Morphological, structural, microhardness and electrochemical characterisations of electrodeposited Cr and Ni–W coatings

Electrochimica Acta 55 (2010) 2078–2086

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Morphological, structural, microhardness and electrochemical characterisations of electrodeposited Cr and Ni–W coatings Pedro de Lima-Neto a,∗ , Adriana N. Correia a , Renato A.C. Santana a , Regilany P. Colares a , Eduardo B. Barros b , Paulo N.S. Casciano a , Gustavo L. Vaz a a b

Departamento de Química Analítica e Físico-Química, Universidade Federal do Ceará, Campus do Pici, Bloco 940, 60455-960, Fortaleza, Ce, Brazil Departamento de Física, Universidade Federal do Ceará, Campus do Pici, Bloco 922, 60455-960, Fortaleza, Ce, Brazil

a r t i c l e

i n f o

Article history: Received 17 August 2009 Received in revised form 9 November 2009 Accepted 12 November 2009 Available online 18 November 2009 Keywords: Ni–W Cr Electrodeposition Corrosion Heat treatment Microhardness

a b s t r a c t This study examines the corrosion of electrodeposited Cr and of two electrodeposited Ni–W coatings in 0.1 mol L−1 NaCl solution, as well as the influence of heat treatment on the crystallographic structure and microhardness properties of these coatings. Physical characterisation is carried out using scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. Electrochemical characterisation is carried out using both the potentiodynamic linear polarization technique and open circuit measurements during long-term immersion tests. The corrosion products on the coating surfaces are characterised by ex situ Raman spectroscopy. As-electrodeposited Ni–W samples do not present defects, and the surface evolves from fine globular grains to rough polycrystalline morphology with decreasing electrodeposition current density. All the studied coatings corrode in the chloride medium and the corrosion is non-uniform for the Ni–W coatings. Raman analyses carried out after the immersion tests reveal Cr2 O3 and Cr(OH)2 corrosion products on the Cr coating surface, and Ni(OH)2 , NiO and WO3 corrosion products on the Ni–W coating surfaces. Ni, Ni4 W and Ni–W phases are formed after heat treatment of the Ni86 W14 coating at 600 ◦ C. Although all the annealed Ni–W layers are cracked, their microhardness increases as the annealing temperature increases, suggesting that Ni–W coatings are potential substitutes for chromium in industrial applications in which good microhardness properties and stability at temperatures higher than 100 ◦ C are required. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Despite their excellent mechanical and corrosion resistance properties, hard chromium coatings are becoming less popular in industrial applications. This is due to environmental and toxicological restrictions on hard chromium industrial electroplating, which requires the use of carcinogenic Cr6+ ions in the chromium plating bath. Additionally, Cr coatings are known to deteriorate in hardness and corrosion resistance under operating temperatures higher than 205 ◦ C [1,2]. Previously, it was demonstrated that this deterioration is related to the presence of cracks that impair the mechanical and corrosion resistance properties of Cr layers [3,4]. In recent years, these difficulties with Cr coatings have spurred research toward finding alternatives. Tungsten, also a group VI metal, is a candidate to replace chromium, because it is expected to have similar chemical and electrochemical properties. Additionally, it is only mildly toxic [5], its toxicity for aquatic species is low

∗ Corresponding author. E-mail address: [email protected] (P. de Lima-Neto). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.11.037

[6], and the industrial production of these coatings will produce environmentally harmless wastewater. Unfortunately, tungsten cannot be electrodeposited alone from aqueous solution. Nevertheless, its electrodeposition can be induced in the presence of Fe, Co and Ni ions. Among these metals, nickel appears to be the best choice for codeposition because it has good corrosion resistance to aqueous media, and it is known to improve high temperature oxidation resistance in ferrous-based alloys, such as stainless steel [7]. Although Brooman suggests using Ni–W coatings as replacements for industrial hard chromium coatings [8,9], only the operational electroplating parameters and electrodeposition mechanisms for depositing Ni–W layers have been reported in the literature. Gileadi and co-workers [10–15] thoroughly investigated the electroplating of Ni–W coatings from a citrate-based plating solution. They studied the effects of additives on the electroplating and examined the effects of plating solution composition and operating conditions on crystal structure, coating composition, surface morphology, and hardness. Mizushima et al. studied the effects of complexing agents, such as citrate, glycine and triethanolamine, on the electroplating process for Ni–W coatings [16]. Obradovic´ et al. [17] compared the use of pulse current

