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A comparative study of the physicochemical and electrochemical properties of Cr and Ni–W–P amorphous electrocoatings
Electrochimica Acta 51 (2006) 4928–4933
A comparative study of the physicochemical and electrochemical properties of Cr and Ni–W–P amorphous electrocoatings Pedro de Lima-Neto ∗ , Gec´ılio P. da Silva, Adriana N. Correia Departamento de Qu´ımica Anal´ıtica e F´ısico-Qu´ımica, Universidade Federal do Cear´a, C.P. 6035, 60455-970 Fortaleza, Cear´a, Brazil Received 1 September 2005; received in revised form 18 January 2006; accepted 21 January 2006 Available online 28 February 2006
Abstract A comparative study of the physicochemical and electrochemical properties of Cr and amorphous Ni–W–P electrocoatings is presented here. Amorphous Ni–W–P alloys were successfully produced by electrodeposition at 70 ◦ C on copper substrate under galvanostatic control in the range of 50–400 mA cm−2 and constant loads of 500 and 1600 C, using a solution containing 0.20 mol L−1 Na2 WO4 .2H2 O; 0.02 mol L−1 NiSO4 ·6H2 O; 0.02 mol L−1 NaPH2 O2 ; 0.02 mol L−1 H3 BO3 ; 0.07 mol L−1 (NH4 )2 SO4 ; 0.20 mol L−1 Na3 C6 H5 O7 ·2H2 O; 0.0001 mol L−1 CH3 (CH2 )10 ·CH2 OSO3 Na. Cr electrocoatings were obtained from an industrial plating solution. The physicochemical characterization of the as-electrodeposited and as-annealed samples was carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were carried out at room temperature in 10−1 mol L−1 NaCl aqueous solutions, using potentiodynamic linear polarization (PLP). Among the various Ni–W–P electrocoatings studied here, the Ni65 W20 P15 layer presented the best corrosion behavior and a slightly superior corrosion potential than the Cr electrocoating. Heat treatments gave rise to a cracked surface morphology in the Cr layers, while the surface morphology of the Ni65 W20 P15 layers remained homogeneous and devoid of cracks. Heat treatments at 400 and 600 ◦ C led to crystallization of the Ni–W–P layer, with precipitation of the Ni3 P, Ni and Ni–W phases and increasing hardness of the Ni–W–P layer as the heat treatment temperature rose. All the annealed Cr layers showed cracked surfaces and their hardness diminished as the annealing temperature increased. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni65 W20 P15 layer is a potential candidate to replace Cr in industrial applications, mainly at operational temperatures that exceed room temperature. © 2006 Elsevier Ltd. All rights reserved. Keywords: Ni–W–P; Cr; Electrodeposition; Corrosion; Amorphous electrocoatings
1. Introduction Chromium electrocoating is widely used for both decorative and functional purposes thanks to its excellent properties of hardness, good wear resistance, low coefficient of friction and great corrosion resistance. However, its industrial application has been limited by two factors, namely, environmental restrictions against conventional industrial Cr galvanization processes, which require the use of carcinogenic Cr6+ ions in the chromium plating bath, and the fact that Cr hardness decreases at temperatures in the range of 20–400 ◦ C [1]. These two factors, allied with public health and environmental laws, have led to an animated discussion in the literature about environmentally acceptable alternatives to chromium elec-
∗
Corresponding author. Tel.: +55 85 40089956; fax: +55 85 40089982. E-mail address: [email protected] (P. de Lima-Neto).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.01.036
trocoatings and a review is given in [2,3]. Among the possible alternatives, tungsten-containing electrocoatings appear to be an interesting and safe choice, for tungsten is located in the same group of the periodic table (group VI) and is thus expected to possess chemical properties similar to those of chromium. Additionally, it is only mildly toxic [4], its toxicity for aquatic species is low [5], and the industrial production of these coatings produces environmentally harmless wastewater. On the other hand, it is well known that tungsten cannot be deposited from an aqueous solution of a soluble compound containing this element. However, the co-electrodeposition of tungsten with elements of the iron group metals (Fe, Ni, Co) is successful and produces coatings with good corrosion resistance which can be highly durable, hard and resistant to high temperatures, as reported by Doten et al. [6]. Our study included amorphous Ni–P-based electrocoatings because this class of material has shown interesting mechanical properties and good corrosion properties, with a greater
P. de Lima-Neto et al. / Electrochimica Acta 51 (2006) 4928–4933 Table 1 Composition of the plating solution and operational parameters of Ni–W–P deposition Reagent
Concentration (mol L−1 )
Na2 WO4 ·2H2 O NiSO4 ·6H2 O H3 BO3 NaH2 PO2 Na3 C6 H5 O7 ·2H2 O (NH4 )2 SO4 CH3 (CH2 )10 ·CH2 OSO3 Na
0.20 0.02 0.02 0.02 0.20 0.07 0.0001
Operational deposition parameters: pH 8.5; T = 70 ◦ C; id = 50, 100, 200, 300 and 400 mA cm−2 ; total charge = 500 and 1600 C.
