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Corrosion resistance of electrolessly deposited Ni–P and Ni–W–P alloys with various structures
Materials Science and Engineering A 381 (2004) 98–103
Corrosion resistance of electrolessly deposited Ni–P and Ni–W–P alloys with various structures Y. Gao∗ , Z.J. Zheng, M. Zhu, C.P. Luo College of Mechanical Engineering, South China University of Technology, Guangzhou 510641, PR China Received 29 October 2003; received in revised form 29 March 2004
Abstract As-plated binary Ni–P and ternary Ni–W–P alloy films with either nanocrystalline, amorphous or mixture of nanocrystalline and amorphous (denoted as mix-structure) structures were prepared by electroless deposition. Single-phase nanocrystalline Ni–P and Ni–W–P alloys were also synthesized by crystallization of their mix-structure counterparts. Corrosion behaviors of the obtained deposits in a 0.5 M sulfuric acid solution were investigated. It was found that the as-plated nanocrystalline deposits, whether the binary Ni–P or the ternary Ni–W–P ones, and the annealed binary Ni–P alloy films, all possessed a corrosion resistance much lower than that of their amorphous counterparts. The annealed ternary Ni–W–P alloys, however, had a corrosion resistance higher than that of their amorphous counterpart. This result was attributed to the formation of a dense tungsten oxide film on the surface during annealing process, which was favored strongly by the high density diffusion paths provided by the large fraction of grain boundaries present in the nanocrystalline deposits. © 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion resistance; Electroless deposition; Nanocrystalline; Amorphous structure
1. Introduction Nanocrystalline materials, as a result of the considerable reduction of their grain size and high volume fraction of grain boundaries and triple junctions, have exhibited many unusual mechanical, chemical and electrical properties [1]. For example, remarkable decrease in grain size has been previously shown to lead to a significant improvement of the wear resistance of nanocrystalline materials [2,3]. According to the classic corrosion theory, nanocrystalline materials should have very poor corrosion resistance because their huge quantity of grain boundaries, acting as the preferential corrosion paths, can accelerate corrosion by forming large number of micro electrochemical cells with the matrix. It is thus of interest to assess the effect of nanostructure formation on the corrosion resistance of nanocrystalline materials. There are already some research reports on the effect of nanostructure on corrosion resistance in magnetic materials [4–9] and some other materials [10,11], but the results are ∗
Corresponding author. Fax: +86 20 8711 4484. E-mail address: [email protected] (Y. Gao).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.04.077
quite controversial. Moreover, only a few reports have been focused on the corrosion resistance of electrolessly plated nanocrystalline deposits [12,13]. It is usually thought that the higher wear resistance of electrolessly plated nanocrystalline deposits, if any, as compared to their amorphous counterpart, might have been obtained at the expense of their corrosion resistance. Therefore further investigations on the corrosion resistance of nanocrystalline materials, especially that of the nanocrystalline deposits by electroless plating, are needed. In the present study, single-phase nanocrystalline Ni–P and Ni–W–P deposits were made for investigating the effect of the nanostructure itself on corrosion resistance without the influence of the second phase Ni3 P. These single-phase nanocrystalline deposits were synthesized in the following two ways: (1) they were directly deposited by electroless plating; (2) they were made by crystallization of their as-plated mix-structure deposits. For comparison, Ni–P and Ni–W–P alloy films with an amorphous structure were also prepared. The addition of passivating element W was intended to improve the corrosion resistance of the deposits, which might have decreased due to the nanostructure formation.
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103 Table 1 Formulation of the plating bath and deposition conditions for Ni–P deposits Chemical compounds
NiSO4 ·6H2 O Na2 H2 PO2 ·H2 O Main complex salt Auxiliary complex salt PH Temperature (◦ C)
Concentration (M) Deposit1: amorphous
Deposit 2: mix-structure
Deposit 3: nanocrystalline
0.11–0.12 0.28–0.29 0.1–0.3 0.1–0.2
0.08–0.12 0.15–0.18 0.2–0.3 0.032–0.064
0.17–0.18 0.20–0.30 0.20–0.22 0.128–0.192
5 90 ± 2
9 90 ± 2
10 70 ± 2
2. Materials and experimental procedures The substrates for the plating were plain carbon steel sheets with a size of 10 × 10 mm2 . They were finally ground with emery paper of 600 grit, cleaned in dilute NaOH and rinsed with de-ionized water. As-plated deposits of nanocrystalline, mix-structure and amorphous structures were electrolessly made for both the binary Ni–P and the ternary Ni–W–P alloys by altering the chemistry and pH value of the bath used in the electroless plating. Totally six types of as-plated deposits were obtained. The compositions of the bath and the deposition conditions for the three structures are shown in Table 1 for the Ni–P deposits and Table 2 for the Ni–W–P deposits. The six resultant deposits were marked as deposits 1, 2, 3, 4, 5 and 6, with a thickness ranging from 25 to 35 m. Before annealing the as-plated mix-structure Ni–P and Ni–W–P deposits to obtain single-phase Ni nanocrystalline alloys, a differential scanning calorimeter (DSC STA409) was used to determine the crystallization temperatures of Ni + Ni3 P of the mix-structure Ni–P and Ni–W–P deposits. Then the mix-structure deposits were annealed for various periods of time at a temperature slightly lower than their corresponding crystallization temperature to obtain single-phase Ni nanocrystalline alloys. The annealing temperatures so chosen were 300 ◦ C for the Ni–P mix-structure deposit and 350 ◦ C for the Ni–W–P mix-structure deposit.
