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Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process
Accepted Manuscript Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process
Jie Chen, Guanlin Zhao, Kenji Matsuda, Yong Zou PII: DOI: Reference:
S0169-4332(19)31135-3 https://doi.org/10.1016/j.apsusc.2019.04.142 APSUSC 42450
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
18 February 2019 18 March 2019 11 April 2019
Please cite this article as: J. Chen, G. Zhao, K. Matsuda, et al., Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.04.142
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ACCEPTED MANUSCRIPT
Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process
Jie Chen1, Guanlin Zhao2*, Kenji Matsuda3, Yong Zou1*
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1. MOE Key Lab for Liquid–Solid Structure Evolution and Materials Processing, Institute of
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Materials Joining, Shandong University, Jinan 250061, China.
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2. School of Materials Science and Engineering, Shandong University, Jinan 250061, China. 3. Faculty of Sustainable Design, Department of Materials Design and Engineering (MaDE),
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Corresponding author: Guanlin Zhao, Email: [email protected], Tel: +8653188399872; Yong
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Zou, Email: [email protected], Tel: +8653188399872.
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University of Toyama, 3190, Gofuku, Toyama.
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ACCEPTED MANUSCRIPT Abstract Ni–Cu–P alloy was produced by electroless deposition, and the different microstructures were obtained by low–temperature heat treatment. Effect of element Cu on microstructure evolution and corrosion resistance of Ni–Cu–P alloy was studied by X–ray diffraction (XRD), transmission electron microscope (TEM) and electrochemical examination. The results
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showed that the crystalline behavior of Ni–Cu–P amorphous alloy was a gradual process during the heat treatment process. However, the corrosion resistance of Ni–Cu–P alloy was
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firstly enhanced and then deteriorated during the heating process. The corrosion resistance of
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the alloys was not only affected by the crystalline precipitates, but also influenced by the
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alloying elements in the coatings. In Comparison with the Ni–P samples under the same states, the incorporated copper enhanced the thermal stability and corrosion resistance of the Ni–Cu–
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P samples. The Mott Schottky (M–S) results showed that the passive layer on ternary Ni–Cu– P had p–n bipolar semiconductor characteristic, however the binary Ni–P passivation film
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showed only one p type semiconductor characteristic in 3.5 wt.% NaCl solution.
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Keywords: Amorphous coatings; Microstructure evolution; Corrosion resistance; Passive
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film; XPS
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ACCEPTED MANUSCRIPT 1. Introduction Ni–P amorphous alloys applicated in industrial as corrosion protective coatings can be dated back to 1946 [1, 2]. More recently, other metal elements, such as Cu, was introduced to produce Ni–Cu–P ternary alloy to improve the properties of binary Ni–P alloy [3-8]. Studies have demonstrated that the functional properties of Ni–Cu–P alloy, such as outward
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appearance [8], wear [5], corrosion resistance [4, 6-8] and thermal stability [4, 9] were
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superior to those of Ni–P alloy. One of the most important functional properties of Ni–Cu–P
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alloy is its excellent corrosion resistance, which have generated great interest. The corrosion resistance mechanism of Ni–P based alloys has been studied extensively
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and different explanations have been put forward [9]. Many researchers attributed it to the amorphous structure, that is, without the typical defects in crystalline alloys like grain
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boundary, dislocation, and segregation [10]. Nevertheless, the local atomic structure of the amorphous alloys is not fully disorder. There exist short range order clusters in the amorphous
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alloy [11]. In our previous studies, the effect of local atomic structure on corrosion resistance
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of Ni–P based alloys have been reported [4, 12]. The Ni–P amorphous alloys are thermodynamically unstable, and its corrosion resistance are strongly affected by the
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microstructure transition during the heating process. Some studies reported that the amorphous coatings showed superior corrosion resistance to those of the coatings with
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microcrystalline structure [13, 14]. On the other hand, the heat–treated specimens (400 ºC [14-17] or 500 ºC [18]) showed higher corrosion resistance than those of unheated coatings. However, there is no detailed mechanistic explanation for the corrosion resistance variation during crystallization in the literature. Recently, the passivation film theory has been involved to analysis the excellent corrosion resistance performance of Ni–P coating [19], which has been used effectively to explain the superior corrosion resistance of stainless steel and other alloys [20-23]. However, reports on properties of passivation film formed on Ni–P alloy were concentrated mainly on 3
ACCEPTED MANUSCRIPT the valence state of each element and chemical constitution of the passivation [19]. Data on the semiconducting performances of Ni–P passive films are scarce. The aim of this study is to analyze the details of the microstructural evolution and corrosion resistance performance of the Ni–Cu–P amorphous alloy during the crystallization process and compare them with Ni–P amorphous alloy with the same phosphorus proportion.
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Both of the coatings were fabricated in the amorphous state by electroless plating and then
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heat treated from room temperature to 500 ºC to obtain different microstructures. The
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microstructure characteristic and phase information of the coatings at as–deposited and heat– treated states were studied through transmission electron microscope (TEM) and X–ray
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diffraction (XRD). The corrosion resistance of the coatings was analyzed by electrochemical measurement. Mott Schottky (M–S) curves and X–ray photoelectron spectroscopy (XPS)
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spectrum were applied to study the passive film formed on the coatings in 3.5 wt.% NaCl solution. Eventually, the corrosion resistance mechanism during the crystallization process
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was revealed by a combined analysis of these results. The correlation between microstructure
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and corrosion resistance of Ni–Cu–P alloy was also discussed.
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2. Material and methods
2.1 Preparation of Ni–Cu–P and Ni–P electroless coatings
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The material used as the substrate for plating was low–carbon steel sheets with the size of 10mm×10mm×1mm. The low–carbon steel sheets were polished and cleaned intensively before electroless plating. Ni, P and Cu in the obtained electroless coating were originated from nickel sulfate, sodium hypophosphite and copper sulfate, respectively. Anhydrous sodium, glycine, sodium citrate and acetate applied as additives in the plating bath. The electroless plating was conducted at about 85 ºC for 3 hours, and the PH value of the electroless plating solution was about 4.7. Some of the coatings were heat treated in an argon atmosphere at 200, 300, 400, and 500 ºC for 1h, respectively. 4
ACCEPTED MANUSCRIPT 2.2 Composition, morphology, and microstructure characterization The composition of the samples was characterized by EDS attached to a scanning electron microscope (SEM; ZEISS EVO MA 10). Accordingly, the compositions of the samples were Ni0.79P0.21 and Ni0.75Cu0.02P0.23 (at.%). The microstructures of samples were evaluated by the means of XRD (Bruker D8
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Discovery) with Cu Kα radiation (λ=1.5406 Å). The work current, voltage and scanning
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speed were fixed at 40 mA, 40 KV, and 1 °/ min. A 200 KV TEM (TOPCON–002B) was also
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employed for the analysis of morphologies and microstructures of the coatings. To avoid the electron beam damage, work voltage was set at 120 kV. TEM samples of the coatings were
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obtained using twin jet thinning instrument, and the work current was 40–80 mA. The work
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temperature was about –30 ºC. The electrolyte was the perchloric acid and methanol mixture.
2.3 Thermal property characterization
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The thermal properties of the coatings were investigated by differential scanning
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calorimetry (DSC; Labsys–TG–DSC 1600ºC) from room temperature to 600 ºC in pure argon atmosphere, and the reference sample was aluminum oxide. The continuous heating rates
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were set at 10, 20, 40 and 60 ºC /min. The following Kissinger equation was applied to evaluate the activation energy of the samples during the heat treatment process [24]: (1)
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ln(𝑣⁄𝑇 2 ) = − 𝛥𝐸 ⁄𝑅𝑇+ C
Where 𝑣 is the annealing rate (K/min); T is the characteristic temperature (K); E is the activation energy (KJ/mol); R is the universal gas constant (kJ mol–1 K–1) and C is a constant.