better corrosion resistance than the corresponding crystallized coatings [7–9]. The physicochemical processing and technical applications of amorphous metallic alloys have been reviewed by various authors [10–13]. Much interest has been evinced in the physicochemical and electrochemical properties of amorphous W-alloy electrocoatings with a view to use this material in place of industrial hard chromium electrocoatings. However, little is yet known about the properties of these amorphous Ni–W–P electrocoatings, although some reports in the literature discuss the structural evolution and microhardness during annealing [14–19] as well as the corrosion behavior of electroless and sputtered Ni–W–P [20]. Thus, this work purported to compare the physicochemical and electrochemical properties of Cr and amorphous Ni–W–P electrocoatings. Our study included an analysis of the influence of thermal treatments on the structure, surface morphology, and microhardness of these electrocoatings. 2. Experimental details 2.1. Electrodeposition The electrodeposition baths and corrosion testing solutions were prepared with Milli-Q purified water, and the chemical reagents used were of analytical grade. All the electrocoatings were deposited on disc-shaped Cu substrate embedded in epoxy resin, with a geometric area of approximately 1 cm2 of exposure. The electrodeposition was done in a single-compartment Pyrex glass cell with a Teflon cover and holes in which to fix the electrodes, keeping a parallel plane between the Cu cathode and the Pt foil anode with geometric area of 2 cm2 . Prior to the 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 plating solution and the operational parameters used to deposit the Ni–W–P electrocoatings. The Cr layers were deposited under galvanostatic control at 135 mA cm−2 from an industrial plating bath containing 0.5 mol L−1 CrO3 and 0.02 mol L−1 H2 SO4 at 60 ◦ C with a total charge of 500 C.
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2.2. Physicochemical characterization The structures of the as-electrodeposited and as-annealed Ni–W–P and Cr layers were assessed by X-ray diffraction (XRD) (Phillips X Pert-Pro) with Cu K␣ at 40 KV and 40 mA and an incident angle of 3◦ . Surface and cross-section morphologies of the samples were analyzed with a Philips XL-30 scanning electron microscope (SEM). The chemical composition of the electrodeposits was determined using a Link Analytical QX2000 X-ray dispersive analyzer attached to the SEM microscope. Ni–W–P and Cr free films, obtained through the removal of the Cu substrate by immersing the as-electrodeposited samples in a 38% (w/v) FeCl3 aqueous solution, were analyzed by differential scanning calorimetry (DSC). DSC curves were obtained at a heating rate of 10 ◦ C min−1 with a N2 flux of 50 mL min−1 , using a SHIMADZU DSC-50 system. Aselectrodeposited Cu/Ni–W–P and Cu/Cr samples were annealed in a N2 atmosphere at temperatures selected from the DSC curves, using a heating rate of 10 ◦ C min−1 from room temperature to the desired temperature plateau, where they were held for 1 h and then cooled to room temperature. The microhardness of the as-electrodeposited and as-annealed samples was measured with a Shimadzu model HMV-2 Micro Hardness Tester operating with an applied load of 10 g (98.07 mN), which was maintained for 30 s. To prevent the substrate from affecting the microhardness measurements, samples were obtained with a total electrodeposition charge of 1600 C, producing electrocoatings with a thickness of at least at 20 m. 2.3. Electrochemical measurements All the electrochemical corrosion tests involved at least triplicate samples and were conducted at room temperature (∼ =27 ◦ C) −1 −1 in an aerated 10 mol L NaCl aqueous solution, using a conventional three-electrode cell. The auxiliary and reference electrodes were, respectively, Pt foil and saturated calomel (SCE). Potentiodynamic polarization experiments were carried out at room temperature and at a scan rate of 1 mV s−1 . A potentiostat/galvanostat AUTOLAB PGSTAT 30, linked to a PC microcomputer and controlled by the GPES and FRA software, was used for the acquisition and analysis of electrochemical data. 3. Results and discussion The thickness and cathodic efficiency of the Cr electrodeposition were 7 m and 10.5%, respectively. The cathodic efficiency was highly congruent with that reported in the literature [20], which is about 12% for the conventional baths. Table 2 shows the influence of deposition current density on the composition, thickness and cathodic efficiency of the Ni–W–P electrocoating. As can be seen, the coating obtained at 50 mA cm−2 presented the highest Ni content (65 at.%), and this content remained approximately constant at about 61 at.% in coatings produced at higher deposition current densities. The W and P contents increased slightly as the deposition current density increased from 50 to 100 mA cm−2 , remaining approximately constant ay higher deposition current densities.
