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Metal finishing Q&As since 1989

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Oxide Finishing

Q. I have a question about oxide finishing. I had some tools that were rusted due to HCl vapors. I brushed the rust off and soon after the rust started again, but very lightly. I then brushed them off again and sprayed them with WD40 and stood them up in a tall plastic container. Because of all the tool that were placed in there, I had a 1/2" of oil that covered the tips of the tools.

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Weeks later when I pulled the tools out and wiped them there remained a 1/2 black oxide finish on the tip of the tools. Could someone with more knowledge explain to me what happened and could it be repeatable?

Thank you.
David

David Juarez
- Corona, California
February 29, 2024

A. There are many old gunsmith black finishes. All of them are based on formation of very thin rust layer. Probably some of your tools are based on more corrosion resistant steel alloy. Simplest formula for gunsmith's black are ferric chloride (or ferrous too) based (1,5% water solution -- according to Angier's book "Firearm Bluing and Browning" [on eBay , Amazon, AbeBooks] , Onslow County 1936, p.78 ) ... Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia
March 2, 2024

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