Annealing was carried out in a muffle furnace in air atmosphere. The microstructures of the deposits were characterized by X-ray diffraction (XRD) using Philips X’pert MRD diffractometer with monochromatic Cu-K␣ radiation. Deposit compositions were determined by Energy Dispersive X-ray Spectrometry (EDX) attached to a LEO1530 VP Scanning Electron Microscope. Anodic potentiodynamic polarization in a 0.5 M H2 SO4 solution was carried out at a scan rate of 2 mv s−1 to study the corrosion behaviors of the deposits at room temperature, using an AUTOLAB PGSTAT30 of ECO Chemie. A conventional three-compartment glass cell, with a graphite plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, was employed for the electrochemical investigations. Samples with a defined area of 1 cm2 were exposed to the electrolyte.
3. Results and discussion 3.1. Deposit compositions and structures The elemental concentrations of the Ni–P and Ni–W–P alloy deposits determined by EDX are listed in Table 3. Fig. 1 shows the XRD spectra of the as-plated Ni–P deposits. It is clear from the spectra that the structure of the as-plated Ni–P deposits 1 is amorphous and deposit 3 is a crystalline phase of fcc structure. As the composition of deposit 3 is Ni–3.48%P, the crystalline phase should be a supersaturated solid solution of P in Ni, with a grain size of about 13 nm as estimated from the peak broadening of its X-ray pattern. The structure of the as-plated Ni–P deposit 2 is neither complete amorphous nor complete nanocrystalline phase, but a mixture of amorphous and nanocrystalline phases, and is thus termed as mix-structure in this paper. For the X-ray pattern of deposit 2, the first peak shows the superimposition of a strong sharp peak on a broad background peak. The sharp peak is from the (1 1 1) diffraction of nanocrystalline Ni and the broad background peak is from the diffraction of amorphous phase in the deposit,
Table 2 Formulation of the plating bath and deposition conditions for Ni–W–P deposits Chemical compounds
NiSO4 ·6H2 O Na2 H2 PO2 ·H2 O Na2 WO4 ·2H2 O Main complex salt Auxiliary complex salt (ml l−1 ) PH Temperature (◦ C)
Concentration (M) Deposit 4: amorphous
Deposit 5: mix-structure
Deposit 6: nanocrystalline
0.08–0.12 0.18–0.19 0.10–0.30 0.32–0.36 3–7
0.09–0.11 0.18–0.22 0.16–0.20 0.44–0.46 8–12
0.12–0.18 0.10–0.30 0.15–0.21 0.13–0.14 15–25
9 90 ± 2
9 90 ± 2
10 75 ± 2
99
Fig. 1. X-ray diffraction spectra of the as-plated Ni–P deposits.
100
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103
Table 3 Compositions of the six deposits wt.%
Deposit 1
Deposit 2
Deposit 3
Deposit 4
Deposit 5
Deposit 6
Ni P W
87.87 12.13 –
92.5 7.5 –
96.52 3.48 –
88.68 7.53 3.79
93.18 4.48 2.34
95.68 3.43 0.89
Table 4 The estimated average grain sizes of the as-plated deposits and the deposits annealed at 300 ◦ C for Ni–P alloys and 350 ◦ C for Ni–W–P alloys for various times from their mix-structure counterparts Grain size (nm)
As-plated nanocrystalline
As-plated mix-structure
Annealed for 30 min
Annealed for 50 min
Annealed for 100 min
Annealed for 130 min
Annealed for 150 min
Ni–P Ni–W–P
12.6 16.6
4.7 6.2
8.8 7.8
9.3 8.3
9.4 9.4
9.5 9.6
9.5 10.7
thus deposit 2 is likely composed of amorphous phase and nanocrystalline phase. The Ni (2 0 0) and Ni (2 2 0) peaks should also have appeared in deposit 2 because of the existence of Ni nanocrystalline phase. However, these two peaks can hardly be visible in the diffraction pattern. This may be owing to the facts that compared to deposit 3, the smaller grain size in deposit 2 (please see Table 4) further widened the peaks and increased the random scattering intensity, and the limited percentage of Ni nanocrystalline phase in deposit 2 further weakened the peaks, thus leading to the disappearance of the Ni (2 0 0) and Ni (2 2 0) peaks in deposit 2. These three types of structures were also produced in the as-plated Ni–W–P deposits and their XRD spectra are shown in Fig. 2. The crystalline phase in the Ni–W–P deposits is a supersaturated solid solution of P and W in Ni. For simplicity in description, “Ni” is used to represent both the Ni (P) and Ni (P, W) solid solutions throughout the text. Figs. 3 and 4 show the XRD spectra of the mix-structure Ni–P and Ni–W–P deposits annealed at 300 ◦ C and 350 ◦ C, respectively for various times. The mix-structure, instead of the amorphous one, was employed to obtain the single-phase Ni nanocrystalline by crystallization, because it was found in the experiments that it was very difficult
Fig. 2. X-ray diffraction spectra of the as-plated Ni–W–P deposits.