2.4 Electrochemical measurements Corrosion resistance of the samples were investigated through potentiodynamic polarization on an Autolab (Aut84886) workstation. The electrochemical tests were conducted in a classic three–electrode system. The platinum plate, Ag/AgCl and electroless coating acted 5
ACCEPTED MANUSCRIPT as counter electrode, reference electrode and working electrode respectively. The scanning rate of polarization tests was 0.5 mV/s. The scanning range was –0.3 to 1 V relative to the open circuit potential (OCP). The corrosion potential (Ecorr) and the corrosion current density (Icorr) were deduced from the polarization curves by Tafel fitting. Semiconductor characteristics of the passive film were studied by M–S curves. The scanning range of M–S
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measurement was –0.2 to 0.2 V (relative to the reference potential), frequency was 1000HZ,
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and the amplitude was 10 mV. To steady the OCP, all of the samples were soaked in the
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corrosive solution (3.5 wt.% NaCl) for 20 min before testing.
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2.5 Corrosion products characterization
The composition of the corrosion products produced on the samples was investigated
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through XPS (ESCALAB 250 Xi). The working condition of XPS was Al Kα, 150 W and 15 kV. The scanning angle of the test was 45°. The broad XPS spectrum was scanned from 0 to
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1200 eV. The detailed XPS spectrum of each element was also employed to compare the
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element differences of different samples. The peak assignment was carried out according to
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the data gathered from published nickel, copper and phosphorus compounds in the literature.
3. Results and discussion
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3.1 Microstructural analysis of the coatings Figure 1 exhibits the XRD curves of the as–deposited and annealed Ni–Cu–P and Ni–P alloys. The only one single broad peak at the position of 2θ = 40~50° of the as–plated coatings was attributed to their amorphous structure [14, 25]. As shown in Fig.1 (a), there was no evident variation in XRD spectrums observed for the Ni–Cu–P sample as the treatment temperature was below 300 ºC, indicating no obvious crystallization occurred at these temperatures. However, as shown in Fig.1 (b), another broad peak appeared at the position of 2θ = 50~55° in the Ni–P sample, as the annealing temperature was 300 ºC. This phenomenon 6
ACCEPTED MANUSCRIPT indicated that the Ni–P sample began to crystallize as heat treated at 300 ºC. When the coating was annealed at 400 ºC, the other peaks of Ni, and Ni3P phases emerged in the broad amorphous profile matrix of both coatings. Besides, the metastable Ni8P3 peak was also appeared in the broad amorphous profile matrix of Ni–P sample. However, the other metastable NixPy phases such as Ni12P5 [16, 26-29] and Ni5P2 [18, 28-30] were not detected by
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XRD in present study, which were reported in the literate. This phenomenon may be related to
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the small amount of metastable NixPy phases formed in the crystallization process, which
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cannot be detected by XRD. Therefore, TEM observation should be further performed to determine the existence of metastable NixPy phases. As the coating was annealed at 500 ºC,
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the broad amorphous peaks in both of the coatings disappeared, instead of the sharp peaks of Ni and Ni3P. This indicated that the coatings were fully crystallized to the equilibrium two–
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phase Ni/Ni3P, resulting in the increase in peaks intensity of the Ni and Ni3P phases.
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3.2 Corrosion behaviors of the coatings
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Corrosion resistance of the samples in neutral sodium chloride solution was studied by potentiodynamic polarization as demonstrated in Fig. 2. As shown in Fig. 2a, Ni–Cu–P samples exhibited a passivation stage between –0.1 and 0.2 V as the heat treatment
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temperature was below 300 ºC. The passivation phenomenon was evidently weakened as the
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Ni–Cu–P amorphous coating was heat treated at the temperature of 400 to 500 ºC. The anodic polarization behavior of the sample annealed at 400 ºC was changed from passive to active with respect to the as–plated conditions, which was consistent with the study results of D. Ahmadkhaniha [31]. As shown in Fig. 2b, the Ni–P coatings showed similar passivation performance during crystallization. The corrosion resistance parameters were obtained from the polarization curves by Tafel fitting and curved in Fig.3. It could be discovered that the Icorr of Ni–Cu–P coating was lower than that of Ni–P coating regardless of the state. In other words, the corrosion resistance of Ni–Cu–P plating was better than that of Ni–P plating.
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ACCEPTED MANUSCRIPT Besides, as shown in Fig. 3, the corrosion resistance of the two alloys was increased firstly with the increase of heat treatment temperature, and showed best corrosion resistance when the samples were annealed at 300 ºC. Then, corrosion resistance of the alloys was reduced when the coatings were crystallized (400 to 500 ºC).
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3.3 TEM analysis of the coatings
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To clarify the influence of heat treatment temperature on corrosion resistance of Ni–Cu–
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P and Ni–P alloys, the microstructure evolution of the coatings during heat treatment were firstly analyzed. Fig. 4 exhibits the TEM results of ternary Ni–Cu–P alloys tempered at 200
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and 300 ºC for 1 h. The bright field image (Fig. 4a) shows that the matrix of annealed Ni–Cu– P at 200 ºC was uniform and showed amorphous structure. However, the corresponding
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selected area diffractions patterns (SADP) was consisted of a halo ring originated from the amorphous Ni–Cu–P matrix as well as some rings originated from nanometer–sized fine
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particles, as shown in Fig. 4b. The plane distances of the diffraction rings were measured and
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affirmed the existence of nanocrystalline Ni and Ni12P5 particles. HRTEM image (Fig. 4c) confirmed the occurrence of nanocrystals with the size of 3–5nm within the amorphous
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matrix. After further crystallization (300 ºC), although the bright field image (Fig. 4d) of Ni– Cu–P coatings still showed the homogeneous amorphous structure, the SADP (Fig. 4e) and
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HRTEM image (Fig. 4f) confirmed the presence of nanocrystalline Ni, Ni5P4 and Ni12P5 with the size of 3–8 nm within the amorphous matrix. As a result of comparison, Fig. 5 exhibits the TEM results of Ni–P coating when annealed at 200 and 300 ºC for 1 h. In the bright field image at 200 ºC (Fig. 5a), dark contrast precipitates in Ni–P sample could be distinguished, which were homogeneous nanometer–sized fine particles with the size of 2~10 nm. The SADP showed in Fig. 5b, implied a large amount of grains differently oriented within the amorphous matrix. The analysis of SADP confirmed the presence of Ni, Ni5P4 and Ni12P5. The HRTEM image of the sample also confirmed the presence of Ni8P3, as shown in Fig. 5c. 8
ACCEPTED MANUSCRIPT As exhibited in Fig. 5d, the nanoparticles in Ni–P coating annealed at 300 ºC were evidently bigger (varying between 10~40 nm) and denser than those in the coating tempered at 200 ºC. In the analysis of SADP (Fig. 5e) and HRTEM image (Fig. 5f), the metastable Ni5P2 phase and stable Ni3P precipitates were also occurred in the amorphous matrix. According to the XRD and TEM analysis, it could be concluded that the Ni–Cu–P and Ni–P samples were
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mainly amorphous as the annealing temperature was below 300 ºC, wherein existed a very
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small amount of metastable crystalline Ni8P3, Ni5P2, Ni5P4, Ni12P5, and stable Ni3P
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nanocrystallines. Besides, the coatings were crystallized to the equilibrium Ni and Ni3P phases as the annealing temperature was changed to 400 and 500 ºC.
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Through comparative analysis, it could be discovered that the amount and sizes of the nanoparticles in the annealed Ni–Cu–P samples were relatively smaller than those in Ni–P
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coatings. This phenomenon indicated that the fine nanocrystalline precipitates in amorphous Ni–P were effectively retarded by alloying a small fraction of copper. In other words, the
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stability of Ni–Cu–P amorphous coating was significantly enhanced through the incorporation
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of Cu. This is because the nucleation and growth of intermetallic compounds was accomplished by the movement of atoms. The movement and aggregation of phosphorus
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atoms in the Ni–Cu–P coating was effectively prevented by copper atoms, resulting in the decrease in the amount and size of the nanophases in the amorphous matrix.
