What surface treatment can turn nickel black?

Question

Questions:

• The detailed surface treatment to turn nickel plated steel black.
• The reaction mechanism of the process.

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Methods:

• (preferred) Anodizing, oxidizing : The only black nickel oxide is $\ce{Ni2O3}$ but I'm not sure if it's suitable for surface protection. I do not know the color or other Ni(II) or Ni(III) compound. There are nickel blackening solution for sale, but I don't know their ingredients.

• (not preferred) Electroplating onto the existing nickel: US military have a standard to electroplate black nickel (MIL-P-18317). However this process is quite a fuss.

Answer 1

The article provided by Karl has answered the question with electroplating method, which is not preferred but this is the only answer I have.

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• The method is based on MIL-P-18317. The electroplating solution consists of $\ce{NiSO4, Ni(NH4)2(SO4)2, ZnSO4}$ and $\ce{NaSCN}$

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• The black nickle coating are $\ce{NiS}$ and $\ce{ZnS}$.
• $\ce{NiS}$ and $\ce{ZnS}$ are deposited by $\ce{M^2+ + H2S -> MS + 2 H+}$.
• $\ce{H2S}$ is generated by $\ce{SCN- + 3H+ + 2e- -> H2S + HCN}$

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• Metal $\ce{Ni}$ and $\ce{Zn}$ are also deposited $\ce{M(NH3)2^2+ + 2e- -> M + 2NH3}$.
• The role of $\ce{NH4+}$ is to generate $\ce{M(NH3)2^2+}$ prior to metal deposition.

Answer 2

You can use PVD or CVD method. You will get a nano coating of Ni oxides. In PVD chamber you can heat Ni steel with $\ce{N2 + O2}$ or $\ce{Ar + O2}.$