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3czasopismo naukowo-techniczne

redagowane przy współudzialePOLSKIEGO

TOWARZYSTWAMATERIAŁOZNAWCZEGO

NR 3 (193) ROK XXXIVMAJ–CZERWIEC 2013

ORGAN NACZELNEJ ORGANIZACJI TECHNICZNEJ

AGNIESZKA bIGOS, EWA bEŁTOWSKA-LEHMAN, bOGUSZ KANIA, MACIEJ SZCZERbA

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

Agnieszka bigos M.Sc. (a.bigos@imim.pl), Assoc. Prof. Ewa bełtowska-Lehman, bogusz Kania, Maciej Szczerba Ph.D. Eng. – Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, Kraków

INTRODUCTION

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 dif-ficult to obtain by conventional thermal methods, what is caused by the large difference in metals melting points (Ni – 1455°C, Mo – 2620°C) 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 electrodeposi-tion technique. It enables uniform surface covering with simultane-ous control of thickness and microstructure and thus allow to influ-ence 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 con-tain 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 decomposi-tion of heteropolymolybdates and the formation of the electroac-tive molybdenum [MoO4(Cit)H]4– and then nickel [NiCit]– citrate

complexes (Cit = C6H5O73–) [9]. It results in an improvement of the

quality and an increase of the molybdenum content in the deposits.Electrodeposition of the homogenous, crack-free and adherent to

substrate Ni-Mo alloys is a complex process, affected by many fac-tors and their interaction. Thus, the control of electrolyte solution composition and electroplating parameters is necessary to obtain (with high current efficiency) deposits with desirable composition, microstructure and mechanical properties. The main aim of this work was to establish the influence of chosen electroplating pa-rameters: pH of the bath and temperature as well as hydrodynamic conditions on chemical and phase composition, microstructure and crystallite size of Ni-Mo alloys obtained from a citrate-ammonia electrolyte solution of previously determined optimal concentration ratio of reagents.

EXPERIMENTAL DETAILS

All of the binary Ni-Mo alloys were electrochemically deposited from aqueous solution containing analytical grade purity chemicals: 0.2 M NiSO4, 0.006 M Na2MoO4 and 0.3 M Na3C6H5O7. No bright-eners, detergents and wetting agents were used. Bath pH in the range from 4 to 10 was adjusted by an addition of sulphuric acid or am-monia. The electrodeposition was conducted in a model system with a rotating disk electrode (RDE) (without and in the rotation speed range: 130÷640 rpm) in the galvanostatic regime, at temperature from 20 to 60°C. Low carbon steel disks (~0.028 dm2) were used as a cathode, which prior to electrodeposition were chemically pol-ished in the solution of hydrogen peroxide and oxalic acid. As an an-ode a platinum wire (~0.5 dm2) was used. The chemical composition was analyzed by energy dispersive X-ray spectroscopy and a mean value was calculated from a minimum of five measurements. The surface and cross-section morphologies of coatings were examined by scanning electron microscopy (ESEM FEI XL-30). X-ray diffrac-tion technique (Bruker D2 Phaser with a copper anode) was used to determine phase composition and an average crystallite size of de-posits (calculated using Rietveld analysis in MAUD software) [10].

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Fig. 1. Molybdenum content (wt %) in Ni-Mo alloys and cathod-ic current efficiency as a function of the electrolyte solution pH (j = 2.5 A/dm2, rotation speed of RDE 640 rpm)Rys. 1. Zawartość molibdenu (% mas.) w stopach Ni-Mo oraz katodo-wa wydajność prądowa w funkcji pH roztworu elektrolitu (j = 2,5 A/dm2, prędkość wirowania WED 640 obrotów/minutę)

Fig. 2. Surface morphology of Ni-Mo coatings electrodeposited from the electrolyte solution of different pH: 4, 7 and 9 (j = 2.5 A/dm2, rota-tion speed of RDE 640 rpm); SEMRys. 2. Morfologia powierzchni powłok Ni-Mo elektroosadzonych z roz-tworu elektrolitu o różnym pH: 4, 7 i 9 (j = 2,5 A/dm2, prędkość wirowa-nia WED 640 obrotów/minutę); SEM

RESULTS AND DISCUSIONElectroplating of alloys is generally affected by the type and com-position of galvanic bath as well as by operating parameters of elec-trolysis. Based on the previous studies, it was established that in the analyzed system it is optimum when in the electrolyte solution concentration of citrate ions is slight higher than the total concen-tration of the metal ions and also when the concentration of Ni(II) is significantly higher than Mo(VI) ions [11, 12]. In this conditions the partial current density of molybdenum is affected by the rate of deposition of nickel, what proves the inducing effect of nickel on the refractory metal deposition. Hence, all alloys where deposited from the electrolyte solution with the same optimized composition in order to better understand the influence of chosen electroplating parameters: pH of the bath and temperature as well as hydrody-namic conditions on properties of Ni-Mo alloys.

