Letter 19004

Passivation of Carbon/Tool Steel [Ohio] 

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We mold various rubber and plastic parts. Some of the molds we can sand blast, but some must be cleaned using a combination of water and beads. The molds are carbon steel/tool steel, and after cleaning them, we use chromic acid to "passivate" the surface to prevent corrosion during storage.

We would very much like to find a replacement for the chromic acid. Would citric acid or nitric acid work for this application? Is there anything else that someone can recommmend. We cannot use anything that leaves a coating on the mold.

Thank you in advance.

Joyce Wgner
- Ashland, Ohio, USA

Carbon steel cannot be "passivated", unfortunately. You can search this site to see ways to improve the corrosion resistance of carbon steel, but citric or nitric acid are not going to help you. You can get in real trouble if you put it in nitric acid!!!!

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA

Your procedures are a bit unusual and have maybe discouraged suggestions. Are you abrasive blasting to remove rubber residue or rust? I have never heard of chromic acid being used on these materials. Two traditional coatings for this purpose are hard chromium and electroless nickel (heat treated for hardness) plating. Either will reduce wear, adhesion and rusting.

Ion implantation using Mo or Ta was recommended for improving the corrosion resistance and hardness of S7 tool steel in an ASM Int. forum. [Nitrogen implantation is commonly used to improve hardness, but usually only on specific alloys and not for corrosion resistance.] For uncoated molds, a dilute phosphoric acid-based cleaner should remove rust and slightly passivate the surface. Coated surfaces don’t require any acid. In any event, quit sand blasting; it encourages adhesion of rubber and rusting of uncoated steel (microscopic roughness) and might damage any coating. The water+ beads blast is probably OK, or its pressure can be adjusted to only remove rubber. Maybe examine surfaces under magnification to verify.

The links below should provide you with a choice of metal finishing jobshops and chemical suppliers within a reasonable distance.

Ken Vlach - Goleta, California

About 20 years ago I was involved in "passivating" carbon steel. We did the work for the nuclear industry. The process details were provided by the customer. I don't have any information left in my records, but from memory I recall the process used a solution containing sodium nitrate and sodium hydroxide and was heated. Maybe somebody else has more information about this process.

Thomas Kemp
- Erie, PA, USA

If mold is not very big it can be passivated by high-voltage electropolishing process for few minutes in special solution of salts. The surface will become slightly yellow due to oxide film. Our experiments show that such film protects surface agains rusting.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania

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