Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Randy Taylor, Advanced Tooling Corporation
October, 2007

Acceptable Levels of Impurities

Q. In reference to december 2006 question "Micro Cracks in Hard Chrome" It references "concentration of bath impurities (iron, copper, zinc, nickel, trivalent chromium,". I have been trying to find an answer to the same question reguarding cylinder O.D. on 4140 steel. I have been testing my solution and running a porous pot, but I'm not sure what level of impurities are acceptable. Can you give me any recommendations?

A. The acceptable range varies, depending on what you are plating and other factors. I think a good maximum level is 7.5 grams per liter (same as 1 oz/gal) combined cations. That includes Cr+3 and tramp metals (Fe, Al, Cu, etc.). However, I've seen plenty of baths operated well at levels above 10 g/l. I'm sure they use more electricity at that level due to high resistance.

Porous pot's don't do much good until the level reaches 10 g/l. If someone wants to keep their bath below 10 g/l and they have a lot of contamination, they need to look at ion exchange or bleed and feed (decanting).