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Table 2 Influence of deposition current density on the composition, thickness and cathodic efficiency i (mA cm−2 )
Composition (at.%)
Thickness (m)
Efficiency (%)
50 100 150 200
Ni65 W20 P15 Ni60 W23 P17 Ni62 W22 P16 Ni61 W22 P17
3.8 3.0 2.2 2.0
3.0 2.0 1.4 1.2
On the other hand, the thickness and cathodic efficiency declined with the deposition current density, likely due to the increase in hydrogen evolution, which followed along with the rise in deposition current density. A possible explanation for the low cathodic efficiency is the presence of citrate, which acts as a complexing agent of both nickel and tungsten ions, forming stable complexes that hinder the alloy’s deposition. These results are in close agreement with those reported by Gileadi and co-workers [21,22], who studied the electrodeposition of Ni–W alloys. In these reports, the authors showed that the alloy’s cathodic efficiency and tungsten content are influenced by the composition and temperature of the plating solution and by the deposition current density, with cathodic efficiency varying from 1 to 36% and tungsten content in the alloy varying from 1 to 65 at.%. Fig. 1 contains a typical X-ray diffractogram obtained for the as-electrodeposited Ni–W–P electrocoatings. All the diffractograms displayed a broad peak at around 2θ = 45◦ , indicating that amorphous Ni–W–P electrocoatings were successfully produced under every operational condition studied here, for the characteristic peaks shown in Fig. 1 are related with Cu substrate. Fig. 2 shows the evolution of the surface morphology of the as-electrodeposited Ni–W–P with the applied deposition
Fig. 1. Typical X-ray diffractogram obtained for as-electrodeposited Ni–W–P layers. Shown here is Ni65 W20 P15 .
current density. As can be observed, increasing the deposition current density caused the surface morphology to evolve from a homogeneous granular morphology to a cracked surface. The appearance of cracks at higher current densities can be explained by the coating’s internal stress developed during its formation in response to the evolution of gas. The surface morphology of the Cr electrocoating (Fig. 2d) was characterized by a dendritic microstructure, showing a homogeneous, compact layer devoid of microcracks. Fig. 3 shows the potentiodynamic polarization curves of the amorphous Ni–W–P and the Cr electrocoatings in 10−1 mol L−1 NaCl aqueous solution. The Ni–W–P electrocoating obtained at 50 mA cm−2 (Ni65 W20 P15 ) showed the noblest corrosion potential, while the others Ni–W–P coatings showed a similar electrochemical behavior. The Cr electrocoating presented a nobler
Fig. 2. SEM micrographs of the Ni–W–P electrocoatings obtained at 50 mA cm−2 (a), at 100 mA cm−2 (b) and at 150 mA cm−2 (c). For comparison, SEM micrograph of the Cr electrocoating (d) is also presented.
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Fig. 3. Potentiodynamic polarization curves of the amorphous Ni–W–P and the Cr electrocoatings obtained at 10−1 mol L−1 in NaCl aqueous solution.
corrosion potential than the Ni–W–P electrocoatings obtained at current densities of more than 50 mA cm−2 . Additionally, a comparison of these results with the SEM micrograph (Fig. 2) indicates that the cracked amorphous Ni–W–P electrocoatings showed a poorer corrosion behavior than the noncracked amorphous Ni–W–P ones. This same finding was reported by Souza et al. [23], who found that the presence of numerous microcracks decreases the corrosion resistance in comparison with electrocoatings with less microcracking. Another point worth mentioning is that the Ni content appears to play a major role in the corrosion behavior of Ni–W–P electrocoatings, for even a slight increase in Ni content in the layer leads to higher values of corrosion potential, as shown in Table 3. This table also shows that the polarization resistance values of the cracked sample were constant and lower than that of the non-cracked sample. Based on these results, the amorphous Ni–W–P electrocoating obtained at 50 mA cm−2 was selected for a comparison of its thermal stability and mechanical properties with those of the Cr electrocoating. Fig. 4 shows the DSC plots for the electrodeposited Ni65 W20 P15 and Cr free films. The thermal profile of Ni–W–P free film reveals two exothermic peaks at 324 and 411 ◦ C, which are typical for the crystallization of hypoeutectic alloys [24]. The DSC diagram of the Cr layer showed only one endothermic peak at 118 ◦ C, probably connected with desorption of hydrogen which is added to the electrocoating during the plating process. To evaluate structural modifications as a function of the annealing temperature, Cu/Ni65 W20 P15 -coated and Cu/CrTable 3 Dependence of the electrochemical parameters derived from polarization curves on the Ni content in the layer Ni content (at.%)
Ecorr /V vs. SCE
Rp (k cm2 )
60 61 62 65
−0.752 −0.725 −0.723 −0.324
12.2 2.4 2.4 2.4
Fig. 4. DSC plot of the amorphous Ni65 W20 P15 (a) and Cr electrocoatings (b).
coated samples were annealed at temperatures selected from the DSC plots. Fig. 5 shows the evolution of the X-ray diffractograms with the annealing temperature. The Cu/Ni65 W20 P15 coated sample heat-treated at 100 and 200 ◦ C presented a broad peak characteristic of an amorphous structure. In addition, the diffractogram of the sample heat-treated at 400 ◦ C also displayed a broad peak, but a characteristic relating to Ni3 P precipitation is visible in this diffractogram. This suggests that the first peak in the DSC diagram of the Ni–W–P film represented the precipitation of this stable phase and that the structure was a mixture of amorphous matrix and Ni3 P. Additionally, more well-defined peaks are visible in the diffractogram of the sample heat-treated at 600 ◦ C, indicating the precipitation of new crystalline phases possibly related to Ni and Ni–W phases. These findings are in close agreement with those previously reported for Ni–W–P coating deposited on AISI 420 stainless steel by electroless plating [16,18]. The XRD diffractograms of Cu/Cr-coated samples annealed from 100 to 600 ◦ C were identical to that obtained for the as-electrodeposited Cu/Cr-coated sample. All the XRD
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Fig. 6. Evolution of the microhardness of the Ni65 W20 P15 and Cr electrocoatings as a function of annealing temperature.
