Summary for black oxide inconel Discussion
black oxide Inconel is generally not considered a workable true conversion coating, according to the thread. Kendrick Kim asked whether Inconel 718 could be black oxide plated after multiple shops said no. Ted Mooney explained that black oxide is normally a steel process, relying on the ability of boiling caustic oxidizers to convert iron-rich surfaces into a black oxide layer; Inconel 718 is mostly nickel and chromium with relatively little iron, so it resists that reaction. He agreed with the platers that it will not work in the usual sense. Ted also noted that Inconel can sometimes be discolored blue to purple by heating, and suggested that if the goal is simply a black appearance, a thin black coating from a PVD coater or possibly a selenium-based cold blackening test might be worth exploring, depending on adhesion and requirements.
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Q. Very simply, can Inconel be black oxided? I’m guessing that we’d have to use a bath similar to that used for black on stainless.
MIL-T-4 Stevenson
Syracuse, New York
May 11, 2023
A. Hi Milt.
That question in different form has been online here for 17 years without any encouraging answers.
It apparently can be discolored to a blue to purple color by heating, as discussed in thread 46264, “Discolorations on Inconel 718” though. Further, it would seem that cold blackening with a selenium-bearing solution ought to work if the adhesion is sufficient — a dab of gun blue / cold blue might be informative.
But as for a true black oxide conversion coating, it seems that inconel has too little iron in it, and too much nickel, chrome, and other stuff to form black rust.
Scholar.google.com lists a number of articles on the subject. As far as I can see, all of them are behind the Elsevier paywall, but we can at least read the abstracts.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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⇩ Similar, related, Q&As — oldest first ⇩
Q. Hello,
I am new to the wire industry and would like to know how to continuous oxide Inconel wire in small diameters.
Thanks,
George Potter
Bob Martin Company – So. el Monte, California, USA
2005
A. Sorry, George, I am not familiar with black oxiding of Inconel being possible. But what properties are you seeking and why? Is it just for color? Might cold blackening or anodizing be acceptable? Thanks.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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Is Black Oxide plating on Inconel 718 possible?
Q. Hi, I sent out an Inconel 718 part for black oxide plating and the platers told me that they cannot black oxide Inconel 718. They tried to multiple times, but it will not work. Multiple other platers told me the same. Anyone here know if black oxide is possible on Inconel 718? Thank you!
Kendrick Kim
– Union City, California USA
March 22, 2018
A. Hi Kendrick. Black oxide, according to the common vernacular, is a process for building an extremely thin (no dimensional change) black finish on steel. It happens because nitrite & other oxidizers in the boiling hot caustic solution are able to convert the surface of steel parts to this black oxidized condition. But Inconel 718 is extremely resistant to such oxidation, being over half nickel, plus 19% or so chromium, another something like 10% between molybdenum, columbium, titanium & cobalt — leaving less than 20% iron … so I would agree with the plating shops you’ve contacted.
Depending on exactly why you want/need a black finish, maybe you should talk to PVD coaters about applying a thin, hard, black layer? Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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Hi Ted, understood! That’s exactly the conclusion I came to myself, but wanted to confirm that here with the pros. I am confident now in stating this to the customer. Cheers.
Kendrick Kim [returning]
– Union City, California USA
Inconel 625 black finishes?
Q. Hello, I am interested in having a black finish applied to a machined inconel 625 part. Much like black anodize for aluminum. Needs to be thin and durable. The less reflective, the better. Suggestions?
Scott Rowe
Rowe Technical Design, Inc. – Dana Point, California USA
November 15, 2018