I am working in the field of nickel-cobalt phosphate electrodeposition. I have some questions. Please help me if possible.
1. Sodium hypophosphite and phosphorous acid are used as sources of phosphorus. Which one is better?
2. Many articles have used phosphorous acid. Phosphorous acid is present in either a solid white 98% (for example, from DAEJUNG company) or 30% phosphorus solution (eg, Merck company). The articles do not mention which one of them. How do I figure out which one?
3. Which of the two phosphorus acids did you use in your articles?
4. Is Ni-Co-P electrolyte unstable? And is Phosphorous acid oxidiezed to phosphoric acid?
5. Should I prepare the solution exactly before the electroplating, or can I use a prepared solution (for example, 1-2 weeks ago)?
6. What should be the current density range? Is the higher current density range such as 10-20 A/dm2 better (I think higher current density reduces the oxidation of phosphorous acid)?
7. What is the deposition rate of the coatings? How can I increase the deposition rate?
8. Is H3PO4 ib better than H3BO3 as buffering agent?
9. What is the better pH range for Ni-Co-P?
I don't have experience with Ni-Co-P, but generally I can answer that:
- All concentrations [g/l or mol/l], which are given in literature, should refer to the concentration of the pure component. So, you need to take the white powder, if you want to apply these concentrations directly. Otherwise, you can also calculate the amount of 30% solution, that you need (except if you need higher concentrations than 30%, which is unlikely the case). If you dissolve the white powder in water or dilute the 30% solution, should not make a difference.
- The current density as well as pH will depend a lot on what you want. For Fe-P, the phosphorus content of the deposit decreases with increasing current density and increasing pH, and I guess it is the same for Ni-Co-P. Besides, for this ternary alloy, also the ratio of Ni and Co will be affected by these parameters.
- The deposition rate is directly linked with the current density of the deposition process (Faraday's law). Thus, there are two possibilities to increase the deposition rate: 1. increase the applied current density and 2. reduce the amount of side reactions. Often, the first way works easily within certain limits. However, if current density is too high, side reactions become stronger and / or deposits become dendritic or even powdery. In this case, other deposition parameters (bath composition, temperature, ...) have to be modified either to reduce side reactions or to allow even higher current densities.
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