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In general, galvanic corrosion occurs as two different alloys are in contact in a corrosive environment. Different alloys show different self–corrosion potential. The corrosion rate of the alloy with negative original self–corrosion potential increased, while the corrosion rate of the alloy with positive original self–corrosion potential decreased. TEM and XRD results showed that various metastable nanoparticles precipitated in the Ni–Cu–P and Ni–P coatings during crystallization. The composition and microstructure difference between metastable crystalline and the amorphous matrix resulted in corrosion potential differences, which lead to micro-galvanic corrosion. Besides, the galvanic corrosion could be increased as 9
ACCEPTED MANUSCRIPT the sizes and total amounts of precipitated phases increased. As the material is totally crystallized, the grain boundaries show some characteristics with thermodynamic instability and electrochemical inhomogeneity. Thus, the grain boundaries are active positions for intergranular corrosion attack. The corrosion starts from the surface and develops inward along the grain boundary. Eventually, many irregular polygonal corrosion cracks formed
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along the grain boundaries. In conclusion, the metastable crystalline and the crystal structure
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in the amorphous matrix is detrimental to the inhibition of corrosion.
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However, in this study, it is interesting that the Tafel fitting results (as seen in Fig. 3) exhibited that the corrosion resistance of the coatings was not in proportion to the heat
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treatment temperature. With the increase of the heat treatment temperature, corrosion resistance of the coatings was initially improved and then deteriorated and showed best
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corrosion resistance at 300 ºC. Besides, although the Ni–Cu–P and Ni–P alloys were significantly crystallized (400 to 500 ºC), the increase in corrosion current densities were
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relatively small. This is because the corrosion resistance variation during crystallization was
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affected by the two aspects of the microstructure and the chemical composition. The heat treatment at low temperature stage (below 300 ºC) was not the main stage of crystallization.
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In this condition, the matrix of the coatings still kept amorphous state with small amount of the small size nanocrystallines. Under this condition, the element distribution in the coating
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was still continuous and uniform, which was conducive to the uniformity and integrity of the formed passivation film. The effect of galvanic corrosion between the amorphous matrix and the nanoparticles was limited. On the other hand, the residual stress in the as–deposited coatings could be effectively eliminated through the low–temperature heat treatment [12]. The vacancies in the amorphous matrix were filled by the accelerated movement of atoms, which resulted in the better uniformity of the matrix. Therefore, the corrosion resistance of alloys was improved as the temperature was below 300 ºC. As the heat treatment temperature was increased to 400 ºC, corrosion resistance of the samples reduced due to the influence of 10
ACCEPTED MANUSCRIPT intergranular corrosion. However, the increase in corrosion current densities were relatively small because of the protection of the passivation film formed on the surface of the coatings. In general, corrosion resistance of samples can be improved by changing the composition and semiconductor properties of the passivation film. Therefore, the semiconductor characteristics of the passivation films formed on the two kinds of coatings in corrosive liquid were tested
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separately, and the results will be discussed in the following section.
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3.4 The thermal properties of the coatings
To study the phase transformation performance of the Ni–Cu–P and Ni–P deposits, DSC
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measurements were conducted. Fig. 6(a)–(b) shows the DSC thermograms of the samples. The major exothermic peaks in the DSC curves of the Ni–Cu–P and Ni–P deposits were
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corresponded to the long range atomic shift, which was the precipitation of nickel and nickel phosphide (Ni3P) phases [32]. Besides, it could be discovered that the peak intensity of the
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coatings increased with the increase of scanning rates. DSC thermogram of the Ni–Cu–P
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amorphous alloy (Fig. 6(a)) exhibited higher exothermic temperatures than those of binary Ni–P coating (Fig. 6(b)). In conclusion, the phase transformation temperature of the Ni–Cu–P
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amorphous coating was strongly increased by the incorporation of a small amount of Cu. Crystallization activation energies of the electroless deposits were calculated according
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to plots of ln(𝑣 /T2) vs 1/T originated from the Kissinger equation (Fig. 6(c) and (d)) using the onset temperatures (Tx) and peak temperatures (Tp ) of the major exothermic peak in the DSC plots, at different heating rates (𝑣). The activation energy of Ni–Cu–P coating was higher than that of Ni–P coating, which implied that the thermal stability of Ni–Cu–P amorphous coating was enhanced by alloying a small amount of Cu. The DSC results was consistence with the results of XRD and TEM.
3.5 Semiconducting properties of the passive films 11
ACCEPTED MANUSCRIPT It is well known that the Mott–Schottky measurement can be effectively used to characterize the semiconducting characteristics of passivation films. In this study, the semiconductor type of passivation thin films was determined according to the slope of the M– S plots. The positive and negative slopes of M–S plots indicate the n–type and p–type semiconductor characteristic of passive film, respectively. Fig.7 shows the M–S plots of Ni–
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Cu–P and Ni–P samples passivated at 0V for 1h in 3.5% NaCl solution. In Fig. 7 (a), the M–S
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curve of Ni–Cu–P passivation film showed two linear segments, which was the negative slop
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at the potential from –0.2 to –0.06 V and the positive slop at the potential range from –0.06 to 0.1V. Therefore, the passive layer generated on Ni–Cu–P sample showed bipolar
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semiconductor characteristic. However, as exhibited in Fig. 7 (b), the Ni–P sample showed only one p–type semiconductor characteristic. The better corrosion resistance of Ni–Cu–P
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sample was related to the passivation film semiconductor property. Passive film theory has been effectively applicated to interpret the better corrosion resistance mechanism of some
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alloys such as stainless steels [33, 34]. The passivation film formed on samples acted as the
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barrier layer as electrons/ions transport in the interface of passive film and corrosion solution. Formation and destruction mechanism of passivation films has been investigated and several
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kinds of models have been put forward. The most popular models among them were the point defect model and the solute-vacancy interaction model. According to the passive film theory,
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the passivation film with bipolar characteristic display excellent corrosion resistance, which could prevent the positive ions out as well as hinder the negative ions in. Therefore, the protective efficacy of the passivation films can be reinforced. In this study, the Ni–Cu–P passive film with p–n bipolar semiconducting characteristic could not only prevent the negative ions in but also restrain the positive ions out, which increased the protectiveness and corrosion resistance of Ni–Cu–P passivation film. In comparison, the passivation layer developed on Ni–P sample showed only p type semiconducting characteristic, which was in a unidirectional conducting condition. The preventive effect of the Ni–P passive layer was 12
ACCEPTED MANUSCRIPT deteriorated. The different semiconductor performance of passive layers formed on Ni–Cu–P and Ni–P samples was determined by the composition difference.
3.6 Compositions of the passive films The element distribution characteristics of the passivation layers produced on Ni–Cu–P
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and Ni–P deposits were analyzed through XPS. The normalized XPS spectrum of the passivated samples are shown in Fig. 8. For reference, the binding energies of some Ni, Cu
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and P compounds were abstracted from the literature and listed in Table 1. Typical element
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distribution positions in the spectrum were signed in Fig. 8. The two curves in Fig. 8 exhibited
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no evident difference, which revealed that the passive films formed on the coatings were mainly consisted of copper, nickel, oxygen, carbon and phosphorus. O2 and C in the spectrum
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were originated from surface contamination [35]. Nevertheless, the peaks intensity and location of the elements were a little different. Therefore, high resolution XPS spectra (Fig. 9)
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of the Ni, Cu, O and P were employed to analysis the difference between two samples.
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Considering the difficulty in determining the main states of Ni and P in some broad peaks from Fig. 9, the curves of Ni 2p 3/2 and P 2p 3/2, were divided into different contributions
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(Fig. 10).
