Metal Treatment For Electroplating And Degreasing Solutions Composition
● The Metal Treatment Practice
The final operation in the preparation of parts prior to electroplating is pickling.
Parts which are to be pickled for the purpose of cleaning their surface before electroplating are subjected to the following treatments.
1. Degreasing
2. Rinsing in running water
3. Electrolytic pickling
4. Rinsing in running water
5. Neutralization
6. Rinsing in running water
7. Drying
8. Heating
All parts that are to be electroplated or pickled must be degreased first.
Even a very thin film of grease on any portion of the surface of the metal insulates it from the action of the electrolyte, and this may result in non-uniform pickling and lead to attack on the surface of the metal.
Parts contaminated with mineral oil are degreased with an organic solvent such as petrol or Dichloroethane and dried at room temperature until all traces of the solvent disappear.
● Hot Alkaline Degreasing solution for vegetable oils or animal fats
Parts contaminated with vegetable oils or animal fats are degreased in a hot alkaline solution of the following composition.
Caustic soda = 20-30 g/litre
Trisodium phosphate = 25-30 g/litre
For degreasing copper and its alloys a solution of 50-100 gram potasium carbonate per litre of water may be used.
For degreasing steel 5-10% solution of sodium silicate may be used.
After chemically degreasing, the parts are rinsed first in hot water and then in cold running water. They are then mounted on racks which are loaded in the electrolytic pickling vats.The racks are usually made of carbon steel.
A good contact between the work and the racks should be provided in the case of anodic pickling.
Unsatisfactory contacts may lead to local heating of the solution and attack on the metal in the region of the contact between the work and the rack.
To improve the uniformity of pickling over the surface of the work, the area of the counter electrode (cathode or anode, as the case may be) must be three to four times greater than the surface area of the work.
After pickling in acid solution, the parts are thoroughly rinsed in running water.
My sludge left on the surface of the work is removed by scrubbing with a brush under running water.
To neutralise residues ofacid, the parts, after rinsing in water, are immersed for 5-10 minutes in a 3-5% solution of sodium carbonate. Such a treatment is particularly necessary for castings and forgings having surface
imperfections.
Any acid remaining in these imperfections and pores may cause corrosion of the metal during subsequent storage.
After the neutralizing treatment, the parts are rinsed in water and dried.
For all steel parts that have been cathodically pickled in acid solutions, the final operation consists of heating the parts for two to three hours in a drying oven at 180-200’C.
A heat treatment of this kind will assist the removal of hydrogen absorbed by the metal and the restoration of the mechanical properties of the metal, which may have been affected as a result of the pickling treatment.
● Solution formulation for anodic degreasing
Before pickling, it is advisable to subject the work to anodic degreasing.
For this purpose, use may be made of a solution having the following composition:
Caustic soda = 85 gram
Trisodium phosphate = 30 gram
Water = 1 litre
The solution is used at a temperature of 70-80’C with an anodic current density of 50-100 amp/sq.ft., the treatment is carried out for about 10-15 minutes.
Vats for the storage of pickling solutions are made of iron. Vats for acid solutions are lined with vinyl plastics, acid-resisting ceramic tiles or rubber. Such linings stand heating up to 60°C.
The solutions are heated with steam in lead pipes laid along the bottom of the vats.
Electric heating can also be used.
The immersion electric heating elements may be protected by glass or ceramic sheaths.
The direct current for pickling is obtained from either generator sets or rectifiers giving a rectified current at 6V.
In the case of anodic pickling in concentrated solutions of sulphuric acid, prior to electroplating, direct current at a potential of 12V is required.
Alternating current for pickling can be taken from the mains through a step down transformer which reduces the voltage to 25 V.
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