Fig. 3. XRD spectra of the mix-crystalline Ni–P alloy deposits annealed at 300 ◦ C for various times.
Fig. 4. XRD spectra of the mix-structure Ni–W–P alloy deposits annealed at 350 ◦ C for various times.
i/A
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103
1.00x10 0 1.00x10 -1 1.00x10 -2 1.00x10 -3 1.00x10 -4 1.00x10 -5 1.00x10 -6 1.00x10 -7 1.00x10 -8 -0.75
101
Ni-P 0.5M H2SO4
as-plated amorphous as-plated mixcrystalline as-plated nanocrystalline annealed for 50min annealed for 100min annealed for 150min -0.50
-0.25
0
0.25
0.50
0.75
1.00
1.25
1.50
E/V Fig. 5. Anodic polarization curves of the as-plated and annealed Ni–P deposits in a 0.5 M H2 SO4 solution.
to obtain a single-phase Ni nanocrystalline by crystallization of an amorphous structure, whereas crystallization of a mix-structure could easily result in the formation of the single-phase Ni nanocrystalline. The research on the mechanism of this difference is under way. It is seen from Figs. 3 and 4 that the as-plated mix-structure Ni–P and Ni–W–P deposits have transformed into the single-phase nanocrystalline structure by annealing, with Ni (2 0 0) and Ni (2 2 0) peaks occurring at the very beginning of annealing. There was no Ni3 P diffraction peak occurring until the sample was annealed for 180 min for the Ni–P deposit and 150 min for the Ni–W–P deposit in the experiments. The grain sizes of the Ni nanocrystalline deposits were estimated using the Scherrer formula. The peaks were fitted with a Pseudo-Voigt profit function using Profit Program after subtracting the background. The average grain sizes so obtained for the Ni–P and Ni–W–P deposits are shown in Table 4. It was found that the grain sizes of the annealed nanocrystalline deposits didn’t change much with the increasing annealing time until the Ni3 P phase precipitated, indicating that the formed nanostructures have very high
thermal stability. This result is in good agreement with the previous finding by Boylan et al. [14]. 3.2. Corrosion behaviors Figs. 5 and 6 show the anodic potentiodynamic polarization curves of the Ni–P and Ni–W–P deposits respectively in a 0.5 M sulfuric acid solution. The tested deposits included the annealed nanocrystalline deposits and the as-plated deposits with different structures. The values of the corrosion potential Ecorr and the anodic current density icorr were calculated from the intersection of the cathodic and anodic Tafel curves extrapolated from the cathodic and anodic potentiodynamic polarization curves. The values of the polarization resistance Rp were calculated from the linear polarization curves. The calculated results are shown in Tables 5 and 6. It can be seen from Figs. 5 and 6 and Tables 5 and 6 that the binary Ni–P and ternary Ni–W–P deposits of various structures show a similar corrosion trend. The as-plated Ni nanocrystalline deposit has the highest icorr , as well as the lowest Ecorr and Rp , and thus potentially the worst corrosion resistance compared with its amorphous,
Fig. 6. Anodic polarization curves of the as-plated and annealed Ni–W–P deposits in a 0.5 M H2 SO4 solution.
