Dear Tahereh Gholizadeh ;

this caused by the high molybdate concentration. you can add NaH2PO2 which hinders the discharge of molybdenum and promotes the evolution of hydrogen, resulting in a decrease in cathodic current efficiency. as detailed in the reference;

https://www.nmfrc.org/pdf/p1095b.pdf

Dear Tahereh Gholizadeh

Are you sure that deposition occurred on anode? In that case pH near the anode increase due to the oxygen evolution, and the formation of NiO powder is possible.

Dear Dr. Mahmood H. M.

Thank you alot for your response, I use 0.2 M Nickel sulfate, 0.003 M Sodium molybdate and 0.05M sodium citrate. pH is adjusted to 9 by adding ammonia. Is this salts ratio is OK? I think if I add NaH2PO2, the electrodeposited coating will have P in the composition and this is not my goal to achieve.

Many thanks to you dear Dr Branimir N. Grgur for your answer, as I mentioned a black layer forms of anode and the composition of the coating on the cathode is 99-100 wt.% nickel. (Anode is positive and the cathode is negative terminal in the setup.) It means that the molybdate ions would not reduced.

In my opinion you can try little addition and see the result.

With regards

Please look at the attached paper about electrodeposition of Fe-Mo coating.

Dear Tahere

During the electrowinning of nickel in a sulfate solution, a very thin black film is usually formed on the lead-based anodes. The thin film has no enough adhesion and can be easily removed from the surface. The reason of its formation in only the process of oxygen release (due to the anodic reaction) from the anode. In more detailed terms, the following reactions take place during the electrowinning form sulfate electrolytes:

Cathodic: Ni(2+) + 2e ---> Ni

Anodic: H2O ---> 2H(+) + 2e + 1/2 O2

Best of luck,

Amirhossein

Dear Amirhossein Alizadeh Many thanks for your answer.

Tahereh Gholizadeh You're welcome.