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Element Ni Ni Ni Ni Cu Cu Cu P P O
Table 1 Spectral line Compound 2p3/2 Ni(OH)2 2p3/2 Ni2+ 2p3/2 Ni3(PO4)2 2p3/2 Ni 2p1/2 Cu 2p3/2 Cu2O 2p3/2 Cu 2p3/2 P in bulk 2p3/2 Ni3(PO4)2 1s O2–
Energy (eV) 861.7 857.0 856.1 853.8 952.4 932.0 932.2 129.7 133.3 531.2
Ref. [36] [37] [36] [38] [39] [40] [41] [2] [36] [36]
From Fig. 9a and Fig. 10a–b, the XPS spectrum of Ni 2p3/2 in the passive films could be decomposed into four peaks. According to the reference, as shown in Table 1, the peaks observed at 855.7±0.1 eV and 856.8±0.1 eV were appointed to Ni2+ in nickel phosphate. 13
ACCEPTED MANUSCRIPT Besides, the peaks at 861.6±0.1 eV and 853.6±0.1 eV were attributed to Ni2+ in Ni(OH)2 and Ni in the coating, respectively. The high–resolution curves of P 2p3/2 (Fig. 9b and Fig. 10c–d) were consisted of two peaks at 130.2±0.1 eV and 133.3±0.2 eV. The peaks at 133.3±0.2 eV was assigned to Ni3(PO4)2 [36], and the peaks at 130.2±0.1 eV was appointed to P in the coating [2]. The spectrum of Cu 2p are exhibited in Fig. 9c. The most intense peak at 932.9
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eV was Cu or Cu2O, followed by Cu (952.3 eV). The O ls spectrum (Fig. 9d) shows a broad
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curve consist of numerous oxygenated compounds. From the XPS results, it could be
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confirmed that the passivation film of the Ni–P plating was mainly consist of nickel phosphate, Ni(OH)2, Ni and P. In comparison with the Ni–P passivation film, peaks of copper appeared in
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the Ni–Cu–P passive layer, corresponding to Cu2O and Cu.
As shown in Fig. 2 and Fig. 3, the electrochemical test results indicated the better
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corrosion resistance of Ni–Cu–P coating than that of Ni–P coating. This is because the passivation film formed on the surface of Ni–Cu–P coating has better protection performance
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than that of the Ni–P passivation film. The protective efficacy of passivation films in a certain
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environment is determined by its composition. The influence of the composition on corrosion resistance of a passivation film is reflected in its semiconductor characteristics. In the other
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words, semiconductor property of a passivation film is affected by its composition. The semiconducting characteristics of different oxide films formed on alloys were determined by
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the main defects in the passivation films [42]. Therefore, in order to study the influence of Cu element on the corrosion resistance of the Ni–Cu–P and Ni–P passivation films, it is of great significance to study the oxide types and contents of Ni, Cu and P in the passivation films. In the Ni–Cu–P and Ni–P passivation films, element P exhibited n–type semiconducting characteristic [43], however NiO2, NiO and Cu2O showed p–type semiconducting characteristic [44-46]. As shown in Fig. 9(b), the most intense peak of P detected in the binary Ni–P passivation film was the elemental P originated from the plating matrix. This phenomenon indicated that the passivation layer produced on Ni–P coating was limited. 14
ACCEPTED MANUSCRIPT However, the most intense peak of P in the ternary Ni–Cu–P passivation layer was nickel phosphate. Besides, the total amount of phosphorus and nickel phosphate in Ni–Cu–P passivation layer was relatively higher than that of Ni–P passivation layer, according to the peak area of P2p in Fig. 9(b). In conclusion, the introduced element Cu in Ni–Cu–P alloy increased the phosphorus and nickel phosphate proportion in the passivation film. The large
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amount of phosphorus and nickel phosphate in Ni–Cu–P passivation layer resulted in the
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evident n–type semiconductor feature. However, the Ni–P passivation film with a small
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amount of P and nickel phosphate was not exhibited the n–type semiconductor characteristics. Since both of the alloys showed a large amount of nickel hydroxide and nickel oxide in the
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passivation films (as seen in Fig. 9(a)), both of the coatings showed p–type semiconducting characteristic. The promotion effect of copper on the enrichment of phosphorus and nickel
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phosphate in the Ni–Cu–P passive layer can be analyzed as follows. The self–corrosion potential of copper is noble than that of nickel, which resulted in the preferential dissolution
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of nickel and the enrichment of phosphorous and copper in the Ni–Cu–P passive layer [9],
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that is, Cu acted as a catalyst for the production of nickel phosphate. The Ni–Cu–P passivation film showed the n–type semiconductor characteristic due to the formation of a
oxides.
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large amount of nickel phosphate, as well as the p–type semiconductor characteristic of nickel
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From the above results, it could be discovered that Cu improved the thermal stability of Ni–Cu–P amorphous deposit and effectively inhibited the crystallization of Ni–Cu–P amorphous plating during heat treatment process. Furthermore, the introduced element Cu in Ni–Cu–P alloy increased the phosphorus and nickel phosphate proportion in the Ni–Cu–P passivation layer. After the incorporation of copper into Ni–P coating, the semiconductor type of passive films was transformed from p–type to p–n type, which was responsible for the better corrosion resistance of Ni–Cu–P sample.
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ACCEPTED MANUSCRIPT 4. Conclusions In this study, the microstructural evolution, thermal properties and corrosion resistance performances of Ni–Cu–P electroless deposits during crystallization process were systematically studied and compared to those of Ni–P plating. The conclusions could be drawn as follows:
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(1) Ni–Cu–P amorphous alloy showed higher crystallization temperature as well as better
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thermal stability than those of Ni–P amorphous alloy. Besides, the amounts and sizes of
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nanoparticles in the Ni–Cu–P alloys were relatively smaller than those in Ni–P coatings under same heat treatment condition.
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(2) The corrosion resistance of Ni–Cu–P and Ni–P alloys were firstly improved and then deteriorated as the increase of the heat-treatment temperature. The corrosion resistance was
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mainly affected by the kinds of alloying element and their distribution uniformity as the temperature was below 300 °C, since there was little amount of precipitates in the plating.
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crystallized (400 to 500 ° C).
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However, it was significantly affected by galvanic corrosion after the coating was completely
(3) The corrosion resistance of Ni–Cu–P alloy was better than that of Ni–P alloy. The
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passive layer produced on ternary Ni–Cu–P showed p–n type bipolar semiconductor characteristic, while the binary Ni–P passivation film showed only one p–type semiconductor
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characteristic in 3.5 wt.% NaCl solution.
Acknowledgements The authors are grateful for the support of the National Natural Science Foundation of China (No.51271099).
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. XRD curves of as–deposited and annealed alloys: (a) Ni–Cu–P and (b) Ni–P. Fig. 2. Electrochemical polarization plots of as–deposited and annealed alloys in 3.5 % NaCl solution: (a) Ni–Cu–P and (b) Ni–P. Fig. 3. Tafel fitting results of the polarization curves.
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Fig. 4. TEM results of Ni–Cu–P alloy after annealed at 200 ºC: (a) bright field micrograph, (b) the corresponding SAED, and (c) HR–TEM image. And, TEM results of Ni–Cu–P alloy after
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annealed at 300 ºC: (d) bright field micrograph, (e) the corresponding SAED, and (f) HR– TEM image.
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Fig. 5. TEM results of Ni–P alloy after annealed at 200 ºC: (a) bright field micrograph, (b) the corresponding SAED, and (c) HR–TEM image. And, TEM results of Ni–P alloy after
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annealed at 300 ºC: (d) bright field micrograph, (e) the corresponding SAED, and (f) HR– TEM image.
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Fig. 6. DSC results of Ni–Cu–P and Ni–P alloys: (a), (b) DSC plots, (c) and (d) curves of ln(𝑣 /T2) vs 1/T originated from the Kissinger equation.
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3.5 % NaCl solution.
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Fig. 7. Mott–Schottky plots of passive layers produced on (a) Ni–Cu–P and (b) Ni–P alloys in
Fig. 8. Wide scan XPS spectra for Ni–Cu–P and Ni–P electroless alloys in 3.5 % NaCl solution.
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Fig. 9. High resolution XPS spectra of (a) Ni 2p, (b) P 2p, (c) Cu 2p and (d) O 1s for Ni–Cu– P and Ni–P amorphous alloys in 3.5 % NaCl solution.
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Fig. 10. The detailed XPS curves of Ni 2p 3/2 and P 2p 3/2 of Ni–Cu–P and Ni–P corroded alloys in 3.5 % NaCl solution.
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ACCEPTED MANUSCRIPT Table caption
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Table 1 Ni 2p, Cu 2p, P 2p and O 1s binding energy for published nickel, copper and phosphorus compounds.
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ACCEPTED MANUSCRIPT Highlights The additional Cu decreased the amount and sizes of nanophases in amorphous coating. The addition of Cu increased the thermal stability of amorphous Ni–Cu–P coating. Element Cu in Ni–Cu–P coating improved the production of Ni3(PO4)2 in NaCl solution.