P. de Lima-Neto et al. / Electrochimica Acta 55 (2010) 2078–2086 Table 1 Composition of the bath used for electrodeposition (pH = 9). Reagent

Concentration/mol dm−3

Na2 WO4 ·2H2 O NiSO4 ·H2 O H3 BO3 Na3 Cit.·2H2 O CH3 (CH2 )10 CH2 OSO3 Na (NH4 )2 SO4

0.240 0.024 0.182 0.422 0.0001 0.0681

Operational deposition parameters. pH = 9 (adjusted with ammonia). Plating temperature = room temperature (≈27 ◦ C); deposition current density = 20, 30, 40 and 50 mA cm−2 ; deposition charge = 250 and 1500 C.

and direct current during the electrodeposition of Ni–W coatings from ammonia–citrate electrolyte. Electrodeposition has also been used to produce nanocrystalline Ni–W alloys, and the hardness, wear and corrosion resistance of these alloys have been evaluated [18,19]. Studies on Ni–W and Ni–W-based coatings have become more frequent in recent years [20–25]. Nevertheless, in the present literature, minimal attention has been paid to the characterisation of the corrosion products of Ni–W coatings after long-term immersion tests in an aqueous medium. Further, little is known about the influence of heat treatment on the crystallographic structure, morphology and hardness of Ni–W coatings. In this work, a comparative study is made of the corrosion and microhardness properties of electrodeposited Cr and two Ni–W coatings (Ni86 W14 and Ni93 W7 ). The influence of thermal treatments on the crystallographic structure, morphology and microhardness properties of these coatings is also investigated. 2. Experimental 2.1. Electrodeposition Solutions were prepared from analytical grade chemicals, dissolved in water purified by a Millipore Milli-Q system. Cr and Ni–W coatings were electrodeposited on disc-shaped Cu substrates embedded in epoxy resin, with a geometric area of approximately 2 cm2 of exposure. The electrodepositions were performed in a single-compartment Pyrex glass cell, with a Teflon cover containing holes to fix the Cu cathode and the platinum mesh anode. 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 finally rinsed again with distilled water. Table 1 gives the composition of the Ni–W plating solution and the operational parameters used to electrodeposit the Ni–W coatings. The Cr layers were deposited under galvanostatic control at 135 mA cm−2 from an industrial plating bath containing 0.5 mol dm−3 CrO3 and 0.02 mol dm−3 H2 SO4 at 60 ◦ C with a total charge of 250 C. 2.2. Heat treatment As-electrodeposited Cu/Ni–W and Cu/Cr samples were annealed in a N2 atmosphere at 100, 200, 400 and 600 ◦ C. Samples were heated from room temperature at a rate of 10 ◦ C min−1 until the desired temperature plateau was reached. This plateau was then held for 1 h. Afterwards, the samples were kept continuously in the N2 atmosphere until the furnace has reached room temperature. 2.3. Physical and chemical characterisations The surface and cross-sectional morphologies of the deposited coatings were analysed using a Philips XL-30 scanning electron microscope (SEM). The coating compositions were analysed by an