Galvanic bath pH

The influence of the electrolyte solution pH on the current efficien-cy of the cathodic process, the molybdenum content and the surface morphology of Ni-Mo alloys electrodeposited galvanostatically at current density of 2.5 A/dm2 are shown in Figures 1 and 2.

An increase of bath pH causes gradual increase of the molybde-num content in Ni-Mo deposits up to 50.4 wt % at pH 7. Below this value considerable decrease of the molybdenum content in the alloy (up to 13.9 wt % for pH 10) is observed, what is related to addi-tion of ammonia necessary to adjustment of pH value. It promotes formation of ammonia complexes of nickel [Ni(NH3)n

2+], which similarly to electroactive citrate complex [NiCit]– could be electro-deposited on the cathode (Cit = C6H5O7

3–) [12]. As seen in Figures 2 and 3, this dependence corresponds to refinement of the average crystallite size and improvement of surface morphology of the coat-ings from non-homogenous with visible cracks to more compact and crack-free. XRD analysis revealed that Ni-Mo alloys consist of a face-centered-cubic solid solution of molybdenum in nickel, what has been proved by the absence of peaks related to molybdenum in the diffraction patterns. The finest grains (~2 nm) for coatings with the highest molybdenum content (50.4 wt %) were obtained. De-crease of refractory metal concentration (to ~7.9 wt %) in deposit caused an increase of grain size to about 33 nm.

Moreover, from the electrolyte solution of pH above 7 Ni-Mo coatings with the highest current efficiency were deposited (Fig. 1). As seen, electrodeposition process was the most efficient at pH 10 (about 80%), however an addition of a large amount of ammonia was necessary to reach this pH value. Due to this reason, as the optimal, pH 9 was chosen for further investigation.

Galvanic bath agitation

Change in hydrodynamic conditions was achieved by varying the rotating speed of the disk electrode from 0 to 640 rpm. This proce-dure accelerates the rate of mass transport of metal ions to the cath-ode surface, what has significant influence on the diffusion con-trolled electroreduction process of Mo(VI) species [11, 12]. In this condition a greater amount of molybdate ions reaches the cathode surface. However, when the concentration of molybdenum ions in electrolyte solution is too high, only part of them is reduced to me-tallic form. An excess of Mo(VI) ions forms multivalent oxides lay-er, which blocks the electrode surface, resulting in a decrease of the current efficiency of nickel and molybdenum codeposition process.

Figure 4 illustrates the effect of the disk electrode rotating speed on the chemical composition of Ni-Mo coatings and on cathodic current efficiency of the deposition process.

In the analyzed example, an increase of the rotating disk speed causes growth of the molybdenum percentage in the deposits and cur-rent efficiency of the cathodic process. Ni-Mo layers, with the highest Faraday efficiency (about 85%) were obtained at 640 rpm. These re-sults confirm that the use of the plating bath of the composition given

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Fig. 3. X-ray diffraction patterns (CuKα) of Ni-Mo alloys electrode-posited from electrolyte solution of pH 4, 7 and 9 (j = 2.5 A/dm2, rota-tion speed of RDE 640 rpm)Rys. 3. Dyfraktogramy rentgenowskie (CuKα) stopów Ni-Mo elektro-osadzonych z roztworu elektrolitu o pH 4, 7 i 9 (j = 2,5 A/dm2, prędkość wirowania WED 640 obrotów/minutę)

Fig. 4. Molybdenum content (wt %) in Ni-Mo alloys electrodeposited at 5 A/dm2 and cathodic current efficiency of the deposition process as a function of RDE rotating speedRys. 4. Zawartość molibdenu (% mas.) w stopach Ni-Mo elektroosadzo-nych przy gęstości prądu 5 A/dm2 oraz katodowa wydajność prądowa w funkcji prędkości wirowania WED

previously prevents formation of oxides layers (even at the highest stirring speed) and allow to obtain Ni-Mo alloys with good quality.

Coatings deposited at stationary conditions (without disk rota-tions) revealed rough and irregular surface morphology (Fig. 5). Increase of RDE rotation speed induces beneficial morphology im-provement to more compact and regular. Nevertheless, increase in RDE rotating speed causes the film surface to become rougher, be-cause the solution agitation supplies higher amount of ions, which can deposit on the side face of nodules [13].

The use of higher rotating speed results in the line broadening on X-ray diffraction patterns of Ni-Mo alloys (Fig. 6). This is caused by increase of the refractory metal content in the deposit (from about 1.8 to 12.5 Mo wt %), which acts as a modifier reducing grain size of the alloy (from about 50 to 9 nm) [1, 14].

