Mo-Ni alloy

WC-free cermet compositions such as TiC-Mo2C-Ni and TiC-VC-Ni-Fe-Co were synthesized in the 1930s for metalcutting applications. However, these alloys could not compete with the stronger WC-Co cutting tools. Improvements in tool performance were obtained in the 1950s with the additions of other carbides such as tantalum carbides (TaC) and niobium carbides (NbC) to the TiC-Mo-Ni alloys. 

On the basis of the observations described above, it was postulated that the precursor for the deposition of the Mo-Ni alloy was an adsorbed intermediate mixed-metal complex of the form [Ni-Cit(Mo02)]. This intermediate can be reduced, thus allowing Mo deposition. The Ni + ion, complexed either with Cit " or with NH3 (as discussed earlier for W codeposition), can be reduced in parallel with Mo, following the simple equations... 

Another mechanism for induced codeposition of Mo was suggested by Chassaing et al for electrodeposition of Mo-Ni alloys from citrate-ammonia electrolytes. Electrochemical impedance spectroscopy (EIS) measurements were carried out in order to better understand the different reactions occurring on the electrode surface during deposition. The proposed mechanism is based on a multi-step reduction of molybdate species. A M0O2 layer is formed via reduction of molybdate ion as in Eq. (42). Then, if free Ni is present in solution, this oxide can first combine at low polarization with Ni, following the reduction reaction ... 

Crousier et al. examined the role of hydrogen evolution in the process of deposition of Mo-Ni alloys on different substrates (glassy carbon, Ni and Pd). It was found that on carbon and Ni substrates, bright and smooth deposits were formed, while on Pd no alloy was formed. This observation was related to easy absorption and diffusion of atomic hydrogen into Pd, which prevented its availability for the alloy codeposition process. Hence, it was concluded that hydrogen plays an important role in the codeposition of the alloy. This conclusion of the authors is, however, not convincing. Firstly, it is known that hydrogen atoms can also permeate into Ni to some extent. Secondly, unsuccessful attempt to deposit Mo-Ni alloys on Pd may also be attributed, for example, to kinetic limitations. 

Rhee, W. H. and Yoon, D. N., The grain boundary migration induced by diffusional coherency strain in Mo-Ni alloy, Acta MetalL, 37, 221-28, 1989. 

Typical nodular morphology of the Mo—Ni alloy surface [35] is shown in Fig. 7.24a, while cross section (Fig. 7.24b) revealed the presence of large cracks in the electrodeposit [36]. Such behavior was characteristic for aU Mo—Ni alloys independently of the solution composition and applied current density. 

All abovementioned methods are expensive in comparison with the electrodeposition of Mo-Ni alloy coatings. Although molybdenum cannot be separately electrodeposited from aqueous solutions, it can be codeposited with the iron-group metals (Fe, Co, Ni) in the presence of appropriate complexing agents, by the type of alloy electrodeposition defined by Brenner [13] as induced codeposition. 

It should be emphasized here that only two papers concerning electrodeposition of Mo-Ni alloy powders (actually powders of the system Mo-Ni-O) and their characterization are published so far [14-16]. 

The Mo-Ni alloys possess several useful properties exceptional corrosion and wearing resistance [90-92], high catalytic activity for hydroprocessing of aromatic oils [93], and gas phase hydrogenation of benzene [94], as well as high hardness [95]. Their catalytic activity for hydrogen evolution has been one of the most investigated properties in the literature [96-110]. 

Most of the papers concerning electrodeposition of compact Mo-Ni alloy coatings are dealing with the mechanism of their electrodeposition (mechanism of induced codepositirm), and according to the literature, the most probable mechanism is the one reported by Podlaha and Landolt [117-120]. 

As Stated earlier the most probable mechanism for Ni and Mo codeposition is the one reported by Podlaha and Landolt [117-120] after X-ray fluorescence analysis of the electrodeposited alloy. Their investigations were performed under controlled mass transport conditions (rotating cylinder electrode). The model assumes that the Ni electrodeposition occurs on the surface not covered by the molybdate ions as a reaction intermediate, by direct reduction of nickel species (all of them being complex of Ni " cations with the citrate anions), independently on the molybdate reaction which can occur only in the presence of nickel species [117-120], The model of the Mo-Ni alloy electrodeposition is described by the following reduction reactions ... 

The main research interests of Vas ko were related to the electrochemistry of refractory metals where he was a well-known expert. He developed the process of galvanic coating of W-Ni and Mo-Ni alloys from aqueous electrolytes. Trying to explain the mechanism of this process, he assumed two stages of the formation of such alloys first, the deposition of solid film consisting of low-valency compounds and, second, electrochemical reduction of the film at the inner surface by solid mechanism. Thus, the solid non-metal film on the electrode surface for the first time became an active participant of the electrochemical process rather than simple passivative layer. This was a breakthrough. Further on, this electrochemical film system (EFS) concept was usefully applied not only in the aqueous electrochemistry but in the electrochemistry of molten salts and ionic liquids as well (see [7] for more details). 

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