How can we improve the coverage of our chromium plating in recessed areas? M. H.
This is a question that I am asked quite frequently, and I suspect most of the people asking it are hoping for a simple two-sentence answer! In the case of chromium plating, there is no simple answer. Hexavalent chromium baths have poor throwing covering capabilities when used to plate complex parts. There is no additive that you can add to the bath that will solve this problem. The answer lies in being able to distribute the current you are using during the plating step in a more consistent manner.
How do you do this? By carefully fixturing your parts. Fixturing means using robbers, conforming anodes, auxiliary anodes and shields to help you better distribute the current.
In one of my earlier columns, I listed a number of references that discuss this whole subject in more detail. Here they are again.
There are a number of books that discuss the process in detail. My favorite is The Handbook of Hard Chromium Plating by R. K. Guffie, 1986. The book is out-of-print but you should be able to purchase a used copy at www.amazon.com. The author worked in the “trenches” for many years and takes a very practical approach to the plating and fixturing process. Two other books that contain a lot of information are Nickel and Chromium Plating by J. K. Dennis and T. E. Such, 3rd edition, 1993 and Chromium Plating by R. Weiner and A. Walmsley, 1980. Both books are available from Metal Finishing Publications, (212) 633-3199.
I am aware of two papers that discuss rack design, anode configurations, shield and baffles. Both papers, authored by E. C. Krill, appeared in Plating and Surface Finishing, 76, January 1989, 62-69 and March 1989, 38-45.