Czasopismo | INŻYNIERIA MATERIAŁOWA | Rocznik 2013 - zeszyt 3

Ni-Mo alloys electrodeposited under direct current from citrate-ammonia plating bath

nr katalogowy: 78019

Ni-Mo alloys are characterized by high hardness, wear, thermal and corrosion resistance [1, 2]. Due to this reason they could offer an important alternative to hard chromium coatings, which according to EU directives (2000/53/WE, 2011/37/UE) have to be eliminated from manufacturing processes [3]. However, these alloys are difficult to obtain by conventional thermal methods, what is caused by the large difference in metals melting points (Ni . 1455 C, Mo . 2620C) and the limited mutual solubility. Convenient way to produce these type of coatings, which overcomes above mentioned problems, is a low-temperature and relatively simple electrodeposition technique. It enables uniform surface covering with simultaneous control of thickness and microstructure and thus allow to influence the properties of the layer. The mechanism of Ni-Mo alloys electrodeposition is still not clearly understood, although a few hypotheses are presented in the literature [4?€7]. Nevertheless, it is known that molybdenum (as well as another reluctant elements, such as W, Ge) cannot be deposited alone from aqueous solution of their salts. However, it could be readily co-deposited with iron-group metals (such as Ni, which acts as a catalyst) with an alloy formation. This phenomenon was called induced co-deposition by Brenner [8]. However, Ni-Mo coatings deposited from solution containing only molybdenum and nickel ions are of poor quality and contain high amount of molybdenum oxides. This effect is probably related to the formation of multimolecular heteropolymolybdates, which are difficult to electroreduce. Addition of an appropriate complexing agent, such as sodium citrate (characterized also by buffering, leveling and brightening properties), causes decomposition of heteropolymolybdates and the formation of the electroactive molybdenum [MoO4(Cit)H]4. and then nickel [NiCit]. citrate complexes (Cit = C6H5O7 3.) [9]. It results in an im[...]

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