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energy dispersive X-ray (EDX) apparatus attached to the SEM. The EDX analyses were carried out on the surface and on a cross-section of the Ni–W samples, and the sample compositions were uniform over the two analysed regions. The crystal phase structures of the coatings were analysed by X-ray diffraction (XRD), using a Philips model X’Pert pro diffractometer with Cu k␣ radiation ( = 1.54 Å) at 40 kV and 40 mA and an incident angle of 3◦ . The main peaks in the diffractograms were compared with XRD data from the JCPDS (Joint Committee of Powder Diffraction Standards). The corrosion products on the coating surfaces after the immersion corrosion tests were identified by Raman spectroscopy (RS). The Raman spectra were obtained in the back-scattering geometry using a 532 nm laser excitation. The power was kept below 50 mW. The signal was dispersed using a UHTS 300 Witec spectrometer with a 600 l/mm grating, giving a resolution of 4 cm−1 , and it was detected with a thermo-electrically cooled CCD. Microhardness measurements were obtained using a Shimadzu model HMV-2-Series microhardness tester with a diamond pyramid indenter applied at a loading of 10 g for 30 s. An average of 10 readings were taken to obtain the microhardness values of the coatings. To prevent the substrate from affecting the microhardness measurements, coating samples with a total electrodeposition charge of 1500 C were used. 2.4. Electrochemical corrosion tests The corrosion behaviours of the coatings were evaluated in 0.1 mol dm−3 aqueous NaCl solution with a natural pH of 6.5. This evaluation employed the potentiodynamic linear polarization technique (PLP) with a scan rate of 1 mV s−1 and also long-term immersion tests, which enabled monitoring of the open circuit potential (Eoc ). The electrochemical experiments were conducted with two or more samples at room temperature (∼ =27 ◦ C). A saturated calomel electrode (SCE) was used as the reference electrode, while the counter electrode was platinum. An AUTOLAB PGSTAT 30 potentiostat/galvanostat, linked to a PC microcomputer and controlled by GPES 4.0 software, was used for the acquisition of electrochemical data. 3. Results and discussion 3.1. EDX analyses The influence of electrodeposition current density on coating composition is shown in Table 2. An increase in deposition current density leads to a decrease in tungsten content in the coating. This observation agrees with results reported in the literature by Younes-Metzler and Gileadi [10,13]. These authors studied Ni–W electroplating from ammonia–citrate baths using rotating disc electrodes, and they showed that electroreduction of Ni2+ is kinetically controlled by activation, while electroreduction of WO4 2− is kinetically controlled by diffusion. This picture is consistent with our results: since increasing the plating current density makes the cathode potential more negative, an increase in Ni content in the coating may be expected. Moussa et al. [26] and Cesiulis and Table 2 Influence of the deposition current density on the Ni–W coating composition. j/mA cm−2

W content/at%

20 30 40 50

15 14 7 8

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results are in close agreement with those reported in the literature [28,29]. Slavcheva et al. [28] showed that the size of grains in Ni–W coatings increased with an increase in applied overpotential, while Chassaing et al. [29] showed that alloys with tungsten content under 20 at% did not crack. Finally, these SEM images confirm that the selected electrodeposition parameters were suitable to obtain Ni–W coatings which did not crack. This is an important requirement for samples used in corrosion studies. Surface and cross-sectional SEM micrographs for the Cr coating are shown in Fig. 2. The surface morphology of the Cr coating is characterised by a dendrite microstructure (Fig. 2a), while the cross-sectional micrograph reveals a homogeneous and compact coating without microcracks (Fig. 2b). The Cr coating has a measured thickness of about 7 ␮m. It is well known that proper adjustment of the plating conditions for Cr coatings can result in a crack-free or a cracked morphology [30]. The morphologies shown in these micrographs are typical of electrodeposited Cr coatings obtained under these operational conditions. 3.3. Electrochemical corrosion tests Ni86 W14 and Ni93 W7 samples were selected as representative Ni–W coatings for studies of corrosion in 0.1 mol dm−3 NaCl solution. PLP curves obtained in 0.1 mol L−1 NaCl solution are presented in Fig. 3 for the three studied as-electrodeposited coatings. This figure shows that the Cr coating has the noblest corrosion potential, followed by the Ni–W coating with the higher W content. In addition, for the chromium coating, a current plateau is observed in the

Fig. 1. Surface SEM images of Ni–W coatings obtained at 30 mA cm−2 (a) and 40 mA cm−2 (b). Typical cross-sectional morphologies of the Ni–W coatings are shown in (c).

