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A study of the corrosion resistance of brush-plated Ni-Fe-W-P films
546
Surface and Coatings Technology, 68/69 (1994) 546—551
A study of the corrosion resistance of brush-plated Ni—Fe—W—P films Wen-Hua Huia, Jia-Jun Liu”, Yi-Shung Chauge aDepartment of Mechanical Engineering, Southeast University, Nanjing, China 1’Department of Mechanical Engineering, Tsinghua University, Beijing, China cSummit Scientific Company, 1486 Lodge Court, Boulder, CO 80303, USA
Abstract Electrochemical metallization by means of brush plating has been developed for coating of Ni—Fe—W—P compound films. The hardness of this compound film is about 800—910 HV, which is similar to that of plated chromium film. The corrosion resistance of this compound film in various corrosive environments was studied. Results showed that the corrosion resistance of this compound film was 1.7 times and 1.4 times that of plated chromium film in the corrosive environments of NaCl (1S03768) and H 2S04 respectively. This indicates that Ni—Fe—W—P brush plating is one of the alternatives to substitute chromium plating. X-ray diffraction, transmission electron microscopy and scanning tunneling microscopy were applied to study the microstructure and the fine surface morphology of this compound film. It was found that the film is composed of an Ni-base amorphous and dispersive precipitated intermetallic compound of Ni3Fe. The surface characteristics of this compound film, analyzed by X-ray photoelectron spectroscopy and Auger electron spectroscopy, revealed that a preferential diffusion of W to the surface would be induced in the corrosive environments. The excellent corrosion resistance of brush-plated Ni—Fe—W—P film attributed to its microstructure and surface characteristics will be discussed.
1. Introduction Various coating deposition techniques have been developed and used for tribological applications. In many instances, chromium coatings have been extensively used for many years because of its high hardness level, low friction, and good wear and corrosion resistance. However, the finding that the hexavalent form of chromium is a carcinogen makes chromium compounds hazardous as vapors in coating operations and as waste products. The regulations for chromium coating processes and their waste controls, therefore, have become tightened because of the environmental concerns. This has stimulated an extended search for alternative materials and coating processes to replace chromium coatings, such as ceramics, polymers, physical vapor deposition, laser chemical vapor deposition plasma nitriding and ion-assisted coating [1—8]. Although each alternative has advantages for specific applications, no one process or material can completely replace chromium coatings in terms of wear resistance, corrosion resistance, the complexity of the process and the cost of the process. Electrochemical metallization by means of brush plating is used yearly for the application of metallurgical coatings [9]. Traditionally, brush plating is a tool for repair and salvage. Its applications for the prevention of fretting and seizing, the improvement of brazeability of braze metals on selected areas of a tool, machine parts and electrical or electronic components are being
0257—8972/94/87.00 SSDI 0257-8972(94)08107-A
exploited [10]. Recently, the coating of Ni—Fe—W—P compound films by means of brush plating has been developed [11]. The hardness of this compound film is about 800—9 10 HV, which is similar to that of the electrochemically deposited chromium film [12]. Wear tests on this film also showed excellent wear resistance [12], and thus the applications of brush-plated Ni—Fe—W—P compound film in tribology have been presumed. We present here the study of the corrosion resistance of brush-plated Ni—Fe—W—P compound film in the corrosive environments of NaC1 (1S03768) and H2S04. The attribution of the film’s microstructure and surface characteristics to the corrosion resistance are also studied. The microstructure and the fine surface morphology of the film were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to study the changes in the surface characteristics of the film in a corrosive environment.
2. Experiment The samples for the corrosion tests were Ni—Fe—W—P compound film, chromium plated film and nickel plated film where the chromium plated film and the nickel plated film were used for the comparison of corrosion
© 1994
—
Elsevier Science S.A. All rights reserved
W.-H. Hui et al.
