Electropolishing (Topic: 14052)

The film which is formed under electropolishing conditions is not qualitatively different from that produced in industrial anodising processes.

Brytal process

The Brytal process is one of the commercial electropolishing processes. This process has retained its unique position of being the only commercially successful electrolytic brightener to employ an alkaline electrolyte. The process is particularly suitable for the treatment of super-purity aluminium (99,99%). Nowadays phosphoric acid based chemical brighteners are more and more frequently used. But primarily the Brytal process has survived because with no other method of treatment has it thus far been possible to obtain mirror finishes on large flat areas.

The Brytal process has the following composition [27]:

working range optimum

sodium carbonate 12ñ20%(wt) 15%(wt)

trisodium phosphate 2,5ñ7,5% 5%

temperature 75ñ90ƒC 80ñ82ƒC

voltage 7ñ16 volts 9ñ12 volts

The current density is usually between 2 and 5 A/dm2. The work should be carefully polished by mechanical means before treatment, and when this has been properly done, about 10ñ15 minutes in the Brytal bath is adequate, but treatment times of 30ñ40 minutes are common. There is a great deal of art as well as science involved in successful operation of the Brytal process.

Developments have been done to allow commercial purity aluminium alloys like AW 1050, 1070 and 1370 to be treated. (See also Chemical composition ). This is achieved by operating a modified bath at a lower pH than usual, preferably 9,5ñ10,5 (normally 11,0ñ11,6), and making sure that reaction products are continuously removed from the surface of the work (e.g. air agitation).

The main deleterious impurity in the Brytal bath is chloride which can lead to very pronounced pitting if it exceeds 0,1%.

Phosphoric acid electropolishing processes

These processes are generally characterised by faster dissolution rates than the alkaline baths. They have a more marked macro-smoothing action, but they are inferior in the production of surfaces of high specular reflectivity. Their main function is, therefore, to replace mechanical polishing on commercial aluminium or its alloys, e.g. prior to anodising. One of the best-known processes is based on a phosphoric acid-sulphuric acid solution, frequently with the addition of chromic acid.

These processes have to a great extent been replaced by chemical polishing.