Possible to black oxide Inconel?

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

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.

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,

 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.