Podlaha-Murphy [27] also reported similar trends for the electrodeposition of Ni–W coatings. 3.2. SEM characterisation of the as-electrodeposited coatings The surface morphologies of the coatings as-electrodeposited were analysed by SEM, and selected images are displayed in Fig. 1. The SEM images in Fig. 1 show that the surface morphologies of the Ni–W coatings are globular and that the grain sizes increase with an increase in current density. Furthermore, all the electrodeposited Ni–W coatings are homogeneous and defect-free from top to bottom (Fig. 1c). The coating thickness is about 5 ␮m. These

Fig. 2. SEM images showing the surface (a) and the cross-sectional (b) morphologies of a Cr coating.

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Fig. 3. Potentiodynamic polarization curves for both Ni–W and Cr coatings obtained in 0.1 mol dm−3 NaCl aqueous solution.

range from −0.048 to 0.483 V, followed by an increase in the current for higher applied potentials. By contrast, for the Ni93 W7 coating a short current plateau is observed between −0.80 and −0.65 V, while only a slight current decrease around −0.6 V is observed in the PLP curve of the Ni86 W14 . The plateau is associated with the formation of surface films that can, initially, block the dissolution of the coating. The increase in current that follows the current plateau is related to the breakdown of said surface film, leading to the dissolution of the coating. This demonstrates that the films are unstable, and hence the coatings corrode in chloride medium. The corrosion processes of these coatings were further evaluated over the course of 50 days in the same electrolyte. The evolution of the open circuit potentials for Ni–W and Cr coatings as a function of immersion time is shown in Fig. 4. For both electrodeposited Ni–W coatings, the Eoc becomes nobler with time, whereas the Eoc value for the Cr coating is the noblest and remains approximately constant during the whole immersion test. SEM images of the tested samples after 50 days of immersion are shown in Fig. 5. These images show that after prolonged immersion in the test medium, both Ni–W samples had two separate areas

Fig. 5. SEM micrographs of Ni93 W7 (a), Ni86 W14 (b) and Cr (c) coatings after 60 days of immersion.

Fig. 4. Variation of the open circuit potential with immersion time for both of the Ni–W coatings and the Cr coating in 0.1 mol L−1 NaCl solution.

with strikingly different types of damage (Fig. 5a and b). One area is covered with corrosion products, and the other, while not covered with the corrosion products, is cracked. This means that the corrosion process on these Ni–W coatings is non-uniform. Fig. 5c shows that, after the immersion test, the chromium coating is covered by corrosion products, which are rather porous. Hairline cracks are also visible. It can be observed that the initial values of the open circuit potential shown in Fig. 4 are different of the corrosion potentials obtained from the PLP curves shown in Fig. 3. Similar observation was reported by Cardoso et al. [31] for the study of the corrosion

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Fig. 6. Variation of the open circuit potential with immersion time for a Cu substrate in 0.1 mol L−1 NaCl solution.

resistance of Ni–Fe–Cr alloys in chloride medium. These authors showed that the initial Eoc measured during the immersion test was about −0.250 V, while the corrosion potential derived from the polarization curve was about −0.650 V, which indicated that the samples submitted to the immersion tests were initially passivated due to an air formed oxide film. Thus, the Eoc results, shown in

Fig. 8. X-ray diffractograms for the as-electrodeposited Ni93 W7 coating (a), aselectrodeposited Ni86 W14 coating (b) and Ni86 W14 coating annealed at: 100 ◦ C (c), 200 ◦ C (d), 400 ◦ C (e) and 600 ◦ C (f).

Fig. 7. Raman spectra of corrosion products on Ni–W (a) and Cr (b) coating surfaces after the immersion tests.

