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

by Randy Taylor, Advanced Tooling Corporation
November, 2007

Problem with Taper

Q. I maintain tank chemistry and have a ongoing problem of taper. We plate gun barrels and have a very low tollerance of thickness inside the barrel. We have been getting the right amount at the bottom of the barrel but not enough in the top. We do use robbers. Could tank purity cause this problem or sulfate ratio? Any ideas?

Thank you in advance.

A. Your chromium plating solution may be suffering from chrome ion depletion. It would make sense especially if you are plating barrels vertically. Depending on barrel length, the solution looses precious chemistry as it flow's upward, depositing chrome metal as it goes and eventually exiting out the top. Solution exiting has been well reduced chemically and needs to re-mix with the plating bath to be normal again.

In addition, in a confined area, such as a gun barrel, the hard working chrome solution is forced to mix with unwanted gasses on its path upward causing dilution and further inhibiting plating activity.

Third item to consider is anode size, material and configuration. Long anodes as well as long parts suffer from voltage drop and depending on the circumstance, you may be seeing the effects of a reduction deposit rate.

Resolution? I have some theories about chrome plating long tubular shapes, but suffice to say, I would first, consider slow pumping fresh solution into the barrel from one or possibly both ends. I know that sounds crazy, but there's a way to accomplish this. Secondly, I would review anode design, material and function and make the necessary corrections to optimize its value. Thirdly, If you're not already aware, I would review the attributes of motion plating, i.e., anode motion to maximize uniform current density and thickness.

RT