I answered a question on the above subject in my February 2003 column. Here is the original question and my response.
We receive parts that have been heated treated and quenched. What is the best way to clean these parts prior to plating? All the methods we have tried have failed. N. H.
Sadly, I am not aware of any best way of cleaning these parts after they have been heat-treated. I assume that the parts have oil on the surface, and the oil is not removed prior to heat treatment. The result is that the oil is baked onto the surface and cannot be removed using the usual “wet methods” available in a plating shop.
The best cleaning method is to remove the oil prior to the heat treatment step. If this cannot be done, then the only thing I can suggest is some type of abrasive blasting process. If you are not familiar with this process, you can find information on line in the Products Finishing article “Mass Finishing”. Names of vendors of equipment and supplies can be found in the Products Finishing 2003 Directory & Technology Guide under Abrasive Blasting Equipment or at www.pfonline.com.
Additional Information: One of our loyal readers contacted me and suggested that the best way to clean heat-treated parts is to use the old Bullard-Dunn process or a variation of this process. The Bullard-Dunn process is an electrolytic process using a solution containing sulfuric acid, hydrochloric acid and tin or lead salts. The process not only descales the iron or steel but also prevents rerusting and over pickling of the surface by depositing a thin layer of lead or tin. One such formulation consists of:
|Concentrated sulfuric acid||4.75 g/liter|
|Concentrated hydrochloric acid||10.3 g/liter|
|Sodium chloride||22.5 g/liter|
|Lead or tin salt||1 g/liter|
|Current Density||50 – 80 asf|
The process can also be used without current to get satisfactory results.
In addition scale removal formulations using sulfuric hydrochloric and nitric acid, hydrofluoric acids are used for scale removal. Which formulation should be used depends on the substrate and to a certain extent the amount of scale present. A number of these formulations can be found in the Electroplating Engineering Handbook, 4th edition, edited by Lawrence J. Durney, Chapter 4. The book, which is an excellent reference, can be obtained from Metal Finishing Publications, 212-633-3100.