Anodising in various electrolytes (Topic: 14054)

Sulphuric acid is the most common electrolyte used for anodising, but many others (combination of others) are also used in some extent to exploit special properties.

Phosphoric acid

Phosphoric acid anodising is rarely used to produce decorative or protective finishes, but it is used as a pre-treatment prior to application of organic (inclusive adhesives) or plated coatings. The reason for using phosphoric acid anodising as a pre-treatment is due to the good adhesion achieved. The good adhesion may be caused by the greater pore diameter in the oxide coating produced in phosphoric acid compared to the pore diameter obtained in sulphuric acid electrolyte.

The following process parameters are commonly used for phosphoric acid anodising:

electrolyte concentration: 10ñ30%(wt)

current density: 0,5ñ2,0 A/dm2 (DC)

voltage: 10ñ20 volts

temperature: 15ñ30ƒC

The coating thickness is usual around 3ñ5 µm.

Chromic acid

Chromic acid anodising is applied to obtain a protective coating, and as a pre-treatment step prior to organic coating. The most used chromic acid anodising process is the constant voltage process. The following process parameters are commonly used:

electrolyte concentration: 2ñ10%

current density: 0,3ñ1,8 A/dm2 (DC)

voltage: 30ñ40 volts

temperature: 30ñ40ƒC

The coating thickness of 5 µm is obtained in approximately 20 minutes, and 10 µm oxide film is obtained in 60 minutes. All the alloys in this manual are suitable for chromic acid anodising.

Oxalic acid

Oxalic acid anodising is used for producing relatively hard coatings. The oxalic acid processes are more expensive than the sulphuric acid processes both in chemicals and power consumption, but thicker coatings up to 60 µm may be obtained without the use of special techniques. Oxalic acid has been used to some extent in hard anodising as a protective coating, but may also be used of decorative purposes.

There are four standard processes used for oxalic acid anodising, and the process parameters are as follows:

electrolyte concentration: 3ñ5%

current density: 1ñ3 A/dm2 (DC)

voltage: 20ñ60 volts

temperature: 18ñ35ƒC

The treatment time is usually between 15 and 60 minutes with coating thicknesses between 10 and 60 µm.

The colour of the coatings varies between almost colourless, yellow, and dark, dependent on the process parameters and the alloy. Alloys containing heavy metals get dark.

Oxalic acid anodising is carried out with AC or DC, or a combination of AC and DC. The proportions of AC to DC may be varied according to the hardness and colour required. On aluminium-manganese alloys, e.g. AW 3003 and 3103, the AC/DC -process gives coatings of greater wear resistance than any other. Neither of the processes using AC is in wide use.

The wear resistance of DC-processes is best when the temperature is around 20ƒC and the voltage between 40 and 60 volts.

Agitation, power supplies and cooling are in principle the same as for sulphuric acid anodising, but the much higher voltage rectifiers and higher capacity refrigeration units are required.