Zinc-based coatings can protect steel surfaces through two different mechanisms: the barrier protection and sacrificial protection. Alloying with 9-15% Nickel and the recently developed Zinc-Nickel Compositionally Modulated Multilayer Coatings CMMCs have been identified to offer great enhancement in the performance of electrodeposited zinc-based coatings. However, there are continuing industrial and scientific interests to develop improved protective coatings.
Nickel-Phosphorus coatings have been extensively implemented for industrial parts protection against wear and corrosion. This is reference to their superior attributes. Two methods have been adopted for producing Ni-P coatings; electrodeposition and electroless deposition. Although electroless deposition has been extensively implemented for the deposition of high quality thin coatings, electrodeposition is suitable for developing thick coatings owing to the high deposition rate.
Ni-P coatings can be used together with Zn-Ni sacrificial coatings to come up with multilayer coatings aiming at enhancing barrier attributes. Recently Ni-P/Zn-Ni compositionally modulated multilayer coatings have prepared via dual bath deposition method: Zn-Ni layers were electrodeposited while the Ni-P layers were deposited through the electroless method. The results indicated that the corrosion resistance of the resulted coating had a considerable edge over that of monolayers of Ni-P and Zn-Ni.
Unfortunately, the dual bath deposition process was found to be time consuming and only limited layers could be deposited. In addition, there was a possibility of solution contamination and substrate passivation. Therefore, Behrouz Bahadormanesh and Mohammad Ghorbani at Sharif University of Technology studied the effects of bath composition and deposition variables on the electrodeposition of Zn-Ni-P alloys. This was in a bid to develop a single bath capable of deposition of both Ni-P and Zn-Ni alloys as well as Ni-P/Zn-Ni compositionally modulated multilayer coatings. Their research work is published in peer-reviewed journal, Electrochemistry Communications.
The basis for the development of the bath was a large increase in the nickel deposition rate as compared to that of zinc at low deposition overpotentials in conjunction with the impossibility of codeposition of zinc with phosphorus.
The authors carried out a series of deposition process using different compositions of Zn-Ni and Zn-Ni-P baths and various current densities and bath temperatures. The overall Ni-P/Zn-Ni CMMC should have been sacrificial to the steel substrate and this criterion also was applied for the Zn-Ni layers of the CMMC.
Bahadormanesh and Ghorbani were able to develop a single bath for electrodeposition of Ni-P/Zn-Ni compositionally modulated multilayer coatings and determined appropriate deposition conditions. Although zinc ions concentration in the developed bath was much higher than that of nickel ions, the presence of sodium hypophosphite in the bath along with the deposition of phosphorus, efficiently inhibited codeposition of zinc at low overpotentials. This led to a deposit, which was nearly all Ni-P. Moreover, at higher current densities, the bath deposited a Zn-Ni alloy with 3.2wt% P. The authors found that the overall Ni-P/Zn-Ni compositionally modulated multilayer coatings was also sacrificial to the steel substrate.
Behrouz Bahadormanesh and Mohammad Ghorbani. Electrodeposition of Zn–Ni–P compositionally modulated multilayer coatings: An attempt to deposit Ni–P and Zn–Ni alloys from a single bath. Electrochemistry Communications, volume 81 (2017), pages 93–96.
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