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Table 5 Corrosion potential Ecorr , corrosion current density icorr and polarization resistance Rp for the as-plated and annealed Ni–P deposits Ni–P deposits
Ecorr (mV)
icorr ( A cm−2 )
Rp ( cm−2 )
As-plated As-plated As-plated Annealed Annealed Annealed
−154 −437 −430 −414 −396 −410
2.879 195.7 1198 100 67.51 143.8
3289 58.68 23.12 50.26 159.2 44
amorphous mix-structure nanocrystalline for 50 min for 100 min for 150 min
mix-structure and annealed counterparts. The icorr of the as-plated mix-structure deposit is much smaller than that of the as-plated nanocrystalline deposit, and higher than that of the amorphous deposit. This result further confirms the XRD analysis that the structure of the as-plated deposit 2 is a mix-structure of amorphous and nanocrystalline phases. The annealed nanocrystalline deposits, on the whole, have much lower icorr and higher Rp than the as-plated nanocrystalline deposits. This indicates that the present annealing treatment can greatly improve the corrosion resistance of the electroless deposits, probably due to the reason that annealing can eliminate deposit defects through stress relief and porosity mergence. It is also seen from Fig. 5 and Table 5 that the binary nanocrystalline Ni–P deposits, whether as-plated or annealed, have much higher icorr , lower Rp and Ecorr than their amorphous counterpart, suggesting that when there is no addition of the third passivation elements, the nanostructure formation itself is harmful to the corrosion resistance of the deposits even in the absence of deposit defects. This result is in agreement with the classic corrosion theory that nanostructure should accelerate corrosion by forming much more micro electrochemical cells between the huge amount of grain boundaries and the matrix while the amorphous structure should exhibit inherently high corrosion resistance because of its extreme structural homogeneity without preferential corrosion paths like grain boundaries or other structural defects. In Fig. 6 and Table 6, however, although the as-plated nanocrystalline Ni–W–P deposit still has higher icorr and lower Rp than its amorphous counterpart, the annealed nanocrystalline Ni–W–P deposits have lower icorr and higher (or similar) Ecorr than their amorphous Table 6 Corrosion potential Ecorr , corrosion current density icorr and polarization resistance Rp for the as-plated and annealed Ni–W–P deposits Ni–W–P deposits
Ecorr (mV)
icorr ( A cm−2 )
Rp ( cm−2 )
As-plated As-plated As-plated Annealed Annealed Annealed
−216 −344 −378 −184 −208 −219
7.498 12.49 181.2 3.101 1.446 6.943
4,196 383.3 110.3 1,871 28,920 4,052
amorphous mix-structure nanocrystalline for 50 min for 100 min for 150 min
counterpart. This result suggests that the annealed nanocrystalline Ni–W–P deposits have potentially better corrosion resistance than (or similar to) their amorphous counterpart. It is obvious in Table 3 that the P and W contents of the mix-structure Ni–W–P deposit (deposit 5) are both lower than those of the amorphous Ni–W–P one (deposit 4), wherein both P and W are considered being beneficial to the corrosion resistance of the alloys [12]. So the better corrosion resistance of the annealed mix-structure Ni–W–P deposits with a resulting nanocrystalline structure could only be attributed to the passivation element W which diffused more easily to the deposit surface to form a dense WO3 film [15] during the annealing process. Obviously, the large volume fraction of grain boundaries present in the nanocrystalline structure, serving as diffusion paths, would have markedly facilitated the diffusion of the W atoms from the interior to the surface of the deposit. This WO3 film is reported to be thermodynamically stable in dilute H2 SO4 [12]. Consequently, the corrosion resistance of the annealed nanocrystalline deposits is improved markedly. In Lu and Zangari’s report [12], W addition to nanocrystalline Ni–P alloys only slightly improved their corrosion resistance. The present study, however, shows that W addition to the nanocrystalline Ni–P deposits has striking influence on their corrosion resistance. The difference between Lu and Zangari’s work [12] and this study in corrosion resistance of the nanocrystalline Ni–W–P deposits was attributed to the following two factors. (1) The structures of the as-deposited nanocrystalline Ni–W–P in the two works are likely different. Only one strong Ni (1 1 1) peak was found in the XRD spectra in Liu’s work, while all the three strong peaks of Ni (1 1 1), (2 0 0) and (2 2 0) appeared in this study. (2) The Ni–W–P deposits obtained in Lu and Zangari’s work didn’t undergo annealing. Similar finding has been reported by Wang and co-workers [16,17]. While investigating the oxidation performance of micro-crystalline high temperature alloys, these investigators found that the formation of micro-crystalline structure remarkably improved the oxidation property by forming dense oxide films with passivating elements accumulated on the surface through micro-crystalline grain boundary diffusion. From the anodic behaviors of Ni–P and Ni–W–P deposit shown in Figs. 5 and 6, it is revealed that there exists an active to passive transition for all the alloys investigated, but the current density in the passive region is very high, as compared with that in the active region. No definite trend has been found with varying structures or annealing treatment. It should be noted that in the nanocrystalline Ni–W–P deposit annealed at 350 ◦ C for 150 min, there already exists small amount of the second Ni3 P phase as it can be seen in Fig. 4, but its corrosion resistance is still similar to that of its amorphous counterpart as shown in Table 6. This is a very prospective result which implies that as long as certain amount of passivation element, like W in this study, is
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103
present, even a dual-phase nanocrystalline alloy can exhibit excellent corrosion resistance.
4. Conclusions 1. The binary Ni–P and ternary Ni–W–P nanocrystalline deposits can be obtained directly by electroless plating. 2. The as-plated nanocrystalline deposits, whether binary Ni–P or ternary Ni–W–P alloys, and the annealed binary Ni–P nanocrystalline deposits have much higher icorr , lower Rp and Ecorr , and thus much lower corrosion resistance than their amorphous counterparts. 3. The annealed ternary nanocrystalline Ni–W–P alloys have lower icorr , higher Ecorr , and thus potentially higher corrosion resistance than their amorphous counterpart. 4. The nanostructure itself, at the absence of the third passivation elements, is harmful to the corrosion resistance of Ni–P alloys. But the addition of a third passivation element, like W used in this study, can markedly improve the corrosion resistance of the alloys due to the huge number of grain boundary diffusion paths provided by the nanostructure, which favor the formation of the dense tungsten oxide film on the surface.
103
Acknowledgements This work was supported by the National Natural Science Foundation of China under project no. 59925102, 50371027 and Provincial Natural Science Foundation of Guangdong under project no. 020912.
