Chemical polishing (Topic: 14053)

The chemical polishing processes are relatively of more recent origin than electropolishing and have replaced them to a considerable extent. Their primary attraction is the comparatively much smaller outlay in plant required, not only for electrical power, but also for tanks and fixtures.

Phosphoric acid based processes

Most of the solutions employed in industry for the chemical polishing of aluminium are based on phosphoric acid. Concentrated phosphoric acid at a temperature above 80°C will attack aluminium vigorously with the release of hydrogen to produce a surface which is very bright and without revealing the grain structure of the metal, but the finish is usually quite diffuse. When a proportion of nitric acid is added to this, the mode of attack is at once modified to produce a more specular surface. Specular surfaces are generally produced in mixtures of phosphoric and nitric acids to which numerous other additions can be made to secure a greater levelling action. Sulphuric acid is very often added (e.g. 10ñ40% by volume) to the mixture to remove or minimise the die lines.

The following composition is an example of a phosphoric-nitric acid process [27]:

optimum range typical solution

phosphoric acid 73ñ83%(vol.) 80,5%(vol.)

nitric acid 2ñ5% 3,5%

water 14ñ23% 16%

temperature approx. 90ƒC approx. 90ƒC

This solution is suitable for use on commercial aluminium, and on wrought alloys containing magnesium and manganese. The treatment time varies between 15 seconds and 5 minutes. Copper is frequently added to this solution to improve the finish.

Nitric acid - fluoride-type solutions

A number of solutions have been developed which contain nitric acid in rather more dilute solution with the addition of fluorides and sometimes other compounds. Best known of these processes is the E.W. (Erftwerk) process. This process is applicable only to high-purity aluminium and the high-purity AlMg or AlMgSi alloys which have been developed particularly for electrolytic and chemical polishing.

Alkaline solutions

A number of alkaline chemical polishing baths has been proposed, but these are now little used in industry as the finish is no better than that obtained by the acid solutions.