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

by Larry Zitko, ChromeTech, Inc.
March, 2003

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 attack.

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 experiment:

• 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!