Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Chrome Plating Thickness Control
I have a carbon steel shaft 7.500" od, 6.660" id, 16.50" L.
I am specifying .002" thk chrome. I require a tight tolerance on the o.d. The part is meeting the .002" thick in the center, but from 1.00" at
both ends, the plating thickness is growing to about .005" thick, bringing my part out of tolerance on both ends. Is this thickness difficult to control or do I have a bad plating supplier?
I am also getting "feathering" buildup on the ends. I do not have a chamfer or radius on the ends. Would a chamfer or a radius prevent this condition?
Thanks in advance.
The problems you describe are quite typical of hard chromium
electroplating, which is well known for its low cathodic efficiency and uneven deposits. Unlike electroless nickel, which deposits more or less uniformly,
electrodeposited chromium will always tend to deposit at faster rate on regions of the part where localized current density (CD) is higher, and at
a slower rate where the CD is lower. The cylindrically-shaped shaft that you describe will naturally have
higher localized CD at its ends than in the middle region. The terms "dogbone" and "hourglass" are often used to describe the resultant shape after plating. The heavier the buildup of chromium, the more exaggerated the deviation in thickness will become. The feathering that you describe is commonly
referred to as "treeing" and are called "chrome trees". With heavy buildups, the chrome trees can grow to a inch or more in length.
Typically, parts are plated with excess chromium, then polished or ground after plating to achieve the desired dimensional tolerances. However,
there are some fixturing techniques that can be used to minimize the thickness deviation:
- Use conforming anodes, instead of "stick" tank anodes. That is,
place the shaft in a cylindrically-shaped anode. Make sure the part is
equidistant from and centered in the anode.
- If the anode is longer in length than the part, use a plastic
centering disk, attached to the bottom of the shaft, to shield the lower
region of the part from "seeing" the portion of the anode which is below
- You can use a conforming anode that is shorter in length than the
shaft being plated. Keep the top of the anode about an inch or two below
the top of the plated area of the shaft. Likewise, keep the bottom of the
anode above the part bottom by an inch or two. This technique really works well.
- If attachment is possible, install temporary steel "shaft
extensions" on the top and bottom of the shaft during plating. These extensions would
be the same O.D. as your shaft. Let all the irregular plating occur on these
sacrificial extensions, so that the plated area of your shaft comes out
better. These extensions will have to be periodically stripped of chromium