PRESENTATION

ASSEMBLIES

It is advisable that assemblies which have been bolted or riveted together should not be sulphuric acid anodised because of the difficulty of rinsing out corrosive process solutions. Welds always show because of metallurgical differences between parent and weld metal. If it is necessary to anodise a welded assembly care should be taken with the selection of filler alloy in order to achieve reasonably uniform appearance, corrosion resistance etc, comparable with the parent metal. Advice should be sought from the anodiser with regard to welded constructions at the design stage, as there are numerous pitfalls.

CORNERS

Where corrosion resistance is a requirement of the anodising it is important that exposed edges be chamfered whenever possible to a radius of not less than 0.2mm, for two reasons:

• Since the coating is brittle a slight knock on a sharp edge can cause spoiling or chipping of the coat, whereas the same impact on the larger area of a radius would cause less damage.
• The mode of growth of the coating is normal to the surface; this means that there is likely to be a discontinuity in the coating at the sharp edge.

INSERTS

Non-aluminium inserts must be masked unless they are titanium, otherwise erosion will occur.

HOLES

Unless special precautions are taken, the coating tapers off down long holes, although the effect is less marked than with electroplating. Nevertheless, a useful coating can often be achieved, subject to the geometry of the workpiece. Where full coating properties are mandatory to the full depth, this can be achieved by additional work.

It is advisable to plug dowel holes and female threads but if this is not to be done it is important to allow manufacture for the decrease in diameter as the coating is a killer for taps and reamers.

THREADS

Care must be exercised when calculating effective growth on threads and the following guidelines should be noted. (see coating thickness & growth)

SURFACE FINISHES

The surface finish produced on components prior to anodising should always be better than that required after anodising. For a film thickness up to 10 microns, the surface roughening is not very significant but at a film thickness of 25 microns the surface roughness can, at least, double. It is advisable that samples should be processed to determine the extent of this effect.