Pickling

Pickling

Write a short note on pickling.
Answer:

After removing the investment the surface of the casting appears dark with oxides and tarnish. Such surface film can be removed by a process known as “pickling” which consists of heating the discolored casting in an acid.

• One of the best pickling solutions for gypsum-bonded investment is a 50% hydrochloric acid solution.
• Hydrochloric acid aids in the removal of any oxide coating as well as any residual investment.
• The disadvantage of HCl is that the fumes from the acid are likely to corrode laboratory metal furnishing.
• In addition, these fumes are a health hazard.
• Ultrasonic devices are useful for cleaning the casting, as are commercial pickling solutions made of acid salt.
• Abrasive blasting devices are also useful for cleaning the surface of the casting.
• The best method for pickling is to place the casting in a test tube or dish and pour the acid over it.
• It may be necessary to heat the acid, but boiling should be avoided because of the considerable amount of acid fumes involved.
• After pickling the acid is poured off and the casting is removed.