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Corrosion resistance of electrolessly deposited Ni–P and Ni–W–P alloys with various structures Y. Gao∗ , Z.J. Zheng, M. Zhu, C.P. Luo College of Mechanical Engineering, South China University of Technology, Guangzhou 510641, PR China Received 29 October 2003; received in revised form 29 March 2004
Abstract As-plated binary Ni–P and ternary Ni–W–P alloy films with either nanocrystalline, amorphous or mixture of nanocrystalline and amorphous (denoted as mix-structure) structures were prepared by electroless deposition. Single-phase nanocrystalline Ni–P and Ni–W–P alloys were also synthesized by crystallization of their mix-structure counterparts. Corrosion behaviors of the obtained deposits in a 0.5 M sulfuric acid solution were investigated. It was found that the as-plated nanocrystalline deposits, whether the binary Ni–P or the ternary Ni–W–P ones, and the annealed binary Ni–P alloy films, all possessed a corrosion resistance much lower than that of their amorphous counterparts. The annealed ternary Ni–W–P alloys, however, had a corrosion resistance higher than that of their amorphous counterpart. This result was attributed to the formation of a dense tungsten oxide film on the surface during annealing process, which was favored strongly by the high density diffusion paths provided by the large fraction of grain boundaries present in the nanocrystalline deposits. © 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion resistance; Electroless deposition; Nanocrystalline; Amorphous structure
1. Introduction Nanocrystalline materials, as a result of the considerable reduction of their grain size and high volume fraction of grain boundaries and triple junctions, have exhibited many unusual mechanical, chemical and electrical properties [1]. For example, remarkable decrease in grain size has been previously shown to lead to a significant improvement of the wear resistance of nanocrystalline materials [2,3]. According to the classic corrosion theory, nanocrystalline materials should have very poor corrosion resistance because their huge quantity of grain boundaries, acting as the preferential corrosion paths, can accelerate corrosion by forming large number of micro electrochemical cells with the matrix. It is thus of interest to assess the effect of nanostructure formation on the corrosion resistance of nanocrystalline materials. There are already some research reports on the effect of nanostructure on corrosion resistance in magnetic materials [4–9] and some other materials [10,11], but the results are ∗
Corresponding author. Fax: +86 20 8711 4484. E-mail address: [email protected] (Y. Gao).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.04.077
quite controversial. Moreover, only a few reports have been focused on the corrosion resistance of electrolessly plated nanocrystalline deposits [12,13]. It is usually thought that the higher wear resistance of electrolessly plated nanocrystalline deposits, if any, as compared to their amorphous counterpart, might have been obtained at the expense of their corrosion resistance. Therefore further investigations on the corrosion resistance of nanocrystalline materials, especially that of the nanocrystalline deposits by electroless plating, are needed. In the present study, single-phase nanocrystalline Ni–P and Ni–W–P deposits were made for investigating the effect of the nanostructure itself on corrosion resistance without the influence of the second phase Ni3 P. These single-phase nanocrystalline deposits were synthesized in the following two ways: (1) they were directly deposited by electroless plating; (2) they were made by crystallization of their as-plated mix-structure deposits. For comparison, Ni–P and Ni–W–P alloy films with an amorphous structure were also prepared. The addition of passivating element W was intended to improve the corrosion resistance of the deposits, which might have decreased due to the nanostructure formation.
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103 Table 1 Formulation of the plating bath and deposition conditions for Ni–P deposits Chemical compounds
NiSO4 ·6H2 O Na2 H2 PO2 ·H2 O Main complex salt Auxiliary complex salt PH Temperature (◦ C)
Concentration (M) Deposit1: amorphous
Deposit 2: mix-structure
Deposit 3: nanocrystalline
0.11–0.12 0.28–0.29 0.1–0.3 0.1–0.2
0.08–0.12 0.15–0.18 0.2–0.3 0.032–0.064
0.17–0.18 0.20–0.30 0.20–0.22 0.128–0.192
5 90 ± 2
9 90 ± 2
10 70 ± 2
2. Materials and experimental procedures The substrates for the plating were plain carbon steel sheets with a size of 10 × 10 mm2 . They were finally ground with emery paper of 600 grit, cleaned in dilute NaOH and rinsed with de-ionized water. As-plated deposits of nanocrystalline, mix-structure and amorphous structures were electrolessly made for both the binary Ni–P and the ternary Ni–W–P alloys by altering the chemistry and pH value of the bath used in the electroless plating. Totally six types of as-plated deposits were obtained. The compositions of the bath and the deposition conditions for the three structures are shown in Table 1 for the Ni–P deposits and Table 2 for the Ni–W–P deposits. The six resultant deposits were marked as deposits 1, 2, 3, 4, 5 and 6, with a thickness ranging from 25 to 35 m. Before annealing the as-plated mix-structure Ni–P and Ni–W–P deposits to obtain single-phase Ni nanocrystalline alloys, a differential scanning calorimeter (DSC STA409) was used to determine the crystallization temperatures of Ni + Ni3 P of the mix-structure Ni–P and Ni–W–P deposits. Then the mix-structure deposits were annealed for various periods of time at a temperature slightly lower than their corresponding crystallization temperature to obtain single-phase Ni nanocrystalline alloys. The annealing temperatures so chosen were 300 ◦ C for the Ni–P mix-structure deposit and 350 ◦ C for the Ni–W–P mix-structure deposit.