Fig. 5. XRD diffraction patterns of the Cu/Ni65 W20 P15 -coated samples annealed at different temperatures for 1 h.
diffractograms displayed a single characteristic peak at 2θ = 65◦ relating to the Cr (1 1 0) phase. Fig. 6 shows the hardness of Cu/Ni65 W20 P15 -coated and Cu/Cr-coated samples in the as-electrodeposited and in the as-annealed conditions. The as-electrodeposited Cu/Cr-coated sample displayed greater hardness than the as-electrodeposited Cu/Ni65 W20 P15 -coated sample. However, the Cu/Cr-coated lost hardness rapidly as the annealing temperature rose and was less hard than the Cu/Ni65 W20 P15 -coated sample annealed at above 100 ◦ C. On the other hand, the hardness of the Cu/Ni65 W20 P15 coated remained approximately constant for samples annealed up to 200 ◦ C, increasing slightly on the sample annealed at
Fig. 7. SEM micrographs of the surface morphology of Ni65 W20 P15 coatings annealed at 200 ◦ C (a), 400 ◦ C (b), and 600 ◦ C (c), and of the Cr coating annealed at 100 ◦ C (d).
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ined here. Heat treating led to cracked surface morphologies in the Cr layers, while the surface morphology of the Ni65 W20 P15 layers remained homogeneous and devoid of cracks. The heat treatments at 400 and 600 ◦ C caused crystallization of the Ni–W–P layer and precipitation of Ni3 P, Ni and Ni–W phases, and to increasing hardness of the Ni–W–P layer at higher heat treatment temperatures. Cracked surfaces were observed in all the annealed Cr layers, whose hardness decreased as the annealing temperature rose. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni65 W20 P15 coatings can potentially replace Cr coatings in industrial applications, particularly at operational temperatures above room temperature. Acknowledgments The authors thank CNPq, CNPq-CTPETRO (Proc. 460033/01-8), CAPES, FUNCAP and FINEP (Proc. 22.01.0762.00), Brazil, for their financial support of this work. Gec´ılio P. da Silva gratefully acknowledges FUNCAP for his doctoral grant. References
Fig. 8. SEM micrographs of the cross-section morphology of Ni65 W20 P15 coating annealed at 600 ◦ C (a), and of Cr coating annealed at 100 ◦ C (b).
400 ◦ C, and rapidly on the sample annealed at 600 ◦ C. The increase in the hardness of the sample annealed at 400 ◦ C was caused by precipitation of the Ni3 P phase. Moreover, the increased hardness of the sample annealed at 600 ◦ C was attributed to the formation of Ni–W phase. Fig. 7 shows the SEM micrographs of the surface of the Ni65 W20 P15 and Cr layers after heat treatments at the temperatures applied in this study. As can be seen, Ni65 W20 P15 presented a homogeneous surface devoid of cracks, while the surfaces of the Cr layers developed a cracked morphology (Fig. 7d). The presence of cracks was likely caused by internal stress of the coating developed during the heat treatment. Fig. 8 shows typical SEM micrographs of cross-sections of the heat-treated samples. The cross-sections of Ni65 W20 P15 layers showed no cracking, but the Cr layer showed cracks from the top down to the substrate. These analyses indicate that the Cr layer’s decrease in hardness in response to increasing heat treatment temperatures gave rise to fairly extensive cracking. 4. Conclusions Amorphous Ni–W–P alloys were successfully produced by electrodeposition. The Ni65 W20 P15 layer presented the best corrosion behavior of the various Ni–W–P electrocoatings exam-
[1] S. Hoshino, H.A. Laitener, G.B. Hoflund, J. Electrochem. Soc. 133 (1986) 681. [2] E.W. Brooman, Met. Finish. 102 (2004) 42, and references cited therein. [3] E.W. Brooman, Met. Finish. 102 (2004) 75, and references cited therein. [4] J. Emsley, The Elements, 3rd ed., Clarendon Press, Oxford, England, 1998, p.218. [5] S.Z. El Abedin, J Appl. Electrochem. 31 (2001) 711. [6] M. Doten, H. Cesiuls, Z. Stojek, Electrochim. Acta 45 (2000) 3389. [7] P. de Lima-Neto, F.J.B. Rabelo, A.M.M.M. Adam, E.R. Gozalez, L.A. Avaca, Quim. Nova 16 (1993) 101. [8] P. de Lima-Neto, F.J.B. Rabelo, A.M.M.M. Adam, E.R. Gozalez, L.A. Avaca, Quim. Nova 19 (1996) 345. [9] M.M.V. Parente, O.R. Mattos, S.L. Diaz, P. Lima-Neto, F.J. Fabri Miranda, J. Appl. Electrochem. 31 (2001) 677. [10] Y. Waseda, K.T. Aust, J. Mater. Sci. 16 (1981) 2337. [11] F.E. Luborsky (Ed.), Amorphous Metallic Alloys, Butterworths, London, 1983. [12] M.D. Archer, C.C. Corke, B.H. Hargi, Electrochim. Acta 32 (1987) 13. [13] P.C. Searson, P.V. Nagarkar, R.M. Latanision, in: R.E. White, J. O’M Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 21, Plenum Press, New York, 1990, p. 121. [14] S.K. Teng, J.G. Duh, Y.I. Chen, Thin Solid Films 469/470 (2004) 333. [15] S.K. Teng, J.G. Duh, Solid Films 469/470 (2004) 268. [16] W.Y. Chen, S.K. Tien, F.B. Wu, J.G. Duh, Surf. Coat. Technol. 182 (2004) 85. [17] F.B. Wu, S.K. Tien, W.Y. Chen, J.G. Duh, Surf. Coat. Technol. 177/178 (2004) 312. [18] Y.Y. Tsai, F.B. Wu, Y.I. Chen, P.J. Peng, J.G. Duh, S.Y. Tsai, Surf. Coat. Technol. 146/147 (2001) 502. [19] G. Lu, G. Zangari, Electrochim. Acta 47 (2002) 2969. [20] F.A. Lowenheim, Electroplating, McGraw-Hill Book Company, New York, 1978. [21] O. Younes-Metzler, L. Zhu, E. Gileadi, Electrochim. Acta 48 (2003) 2551. [22] N. Eliaz, T.M. Sridhar, E. Gileadi, Electrochim. Acta 50 (2005) 2893. [23] C.A.C. Souza, J.E. May, A.T. Machado, A.L.R. Tachard, E.D. Bidoia, Surf. Coat. Technol. 190 (2005) 75. [24] I. Bakonyi, A. CziraKi, I. Nagy, M. Hosso, Z. MetaIlkd. 77 (1986) 425.
