June 28, 2008

Degreasing and Etching before Electroplating

Sound, continuous, adherent electroplated coating can only be obtained on a chemically clean and firm metal foundation. Therefore the degreasing, thorough cleaning and etching of a metal article before electroplating is a vital preliminary step. Similarly, through cleaning is necessary before most wet finishing processes, and also to some extent before painting and other dry finishing processes.

The first part of the preparation is the removal of all traces of grease or oil, so that the metal article is freely wetted by the electroplating solution. Indeed the test for the satisfactory completion of the degreasing process is that the metal surface remains completely wetted all over, without any visible 'water-break', even after dipping in a slightly acid solution.

Metal article will often be lubricated with oil or grease during their manufacture, on smoothed articles traces of greasy or oily films to the articles. Metal surfaces have types of organic compounds, such as fatty acids and fatty alcohols, which are common polar groups attach themselves tenaciously to a metal surface, whilst the long tails of carbon atoms form a thick greasy barrier to the approach of any other substance to the metal. Deliberate use is made of this property in lubricants by including these polar substances and dirt. Sometimes, by the action of heat, light or pressure, the organic material is further changed, or polymerized, on the surface to resinous or tarry materials, which are even more difficult to dislodge. The purpose of the degreasing step is to different in principle from those used domestically for dry-cleaning, clothes washing or dish washing except that they are more through.

To remove of heavy film of oil or grease is most conveniently done by dissolving them in an organic solvent. Normal hydrocarbon solvent such as petrol, paraffin and white spirit present too great a fire hazard; their light vapors form an explosive mixture with air, and spread readily. Heavy, non-inflammable chlorinated hydrocarbons are therefore used, principally trichlorethylene (C2HCl2) (colloquially known to the plater as 'tri'), and perchlorethylene (C2Cl4).

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