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Ni–Cu–P passive film showed p–n bipolar semiconductors characteristics.
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Jie Chen, Guanlin Zhao, Kenji Matsuda, Yong Zou PII: DOI: Reference:
S0169-4332(19)31135-3 https://doi.org/10.1016/j.apsusc.2019.04.142 APSUSC 42450
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
18 February 2019 18 March 2019 11 April 2019
Please cite this article as: J. Chen, G. Zhao, K. Matsuda, et al., Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.04.142
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Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process
Jie Chen1, Guanlin Zhao2*, Kenji Matsuda3, Yong Zou1*
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1. MOE Key Lab for Liquid–Solid Structure Evolution and Materials Processing, Institute of
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Materials Joining, Shandong University, Jinan 250061, China.
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2. School of Materials Science and Engineering, Shandong University, Jinan 250061, China. 3. Faculty of Sustainable Design, Department of Materials Design and Engineering (MaDE),
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Corresponding author: Guanlin Zhao, Email: [email protected], Tel: +8653188399872; Yong
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Zou, Email: [email protected], Tel: +8653188399872.
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University of Toyama, 3190, Gofuku, Toyama.
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ACCEPTED MANUSCRIPT Abstract Ni–Cu–P alloy was produced by electroless deposition, and the different microstructures were obtained by low–temperature heat treatment. Effect of element Cu on microstructure evolution and corrosion resistance of Ni–Cu–P alloy was studied by X–ray diffraction (XRD), transmission electron microscope (TEM) and electrochemical examination. The results
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showed that the crystalline behavior of Ni–Cu–P amorphous alloy was a gradual process during the heat treatment process. However, the corrosion resistance of Ni–Cu–P alloy was
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firstly enhanced and then deteriorated during the heating process. The corrosion resistance of
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the alloys was not only affected by the crystalline precipitates, but also influenced by the
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alloying elements in the coatings. In Comparison with the Ni–P samples under the same states, the incorporated copper enhanced the thermal stability and corrosion resistance of the Ni–Cu–
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P samples. The Mott Schottky (M–S) results showed that the passive layer on ternary Ni–Cu– P had p–n bipolar semiconductor characteristic, however the binary Ni–P passivation film
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showed only one p type semiconductor characteristic in 3.5 wt.% NaCl solution.
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Keywords: Amorphous coatings; Microstructure evolution; Corrosion resistance; Passive
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film; XPS
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ACCEPTED MANUSCRIPT 1. Introduction Ni–P amorphous alloys applicated in industrial as corrosion protective coatings can be dated back to 1946 [1, 2]. More recently, other metal elements, such as Cu, was introduced to produce Ni–Cu–P ternary alloy to improve the properties of binary Ni–P alloy [3-8]. Studies have demonstrated that the functional properties of Ni–Cu–P alloy, such as outward
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appearance [8], wear [5], corrosion resistance [4, 6-8] and thermal stability [4, 9] were
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superior to those of Ni–P alloy. One of the most important functional properties of Ni–Cu–P
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alloy is its excellent corrosion resistance, which have generated great interest. The corrosion resistance mechanism of Ni–P based alloys has been studied extensively
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and different explanations have been put forward [9]. Many researchers attributed it to the amorphous structure, that is, without the typical defects in crystalline alloys like grain
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boundary, dislocation, and segregation [10]. Nevertheless, the local atomic structure of the amorphous alloys is not fully disorder. There exist short range order clusters in the amorphous
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alloy [11]. In our previous studies, the effect of local atomic structure on corrosion resistance
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of Ni–P based alloys have been reported [4, 12]. The Ni–P amorphous alloys are thermodynamically unstable, and its corrosion resistance are strongly affected by the
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microstructure transition during the heating process. Some studies reported that the amorphous coatings showed superior corrosion resistance to those of the coatings with
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microcrystalline structure [13, 14]. On the other hand, the heat–treated specimens (400 ºC [14-17] or 500 ºC [18]) showed higher corrosion resistance than those of unheated coatings. However, there is no detailed mechanistic explanation for the corrosion resistance variation during crystallization in the literature. Recently, the passivation film theory has been involved to analysis the excellent corrosion resistance performance of Ni–P coating [19], which has been used effectively to explain the superior corrosion resistance of stainless steel and other alloys [20-23]. However, reports on properties of passivation film formed on Ni–P alloy were concentrated mainly on 3
ACCEPTED MANUSCRIPT the valence state of each element and chemical constitution of the passivation [19]. Data on the semiconducting performances of Ni–P passive films are scarce. The aim of this study is to analyze the details of the microstructural evolution and corrosion resistance performance of the Ni–Cu–P amorphous alloy during the crystallization process and compare them with Ni–P amorphous alloy with the same phosphorus proportion.
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Both of the coatings were fabricated in the amorphous state by electroless plating and then
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heat treated from room temperature to 500 ºC to obtain different microstructures. The
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microstructure characteristic and phase information of the coatings at as–deposited and heat– treated states were studied through transmission electron microscope (TEM) and X–ray
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diffraction (XRD). The corrosion resistance of the coatings was analyzed by electrochemical measurement. Mott Schottky (M–S) curves and X–ray photoelectron spectroscopy (XPS)
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spectrum were applied to study the passive film formed on the coatings in 3.5 wt.% NaCl solution. Eventually, the corrosion resistance mechanism during the crystallization process
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was revealed by a combined analysis of these results. The correlation between microstructure
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and corrosion resistance of Ni–Cu–P alloy was also discussed.
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2. Material and methods
2.1 Preparation of Ni–Cu–P and Ni–P electroless coatings
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The material used as the substrate for plating was low–carbon steel sheets with the size of 10mm×10mm×1mm. The low–carbon steel sheets were polished and cleaned intensively before electroless plating. Ni, P and Cu in the obtained electroless coating were originated from nickel sulfate, sodium hypophosphite and copper sulfate, respectively. Anhydrous sodium, glycine, sodium citrate and acetate applied as additives in the plating bath. The electroless plating was conducted at about 85 ºC for 3 hours, and the PH value of the electroless plating solution was about 4.7. Some of the coatings were heat treated in an argon atmosphere at 200, 300, 400, and 500 ºC for 1h, respectively. 4
ACCEPTED MANUSCRIPT 2.2 Composition, morphology, and microstructure characterization The composition of the samples was characterized by EDS attached to a scanning electron microscope (SEM; ZEISS EVO MA 10). Accordingly, the compositions of the samples were Ni0.79P0.21 and Ni0.75Cu0.02P0.23 (at.%). The microstructures of samples were evaluated by the means of XRD (Bruker D8
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Discovery) with Cu Kα radiation (λ=1.5406 Å). The work current, voltage and scanning
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speed were fixed at 40 mA, 40 KV, and 1 °/ min. A 200 KV TEM (TOPCON–002B) was also
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employed for the analysis of morphologies and microstructures of the coatings. To avoid the electron beam damage, work voltage was set at 120 kV. TEM samples of the coatings were
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obtained using twin jet thinning instrument, and the work current was 40–80 mA. The work
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temperature was about –30 ºC. The electrolyte was the perchloric acid and methanol mixture.
2.3 Thermal property characterization
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The thermal properties of the coatings were investigated by differential scanning
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calorimetry (DSC; Labsys–TG–DSC 1600ºC) from room temperature to 600 ºC in pure argon atmosphere, and the reference sample was aluminum oxide. The continuous heating rates
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were set at 10, 20, 40 and 60 ºC /min. The following Kissinger equation was applied to evaluate the activation energy of the samples during the heat treatment process [24]: (1)
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ln(𝑣⁄𝑇 2 ) = − 𝛥𝐸 ⁄𝑅𝑇+ C
Where 𝑣 is the annealing rate (K/min); T is the characteristic temperature (K); E is the activation energy (KJ/mol); R is the universal gas constant (kJ mol–1 K–1) and C is a constant.