A comparative study of the physicochemical and electrochemical properties of Cr and Ni–W–P amorphous electrocoatings Pedro de Lima-Neto ∗ , Gec´ılio P. da Silva, Adriana N. Correia Departamento de Qu´ımica Anal´ıtica e F´ısico-Qu´ımica, Universidade Federal do Cear´a, C.P. 6035, 60455-970 Fortaleza, Cear´a, Brazil Received 1 September 2005; received in revised form 18 January 2006; accepted 21 January 2006 Available online 28 February 2006
Abstract A comparative study of the physicochemical and electrochemical properties of Cr and amorphous Ni–W–P electrocoatings is presented here. Amorphous Ni–W–P alloys were successfully produced by electrodeposition at 70 ◦ C on copper substrate under galvanostatic control in the range of 50–400 mA cm−2 and constant loads of 500 and 1600 C, using a solution containing 0.20 mol L−1 Na2 WO4 .2H2 O; 0.02 mol L−1 NiSO4 ·6H2 O; 0.02 mol L−1 NaPH2 O2 ; 0.02 mol L−1 H3 BO3 ; 0.07 mol L−1 (NH4 )2 SO4 ; 0.20 mol L−1 Na3 C6 H5 O7 ·2H2 O; 0.0001 mol L−1 CH3 (CH2 )10 ·CH2 OSO3 Na. Cr electrocoatings were obtained from an industrial plating solution. The physicochemical characterization of the as-electrodeposited and as-annealed samples was carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were carried out at room temperature in 10−1 mol L−1 NaCl aqueous solutions, using potentiodynamic linear polarization (PLP). Among the various Ni–W–P electrocoatings studied here, the Ni65 W20 P15 layer presented the best corrosion behavior and a slightly superior corrosion potential than the Cr electrocoating. Heat treatments gave rise to a cracked surface morphology in the Cr layers, while the surface morphology of the Ni65 W20 P15 layers remained homogeneous and devoid of cracks. Heat treatments at 400 and 600 ◦ C led to crystallization of the Ni–W–P layer, with precipitation of the Ni3 P, Ni and Ni–W phases and increasing hardness of the Ni–W–P layer as the heat treatment temperature rose. All the annealed Cr layers showed cracked surfaces and their hardness diminished as the annealing temperature increased. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni65 W20 P15 layer is a potential candidate to replace Cr in industrial applications, mainly at operational temperatures that exceed room temperature. © 2006 Elsevier Ltd. All rights reserved. Keywords: Ni–W–P; Cr; Electrodeposition; Corrosion; Amorphous electrocoatings
1. Introduction Chromium electrocoating is widely used for both decorative and functional purposes thanks to its excellent properties of hardness, good wear resistance, low coefficient of friction and great corrosion resistance. However, its industrial application has been limited by two factors, namely, environmental restrictions against conventional industrial Cr galvanization processes, which require the use of carcinogenic Cr6+ ions in the chromium plating bath, and the fact that Cr hardness decreases at temperatures in the range of 20–400 ◦ C [1]. These two factors, allied with public health and environmental laws, have led to an animated discussion in the literature about environmentally acceptable alternatives to chromium elec-
∗
Corresponding author. Tel.: +55 85 40089956; fax: +55 85 40089982. E-mail address: [email protected] (P. de Lima-Neto).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.01.036
trocoatings and a review is given in [2,3]. Among the possible alternatives, tungsten-containing electrocoatings appear to be an interesting and safe choice, for tungsten is located in the same group of the periodic table (group VI) and is thus expected to possess chemical properties similar to those of chromium. Additionally, it is only mildly toxic [4], its toxicity for aquatic species is low [5], and the industrial production of these coatings produces environmentally harmless wastewater. On the other hand, it is well known that tungsten cannot be deposited from an aqueous solution of a soluble compound containing this element. However, the co-electrodeposition of tungsten with elements of the iron group metals (Fe, Ni, Co) is successful and produces coatings with good corrosion resistance which can be highly durable, hard and resistant to high temperatures, as reported by Doten et al. [6]. Our study included amorphous Ni–P-based electrocoatings because this class of material has shown interesting mechanical properties and good corrosion properties, with a greater
P. de Lima-Neto et al. / Electrochimica Acta 51 (2006) 4928–4933 Table 1 Composition of the plating solution and operational parameters of Ni–W–P deposition Reagent
Concentration (mol L−1 )
Na2 WO4 ·2H2 O NiSO4 ·6H2 O H3 BO3 NaH2 PO2 Na3 C6 H5 O7 ·2H2 O (NH4 )2 SO4 CH3 (CH2 )10 ·CH2 OSO3 Na
0.20 0.02 0.02 0.02 0.20 0.07 0.0001
Operational deposition parameters: pH 8.5; T = 70 ◦ C; id = 50, 100, 200, 300 and 400 mA cm−2 ; total charge = 500 and 1600 C.
