Summary for reddish brown coating Discussion
Reddish brown coating on hand tools is hard to identify from appearance alone, and the thread cautions against guessing a finish without more information. The question from Sandip Sanagar asked about the coating on Allen keys or screw bits. The reply notes that one possibility is Corten weathering steel, which is intentionally made to rust, though only to a limited extent. It also mentions black oxide, the same as gun bluing, as another example of a controlled rust finish formed by immersion in a very hot caustic solution with nitrites/nitrates. The practical takeaway is that a reddish-brown look does not by itself prove the coating type, and more context or testing would be needed to identify it reliably.
Quickstart:
Unfortunately, it’s difficult to guess what a finish actually is based just on its appearance.
This thread mentions ‘Corten’ weathering steel which is designed to rust (but only to a limited extent), and ‘black oxide’ (which is the same thing as gun bluing), a black rust formed by immersion into very hot caustic solution with nitrites/nitrates.
⇦ Tip: Readers like to learn from others’ situations;
some readers skip abstract questions.
Q. What is Reddish brown coating on Allen keys or screw bits
Sandip Sanagar
Owners of a electroplating ind. – Nashik , Maharashtra, India
September 24, 2025
A. Hi Sandip
Sorry but I don’t know. Please <send pics for posting> and additional descriptive info about your situation.
I suppose it’s possible that cheap tools could be made of Corten weathering steel ⇦ huh? , but it doesn’t seem like you could make self-weathering tool steels because they are heat treated high carbon steels.
Poor quality black oxide could be reddish brown, but I doubt that that is what your are interested in either.
Are you sure you’re not just looking at lightly rusted items?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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