Integral colouring (Topic: 14067)

Integral colour anodising processes are those in which colour is produced during the anodising step itself by use of special organic-acid based electrolytes. They are also sometimes referred to as "self-colour anodising" or "hard colour anodising" processes. They are all used at high voltages (up to 100V or more) and, as a result, the hardness and abrasion resistance are higher than that of films produced in a sulphuric acid electrolyte, hence the name "hard colour". Most processes produce ranges of bronze, gold, grey and black finishes, according to the alloys and anodising conditions used. In many cases special alloys have been developed for use with these processes.

One of the best known processes for integral colour anodising is the Kalcolor of Kaiser Aluminium. The main constituent of the electrolyte is sulphosalicylic acid (70ñ150 g/l) to which is added a small proportion of sulphuric acid (3ñ40 g/l) or metal sulphates in order to increase the conductivity of the electrolyte and hence reduce the operating voltage. The sulphuric acid content should be less than 10 g/l for coloured films. The operating temperature is around 22°C, current density between 2ñ3 A/dm2, voltage between 25ñ70V and processing time from 20 to 45 minutes. Under these conditions amber colour is obtained in 30 minutes and light brown colour in 45 minutes for an AW 6063 alloy.

The colours obtained with the Kalcolor process depend on the basis metal composition, current density, voltage, film thickness and electrolyte composition, particularly the sulphate and aluminium contents. It is necessary therefore to control all these factors carefully in order to obtain a consistent colour in the finished product. Depending on colour, the thickness of the Kalcolor coatings varies from approximately 20 to 30 µm.

Investigations have shown that three factors contribute to the colour of integral coatings. The first is the presence of intermetallic particles, or their reaction products, dispersed in the oxide. In general the more fine particles that remain trapped in the anodic coating the darker the colour produced for any particular anodising condition. Secondly colouring of the oxide matrix by ions such as chromium, copper or manganese, which are present in solid solution in the alloy, also occurs. Finally the oxide matrix can be coloured by the electrochemical process and it is this reaction that is often dominant with integral colour anodising processes. It has been shown that the third reaction involves the presence of metallic aluminium in the oxide.

The equipment for an integral colour anodising bath is considerably more expensive than that for a conventional sulphuric acid anodising process, and it is this high cost (and the high energy consumption during operation) that makes electrolytic colouring more advantageous.