Fig. 4, suggest that the samples submitted to the immersion tests were previously passivated due to the presence of an air formed oxide film on the coating surfaces. On the other hand, the presence of cracks in the coatings means that the Eoc values, shown in Fig. 4, may be strongly influenced by the copper substrate, since the electrolyte can access the substrate through the cracks. To improve the understanding of the evolution of Eoc towards nobler potentials with immersion time, immersion tests were also carried out on the Cu substrate for 48 h. The evolution of Eoc values for copper with immersion time in the test electrolyte is shown in Fig. 6. This figure shows that Eoc values for copper shift to more negative potentials and tend to stabilise after 24 h of immersion at about −0.21 V. This is very close to the final Eoc values presented by the tested coatings. The Eoc values indicate that Cu corrodes actively in chloride solution, in accordance with results previously reported in the literature [32]. Since the electrolyte has access to the Cu substrate through the cracks, the measured Eoc values likely correspond to a mixed potential containing contributions of the coating and of the substrate. RS analyses were carried out to identify corrosion products formed on the Cr and Ni–W surfaces during the immersion tests. Fig. 7 shows the ex situ Raman spectra of the corrosion products

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Fig. 9. Microhardness of Ni86 W14 and Cr coatings annealed at different temperatures.

on the surface of the Cr and Ni–W coatings after the immersion tests. The spectrum in Fig. 7a shows peaks at 304 and 342 cm−1 , which may be attributed to the Eg symmetry of the Cr2 O3 . The peak at 541 cm−1 is attributable to the A1g symmetry of the Cr2 O3 [33]. On the other hand, based on the literature the assignment of the broad peak at about 662 cm−1 is not clear. Oblonsky and Devine [34] used RS to describe the passivity of Cr in borate buffer solution (pH = 8.4), and they attributed the Cr(OH)2 -like species to a peak observed at 600 cm−1 . They also pointed out that the A1 symmetric stretch mode for Cr(OH)2 can exist between 600

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and 700 cm−1 . Thus, the broad peak observed at about 662 cm−1 (Fig. 7a) may be attributed to the presence of Cr(OH)2 in the film. However, according to the Pourbaix diagram for chromium [35], Cr(OH)2 is unstable in water. For this reason, Oblonsky and Devine [34] postulated the existence of a duplex passive film formed by a barrier layer of Cr3+ species above the Cr(OH)2 species. Bjornkvist and Olefjord [36] corroborated the existence of this duplex film by their electrochemical study of chromium in acidic chloride solution. Another possible source of the broad peak at 662 cm−1 is CrOOH, since the formation of CrOOH is preceded by the formation of Cr(OH)2 and, according to Oblonsky and Devine [34], a chromium–oxygen or chromium–hydroxide complex could be the source of the vibration. However, a reference Raman spectrum for CrOOH does not exist. Thus, the corrosion products on the Cr coating surface (Fig. 5c) are thought to be related to formation of the duplex film (Cr2 O3 + Cr(OH)2 ) during the immersion tests. In addition, chromium oxide dissolves into HCrO4 − ions in water [35], which may explain the appearance of the hairline cracks shown in Fig. 5c. The ex situ Raman spectrum of the corrosion products on the Ni–W coating surface is shown in Fig. 7b. The peak observed at about 367 cm−1 is attributable to the Ni–OH stretching mode [34,37]. The peak at about 535 cm−1 is related to the Ni–O stretching mode [38]. The peak located at about 770 cm−1 may be attributed to the O–W6+ –O bonds, while the peak at 960 cm−1 is attributable to the W6+ = O stretching mode of terminal oxygen atoms, possibly on the surfaces of the clusters in the film [37–39]. The peak observed at 886 cm−1 is not present in the Raman spectrum of WO3 . Lee et al. [40] carried out a RS study of Ni–W oxide thin films and suggested that the peak at 770 cm−1 was related to WO3 . Therefore, the Raman ex situ analyses indicate that Ni(OH)2 , NiO and WO3 are

Fig. 10. SEM micrographs of the Ni86 W14 coating annealed at 100 ◦ C (a), 200 ◦ C (b), 400 ◦ C (c) and 600 ◦ C (d).

