COATING THICKNESS Most people call for hard coatings in the 25-50 micron range; this gives a good compromise between cost and reliable performance, but we can give an indication of the effect of going outside this range in particular cases, It should be noted that the coating is derived in part from dissolution of the metal surface, so that a 50 micron coating causes an increase in surface level of 25 microns only, or an increase in a diameter of 50 microns. It should be noted that increasing thickness does not necessarily confer proportionate improvement in performance. Thicker coatings can be obtained and have been found beneficial where thermal or electrical insulation is required, but their relative wear resistance fails off and cost increases more than proportionately to the film thickness. Useful sulphuric acid coatings can start as low as 5 microns. These thinner films may be preferred to avoid the need to stop-off threads and other mating parts. Thinner coatings will minimise the degradation of the surface finish. When using sulphuric acid anodising to give good resistance to handling and corrosion, and to accommodate black dye in particular, a coating thickness of 25 microns is recommended. Where pale colours only are required or when service conditions are relatively benign, coatings down to 10 microns nominal may be specified. We can advise on this. Chromic acid anodising produces films 2 - 5 microns thick. GROWTH DUE TO ANODISING The anodising process converts aluminium into aluminium oxide Al 2 O 3 . This species has a volume roughly twice that of the aluminium from which it was formed, this gives rise to an approximate growth of half the applied anodic film thickness. For example a 50 micron film will use up 25 microns of aluminium in its formation, thus it will create growth of only 25 microns per face. An internal diameter will decrease by 50 microns if such a film is applied, an external diameter will increase by the same amount. This must be borne in mind when making allowances for growth: Remember-growth is approximately equal to half the applied film thickness per face* A special case that needs discussion is that of growth on threads: Although the film grows on threads in exactly the same way as it does on anything else, the thread angle causes an effective growth, which means that a greater allowance needs to be made for threads. The effective growth on diameter can be calculated by the following formula: EFFECTIVE GROWTH Effective growth = applied film thickness where q is the angle shown on the sketch above. ½ sin q 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. the anodising process about chromate coatings how to specify electroless nickel plating specifications

Anodising and Coating Thickness Considerations

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COATING THICKNESS

Most people call for hard coatings in the 25-50 micron range; this gives a good compromise between cost andreliable performance, but we can give an indication of the effect of going outside this range in particular cases, Itshould be noted that the coating is derived in part from dissolution of the metal surface, so that a 50 microncoating causes an increase in surface level of 25 microns only, or an increase in a diameter of 50 microns.

It should be noted that increasing thickness does not necessarily confer proportionate improvement inperformance. Thicker coatings can be obtained and have been found beneficial where thermal or electricalinsulation is required, but their relative wear resistance fails off and cost increases more than proportionately tothe film thickness.

Useful sulphuric acid coatings can start as low as 5 microns. These thinner films may be preferred to avoid theneed to stop-off threads and other mating parts. Thinner coatings will minimise the degradation of the surfacefinish.

When using sulphuric acid anodising to give good resistance to handling and corrosion, and to accommodateblack dye in particular, a coating thickness of 25 microns is recommended. Where pale colours only are requiredor when service conditions are relatively benign, coatings down to 10 microns nominal may be specified. We canadvise on this.

Chromic acid anodising produces films 2 - 5 microns thick.

GROWTH DUE TO ANODISING

The anodising process converts aluminium into aluminium oxide Al2O3. This species has a volume roughly twice

that of the aluminium from which it was formed, this gives rise to an approximate growth of half the applied anodicfilm thickness.

For example a 50 micron film will use up 25 microns of aluminium in its formation, thus it will create growth of only25 microns per face.

An internal diameter will decrease by 50 microns if such a film is applied, an external diameter will increase by thesame amount.

This must be borne in mind when making allowances for growth:

Remember-growth is approximately equal to half the applied film thickness per face*

A special case that needs discussion is that of growth on threads:

Although the film grows on threads in exactly the same way as it does on anything else, the thread angle causesan effective growth, which means that a greater allowance needs to be made for threads.

The effective growth on diameter can be calculated by the following formula:

EFFECTIVE GROWTH

Effective growth = applied film thickness where q is the angle shown on the sketch above.

½ sin q

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.

the anodising process about chromate coatings how to specify electroless nickel plating specifications