2.4 Electrochemical measurements Corrosion resistance of the samples were investigated through potentiodynamic polarization on an Autolab (Aut84886) workstation. The electrochemical tests were conducted in a classic three–electrode system. The platinum plate, Ag/AgCl and electroless coating acted 5
ACCEPTED MANUSCRIPT as counter electrode, reference electrode and working electrode respectively. The scanning rate of polarization tests was 0.5 mV/s. The scanning range was –0.3 to 1 V relative to the open circuit potential (OCP). The corrosion potential (Ecorr) and the corrosion current density (Icorr) were deduced from the polarization curves by Tafel fitting. Semiconductor characteristics of the passive film were studied by M–S curves. The scanning range of M–S
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measurement was –0.2 to 0.2 V (relative to the reference potential), frequency was 1000HZ,
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and the amplitude was 10 mV. To steady the OCP, all of the samples were soaked in the
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corrosive solution (3.5 wt.% NaCl) for 20 min before testing.
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2.5 Corrosion products characterization
The composition of the corrosion products produced on the samples was investigated
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through XPS (ESCALAB 250 Xi). The working condition of XPS was Al Kα, 150 W and 15 kV. The scanning angle of the test was 45°. The broad XPS spectrum was scanned from 0 to
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1200 eV. The detailed XPS spectrum of each element was also employed to compare the
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element differences of different samples. The peak assignment was carried out according to
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the data gathered from published nickel, copper and phosphorus compounds in the literature.
3. Results and discussion
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3.1 Microstructural analysis of the coatings Figure 1 exhibits the XRD curves of the as–deposited and annealed Ni–Cu–P and Ni–P alloys. The only one single broad peak at the position of 2θ = 40~50° of the as–plated coatings was attributed to their amorphous structure [14, 25]. As shown in Fig.1 (a), there was no evident variation in XRD spectrums observed for the Ni–Cu–P sample as the treatment temperature was below 300 ºC, indicating no obvious crystallization occurred at these temperatures. However, as shown in Fig.1 (b), another broad peak appeared at the position of 2θ = 50~55° in the Ni–P sample, as the annealing temperature was 300 ºC. This phenomenon 6
ACCEPTED MANUSCRIPT indicated that the Ni–P sample began to crystallize as heat treated at 300 ºC. When the coating was annealed at 400 ºC, the other peaks of Ni, and Ni3P phases emerged in the broad amorphous profile matrix of both coatings. Besides, the metastable Ni8P3 peak was also appeared in the broad amorphous profile matrix of Ni–P sample. However, the other metastable NixPy phases such as Ni12P5 [16, 26-29] and Ni5P2 [18, 28-30] were not detected by
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XRD in present study, which were reported in the literate. This phenomenon may be related to
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the small amount of metastable NixPy phases formed in the crystallization process, which
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cannot be detected by XRD. Therefore, TEM observation should be further performed to determine the existence of metastable NixPy phases. As the coating was annealed at 500 ºC,
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the broad amorphous peaks in both of the coatings disappeared, instead of the sharp peaks of Ni and Ni3P. This indicated that the coatings were fully crystallized to the equilibrium two–
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phase Ni/Ni3P, resulting in the increase in peaks intensity of the Ni and Ni3P phases.
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3.2 Corrosion behaviors of the coatings
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Corrosion resistance of the samples in neutral sodium chloride solution was studied by potentiodynamic polarization as demonstrated in Fig. 2. As shown in Fig. 2a, Ni–Cu–P samples exhibited a passivation stage between –0.1 and 0.2 V as the heat treatment
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temperature was below 300 ºC. The passivation phenomenon was evidently weakened as the
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Ni–Cu–P amorphous coating was heat treated at the temperature of 400 to 500 ºC. The anodic polarization behavior of the sample annealed at 400 ºC was changed from passive to active with respect to the as–plated conditions, which was consistent with the study results of D. Ahmadkhaniha [31]. As shown in Fig. 2b, the Ni–P coatings showed similar passivation performance during crystallization. The corrosion resistance parameters were obtained from the polarization curves by Tafel fitting and curved in Fig.3. It could be discovered that the Icorr of Ni–Cu–P coating was lower than that of Ni–P coating regardless of the state. In other words, the corrosion resistance of Ni–Cu–P plating was better than that of Ni–P plating.
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3.3 TEM analysis of the coatings
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To clarify the influence of heat treatment temperature on corrosion resistance of Ni–Cu–
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P and Ni–P alloys, the microstructure evolution of the coatings during heat treatment were firstly analyzed. Fig. 4 exhibits the TEM results of ternary Ni–Cu–P alloys tempered at 200
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and 300 ºC for 1 h. The bright field image (Fig. 4a) shows that the matrix of annealed Ni–Cu– P at 200 ºC was uniform and showed amorphous structure. However, the corresponding
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selected area diffractions patterns (SADP) was consisted of a halo ring originated from the amorphous Ni–Cu–P matrix as well as some rings originated from nanometer–sized fine
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particles, as shown in Fig. 4b. The plane distances of the diffraction rings were measured and
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affirmed the existence of nanocrystalline Ni and Ni12P5 particles. HRTEM image (Fig. 4c) confirmed the occurrence of nanocrystals with the size of 3–5nm within the amorphous
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matrix. After further crystallization (300 ºC), although the bright field image (Fig. 4d) of Ni– Cu–P coatings still showed the homogeneous amorphous structure, the SADP (Fig. 4e) and
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HRTEM image (Fig. 4f) confirmed the presence of nanocrystalline Ni, Ni5P4 and Ni12P5 with the size of 3–8 nm within the amorphous matrix. As a result of comparison, Fig. 5 exhibits the TEM results of Ni–P coating when annealed at 200 and 300 ºC for 1 h. In the bright field image at 200 ºC (Fig. 5a), dark contrast precipitates in Ni–P sample could be distinguished, which were homogeneous nanometer–sized fine particles with the size of 2~10 nm. The SADP showed in Fig. 5b, implied a large amount of grains differently oriented within the amorphous matrix. The analysis of SADP confirmed the presence of Ni, Ni5P4 and Ni12P5. The HRTEM image of the sample also confirmed the presence of Ni8P3, as shown in Fig. 5c. 8
ACCEPTED MANUSCRIPT As exhibited in Fig. 5d, the nanoparticles in Ni–P coating annealed at 300 ºC were evidently bigger (varying between 10~40 nm) and denser than those in the coating tempered at 200 ºC. In the analysis of SADP (Fig. 5e) and HRTEM image (Fig. 5f), the metastable Ni5P2 phase and stable Ni3P precipitates were also occurred in the amorphous matrix. According to the XRD and TEM analysis, it could be concluded that the Ni–Cu–P and Ni–P samples were
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mainly amorphous as the annealing temperature was below 300 ºC, wherein existed a very
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small amount of metastable crystalline Ni8P3, Ni5P2, Ni5P4, Ni12P5, and stable Ni3P
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nanocrystallines. Besides, the coatings were crystallized to the equilibrium Ni and Ni3P phases as the annealing temperature was changed to 400 and 500 ºC.
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Through comparative analysis, it could be discovered that the amount and sizes of the nanoparticles in the annealed Ni–Cu–P samples were relatively smaller than those in Ni–P
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coatings. This phenomenon indicated that the fine nanocrystalline precipitates in amorphous Ni–P were effectively retarded by alloying a small fraction of copper. In other words, the
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stability of Ni–Cu–P amorphous coating was significantly enhanced through the incorporation
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of Cu. This is because the nucleation and growth of intermetallic compounds was accomplished by the movement of atoms. The movement and aggregation of phosphorus
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atoms in the Ni–Cu–P coating was effectively prevented by copper atoms, resulting in the decrease in the amount and size of the nanophases in the amorphous matrix.