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the corrosion products observable on the SEM image for the both tested Ni–W coatings (Fig. 5a and b). 3.4. XRD characterisation of the as-electrodeposited and as-annealed samples The X-ray diffractograms for both Ni–W coatings, aselectrodeposited, are shown in Fig. 8(A and B). Three peaks can be observed in the diffractograms of both Ni–W coatings at 2 angles of about 44◦ , 51◦ and 75◦ . These peaks are related to the nickel diffraction peaks, and none are related to the tungsten. This XRD result can be explained by the formation of a W solid solution in nickel. This consists of tungsten atoms substitutionally dissolved in nickel and it is consistent with reports in the literature for electrodeposited Ni–W coatings [11,16,18,28,41]. Furthermore, XRD peaks at about 51◦ and 75◦ broaden with increasing tungsten content. This is in close agreement with similar XRD analyses as reported by Sriraman et al. [18]. According to these authors, the broadened peaks are related to a reduction in the crystallite size of the alloys, with an increase in the amount of alloying tungsten element. Sriraman et al. [18] also showed by transmission electron micrographs that the Ni–W crystallites are distributed in a Ni–W amorphous matrix. Thus, the XRD analyses suggest that the Ni–W coatings as-electrodeposited present a microstructure consisting of two phases: the crystalline phase and the amorphous phase. The amorphous phase is formed during the electrodeposition process, either due to the formation of mutually incoherent particles that are too small to permit the crystalline phase to be formed, or due to the presence of atoms that do not bond together in the arrangement required for crystalline long-range order [41]. Some authors have described electrodeposited Ni–W coatings from a citrate-based electrolyte and report the detection of another peak by XRD, at the 2 angle of about 41◦ [14,41]. Juˇskenas et al. [41] used XRD, XPS and AFM techniques to characterise this peak, and showed that it is related to the formation of NiWO4 during electrodeposition. Since this peak at 41◦ is not observed in the diffractograms of the as-electrodeposited samples in this study, this analysis suggests that the W co-deposited with Ni from the ammonia–citrate bath is only deposited in its metallic form. Sridhar et al. [14] were the first to observe a diffraction peak at 2 = 50.4◦ in the diffractograms of as-electrodeposited Ni–W coatings. The authors related this peak to the formation of the Ni4 W phase and pointed out that a combination of the plating bath at high temperature and the use of a low electrodeposition current density are expected to yield a relatively low tungsten concentration in the alloy, enabling the formation of the Ni4 W phase. In addition, other crystallographic phases, such as NiW2 and NiW, were observed in the diffractograms of the as-electrodeposited Ni–W coatings when the W content was around 66 and 55 at%, respectively. These differences in W concentration explain why these phases are not present in the diffractograms of the Ni86 W14 and Ni93 W7 coatings shown here. Ni86 W14 samples were annealed at different temperatures to study the evolution of their crystal structure and microhardness properties with annealing temperature. The diffractograms of the annealed samples show that thermal treatment leads to the precipitation of new crystalline phases, even for temperatures as low as 100 ◦ C. As the annealing temperature increases up to 400 ◦ C, the peaks with the 2 angles of about 51◦ and 75◦ become better defined. Also, the intensity of the peak at 45◦ rises while the intensity of the peak observed at 75◦ remains approximately constant. In addition, a new peak appears at about 45◦ and its intensity also rises with the annealing temperature. These three peaks are related to the Ni, and the appearance of this new peak suggest that with the increase of the annealing temperature up to 400 ◦ C, Ni phases precipitate from the Ni–W amorphous matrix. The increase

Fig. 11. Typical SEM micrographs of the surface (a) and cross sectional (b) morphologies of the as-annealed Cr coating. The sample was annealed at 100 ◦ C.

