The anodising process provides a method of electrolytically increasing the thickness of the naturally occurring oxide film on aluminium and its alloys, to give a coating which may be considered in many ways analogous to the case hardening of steel. It is distinguished from ‘decorative’ or ‘architectural’ anodising in that the process is operated in a manner, which gives optimum technical properties without regard to aesthetic effect; thus, the coatings may be grey or tinged with yellow or brown, according to the alloy and coating thickness. Colouring may be carried out by dyeing after anodising, but the colours so obtained will be diluted by the ‘natural’ colour of the coating. Dichromate sealing also imparts a colour to the film. Anodising lowers the fatigue strength of alloys but this reduction is said to be restored to some extent by hot sealing.
It is difficult to present a simple picture in writing of the possibilities and limitations of the process because there are many factors that affect performance of the coatings, but the following will give a general picture that can be expanded in more detail when we know the nature of your interest.
Hard anodising produces darker coatings than the normal ‘decorative’ process, but the exact shade depends upon coating thickness and anodising conditions (which we can control), as well as alloy, thermal history, degree of hot or cold work, etc, over which we have no control. We cannot, therefore, colour match except between components from the same batch of metal. Some specifications allow the anodiser wide tolerances in his process, which must be closed down if matching is to be achieved. Matching must therefore be covered at the time of order.
The effective growth on diameter can be calculated by the following formula:
Effective growth = applied film thickness where q is the angle shown on the sketch above.
This is the allowance that must be used to calculate allowances for growth due to anodising.
Please remember that on any blind deep holes the anodic film thickness will drop off considerably with depth.