Annealing was carried out in a muffle furnace in air atmosphere. The microstructures of the deposits were characterized by X-ray diffraction (XRD) using Philips X’pert MRD diffractometer with monochromatic Cu-K␣ radiation. Deposit compositions were determined by Energy Dispersive X-ray Spectrometry (EDX) attached to a LEO1530 VP Scanning Electron Microscope. Anodic potentiodynamic polarization in a 0.5 M H2 SO4 solution was carried out at a scan rate of 2 mv s−1 to study the corrosion behaviors of the deposits at room temperature, using an AUTOLAB PGSTAT30 of ECO Chemie. A conventional three-compartment glass cell, with a graphite plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, was employed for the electrochemical investigations. Samples with a defined area of 1 cm2 were exposed to the electrolyte.
3. Results and discussion 3.1. Deposit compositions and structures The elemental concentrations of the Ni–P and Ni–W–P alloy deposits determined by EDX are listed in Table 3. Fig. 1 shows the XRD spectra of the as-plated Ni–P deposits. It is clear from the spectra that the structure of the as-plated Ni–P deposits 1 is amorphous and deposit 3 is a crystalline phase of fcc structure. As the composition of deposit 3 is Ni–3.48%P, the crystalline phase should be a supersaturated solid solution of P in Ni, with a grain size of about 13 nm as estimated from the peak broadening of its X-ray pattern. The structure of the as-plated Ni–P deposit 2 is neither complete amorphous nor complete nanocrystalline phase, but a mixture of amorphous and nanocrystalline phases, and is thus termed as mix-structure in this paper. For the X-ray pattern of deposit 2, the first peak shows the superimposition of a strong sharp peak on a broad background peak. The sharp peak is from the (1 1 1) diffraction of nanocrystalline Ni and the broad background peak is from the diffraction of amorphous phase in the deposit,
Table 2 Formulation of the plating bath and deposition conditions for Ni–W–P deposits Chemical compounds
NiSO4 ·6H2 O Na2 H2 PO2 ·H2 O Na2 WO4 ·2H2 O Main complex salt Auxiliary complex salt (ml l−1 ) PH Temperature (◦ C)
Concentration (M) Deposit 4: amorphous
Deposit 5: mix-structure
Deposit 6: nanocrystalline
0.08–0.12 0.18–0.19 0.10–0.30 0.32–0.36 3–7
0.09–0.11 0.18–0.22 0.16–0.20 0.44–0.46 8–12
0.12–0.18 0.10–0.30 0.15–0.21 0.13–0.14 15–25
9 90 ± 2
9 90 ± 2
10 75 ± 2
99
Fig. 1. X-ray diffraction spectra of the as-plated Ni–P deposits.
100
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103
Table 3 Compositions of the six deposits wt.%
Deposit 1
Deposit 2
Deposit 3
Deposit 4
Deposit 5
Deposit 6
Ni P W
87.87 12.13 –
92.5 7.5 –
96.52 3.48 –
88.68 7.53 3.79
93.18 4.48 2.34
95.68 3.43 0.89
Table 4 The estimated average grain sizes of the as-plated deposits and the deposits annealed at 300 ◦ C for Ni–P alloys and 350 ◦ C for Ni–W–P alloys for various times from their mix-structure counterparts Grain size (nm)
As-plated nanocrystalline
As-plated mix-structure
Annealed for 30 min
Annealed for 50 min
Annealed for 100 min
Annealed for 130 min
Annealed for 150 min
Ni–P Ni–W–P
12.6 16.6
4.7 6.2
8.8 7.8
9.3 8.3
9.4 9.4
9.5 9.6
9.5 10.7
thus deposit 2 is likely composed of amorphous phase and nanocrystalline phase. The Ni (2 0 0) and Ni (2 2 0) peaks should also have appeared in deposit 2 because of the existence of Ni nanocrystalline phase. However, these two peaks can hardly be visible in the diffraction pattern. This may be owing to the facts that compared to deposit 3, the smaller grain size in deposit 2 (please see Table 4) further widened the peaks and increased the random scattering intensity, and the limited percentage of Ni nanocrystalline phase in deposit 2 further weakened the peaks, thus leading to the disappearance of the Ni (2 0 0) and Ni (2 2 0) peaks in deposit 2. These three types of structures were also produced in the as-plated Ni–W–P deposits and their XRD spectra are shown in Fig. 2. The crystalline phase in the Ni–W–P deposits is a supersaturated solid solution of P and W in Ni. For simplicity in description, “Ni” is used to represent both the Ni (P) and Ni (P, W) solid solutions throughout the text. Figs. 3 and 4 show the XRD spectra of the mix-structure Ni–P and Ni–W–P deposits annealed at 300 ◦ C and 350 ◦ C, respectively for various times. The mix-structure, instead of the amorphous one, was employed to obtain the single-phase Ni nanocrystalline by crystallization, because it was found in the experiments that it was very difficult
Fig. 2. X-ray diffraction spectra of the as-plated Ni–W–P deposits.