of the Ni peak intensity corresponds to an increase in grain size with annealing temperature. For the sample treated at 600 ◦ C, new characteristic peaks can be observed in the diffractograms, associated with the precipitation of the Ni4 W and NiW phases. Just as a crystallographic structure can arise from an amorphous structure with an increase in annealing temperature, so the formation of Ni4 W and NiW phases after heat treatment at 600 ◦ C may be explained by a thermally induced separation of these crystal phases from the amorphous phase. As previously reported [3,4], the XRD diffractograms of Cr coatings annealed at temperatures from 100 to 600 ◦ C were identical to those obtained for a Cr coating sample examined without annealing. All of the XRD diffractograms displayed a single characteristic peak at 2 = 65◦ for the Cr (110) reflection. 3.5. Microhardness measurements The heat treatment process affects the microhardness of both the Ni–W and the Cr coatings. The evolution of the microhardness of the Ni86 W14 coating with annealing temperature is shown in Fig. 9. The as-electrodeposited Ni–W coatings have microhardness in the range of 350–450 HV. The as-electrodeposited Ni86 W14 coating has a slightly higher microhardness value than the as-electrodeposited Ni93 W7 coating. According to He et al. [42], this behaviour is associated with the enlargement of crystal lattice aberrations caused by tungsten, which leads to a lower likelihood of dislocation. Fig. 9 also shows that the microhardness of both aselectrodeposited Ni–W coatings are smaller than the microhardness of the as-electrodeposited Cr coating, which is about 855 HV.

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Fig. 12. SEM micrographs of the cross sectional morphology of a Ni86 W14 coating annealed at 100 ◦ C (a), 200 ◦ C (b), 400 ◦ C (c) and 600 ◦ C (d).

However, the microhardness of the Ni86 W14 coating increases with increasing annealing temperature. A similar trend was observed by He et al. [42] for Fe–Ni–W coatings obtained by electrodeposition. According to these authors, as the annealing temperature increased, an increasing number of crystals precipitated, and these new crystal phases remained embedded in the remaining amorphous phase, favouring an increase in microhardness. Here, as the annealing temperature increases, Ni, Ni4 W and NiW phases precipitate leading to an increase in the microhardness of the Ni86 W14 coating. 3.6. SEM characterisation of the as-annealed samples Fig. 10 shows the SEM micrograph of a Ni86 W14 coating after heat treatment. The surface morphology of the Ni86 W14 coating evolves from a granular to a cracked morphology as the annealing temperature is increased. Cracks are also observed for as-annealed Cr coatings. A typical SEM image of a heat treated Cr coating is shown in Fig. 11. The SEM images of the cross-section of the Ni86 W14 samples as-annealed (Fig. 12) reveal that the samples heated at 100 ◦ C are devoid of cracks, while those heat treated at higher temperatures have cracks running from the top of the coating down to the substrate. SEM cross-sectional analyses of the as-annealed Cr coating also reveal cracks from the top down to the substrate (Fig. 11b). These cracks are caused by residual stress relieved during the annealing process. 4. Conclusions Ni–W samples as-electrodeposited were uniform, were defectfree from the top to the bottom, and presented a surface morphology that evolved from fine globular grains to rough poly-

crystalline morphology with decreasing electrodeposition current density. All the coatings studied corroded in a chloride medium, and this corrosion was non-uniform for Ni–W coatings. Raman analyses carried out after the immersion tests indicated that Cr2 O3 and Cr(OH)2 were corrosion products on the Cr coating surfaces, while Ni(OH)2 , NiO and WO3 were corrosion products on the Ni–W coating surfaces. Ni, Ni4 W and NiW phases were formed after heat treatment of the Ni86 W14 coating at 600 ◦ C. Although all the annealed Ni–W layers were cracked, their microhardness increased as annealing temperature increased. Overall, the results showed that Ni–W coatings are potential substitutes for chromium in industrial applications when good microhardness properties and tolerance of temperatures higher than 100 ◦ C are required.

Acknowledgments The authors gratefully acknowledge the funding provided by CNPq, CAPES, FINEP and FUNCAP (Brazil). Renato A.C. Santana also thanks FUNCAP for his DCR grant. Eduardo B. Barros acknowledges Funcap/CNPq for financial support. The authors acknowledge the use of the SEM and Raman facilities from IPDI.

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