Characterisation of Electrodeposited and Heat-Treated Ni − Mo − P Coatings

The electrodeposition, hardness and corrosion resistance properties of Ni−Mo−P coatings were investigated. Characterisations of the electrodeposited coatings were carried out using scanning electron microscopy, X-ray diffraction and energy dispersive X-ray analysis techniques. Corrosion tests were performed at room temperature in 10-1 mol dm-3 NaCl solutions and by potentiodynamic linear polarisation. Amorphous Ni−Mo−P coatings were successfully obtained by electrodeposition using direct current. The coating composition showed to be dependent on the bath composition, current density and bath temperature. Both P and Mo contents contribute for the hardness properties of the Ni−Mo−P coatings and the absence of cracks is a requirement to produce electrodeposited Ni−Mo−P coatings with good hardness properties. The hardness values increase with heat-treatment temperature due to the precipitation of Ni, Ni3P and NiMo phases during the heat treatment. The corrosion resistance of the electrodeposited Ni−Mo−P amorphous coatings increases with P content in the layer. Among the electrodeposited Ni−Mo−P amorphous coatings, Ni78Mo10P12 presented the best hardness and corrosion-resistance properties. The results showed that the addition of P is beneficial for the hardness and corrosion resistance properties of the Ni−Mo-based coatings.


Introduction
There is currently great interest in the study of the electrodeposition of Ni−Mo coatings, because they are considered to be corrosion-resistant coatings, [1][2][3][4] they are electrocatalysts for hydrogen and oxygen evolution reactions, [5][6][7][8] and they present good hardness properties. 9ecently, we showed that electrodeposited Ni−Mo coatings presented inferior corrosion-resistance and hardness properties in comparison to electrodeposited Cr coatings. 10However, after heat treatment at temperature higher than 100 °C, these coatings presented superior hardness properties in comparison to the heat-treated Cr coatings.These results indicated that Ni−Mo coatings could be a potential substitutes for chromium coatings in industrial applications in which operational temperatures higher than 100 °C and good hardness properties are required. 10nother point that supports a possible replacement of the Cr coatings by Ni−Mo coatings is that the industrial production of electrodeposited Ni−Mo coatings could produce Vol.23, No. 2, 2012   environmentally harmless wastewater, since molybdenum is a non-toxic metal for the aquatic environment, 11 while the industrial chromium electroplating process requires the use of carcinogenic Cr 6+ ions.However, the electrodeposition of Ni−Mo coatings is limited to a maximum of 17 at.% of Mo in the layer, since electrodeposits with Mo content higher than 17% were cracked, which compromised the corrosion-resistance and hardness properties of the Ni−Mo coatings. 10,12Thus, the challenge in the research of electrodeposition of Ni−Mo-based coatings is to obtain electrodeposits that are non-cracked, even with Mo content in the layer higher than 17 at.%.
On the other hand, amorphous metallic alloys present interesting mechanical, magnetic and electrochemical properties.These materials have shown higher corrosion resistance than crystalline alloys with the same composition, which encourages the interest in producing anticorrosive amorphous coatings.][15][16] This class of coating can be produced by sputtering, electroless and electrodeposition techniques.Among these techniques, electrodeposition is very suitable to produce amorphous coatings because it permits working with substrates in different shapes and geometries and makes it possible to control the thickness and the composition of the coating by only changing the electrodeposition parameters.8][19][20][21][22] Among them, P is the more usual element to be codeposited with the elements of the iron group because it improves the corrosion-resistance properties of the electrodeposited coatings due to the fact that in aqueous medium this element produces a protective surface film formed by the phosphate anion, which kinetically limits the dissolution of the amorphous coatings such as Ni−P and Co−P electrodeposits. 22Another important requirement to obtain amorphous electrodeposits is the composition of coating.For instance, binary Ni−P and Co−P electrodeposits are amorphous if the P content exceeds 14 at.%. 22 Therefore, the codeposition of P with Ni and Mo can produce electrodeposits with good corrosion resistance and good hardness properties.Furthermore, little is known about the influence of operational parameters on the composition of Ni−Mo−P coatings, on their corrosion resistance, or about the influence of heat-treatment temperature on the crystal structure and hardness properties of Ni−Mo−P coatings.
Thus, the aim of this work is to study the effect of P content in the corrosion resistance and microhardness properties of the electrodeposited Ni−Mo−P amorphous coatings.An analysis of the influence of thermal treatments on the crystal structure and on hardness properties of these coatings is also present.

