Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Stripping Hard Chromium From an Aluminum Substrate
Q. I would like to know the best process for removing a hard chromium layer
(100 - 150m) on an aluminum substrate alloy with no or minimum surface
A.I ran up against this problem years ago with a large company in the
textile industry. They manufactured and repaired their own rolls, guides, spindles
etc. for use in the filament processes. The rolls were steel-clad aluminum.
The use of alkaline electrostrip solutions containing sodium hydroxide was
out of the question, due to the rapid attack to the aluminum components,
so historically the old chromium was mechanically removed by grinding operations prior to replating with hard chromium.
I found that alkaline electrostripping could be performed with a sodium
carbonate bath, with minimal attack to the aluminum. This approach may
operate at reduced efficiency, or have shorter bath life, than sodium
hydroxide-based solutions. Here are the parameters, in case you wish to
- Bath composition: 10 oz/gal sodium carbonate (wt/vol)
- Bath temperature: ambient
- Current density: 0.5 - 1 amp/in2.
- Polarity: Part is made anodic (+)
- Cathode: carbon steel
Follow up Q. This "minimum attack" that you have mentioned in your answer--can you be more precise?
Our parts ("two stroke cylinders") have some holes with screws,
that we must to keep in very narrow tolerances.
A. I can't quantify any rates. The task dates back to the mid 1980's, and we
never were required to measure a decrease in mass or size anyway. I have no
way of even knowing the aluminim alloy. I would encourage you to experiment
on some scrap parts with a test bath, then draw your own conclusions. My
previous comments were not meant to prescribe any course of action for your
company, but rather to share with you a chemical method that worked well on
a application that had some similarity (the presence of aluminum). Good luck
with your experiments!