better corrosion resistance than the corresponding crystallized coatings [7–9]. The physicochemical processing and technical applications of amorphous metallic alloys have been reviewed by various authors [10–13]. Much interest has been evinced in the physicochemical and electrochemical properties of amorphous W-alloy electrocoatings with a view to use this material in place of industrial hard chromium electrocoatings. However, little is yet known about the properties of these amorphous Ni–W–P electrocoatings, although some reports in the literature discuss the structural evolution and microhardness during annealing [14–19] as well as the corrosion behavior of electroless and sputtered Ni–W–P [20]. Thus, this work purported to compare the physicochemical and electrochemical properties of Cr and amorphous Ni–W–P electrocoatings. Our study included an analysis of the influence of thermal treatments on the structure, surface morphology, and microhardness of these electrocoatings. 2. Experimental details 2.1. Electrodeposition The electrodeposition baths and corrosion testing solutions were prepared with Milli-Q purified water, and the chemical reagents used were of analytical grade. All the electrocoatings were deposited on disc-shaped Cu substrate embedded in epoxy resin, with a geometric area of approximately 1 cm2 of exposure. The electrodeposition was done in a single-compartment Pyrex glass cell with a Teflon cover and holes in which to fix the electrodes, keeping a parallel plane between the Cu cathode and the Pt foil anode with geometric area of 2 cm2 . Prior to the 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 plating solution and the operational parameters used to deposit the Ni–W–P electrocoatings. The Cr layers were deposited under galvanostatic control at 135 mA cm−2 from an industrial plating bath containing 0.5 mol L−1 CrO3 and 0.02 mol L−1 H2 SO4 at 60 ◦ C with a total charge of 500 C.
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2.2. Physicochemical characterization The structures of the as-electrodeposited and as-annealed Ni–W–P and Cr layers were assessed by X-ray diffraction (XRD) (Phillips X Pert-Pro) with Cu K␣ at 40 KV and 40 mA and an incident angle of 3◦ . Surface and cross-section morphologies of the samples were analyzed with a Philips XL-30 scanning electron microscope (SEM). The chemical composition of the electrodeposits was determined using a Link Analytical QX2000 X-ray dispersive analyzer attached to the SEM microscope. Ni–W–P and Cr free films, obtained through the removal of the Cu substrate by immersing the as-electrodeposited samples in a 38% (w/v) FeCl3 aqueous solution, were analyzed by differential scanning calorimetry (DSC). DSC curves were obtained at a heating rate of 10 ◦ C min−1 with a N2 flux of 50 mL min−1 , using a SHIMADZU DSC-50 system. Aselectrodeposited Cu/Ni–W–P and Cu/Cr samples were annealed in a N2 atmosphere at temperatures selected from the DSC curves, using a heating rate of 10 ◦ C min−1 from room temperature to the desired temperature plateau, where they were held for 1 h and then cooled to room temperature. The microhardness of the as-electrodeposited and as-annealed samples was measured with a Shimadzu model HMV-2 Micro Hardness Tester operating with an applied load of 10 g (98.07 mN), which was maintained for 30 s. To prevent the substrate from affecting the microhardness measurements, samples were obtained with a total electrodeposition charge of 1600 C, producing electrocoatings with a thickness of at least at 20 m. 2.3. Electrochemical measurements All the electrochemical corrosion tests involved at least triplicate samples and were conducted at room temperature (∼ =27 ◦ C) −1 −1 in an aerated 10 mol L NaCl aqueous solution, using a conventional three-electrode cell. The auxiliary and reference electrodes were, respectively, Pt foil and saturated calomel (SCE). Potentiodynamic polarization experiments were carried out at room temperature and at a scan rate of 1 mV s−1 . A potentiostat/galvanostat AUTOLAB PGSTAT 30, linked to a PC microcomputer and controlled by the GPES and FRA software, was used for the acquisition and analysis of electrochemical data. 3. Results and discussion The thickness and cathodic efficiency of the Cr electrodeposition were 7 m and 10.5%, respectively. The cathodic efficiency was highly congruent with that reported in the literature [20], which is about 12% for the conventional baths. Table 2 shows the influence of deposition current density on the composition, thickness and cathodic efficiency of the Ni–W–P electrocoating. As can be seen, the coating obtained at 50 mA cm−2 presented the highest Ni content (65 at.%), and this content remained approximately constant at about 61 at.% in coatings produced at higher deposition current densities. The W and P contents increased slightly as the deposition current density increased from 50 to 100 mA cm−2 , remaining approximately constant ay higher deposition current densities.