/
Brush-plated Ni—Fe—W—P films
resistance. The Ni—Fe—W—P compound films, approximately 30 pm thick, were brush plated on aluminium substrates. The chemical composition of the compound film was 65 wt.% Ni, 28 wt.% Fe, 6 wt.% W and 1 wt.% P. The chromium plated film and the nickel plated film were coated on aluminum substrates by a conventional bath plating technique. Prior to this study, all the samples were sanded using emery paper (no. 800) to remove the surface oxidic layer and washed by distilled water and underwent acetic elimination by CH2COCH3. An electrochemical method of Tafel line extrapolation was employed for the corrosion test [13]. It was performed in an electrochemical polarization cell where the test sample was immersed in a corrosive solution for 30 mm. The corrosive solutions used in this study were (1) NaC1, 50 ±1 g l’, pH 6.0—7.0 (ISO 3768) and (2) 1 N H2S04. During the test, the polarization current was measured by the potentiostat at each setting of the cathode polarization, i.e. L~E= 0.1, 0.2, 0.5 mY. The data were then treated by the linear regression method and the corrosion current was acquired by extrapolating cathode polarization curve. The corrosion rate was calculated by 2 h’)=~-~x io~ V(gm nF where V is the corrosion rate, A (g mol is the atomic mass, i (A cm 2) is the corrosion current density, n is the ionic valence, and F=26.8 A h mol’ is the Faraday constant. —‘
The crystal structure of the brush-plated compound film was examined by XRD. The XRD measurements were made with a conventional diffractometer (D/MAX-YA) using Cu Ktx radiation. A transmission electron microscope (JEOL JEM-200EX) was employed to study the microstructure of the compound film. The fine surface morphology and the composition of the compound film before and after the corrosive treatment were also examined by STM and energy-dispersive analysis of X-rays (EDAX) respectively. The corrosive treatment was performed by immersing the sample in a corrosive solution in the electrochemical polarization cell until the anode polarization was finished. A passivated layer on the surface of the compound film was prepared by cathode polarization of the sample with + 4 V for 45 mm in the electrochernical polarization cell after the anode polarization was finished. The surface characteristics of the compound film and the passivated layer were examined by XPS. XPS analysis was performed in a Perkin—Elmer model PHI-550 electron spectrometer with a monochromatic Al Kcc X-ray source and a spherical analyzer. During the XPS measurements, an electron flood gun was used to reduce the sample charging effect. The binding energies of the XPS peaks were determined by using the carbon peak at a binding
547
energy of 284.5 eV as the reference position. The cornposition depth profiles of the compound film and the corrosive-treated film were also analyzed by AES. The AES analysis was carried out with a Perkin—Elmer model 600 Auger electron spectrometer equipped with a single-pass cylindrical mirror analyzer and a sputter gun. Based on the predetermined sputter etching rates for the compound film under selected conditions, the AES depth profile analysis was performed in the Auger chamber.
3. Results 3.1. Corrosion tests The corrosion rates of the compound film, the chromium film and the nickel film in the corrosive solutions of NaCl and H2SO4 are shown in Table 1. The cornpound film has excellent corrosion resistance in these corrosive solutions. The corrosion resistance of the cornpound film in the corrosive solution of NaCl is 1.7 times and 5.2 times that of chromium film and nickel film respectively; that in corrosive solution of H2S04 is 1.4 timesrespectively. and 2.7 times that of chromium film and nickel film 3.2. Crystallinity and microstructure analysis Fig. 1 shows the XRD pattern from an Ni—Fe—W—P compound film. The diffraction peaks at 20=43.8° and 51°corresponding to (111) and (200) reflection for Ni 3Fe Table 1 Corrosion rates of the plated films in NaCl (1S03768) and H2S04 Plated film
Corrosion ri~tei1~ Corrosion rat~ein1
Ni—Fe-W—P brush-plated film Cr plated film Ni plated film
0.025
0.084
0.044 0.131
0.116 0.288
2
4
~ ~ ~
Ni~Fe(200)
I 40
50
60
2 Theta Fig. 1. The XRD pattern from Ni—Fe—W—P brush-plated film.
70
W.-H. Hui et al.
548
__~
~_i~
/
Brush-plated Ni—Fe—W—P films
-~—-
a Y(’4LF1i~ii
—
(a)
Fig. 2. TEM image of Ni—Fe—W—P brush-plated film.
zI~~.IwI1rLz~ Fig. 3. Electron diffraction pattern from Ni—Fe—W—P brush-plated film,
(b) Fig. 4. STM images of Ni—Fe—W—P brush-plated film: (b) is at 20 x the magnification of (a).
W-H. Hui et al.