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In general, galvanic corrosion occurs as two different alloys are in contact in a corrosive environment. Different alloys show different self–corrosion potential. The corrosion rate of the alloy with negative original self–corrosion potential increased, while the corrosion rate of the alloy with positive original self–corrosion potential decreased. TEM and XRD results showed that various metastable nanoparticles precipitated in the Ni–Cu–P and Ni–P coatings during crystallization. The composition and microstructure difference between metastable crystalline and the amorphous matrix resulted in corrosion potential differences, which lead to micro-galvanic corrosion. Besides, the galvanic corrosion could be increased as 9
ACCEPTED MANUSCRIPT the sizes and total amounts of precipitated phases increased. As the material is totally crystallized, the grain boundaries show some characteristics with thermodynamic instability and electrochemical inhomogeneity. Thus, the grain boundaries are active positions for intergranular corrosion attack. The corrosion starts from the surface and develops inward along the grain boundary. Eventually, many irregular polygonal corrosion cracks formed
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along the grain boundaries. In conclusion, the metastable crystalline and the crystal structure
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in the amorphous matrix is detrimental to the inhibition of corrosion.
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However, in this study, it is interesting that the Tafel fitting results (as seen in Fig. 3) exhibited that the corrosion resistance of the coatings was not in proportion to the heat
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treatment temperature. With the increase of the heat treatment temperature, corrosion resistance of the coatings was initially improved and then deteriorated and showed best
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corrosion resistance at 300 ºC. Besides, although the Ni–Cu–P and Ni–P alloys were significantly crystallized (400 to 500 ºC), the increase in corrosion current densities were
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relatively small. This is because the corrosion resistance variation during crystallization was
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affected by the two aspects of the microstructure and the chemical composition. The heat treatment at low temperature stage (below 300 ºC) was not the main stage of crystallization.
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In this condition, the matrix of the coatings still kept amorphous state with small amount of the small size nanocrystallines. Under this condition, the element distribution in the coating
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was still continuous and uniform, which was conducive to the uniformity and integrity of the formed passivation film. The effect of galvanic corrosion between the amorphous matrix and the nanoparticles was limited. On the other hand, the residual stress in the as–deposited coatings could be effectively eliminated through the low–temperature heat treatment [12]. The vacancies in the amorphous matrix were filled by the accelerated movement of atoms, which resulted in the better uniformity of the matrix. Therefore, the corrosion resistance of alloys was improved as the temperature was below 300 ºC. As the heat treatment temperature was increased to 400 ºC, corrosion resistance of the samples reduced due to the influence of 10
ACCEPTED MANUSCRIPT intergranular corrosion. However, the increase in corrosion current densities were relatively small because of the protection of the passivation film formed on the surface of the coatings. In general, corrosion resistance of samples can be improved by changing the composition and semiconductor properties of the passivation film. Therefore, the semiconductor characteristics of the passivation films formed on the two kinds of coatings in corrosive liquid were tested
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separately, and the results will be discussed in the following section.
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3.4 The thermal properties of the coatings
To study the phase transformation performance of the Ni–Cu–P and Ni–P deposits, DSC
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measurements were conducted. Fig. 6(a)–(b) shows the DSC thermograms of the samples. The major exothermic peaks in the DSC curves of the Ni–Cu–P and Ni–P deposits were
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corresponded to the long range atomic shift, which was the precipitation of nickel and nickel phosphide (Ni3P) phases [32]. Besides, it could be discovered that the peak intensity of the
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coatings increased with the increase of scanning rates. DSC thermogram of the Ni–Cu–P
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amorphous alloy (Fig. 6(a)) exhibited higher exothermic temperatures than those of binary Ni–P coating (Fig. 6(b)). In conclusion, the phase transformation temperature of the Ni–Cu–P
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amorphous coating was strongly increased by the incorporation of a small amount of Cu. Crystallization activation energies of the electroless deposits were calculated according
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to plots of ln(𝑣 /T2) vs 1/T originated from the Kissinger equation (Fig. 6(c) and (d)) using the onset temperatures (Tx) and peak temperatures (Tp ) of the major exothermic peak in the DSC plots, at different heating rates (𝑣). The activation energy of Ni–Cu–P coating was higher than that of Ni–P coating, which implied that the thermal stability of Ni–Cu–P amorphous coating was enhanced by alloying a small amount of Cu. The DSC results was consistence with the results of XRD and TEM.
3.5 Semiconducting properties of the passive films 11
ACCEPTED MANUSCRIPT It is well known that the Mott–Schottky measurement can be effectively used to characterize the semiconducting characteristics of passivation films. In this study, the semiconductor type of passivation thin films was determined according to the slope of the M– S plots. The positive and negative slopes of M–S plots indicate the n–type and p–type semiconductor characteristic of passive film, respectively. Fig.7 shows the M–S plots of Ni–
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Cu–P and Ni–P samples passivated at 0V for 1h in 3.5% NaCl solution. In Fig. 7 (a), the M–S
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curve of Ni–Cu–P passivation film showed two linear segments, which was the negative slop
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at the potential from –0.2 to –0.06 V and the positive slop at the potential range from –0.06 to 0.1V. Therefore, the passive layer generated on Ni–Cu–P sample showed bipolar
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semiconductor characteristic. However, as exhibited in Fig. 7 (b), the Ni–P sample showed only one p–type semiconductor characteristic. The better corrosion resistance of Ni–Cu–P
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sample was related to the passivation film semiconductor property. Passive film theory has been effectively applicated to interpret the better corrosion resistance mechanism of some
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alloys such as stainless steels [33, 34]. The passivation film formed on samples acted as the
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barrier layer as electrons/ions transport in the interface of passive film and corrosion solution. Formation and destruction mechanism of passivation films has been investigated and several
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kinds of models have been put forward. The most popular models among them were the point defect model and the solute-vacancy interaction model. According to the passive film theory,
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the passivation film with bipolar characteristic display excellent corrosion resistance, which could prevent the positive ions out as well as hinder the negative ions in. Therefore, the protective efficacy of the passivation films can be reinforced. In this study, the Ni–Cu–P passive film with p–n bipolar semiconducting characteristic could not only prevent the negative ions in but also restrain the positive ions out, which increased the protectiveness and corrosion resistance of Ni–Cu–P passivation film. In comparison, the passivation layer developed on Ni–P sample showed only p type semiconducting characteristic, which was in a unidirectional conducting condition. The preventive effect of the Ni–P passive layer was 12
ACCEPTED MANUSCRIPT deteriorated. The different semiconductor performance of passive layers formed on Ni–Cu–P and Ni–P samples was determined by the composition difference.
3.6 Compositions of the passive films The element distribution characteristics of the passivation layers produced on Ni–Cu–P
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and Ni–P deposits were analyzed through XPS. The normalized XPS spectrum of the passivated samples are shown in Fig. 8. For reference, the binding energies of some Ni, Cu
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and P compounds were abstracted from the literature and listed in Table 1. Typical element
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distribution positions in the spectrum were signed in Fig. 8. The two curves in Fig. 8 exhibited
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no evident difference, which revealed that the passive films formed on the coatings were mainly consisted of copper, nickel, oxygen, carbon and phosphorus. O2 and C in the spectrum
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were originated from surface contamination [35]. Nevertheless, the peaks intensity and location of the elements were a little different. Therefore, high resolution XPS spectra (Fig. 9)
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of the Ni, Cu, O and P were employed to analysis the difference between two samples.
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Considering the difficulty in determining the main states of Ni and P in some broad peaks from Fig. 9, the curves of Ni 2p 3/2 and P 2p 3/2, were divided into different contributions
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(Fig. 10).
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Element Ni Ni Ni Ni Cu Cu Cu P P O
Table 1 Spectral line Compound 2p3/2 Ni(OH)2 2p3/2 Ni2+ 2p3/2 Ni3(PO4)2 2p3/2 Ni 2p1/2 Cu 2p3/2 Cu2O 2p3/2 Cu 2p3/2 P in bulk 2p3/2 Ni3(PO4)2 1s O2–
Energy (eV) 861.7 857.0 856.1 853.8 952.4 932.0 932.2 129.7 133.3 531.2
Ref. [36] [37] [36] [38] [39] [40] [41] [2] [36] [36]
From Fig. 9a and Fig. 10a–b, the XPS spectrum of Ni 2p3/2 in the passive films could be decomposed into four peaks. According to the reference, as shown in Table 1, the peaks observed at 855.7±0.1 eV and 856.8±0.1 eV were appointed to Ni2+ in nickel phosphate. 13
ACCEPTED MANUSCRIPT Besides, the peaks at 861.6±0.1 eV and 853.6±0.1 eV were attributed to Ni2+ in Ni(OH)2 and Ni in the coating, respectively. The high–resolution curves of P 2p3/2 (Fig. 9b and Fig. 10c–d) were consisted of two peaks at 130.2±0.1 eV and 133.3±0.2 eV. The peaks at 133.3±0.2 eV was assigned to Ni3(PO4)2 [36], and the peaks at 130.2±0.1 eV was appointed to P in the coating [2]. The spectrum of Cu 2p are exhibited in Fig. 9c. The most intense peak at 932.9
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eV was Cu or Cu2O, followed by Cu (952.3 eV). The O ls spectrum (Fig. 9d) shows a broad
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curve consist of numerous oxygenated compounds. From the XPS results, it could be
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confirmed that the passivation film of the Ni–P plating was mainly consist of nickel phosphate, Ni(OH)2, Ni and P. In comparison with the Ni–P passivation film, peaks of copper appeared in
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the Ni–Cu–P passive layer, corresponding to Cu2O and Cu.