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Table 2 Influence of deposition current density on the composition, thickness and cathodic efficiency i (mA cm−2 )
Composition (at.%)
Thickness (m)
Efficiency (%)
50 100 150 200
Ni65 W20 P15 Ni60 W23 P17 Ni62 W22 P16 Ni61 W22 P17
3.8 3.0 2.2 2.0
3.0 2.0 1.4 1.2
On the other hand, the thickness and cathodic efficiency declined with the deposition current density, likely due to the increase in hydrogen evolution, which followed along with the rise in deposition current density. A possible explanation for the low cathodic efficiency is the presence of citrate, which acts as a complexing agent of both nickel and tungsten ions, forming stable complexes that hinder the alloy’s deposition. These results are in close agreement with those reported by Gileadi and co-workers [21,22], who studied the electrodeposition of Ni–W alloys. In these reports, the authors showed that the alloy’s cathodic efficiency and tungsten content are influenced by the composition and temperature of the plating solution and by the deposition current density, with cathodic efficiency varying from 1 to 36% and tungsten content in the alloy varying from 1 to 65 at.%. Fig. 1 contains a typical X-ray diffractogram obtained for the as-electrodeposited Ni–W–P electrocoatings. All the diffractograms displayed a broad peak at around 2θ = 45◦ , indicating that amorphous Ni–W–P electrocoatings were successfully produced under every operational condition studied here, for the characteristic peaks shown in Fig. 1 are related with Cu substrate. Fig. 2 shows the evolution of the surface morphology of the as-electrodeposited Ni–W–P with the applied deposition
Fig. 1. Typical X-ray diffractogram obtained for as-electrodeposited Ni–W–P layers. Shown here is Ni65 W20 P15 .
current density. As can be observed, increasing the deposition current density caused the surface morphology to evolve from a homogeneous granular morphology to a cracked surface. The appearance of cracks at higher current densities can be explained by the coating’s internal stress developed during its formation in response to the evolution of gas. The surface morphology of the Cr electrocoating (Fig. 2d) was characterized by a dendritic microstructure, showing a homogeneous, compact layer devoid of microcracks. Fig. 3 shows the potentiodynamic polarization curves of the amorphous Ni–W–P and the Cr electrocoatings in 10−1 mol L−1 NaCl aqueous solution. The Ni–W–P electrocoating obtained at 50 mA cm−2 (Ni65 W20 P15 ) showed the noblest corrosion potential, while the others Ni–W–P coatings showed a similar electrochemical behavior. The Cr electrocoating presented a nobler
Fig. 2. SEM micrographs of the Ni–W–P electrocoatings obtained at 50 mA cm−2 (a), at 100 mA cm−2 (b) and at 150 mA cm−2 (c). For comparison, SEM micrograph of the Cr electrocoating (d) is also presented.
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Fig. 3. Potentiodynamic polarization curves of the amorphous Ni–W–P and the Cr electrocoatings obtained at 10−1 mol L−1 in NaCl aqueous solution.
corrosion potential than the Ni–W–P electrocoatings obtained at current densities of more than 50 mA cm−2 . Additionally, a comparison of these results with the SEM micrograph (Fig. 2) indicates that the cracked amorphous Ni–W–P electrocoatings showed a poorer corrosion behavior than the noncracked amorphous Ni–W–P ones. This same finding was reported by Souza et al. [23], who found that the presence of numerous microcracks decreases the corrosion resistance in comparison with electrocoatings with less microcracking. Another point worth mentioning is that the Ni content appears to play a major role in the corrosion behavior of Ni–W–P electrocoatings, for even a slight increase in Ni content in the layer leads to higher values of corrosion potential, as shown in Table 3. This table also shows that the polarization resistance values of the cracked sample were constant and lower than that of the non-cracked sample. Based on these results, the amorphous Ni–W–P electrocoating obtained at 50 mA cm−2 was selected for a comparison of its thermal stability and mechanical properties with those of the Cr electrocoating. Fig. 4 shows the DSC plots for the electrodeposited Ni65 W20 P15 and Cr free films. The thermal profile of Ni–W–P free film reveals two exothermic peaks at 324 and 411 ◦ C, which are typical for the crystallization of hypoeutectic alloys [24]. The DSC diagram of the Cr layer showed only one endothermic peak at 118 ◦ C, probably connected with desorption of hydrogen which is added to the electrocoating during the plating process. To evaluate structural modifications as a function of the annealing temperature, Cu/Ni65 W20 P15 -coated and Cu/CrTable 3 Dependence of the electrochemical parameters derived from polarization curves on the Ni content in the layer Ni content (at.%)
Ecorr /V vs. SCE
Rp (k cm2 )
60 61 62 65
−0.752 −0.725 −0.723 −0.324
12.2 2.4 2.4 2.4
Fig. 4. DSC plot of the amorphous Ni65 W20 P15 (a) and Cr electrocoatings (b).
