Anodising process parameters (Topic: 14064)

In the anodising bath the following parameters are important in controlling the process:

- current density

- voltage

- concentration of the electrolyte

- aluminium concentration

- agitation of the electrolyte

- temperature

Current density

The current density influences the quality of the anodic oxide coating. As shown in figure below the quality of the oxide layer is optimum when the current density is between 1,4 and 1,8 A/dm2 for a conventional electrolyte. If the current density is too low, long anodising time will be necessary leading to excessive electrolyte attack on the oxide layer which in turn gives poor quality of the anodic oxide.

The quality of the anodic oxide coating as a function of current density.

High current density increases the possibility of local heating of the aluminium surface which will reduce the oxide hardness, wear resistance, and corrosion resistance.

Voltage

The current density is usually kept constant during the anodising step, and the voltage will vary due to changes in temperature and the increasing oxide thickness, and hence electrical resistance. In normal sulphuric acid anodising the voltage is between 14 to 18 Volt.

Different alloys have different resistivity in the anodising bath. To achieve a current density of e.g. 1,5 A/dm2, the voltage has to be higher for a 3003 alloy than a 6063 alloy. To avoid this problem, profiles of different alloys should never be anodised at the same jig or in the same tank at the same time.

Electrolyte concentration

The concentration of the sulphuric acid is usually between 160 and 200 g/l. That is a optimum value regarding electric conductivity of the bath, and the electrolyte attack of the oxide coating. Small amounts of oxalic acid is often added to allow higher processing temperature.

Aluminium concentration

A freshly-made anodising bath will not work very well, due to lack of aluminium in the bath. The bath is in good condition when the aluminium content is between 8 and 12 g/l. When the content reaches approximately 15 g/l, regeneration of the bath - or a part of it - has to be done, because build-up of aluminium in the electrolyte gives reduced conductivity and raises the operating voltage required. Too much aluminium in the bath may also give a grey colour shade of the oxide layer. To achieve a good oxide quality the concentration of sulphuric acid and aluminium have to be kept within narrow limits, or constant as much as possible.

Agitation/temperature

The temperature of the profile surface is important to achieve a good quality of the oxide layer. The figure below shows the decrease in oxide hardness with increasing temperature. It is important to have good agitation of the electrolyte to prevent local heating. Even if the temperature of the bath is 20°C, the surface temperature could be much higher. This is illustrated in figure below.

Oxide hardness as a function of electrolyte temperature.

Surface temperature on anodised work as a function of air agitation [29]

By addition of oxalic acid in the anodising bath, higher temperatures (approx. 28°C) are possible during the anodising process without decreasing quality of the anodic film.