and corrosion resistance. Due to the corrosive attack of the electrolyte(e.g. sulphuric acid) on the anodic oxide layer, it is important to carry out the anodising as fast as possible. Increasing of the current density will speed up the anodising process, but will also increase the temperature in the bath. This in turn increases the tendency of the electrolyte to dissolve the anodic layer. The diameter of the oxide pores will get bigger in the outer part of the oxide layer, resulting in a decrease in hardness, wear and corrosion resistance.