coated samples were annealed at temperatures selected from the DSC plots. Fig. 5 shows the evolution of the X-ray diffractograms with the annealing temperature. The Cu/Ni65 W20 P15 coated sample heat-treated at 100 and 200 ◦ C presented a broad peak characteristic of an amorphous structure. In addition, the diffractogram of the sample heat-treated at 400 ◦ C also displayed a broad peak, but a characteristic relating to Ni3 P precipitation is visible in this diffractogram. This suggests that the first peak in the DSC diagram of the Ni–W–P film represented the precipitation of this stable phase and that the structure was a mixture of amorphous matrix and Ni3 P. Additionally, more well-defined peaks are visible in the diffractogram of the sample heat-treated at 600 ◦ C, indicating the precipitation of new crystalline phases possibly related to Ni and Ni–W phases. These findings are in close agreement with those previously reported for Ni–W–P coating deposited on AISI 420 stainless steel by electroless plating [16,18]. The XRD diffractograms of Cu/Cr-coated samples annealed from 100 to 600 ◦ C were identical to that obtained for the as-electrodeposited Cu/Cr-coated sample. All the XRD
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Fig. 6. Evolution of the microhardness of the Ni65 W20 P15 and Cr electrocoatings as a function of annealing temperature.
Fig. 5. XRD diffraction patterns of the Cu/Ni65 W20 P15 -coated samples annealed at different temperatures for 1 h.
diffractograms displayed a single characteristic peak at 2θ = 65◦ relating to the Cr (1 1 0) phase. Fig. 6 shows the hardness of Cu/Ni65 W20 P15 -coated and Cu/Cr-coated samples in the as-electrodeposited and in the as-annealed conditions. The as-electrodeposited Cu/Cr-coated sample displayed greater hardness than the as-electrodeposited Cu/Ni65 W20 P15 -coated sample. However, the Cu/Cr-coated lost hardness rapidly as the annealing temperature rose and was less hard than the Cu/Ni65 W20 P15 -coated sample annealed at above 100 ◦ C. On the other hand, the hardness of the Cu/Ni65 W20 P15 coated remained approximately constant for samples annealed up to 200 ◦ C, increasing slightly on the sample annealed at
Fig. 7. SEM micrographs of the surface morphology of Ni65 W20 P15 coatings annealed at 200 ◦ C (a), 400 ◦ C (b), and 600 ◦ C (c), and of the Cr coating annealed at 100 ◦ C (d).
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ined here. Heat treating led to cracked surface morphologies in the Cr layers, while the surface morphology of the Ni65 W20 P15 layers remained homogeneous and devoid of cracks. The heat treatments at 400 and 600 ◦ C caused crystallization of the Ni–W–P layer and precipitation of Ni3 P, Ni and Ni–W phases, and to increasing hardness of the Ni–W–P layer at higher heat treatment temperatures. Cracked surfaces were observed in all the annealed Cr layers, whose hardness decreased as the annealing temperature rose. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni65 W20 P15 coatings can potentially replace Cr coatings in industrial applications, particularly at operational temperatures above room temperature. Acknowledgments The authors thank CNPq, CNPq-CTPETRO (Proc. 460033/01-8), CAPES, FUNCAP and FINEP (Proc. 22.01.0762.00), Brazil, for their financial support of this work. Gec´ılio P. da Silva gratefully acknowledges FUNCAP for his doctoral grant. References
Fig. 8. SEM micrographs of the cross-section morphology of Ni65 W20 P15 coating annealed at 600 ◦ C (a), and of Cr coating annealed at 100 ◦ C (b).
400 ◦ C, and rapidly on the sample annealed at 600 ◦ C. The increase in the hardness of the sample annealed at 400 ◦ C was caused by precipitation of the Ni3 P phase. Moreover, the increased hardness of the sample annealed at 600 ◦ C was attributed to the formation of Ni–W phase. Fig. 7 shows the SEM micrographs of the surface of the Ni65 W20 P15 and Cr layers after heat treatments at the temperatures applied in this study. As can be seen, Ni65 W20 P15 presented a homogeneous surface devoid of cracks, while the surfaces of the Cr layers developed a cracked morphology (Fig. 7d). The presence of cracks was likely caused by internal stress of the coating developed during the heat treatment. Fig. 8 shows typical SEM micrographs of cross-sections of the heat-treated samples. The cross-sections of Ni65 W20 P15 layers showed no cracking, but the Cr layer showed cracks from the top down to the substrate. These analyses indicate that the Cr layer’s decrease in hardness in response to increasing heat treatment temperatures gave rise to fairly extensive cracking. 4. Conclusions Amorphous Ni–W–P alloys were successfully produced by electrodeposition. The Ni65 W20 P15 layer presented the best corrosion behavior of the various Ni–W–P electrocoatings exam-
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