Experimental
Electrodeposition Solutions were prepared from chemicals of analyticalgrade purity dissolved in water purified with a Millipore Milli-Q system.Ni−Mo−P coatings were electrodeposited on a disc-shaped Cu substrate embedded in epoxy resin, with a geometric area of approximately 2 cm 2 of exposure.The electrodepositions were performed in a single-compartment Pyrex glass cell with a Teflon cover containing holes to affix the Cu cathode and the platinum-mesh anode.Prior to alloy plating, the Cu surfaces were polished with 240, 400 and 600 SiC emery paper, degreased in a hot NaOH solution, rinsed in distilled water, etched in 15% HCl solution and, lastly, rinsed with distilled water.Table 1 gives the composition of the Ni−Mo−P plating solutions and the operational parameters used to electrodeposit these coatings.

Heat treatment
As-electrodeposited Cu/Ni−Mo−P samples were annealed in an N 2 atmosphere at 100, 200, 400 and 600 ºC, using a heating rate of 10 ºC min -1 from room temperature to the desired temperature plateau, where they were held for 1 h.After the annealing process, the samples were cooled under the furnace atmosphere until to room temperature.

Physical and chemical characterisations
The surface morphology of the electrodeposits was analysed by a Philips XL-30 scanning electron microscope (SEM).The coating composition was analysed by energy dispersive X-ray (EDX) apparatus attached to the SEM.The coating crystal phase structure was analysed by X-ray diffraction (XRD) using a Philips model X'Pert pro diffractometer with Cu Ka radiation (l = 1.54 Å) at 40 KV and 40 mA and an incident angle of 3º.The main peaks observed in the diffractograms were compared with XRD data from the JCPDS (Joint Committee of Powder Diffraction Standards).Microhardness measurements were carried out using a Shimadzu model HMV-2-Series microhardness tester with a diamond pyramid indenter at a load of 10 g, which was maintained for 30 s.An average of 10 readings was taken to obtain the hardness values of the coatings.To prevent the substrate from affecting the microhardness measurements, samples were obtained with a total electrodeposition charge of 2000 C.

Corrosion tests
The corrosion behaviour of as-electrodeposited Ni−Mo−P coatings was evaluated by potentiodynamic linear polarisation (PLP) technique with a scan rate of 1 mV s -1 in 0.1 mol dm -3 NaCl aqueous solutions with a natural pH of 6.5.A potentiostat/galvanostat AUTOLAB PGSTAT 30, linked to a PC microcomputer and controlled by GPES and FRA software, was used for the acquisition of the electrochemical data.

Influence of the plating parameters
The dependences of Mo and P contents in the electrodeposited coatings with the operational plating parameters are shown in Figure 1.The values shown in this figure correspond to average values of at least duplicate samples obtained in each operational condition.It can be observed that the increase in the temperature of the plating process leads to an increase in the Mo content in the layer, which decreases with the NaPH 2 O 2 concentration in the plating solution and with the deposition current density.In each bath, the lowest Mo content is observed for the coatings obtained at 30 °C, and at this deposition temperature the Mo content is approximately independent of the deposition current density.For the layers electrodeposited at 50 °C and 70 °C, the Mo content decreases with deposition current density for the layers obtained from bath 1 and bath 2. For the layers obtained from bath 3, in each studied temperature, the Mo content remains approximately constant for the deposition current density values lower than 100 mA cm -2 , followed by a decrease for those coatings electrodeposited at 100 mA cm -2 .The observed dependence of the Mo content in the coating with the deposition current density suggests that the Mo codeposition process is limited by mass transport of the molybdate ion to the electrode surface, which is in close agreement with results reported by Marlot et al. 23 while the dependence with temperature is explained as a consequence of the increase of mass transport due to the increase in the ionic mobility with the rise of temperature.
Figure 1 shows that P content in the electrodeposited coatings increases with the NaPH 2 O 2 concentration in the plating solution.However, distinct dependences for P content in the layer with the temperature and with deposition current density can be observed.Initially, all coatings electrodeposited at 25 mA cm -2 show that P content in the layer decreases with the temperature, while those obtained at higher deposition current density values tend to increase with temperature.In addition, while the deposition current density has little effect on P content in the coatings obtained at 30 °C, it influences the P content in the layers obtained at higher temperatures, since P content in the coatings tends to increase with the deposition current density.
For further study, Ni−Mo−P coatings were selected in order to evaluate the influence of P content on the hardness and corrosion properties of the Ni−Mo−P coatings.The selected samples are listed in Table 2.

