Pickling Before Electroplating And The Formulation Of Steel Pickling Solutions
● What is pickling ?
Pickling is the final operation in the preparation of parts prior to electroplating.
The degree of adhesion of the deposit on the metal depends to a considerable extent on the thoroughness with which the pickling is carried out.
The pickling results in removal of oxide films and the exposure of the metal surface, which improves the plating of the deposit.
A firm adhesion between the metal and the deposit can be obtained on metals which have been subjected to a special passivation.
The pickling is done by chemical or electrolytic treatment of the parts in solutions of acids and alkalies. The time of pickling is between 1/2 and 1 minute.
After pickling, the part should be quickly and thoroughly rinsed in running water and immediately transferred into the plating bath.
There must be no loss of time between pickling, rinsing and plating operations.
● Formulation Of Pickling Solutions
The following solutions are used for anodic pickling of carbon steel.
(1) Sulphuric acid = 700- 800 g/litre
(2) Potasium bichromate = 20-30 g/litre
In bath solutions, the anodic current density should be 100-150 amp/sq.ft.
Steel or lead plates can be used for the cathodes.
For the anodic pickling of Chrome, Nickel and stainless steel, use can be made of a 10-15 parts or % solution of sulfuric acid.
The anodic current density in the process is 100-150 amp/sq.ft. and pickling is continued for 1-2 minutes. The use of more dilute acid increases the risk of attack on the surface of the metal.
Because of this, it is always advisable to use a cathodic treatment in pickling prior to electroplating.
Good results are obtained with cathodic pickling of stainless steel in a 15-20% solution of Sulphuric acid with a cathodic current density of 10-20 amp/sq.ft.
The pickling is done for 1-2 minutes.
Steel parts that are to be Chromium plated are pickled by connecting them as anode in the Chromium plating bath.
The temperature of the bath is maintained at 50-55°C. the anodic current density at 150-200 amp/sq.ft., and the treatment time 0.2-2 minutes.
When Chromium plating is done on cast iron, the time of treatment should not exceed 25-30 seconds with a current density of 200-250 amp/sq.ft.
An increase in current density or longer treatment period roughens the surface of cast iron resulting in poor adhesion of the chromium deposit.
● Copper parts for electroplating are anodically pickled in the solution of following composition:
Potassium cyanide = 30-40 g/litre
Potassium carbonate = 20-30 g/litre
Water = to make 1 litre
The anodic current density should be 30-50 amp/sq.fl., and the treatment should be continued for 1 min.
Recommended article :