Sunday, January 13, 2008

Preparation of Basis Metal

If part are transferred from nickel or other baths to the chromium bath without undue delay, only an acid dip and water rinse may suffice. On the other hand if the nickel is buffed or the parts are handle or stored for a time, further treatment may be necessary before chromium plating.

The cleaning of work to be chromium for bright or decorative finish may be divided into two general classifications, wet cleaning and dry cleaning. Typical wet is discussed in next article. Dry cleaning consist in wiping the work on a buff wheel or by hand, without dipping it in solution of any kind.

Where wet cleaning is feasible, it generally gives better result than dry cleaning. It help to remove any oxide or tarnish on a nickel surface, whether visible or invisible, and result in "activating" the nickel or making it easier to chromium plate. Nickel surfaces are considered "passive" if they are oxidized and difficult to cover with a bright chromium plate. Cathode alkaline cleaning is quite effective in removing this condition it is not too severe. Acid dipping is even more effective. Typical acid dipping procedure to give maximum nickel activation are:
  • Immersion in 30 to 50% (vol) HCl for 30 to 60 sec.
  • Immersion in 20% (vol) H2SO4 for about 5 minutes.
  • Treatment at cathode at 4 to 6 Volt in 5% (vol) H2SO4 for about 15 seconds.
Where wet cleaning is not feasible, the plater must sometimes resort to dry cleaning. The success of this procedure depend on the fact that the chromium plating solution itself serves to some extent as both cleaner and acid dip. The vigorous evolution of gas during plating, together with the strong cleansing action of the hot chromic acid, tends to break down light dirt films. If the dirt, grease, and oxide are excessive, the cleansing action of the plating solution is overtaxed, with the result that the chromium plate is defective.
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