Physical characterisation
Figure 2 shows the X-ray diffractograms obtained for the selected as-electrodeposited Ni−Mo−P coatings.All diffractograms present a broad peak around 2q of 45°, indicating that amorphous Ni−Mo−P coatings were successfully produced under every operational condition studied here.These results are in close accordance with those already published in the literature for electroless Ni−Mo−P amorphous coatings. 24he surface morphologies of the selected as-electrodeposited Ni−Mo−P coatings are shown in Figure 3. Initially, the electrodeposited coating that contains 18 at.% of Mo and 1 at.% of P (Figure 3A) is cracked, while the surface morphologies of the other selected Ni−Mo−P coatings are similar (Figures 3B-3F), presenting spherical nodules distributed on the surface, and no cracks.The existence of cracks in the electrodeposited Ni 81 Mo 18 P 1 layer is related to the surface relaxation of internal tensile stress in the coating, and these cracks certainly compromise the corrosion-resistance performance of this layer, since they allow the permeation of electrolyte to the substrate.The observed spherical nodules are explained as a consequence of the growth of secondary nuclei on top of the first layer that was formed on the substrate surface.It has previously been reported in the literature 10,12 that cracked Ni-Mo electrodeposits were obtained when the Mo content in the layer was higher than 17 at.%.Thus, Figure 3A shows that 1 at.% of P is not enough to avoid the formation of a cracked Ni−Mo−P layer which can compromise the corrosion resistance and hardness properties of this coating.On the other hand, the others SEM images (Figures 3B-3F) show the beneficial effect of the increase of the P content in the coating, since the existence of a cracked morphology is not observed for the coatings containing P higher than 1 at.%, even for those coatings containing Mo higher than 17 at.%.
Finally, these SEM images shows that the addition of P makes possible to overcome one of the problems associated with the electrodeposition of Ni-Mo-based coatings, which is the arisen of a cracked morphology when the Mo content in the coating exceeds 17 at.%.
Hardness measurements were carried out in order to evaluate the influence of the coating composition and the influence of heat-treatment temperature on the hardness properties of the Ni−Mo−P coatings.Figure 4 shows the graph of the relationship between the compositions of the as-electrodeposited Ni−Mo−P layers and the hardness of the coatings.Initially, it can be observed that the lowest hardness value is presented by the Ni 81 Mo 18 P 1 , which shows the deleterious effect of the cracked morphology on the hardness properties.For the no cracked Ni−Mo−P coatings, a change from 190 Hv to 298 Hv is observed for the samples Ni 90 Mo 5 P 5 and Ni 76 Mo 21 P 6 , respectively.For higher P content, little change in the hardness values is observed and the highest hardness value of 309 Hv is presented by the Ni 78 Mo 10 P 12 coating.In addition, coatings with higher Mo content, but with low P content such as Ni 82 Mo 21 P 6 have similar hardness to the layers with lower Mo content but with higher P content such as Ni 78 Mo 10 P 12 .These results suggest that both P and Mo contents contribute for the hardness properties of the Ni−Mo−P coatings and that the absence of cracks is a requirement to produce electrodeposited Ni−Mo−P coatings with good hardness properties.Comparing these hardness values to those reported in the literature for electrodeposited Ni-Mo coatings, 10 the electrodeposited Ni 78 Mo 10 P 12 amorphous coatings present higher hardness values than the as-electrodeposited Ni 87 Mo 13 , which is about 287 Hv.However, the hardness values presented by the aselectrodeposited Ni−Mo−P are lower than that reported in the literature for the as-electrodeposited Cr coating, which is about 800 Hv. 10 From the results shown in Figure 4, the sample Ni 78 Mo 10 P 12 was selected in order to evaluate the influence of the heat treatment in the hardness of the coating and in Table 3 is listed the hardness values for Ni 78 Mo 10 P 12 coatings heat-treated at the selected temperature.This Table shows that the hardness of the Ni 78 Mo 10 P 12 coatings increases with the heat-treatment temperature.Similar trends have previously been reported for Ni−Mo 10 and Ni−W, 25 as well    for similar systems such as Ni−W−P 26 and Ni−Cr−P. 27n order to understand the hardness behaviour of the Ni 78 Mo 10 P 12 coating with the heat-treatment temperature, X-ray diffractograms were obtained for the heat-treated samples, and they are shown in Figure 5.
The diffractograms of the samples heat-treated at 100 °C and 200 °C present similar X-ray diffraction patterns as the as-electrodeposited Ni 78 Mo 10 P 12 .The diffractogram of the sample heat-treated at 400 °C shows that the broad peak around 2q of 45° is better defined, as well as two new broad peaks at around 2q of 51° and 75° and two well-defined peaks around 2q of 35° and 60°.The two broad peaks around 2q of 45° and 51° are related to the nickel diffraction peaks.The broad peak around 2q of 75° is related to Ni-Mo diffraction peak.The peaks at 2q of 35° and 60° are related to the Ni 3 P diffraction peaks.These results are in close agreement with the work of Koiwa et al., 28 that studied the effect of the heat treatment on the electroless-deposited Ni−Mo−P amorphous coatings and showed that the Ni 3 P and Ni−Mo phases were present in the sample heat-treated at 400 °C.Thus, these results suggest that the heat-treatment temperature of 400 °C leads to precipitation of Ni, Ni Finally, the crystallographic structure that arises from the amorphous structure with the increase of the heat-treatment temperature, mainly for temperatures higher than 400 °C, occurs due to separation of these crystal phases during heat treatment from the amorphous phase.
From the X-ray analyses, the behaviour of the hardness with the heat-treatment temperature for the Ni 78 Mo 10 P 12 coatings can be explained by the fact that, with the increase of heat-treatment temperature, an increase occurs in the number of crystals separated from the amorphous matrix, and the new crystal phases that form during the heat treatment favour an increase in hardness.Thus, as the annealing temperature increases, Ni, Ni 3 P and Ni−Mo phases precipitate, leading to an increase in the hardness of the Ni 78 Mo 10 P 12 coating.
Finally, these hardness measurements suggest that Ni−Mo−P coatings can substitute Cr coatings in industrial applications that require high operational temperatures and good hardness properties, since it is reported in the literature 10,[25][26][27] that the hardness of the Cr coatings drops between 550 Hv and 400 Hv when heat-treatment temperature is in the range of 100 °C to 600 °C.