Fig. 3. XRD spectra of the mix-crystalline Ni–P alloy deposits annealed at 300 ◦ C for various times.
Fig. 4. XRD spectra of the mix-structure Ni–W–P alloy deposits annealed at 350 ◦ C for various times.
i/A
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1.00x10 0 1.00x10 -1 1.00x10 -2 1.00x10 -3 1.00x10 -4 1.00x10 -5 1.00x10 -6 1.00x10 -7 1.00x10 -8 -0.75
101
Ni-P 0.5M H2SO4
as-plated amorphous as-plated mixcrystalline as-plated nanocrystalline annealed for 50min annealed for 100min annealed for 150min -0.50
-0.25
0
0.25
0.50
0.75
1.00
1.25
1.50
E/V Fig. 5. Anodic polarization curves of the as-plated and annealed Ni–P deposits in a 0.5 M H2 SO4 solution.
to obtain a single-phase Ni nanocrystalline by crystallization of an amorphous structure, whereas crystallization of a mix-structure could easily result in the formation of the single-phase Ni nanocrystalline. The research on the mechanism of this difference is under way. It is seen from Figs. 3 and 4 that the as-plated mix-structure Ni–P and Ni–W–P deposits have transformed into the single-phase nanocrystalline structure by annealing, with Ni (2 0 0) and Ni (2 2 0) peaks occurring at the very beginning of annealing. There was no Ni3 P diffraction peak occurring until the sample was annealed for 180 min for the Ni–P deposit and 150 min for the Ni–W–P deposit in the experiments. The grain sizes of the Ni nanocrystalline deposits were estimated using the Scherrer formula. The peaks were fitted with a Pseudo-Voigt profit function using Profit Program after subtracting the background. The average grain sizes so obtained for the Ni–P and Ni–W–P deposits are shown in Table 4. It was found that the grain sizes of the annealed nanocrystalline deposits didn’t change much with the increasing annealing time until the Ni3 P phase precipitated, indicating that the formed nanostructures have very high
thermal stability. This result is in good agreement with the previous finding by Boylan et al. [14]. 3.2. Corrosion behaviors Figs. 5 and 6 show the anodic potentiodynamic polarization curves of the Ni–P and Ni–W–P deposits respectively in a 0.5 M sulfuric acid solution. The tested deposits included the annealed nanocrystalline deposits and the as-plated deposits with different structures. The values of the corrosion potential Ecorr and the anodic current density icorr were calculated from the intersection of the cathodic and anodic Tafel curves extrapolated from the cathodic and anodic potentiodynamic polarization curves. The values of the polarization resistance Rp were calculated from the linear polarization curves. The calculated results are shown in Tables 5 and 6. It can be seen from Figs. 5 and 6 and Tables 5 and 6 that the binary Ni–P and ternary Ni–W–P deposits of various structures show a similar corrosion trend. The as-plated Ni nanocrystalline deposit has the highest icorr , as well as the lowest Ecorr and Rp , and thus potentially the worst corrosion resistance compared with its amorphous,
Fig. 6. Anodic polarization curves of the as-plated and annealed Ni–W–P deposits in a 0.5 M H2 SO4 solution.
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Table 5 Corrosion potential Ecorr , corrosion current density icorr and polarization resistance Rp for the as-plated and annealed Ni–P deposits Ni–P deposits
Ecorr (mV)
icorr ( A cm−2 )
Rp ( cm−2 )
As-plated As-plated As-plated Annealed Annealed Annealed
−154 −437 −430 −414 −396 −410
2.879 195.7 1198 100 67.51 143.8
3289 58.68 23.12 50.26 159.2 44
amorphous mix-structure nanocrystalline for 50 min for 100 min for 150 min
mix-structure and annealed counterparts. The icorr of the as-plated mix-structure deposit is much smaller than that of the as-plated nanocrystalline deposit, and higher than that of the amorphous deposit. This result further confirms the XRD analysis that the structure of the as-plated deposit 2 is a mix-structure of amorphous and nanocrystalline phases. The annealed nanocrystalline deposits, on the whole, have much lower icorr and higher Rp than the as-plated nanocrystalline deposits. This indicates that the present annealing treatment can greatly improve the corrosion resistance of the electroless deposits, probably due to the reason that annealing can eliminate deposit defects through stress relief and porosity mergence. It is also seen from Fig. 5 and Table 5 that the binary nanocrystalline Ni–P deposits, whether as-plated or annealed, have much higher icorr , lower Rp and Ecorr than their amorphous counterpart, suggesting that when there is no addition of the third passivation elements, the nanostructure formation itself is harmful to the corrosion resistance of the deposits even in the absence of deposit defects. This result is in agreement with the classic corrosion theory that nanostructure should accelerate corrosion by forming much more micro electrochemical cells between the huge amount of grain boundaries and the matrix while the amorphous structure should exhibit inherently high corrosion resistance because of its extreme structural homogeneity without preferential corrosion paths like grain boundaries or other structural defects. In Fig. 6 and Table 6, however, although the as-plated nanocrystalline Ni–W–P deposit still has higher icorr and lower Rp than its amorphous counterpart, the annealed nanocrystalline Ni–W–P deposits have lower icorr and higher (or similar) Ecorr than their amorphous Table 6 Corrosion potential Ecorr , corrosion current density icorr and polarization resistance Rp for the as-plated and annealed Ni–W–P deposits Ni–W–P deposits
