Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Q. I have read several of Ask the Expert Question-and Answers, it seems to me that the method to strip hard chrome is sodium hydroxide and elctrostripping . To clean our molds we are using Sodium hydroxide and Potassium hydroxide with a 13pH at 165F in an ultrasonic tank without any measurable current . Under these conditions would expect this cleaning process be detrimental to the chrome?
A. Chromium can be electrostripped in sodium hydroxide and potassium hydroxide solutions, but the workpiece is made anodic (attached to the positive pole of a direct current power supply. Typically, we are in the 3 to 12 VDC range. Most alkaline soak clean tanks contain a sodium hydroxide component, and chromium plated regions seem unaffected by simple immersion in the solution, even at elevated temperatures.
I am not sure what effect the ultrasound would have on pre-existing chromium, but if it was plated with good adhesion and microstructure, I wouldn't expect it to delaminate during ultrasonic cleaning. I once designed and installed a hard chrome plating line for a company that manufactures and reconditions transfer rubber mold inserts. They have always used ultrasonic cleaning with good success, although I am not certain if every part is stripped of all chromium before replating.
Follow-up Q. I have been told that since chrome is amphoteric that it will deteriorate with sodium and potassium hydroxide. Do you disagree?
A. I'm not a degreed chemist or metallurgist, just a system designer with lots of tankside experience. I was not aware that electrodeposited chromium would either a) act as a weak acid or base, or b) react as electropositive or electronegative, as your adjective implies. Have you seen visual or physical evidence that the chromium electrodeposit is attacked by the caustic solution or ultrasound?
Follow-up Q. We have seen chrome coming off of our inserts. The cause is what I am seeking.
A. This sounds like an adhesion problem. Without a good bond, the chrome could easily separate from the molds in service.
The best general advice I can give would be to examine the chrome plating cycle for failures in the preplate cleaning or activation processes. I find that adhesion problems are usually caused by ineffective cleaning or improper etch. Peeled chrome can also be caused by DC power supply problems, but this would be evident when the parts are pulled from the plating tank - even before they are put into service.
I solve a lot of these problems on a consulting basis, but it usually requires a site visit. It has been my experience that adhesion problems are less common than other plating defects, and are generally able to be overcome.
Follow-up Q.The main reason I am suspicious of the sodium and potassium hydroxides is that we use four chrome platers and we are having problems with inserts from all four.
A. I researched my email and found that I did get an inquiry once from someone who worked in the cleaning lab at a large corporation. He claimed that ultrasonic cleaning in a 12 pH solution "crazed" the chrome, that it became duller and that the microcracks enlarged.