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Passivation of Carbon/Tool Steel

Q. 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 reccommmend. We cannot use anything that leaves a coating on the mold.

Thank you in advance.

Joyce Wagner
- Ashland, Ohio
2003

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A. 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. McHenry, Illinois 2003 ---- Ed. note: There is a semantics issue here, so we ought to re-order the words in Lee's response to something like "citric acid and nitric acid cannot be used to passivate carbon steel" ... because although those chemicals are used in the ASTM A967 process for "passivation of stainless steel" the word 'passivate' has other meanings to other people: to zinc & cadmium platers and galvanizers in can mean chromate conversion coating, to phosphatizers it can mean a final dip in a chrome-bearing or other post-treatment rinse, to steam boiler engineers it can mean a coating on steel components achieved by proper operating conditions which slows corrosion, etc. A. 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 [deceased] - Goleta, California Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015. Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from. 2003

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Discoloration of Chrome Plated Parts after Citric Passivation

March 11, 2020

Q. First let me describe the part and its processes in detail, and then describe what's happened.

Part: Medical Drill Bit
Material: AISI D2 Steel, Annealed, 48Rc, Chromimum content 11.4% min, ASTM A681 [withdrawn]
Processes Prior to Citric Passivation:
1) Machined (turned)
2) EDM Wire Cut
3) Bending Process to Straighten Parts
4) Ultrasonic Cleaning
5) Hand Deburr
6) Heat-treat (Vacuum Furnace, Hydrogen Atmosphere, Per AMSH6875 and AMS2759/5)
7) Grind
8) Vapor Degrease
9) Glass Bead Blast
10) Electro-deposited Chromium Plating per ASTM B650 (2002)
11) Laser Mark
12) Passivation, Citric, per ASTM A967

The part looked silver and un-blemished prior to step#12 above (citric passivation). However, after the passivation process and prior to the water rinse, there were blackish areas which had appeared on the parts. After the rinsing with water after the citric tank, tan/light brown areas also appeared (maybe rust?). We ran some of these same parts again through citric passivation again and the same thing occurred. The rust-like coloration did disappear after coming out of the citric tank (still and the blackish discoloration) and then, once again, had tan/browning areas re-appear after the water rinse.

Of note, we had an order processed the same day of a similar but different part number. The operations were identical to the problematic parts but they appeared to be fine after completing the passivation. The only differences I could see was that the material lots were not the same (different material ID, heat-treat number, etc) and that the outside-service chromium plating was performed on different days.

Can anybody try to shed some light on why the one lot of bits discolored and the other turned out fine? Could it be either the difference in the raw material lots or maybe the an issue with the chromium plating?

Any ideas would be greatly appreciated. Thanks.

Mark Gilbert
- Naples, Florida, USA