As shown in Fig. 2 and Fig. 3, the electrochemical test results indicated the better
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corrosion resistance of Ni–Cu–P coating than that of Ni–P coating. This is because the passivation film formed on the surface of Ni–Cu–P coating has better protection performance
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than that of the Ni–P passivation film. The protective efficacy of passivation films in a certain
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environment is determined by its composition. The influence of the composition on corrosion resistance of a passivation film is reflected in its semiconductor characteristics. In the other
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words, semiconductor property of a passivation film is affected by its composition. The semiconducting characteristics of different oxide films formed on alloys were determined by
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the main defects in the passivation films [42]. Therefore, in order to study the influence of Cu element on the corrosion resistance of the Ni–Cu–P and Ni–P passivation films, it is of great significance to study the oxide types and contents of Ni, Cu and P in the passivation films. In the Ni–Cu–P and Ni–P passivation films, element P exhibited n–type semiconducting characteristic [43], however NiO2, NiO and Cu2O showed p–type semiconducting characteristic [44-46]. As shown in Fig. 9(b), the most intense peak of P detected in the binary Ni–P passivation film was the elemental P originated from the plating matrix. This phenomenon indicated that the passivation layer produced on Ni–P coating was limited. 14
ACCEPTED MANUSCRIPT However, the most intense peak of P in the ternary Ni–Cu–P passivation layer was nickel phosphate. Besides, the total amount of phosphorus and nickel phosphate in Ni–Cu–P passivation layer was relatively higher than that of Ni–P passivation layer, according to the peak area of P2p in Fig. 9(b). In conclusion, the introduced element Cu in Ni–Cu–P alloy increased the phosphorus and nickel phosphate proportion in the passivation film. The large
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amount of phosphorus and nickel phosphate in Ni–Cu–P passivation layer resulted in the
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evident n–type semiconductor feature. However, the Ni–P passivation film with a small
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amount of P and nickel phosphate was not exhibited the n–type semiconductor characteristics. Since both of the alloys showed a large amount of nickel hydroxide and nickel oxide in the
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passivation films (as seen in Fig. 9(a)), both of the coatings showed p–type semiconducting characteristic. The promotion effect of copper on the enrichment of phosphorus and nickel
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phosphate in the Ni–Cu–P passive layer can be analyzed as follows. The self–corrosion potential of copper is noble than that of nickel, which resulted in the preferential dissolution
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of nickel and the enrichment of phosphorous and copper in the Ni–Cu–P passive layer [9],
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that is, Cu acted as a catalyst for the production of nickel phosphate. The Ni–Cu–P passivation film showed the n–type semiconductor characteristic due to the formation of a
oxides.
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large amount of nickel phosphate, as well as the p–type semiconductor characteristic of nickel
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From the above results, it could be discovered that Cu improved the thermal stability of Ni–Cu–P amorphous deposit and effectively inhibited the crystallization of Ni–Cu–P amorphous plating during heat treatment process. Furthermore, the introduced element Cu in Ni–Cu–P alloy increased the phosphorus and nickel phosphate proportion in the Ni–Cu–P passivation layer. After the incorporation of copper into Ni–P coating, the semiconductor type of passive films was transformed from p–type to p–n type, which was responsible for the better corrosion resistance of Ni–Cu–P sample.
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ACCEPTED MANUSCRIPT 4. Conclusions In this study, the microstructural evolution, thermal properties and corrosion resistance performances of Ni–Cu–P electroless deposits during crystallization process were systematically studied and compared to those of Ni–P plating. The conclusions could be drawn as follows:
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(1) Ni–Cu–P amorphous alloy showed higher crystallization temperature as well as better
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thermal stability than those of Ni–P amorphous alloy. Besides, the amounts and sizes of
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nanoparticles in the Ni–Cu–P alloys were relatively smaller than those in Ni–P coatings under same heat treatment condition.
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(2) The corrosion resistance of Ni–Cu–P and Ni–P alloys were firstly improved and then deteriorated as the increase of the heat-treatment temperature. The corrosion resistance was
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mainly affected by the kinds of alloying element and their distribution uniformity as the temperature was below 300 °C, since there was little amount of precipitates in the plating.
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crystallized (400 to 500 ° C).
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However, it was significantly affected by galvanic corrosion after the coating was completely
(3) The corrosion resistance of Ni–Cu–P alloy was better than that of Ni–P alloy. The
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passive layer produced on ternary Ni–Cu–P showed p–n type bipolar semiconductor characteristic, while the binary Ni–P passivation film showed only one p–type semiconductor
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characteristic in 3.5 wt.% NaCl solution.
Acknowledgements The authors are grateful for the support of the National Natural Science Foundation of China (No.51271099).
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. XRD curves of as–deposited and annealed alloys: (a) Ni–Cu–P and (b) Ni–P. Fig. 2. Electrochemical polarization plots of as–deposited and annealed alloys in 3.5 % NaCl solution: (a) Ni–Cu–P and (b) Ni–P. Fig. 3. Tafel fitting results of the polarization curves.
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Fig. 4. TEM results of Ni–Cu–P alloy after annealed at 200 ºC: (a) bright field micrograph, (b) the corresponding SAED, and (c) HR–TEM image. And, TEM results of Ni–Cu–P alloy after
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annealed at 300 ºC: (d) bright field micrograph, (e) the corresponding SAED, and (f) HR– TEM image.
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Fig. 5. TEM results of Ni–P alloy after annealed at 200 ºC: (a) bright field micrograph, (b) the corresponding SAED, and (c) HR–TEM image. And, TEM results of Ni–P alloy after
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annealed at 300 ºC: (d) bright field micrograph, (e) the corresponding SAED, and (f) HR– TEM image.
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Fig. 6. DSC results of Ni–Cu–P and Ni–P alloys: (a), (b) DSC plots, (c) and (d) curves of ln(𝑣 /T2) vs 1/T originated from the Kissinger equation.
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3.5 % NaCl solution.
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Fig. 7. Mott–Schottky plots of passive layers produced on (a) Ni–Cu–P and (b) Ni–P alloys in
Fig. 8. Wide scan XPS spectra for Ni–Cu–P and Ni–P electroless alloys in 3.5 % NaCl solution.
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Fig. 9. High resolution XPS spectra of (a) Ni 2p, (b) P 2p, (c) Cu 2p and (d) O 1s for Ni–Cu– P and Ni–P amorphous alloys in 3.5 % NaCl solution.
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Fig. 10. The detailed XPS curves of Ni 2p 3/2 and P 2p 3/2 of Ni–Cu–P and Ni–P corroded alloys in 3.5 % NaCl solution.
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ACCEPTED MANUSCRIPT Table caption
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Table 1 Ni 2p, Cu 2p, P 2p and O 1s binding energy for published nickel, copper and phosphorus compounds.
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ACCEPTED MANUSCRIPT Highlights The additional Cu decreased the amount and sizes of nanophases in amorphous coating. The addition of Cu increased the thermal stability of amorphous Ni–Cu–P coating. Element Cu in Ni–Cu–P coating improved the production of Ni3(PO4)2 in NaCl solution.
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Ni–Cu–P passive film showed p–n bipolar semiconductors characteristics.
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Figure 1
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Figure 10