Galvanic bath temperature

It is known, that the rise of temperature of the galvanic bath causes a higher electrolytic conductivity and an improvement in transport

Fig. 5. Surface morphology of Ni-Mo coatings electrodeposited at dif-ferent RDE rotation speed: 0, 390 and 640 rpm and current density 5 A/dm2; SEMRys. 5. Morfologia powierzchni powłok Ni-Mo elektroosadzonych przy różnych prędkościach wirowania WED: 0, 390 i 640 obrotów/minutę i gęstości prądu 5 A/dm2; SEM

Fig. 6. X-ray diffraction patterns (CuKα) of Ni-Mo alloys electrodepos-ited at different RDE rotation speed: 0, 390 and 640 rpm and current density 5 A/dm2

Rys. 6. Dyfraktogramy rentgenowskie (CuKα) stopów Ni-Mo elektroosa-dzonych przy różnych prędkościach wirowania WED: 0, 390 i 640 obro-tów/minutę i gęstości prądu 5 A/dm2

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Fig. 7. Molybdenum content (wt %) in Ni-Mo alloys and cathodic current efficiency as a function of temperature (j = 5 A/dm2, rotation speed of RDE 640 rpm) Rys. 7. Zawartość molibdenu (% mas.) w stopach Ni-Mo oraz katodowa wydajność prądowa w funkcji temperatury (j = 5 A/dm2, prędkość obro-tów WED 640 obrotów/minutę)

Fig. 8. Surface morphology of Ni-Mo coatings electrodeposited at dif-ferent temperature: 20, 40 and 60°C (j = 5 A/dm2, rotation speed of RDE 640 rpm); SEMRys. 8. Morfologia powierzchni powłok Ni-Mo elektroosadzonych w róż-nej temperaturze: 20, 40 i 60°C (j = 5 A/dm2, prędkość wirowania WED 640 obrotów/minutę); SEM

Fig. 9. X-ray diffraction patterns (CuKα) of Ni-Mo alloys electrode-posited at temperature: 20, 40 and 60°C (j = 5 A/dm2, rotation speed of RDE 640 rpm)Rys. 9. Dyfraktogramy rentgenowskie (CuKα) stopów Ni-Mo elektroosa-dzonych w temperaturze: 20, 40 i 60°C (j = 5 A/dm2, prędkość wirowania WED 640 obrotów/minutę)

rate of metal ions to the cathode. Furthermore, the results presented in the literature do not define clearly the influence of this parameter on the induced codeposition of Ni-based coatings from electrolyte solution containing ammonia. It has to be taken into account that described effects of temperature and pH could be partially affected by evaporation of ammonia from solution.

In the analyzed system (bath of pH 9, 640 rpm), considerable increase of the molybdenum content (from about 12.5 to 21 wt %) in the deposits as well as current efficiency of the electrolysis (from about 80% to 90%) with increase of electrolyte solution tempera-ture was observed (Fig. 7).

As seen in Figure 8, the temperature in which the electrodeposi-tion process was conducted has significant influence on the coatings surface morphology. All analyzed deposits were compact and ad-herent to the substrate, however, increase of temperature generated surface irregularities visible as globular surface morphology, which is more pronounced at higher temperature.

Generally, an elevation of electrolyte temperature causes an in-crease of average crystallite size of electrodeposits. However, for binary alloys, obtained according to induced codeposition mech-anism, different tendency could be observed. Similarly to Ni-Mo coatings, deposited at different hydrodynamic conditions, the use of higher temperature causes increase of modifier concentration in layers (from 12.5 to 20.8 wt % Mo), what results in slight decrease of crystallite size (from about 9 to 5 nm). In the measured X-ray diffraction patterns a significant lines broadening with a decrease of their intensities and a shift of the (111) preferred orientation to the lower 2θ values were observed (Fig. 9).

CONCLUSIONS

Electroplating of nanocrystalline Ni-Mo alloys is a complex pro-cess affected by many factors and their interactions, such as pH and temperature of galvanic bath and hydrodynamic conditions in which electrolysis is conducted. However, control of these param-eters allow to obtained coatings with a desirable chemical and phase composition and regular surface morphology, which determine properties of layers.

It was established that in the analyzed system a homogenous, crack-free and adherent to steel substrate Ni-Mo alloys were ob-tained, when bath pH excides 7. In this conditions, formation of posi-tively charged Ni(II) ammonia complexes starts to dominate. There are reduced preferentially on the cathode, in comparison with nega-

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tively-charged citrate complexes of Ni(II), which dominate in solu-tions of lower pH. Due to this reason, the concentration ratio of the electroactive Ni(II) and Mo(VI) complexes is higher in the bath with pH above 7. Thus, alloys obtained in such conditions contain less molybdenum, but they are deposited with a higher current efficiency.

Increase of rotating speed of RDE and temperature of a galvanic bath, cause an increase of mass transport rate of metal ions to the cathode. It results in an increase of surface roughness and signifi-cant increase of molybdenum content, correlated with decrease of average crystallite size of deposits.

ACKNOWLEDGEMENT

The results presented in this paper were supported by the Na-tional Science Centre in the frame of the project No. 2011/01/B/ST8/03974.

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