Electrochemical measurements
The influence of P content on the corrosion behaviour of the Ni−Mo−P coatings is shown in Figure 6.It can be observed that the worst corrosion behaviour is showed by the cracked Ni 81 Mo 18 P 1 coating.For the no cracked Ni-Mo-P samples, it can be observed the beneficial effect of the P content in the corrosion behaviour, since the corrosion potentials shift to more positive values with the increase of P content and the lowest anodic current densities are presented by the Ni−Mo−P amorphous coating with highest P content.The electrochemical parameters derived from the polarisation curves are shown in Table 4.

Figure 4 .
Figure 4. Variation of the hardness properties with the coating composition.
3 P and Ni−Mo phases on the Ni−Mo−P amorphous matrix.The diffractogram of the Ni 78 Mo 10 P 12 sample heat-treated at 600 °C shows the diffraction peaks with better definition, indicating the crystallisation of the Ni 78 Mo 10 P 12 and that the precipitation of new crystalline phases do not occur.

Figure 5 .
Figure 5. Evolution of the X-ray diffractograms for Ni 78 Mo 10 P 12 coatings with heat-treatment temperature.

Table 2 .
List of the selected electrodeposited Ni−Mo−P amorphous coatings

Table 3 .
Hardness values of the Ni 78 Mo 10 P 12 coatings heat-treated at selected temperatures

Table 4 .
Corrosion parameters derived from the potentiodynamic linear polarisation curves