Ecorr (mV)
icorr ( A cm−2 )
Rp ( cm−2 )
As-plated As-plated As-plated Annealed Annealed Annealed
−216 −344 −378 −184 −208 −219
7.498 12.49 181.2 3.101 1.446 6.943
4,196 383.3 110.3 1,871 28,920 4,052
amorphous mix-structure nanocrystalline for 50 min for 100 min for 150 min
counterpart. This result suggests that the annealed nanocrystalline Ni–W–P deposits have potentially better corrosion resistance than (or similar to) their amorphous counterpart. It is obvious in Table 3 that the P and W contents of the mix-structure Ni–W–P deposit (deposit 5) are both lower than those of the amorphous Ni–W–P one (deposit 4), wherein both P and W are considered being beneficial to the corrosion resistance of the alloys [12]. So the better corrosion resistance of the annealed mix-structure Ni–W–P deposits with a resulting nanocrystalline structure could only be attributed to the passivation element W which diffused more easily to the deposit surface to form a dense WO3 film [15] during the annealing process. Obviously, the large volume fraction of grain boundaries present in the nanocrystalline structure, serving as diffusion paths, would have markedly facilitated the diffusion of the W atoms from the interior to the surface of the deposit. This WO3 film is reported to be thermodynamically stable in dilute H2 SO4 [12]. Consequently, the corrosion resistance of the annealed nanocrystalline deposits is improved markedly. In Lu and Zangari’s report [12], W addition to nanocrystalline Ni–P alloys only slightly improved their corrosion resistance. The present study, however, shows that W addition to the nanocrystalline Ni–P deposits has striking influence on their corrosion resistance. The difference between Lu and Zangari’s work [12] and this study in corrosion resistance of the nanocrystalline Ni–W–P deposits was attributed to the following two factors. (1) The structures of the as-deposited nanocrystalline Ni–W–P in the two works are likely different. Only one strong Ni (1 1 1) peak was found in the XRD spectra in Liu’s work, while all the three strong peaks of Ni (1 1 1), (2 0 0) and (2 2 0) appeared in this study. (2) The Ni–W–P deposits obtained in Lu and Zangari’s work didn’t undergo annealing. Similar finding has been reported by Wang and co-workers [16,17]. While investigating the oxidation performance of micro-crystalline high temperature alloys, these investigators found that the formation of micro-crystalline structure remarkably improved the oxidation property by forming dense oxide films with passivating elements accumulated on the surface through micro-crystalline grain boundary diffusion. From the anodic behaviors of Ni–P and Ni–W–P deposit shown in Figs. 5 and 6, it is revealed that there exists an active to passive transition for all the alloys investigated, but the current density in the passive region is very high, as compared with that in the active region. No definite trend has been found with varying structures or annealing treatment. It should be noted that in the nanocrystalline Ni–W–P deposit annealed at 350 ◦ C for 150 min, there already exists small amount of the second Ni3 P phase as it can be seen in Fig. 4, but its corrosion resistance is still similar to that of its amorphous counterpart as shown in Table 6. This is a very prospective result which implies that as long as certain amount of passivation element, like W in this study, is
Y. Gao et al. / Materials Science and Engineering A 381 (2004) 98–103
present, even a dual-phase nanocrystalline alloy can exhibit excellent corrosion resistance.
4. Conclusions 1. The binary Ni–P and ternary Ni–W–P nanocrystalline deposits can be obtained directly by electroless plating. 2. The as-plated nanocrystalline deposits, whether binary Ni–P or ternary Ni–W–P alloys, and the annealed binary Ni–P nanocrystalline deposits have much higher icorr , lower Rp and Ecorr , and thus much lower corrosion resistance than their amorphous counterparts. 3. The annealed ternary nanocrystalline Ni–W–P alloys have lower icorr , higher Ecorr , and thus potentially higher corrosion resistance than their amorphous counterpart. 4. The nanostructure itself, at the absence of the third passivation elements, is harmful to the corrosion resistance of Ni–P alloys. But the addition of a third passivation element, like W used in this study, can markedly improve the corrosion resistance of the alloys due to the huge number of grain boundary diffusion paths provided by the nanostructure, which favor the formation of the dense tungsten oxide film on the surface.
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Acknowledgements This work was supported by the National Natural Science Foundation of China under project no. 59925102, 50371027 and Provincial Natural Science Foundation of Guangdong under project no. 020912.
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