Process of Cleaning Metal Surfaces
For all metal coloring and electroplating a clean metal surface is essential. The cleaning process must remove mineral oils, organic oils and greases as well as traces of chemicals on the surface. It must remove oxidation which might interfere with the metal coloration or plating adhesion and it should possible activate or roughen the metal surface to better receive the treatment.
Cleaning may be accomplished using mechanical (abrasive), chemical (heating, solvents, pickling with acids) and electrolytic (electrocleaning, electrostripping) means. Examples of easily achieved clean surfaces include sandblasted ones and ones scrubbed well twice with rinsing using Fantastic®. A pumice rub followed by scrubbing with dish-washing liquid and ammonia on a toothbrush does pretty well. Best of all is electrocleaning.
A simple procedure is:
- 1.0 Anneal and pickle in suitable acid.
- 1.1 Rinse 3-5 times in running water.
- 1.2 Dip into simmering ammonia and detergent solution and scrub well.
- 1.3 Rinse well 5 – 10 times.
NB: electrocleaning can be used after step 1.3. While steps 1.0-1.3 produce a quite clean surface by themselves if one is electrocleaning it is not a bad idea to pre-clean using steps 1.0-1.3 so as to make the electrocleaning solution last longer and lower it’s work load in cleaning the metal surface.
A much better and surer procedure is:
- 2.0 Remove oxides with pumice.
- 2.1 Rinse well.
- 2.2 Electroclean for 1.5 – 2.5 minutes, object as cathode, then reverse the polarity for a few seconds.
- 2.3 Rinse 3- 5 times in running water.
- 2.4 Dip in 1:10 sulfuric acid to neutralize the electrocleaning solution and activate surface. (15 seconds or so).
- 2.5 Rinse 3 – 5 times in running water.
After cleaning, the metal should be placed in running or circulating water (preferably distilled) to avoid oxidation until it is plated or coloured. One can of course plate or colour immediately after cleaning. In all cleaning methods the piece must either be wired to suspend it in the solution or held with tweezers. It must afterwards be very well rinsed to remove all traces of cleaning chemicals. Do not touch the surface once it is free of grease.
Note: Sand blasting may be substituted for steps 1.0 and 2.0 above.
Sample Electrocleaning Solution:
|Current density: 1-3A/dm Polarity:
1 – 3 minutes Cathodic (of work)
5 – 10 seconds Anodic
|Sodium carbonate||50 grams|
|Trisodium phosphate||25 grams|
|Sodium Hydroxide||25 grams|