(a) Before Corrosion
/
549
Brush-plated Ni—Fe—W—P films
Ni
(a)
Ni(2p~)
Nic
-I
Fe
Ni203
w
>1
“-I
.11 III -~ ____________________________ -
_j
____________
__________________________________________________
a, SI
A
H
B
I
(b) After Corrosion 893
873
W
‘-1
w(4~~
woa
Ni
Fe
J_~I
833
Binding Energy (eV) (b)
—
853
L
~1 .~
arebefore this indicates some possible effect of film: the Fig. (a) 5.broad; EDAX corrosion spectra treatment; from (b) the after Ni—Fe—W—P corrosion size treatment. brush-plated crystalline structure. The diffraction peak at 20 = 51°is not symmetrical because the right-hand side of the peak is much broader. The diffraction peak of (200) reflection for Ni is at 51.8°. In the case of amorphous Ni, this peak should be very broad. It is suggested that the asymmetry of the diffraction peak at 20 = 51° is correlated with the result from the scattering reflection of an Ni-base amorphous material. XRD result indicates that the crystal structure of the compound film is composed of an intermetallic compound of Ni3Fe and Ni-base solid solution. A TEM image from a prepared compound film is shown in Fig. 2. It clearly shows that the particle-like precipitations, having an average dimension of 8 nrn, are distributed in the film. An example of the electron diffraction pattern observed in the prepared film is illustrated in Fig. 3. It consists of some multicrystal circles which are attributed to an ordered f.c.c. substance in the film. The lattice parameter of this ordered f.c.c. substance is determined to be 0.356 nm which is same as that of Ni3Fe. Fig. 4(a) is the STM surface morphology of the corn-
w02
—
AB
H SIw >1 SI ~,
39
I
I
37
35
33
Binding Energy
I
I
31.
29
27
(eV)
2P3
Fig. 6. (a) Ni 12 XPS spectra of the Ni—Fe—W—P brush-plated film before corrosion treatment (spectrum A) and the passivated surface of Ni—Fe—W—P brush-plated film (spectrum B). (b) W 4f712 XPS spectra of the Ni—Fe—W—P brush-plated film before corrosion treatment (spectrum A) and the passivated surface of Ni—Fe—W—P brush-plated film.
pound film. The image shows the roughness pattern of the film where irregular partially curved streaks are displayed across the surface. At 20 x the magnification of Fig. 4(a), some equidistant straight lines are observed, as shown in Fig. 4(b). This indicates that some degree of ordering exists on the surface. It is consistent with the results of XRD and TEM that the compound film is composed of dispersed microcrystals. 3.3. Composition and depth profile analyses The EDAX spectra of the compound film before and after the corrosion treatment are shown in Fig. 5(a) and
W.-H. Hui et al.
550
/
Brush-plated Ni—Fe—W—P films
Fig. 5(b) respectively. It is clear that the corrosion treatment of the compound film enriches its W content, which increases from 6.0 wt.% to approximately
passivated layer, as shown in Fig. 6(b). The result reveals that the W on a passivated surface tends to form an oxide of low valence, i.e. WO2.
56 wt.%. It is evident that the W in the compound film is preferentially induced to migrate toward the surface when the film is exposed to a corrosive environment, Fig. 2P3/2 6(a) and and Fig. W 4f6(b) are comparisons of the spectra of Ni 712 XPS spectra from a surface of the compound film and a surface of a passivated layer respectively. It shows that nickel oxides, i.e. NiO and Ni203, are formed on both surfaces. No significant change is observed in the chemical state of the Ni on the surface of a passivated layer; see Fig. 6(a). However, a pronounced increase in the W02 component in the W 2p XPS spectrum is observed on the surface of a
The composition depth profiles of the compound film and the corrosion-treated sample are shown in Figs. 7(a) and 7(b) respectively. A great amount of oxygen is found on the corrosion-treated sample where the relative Auger element peak intensities of Ni, Fe and W are quite small compared with that of 0, as shown in Fig. 7(b). By calculating from the relative Auger element peak intensities corrected for sensitivity factors, it is suggested that the oxygen on the corrosion-treated surface would be chemically absorbed by Ni, Fe and W to form oxides, but would also be physical adsorbed on the surface in the form of free oxygen.
.2
~
Sputter Time (Mm) .04 (‘j
~
(b)
0
.04
0
I
I
I
20
40
60
Fe 80
100
Sputter Time (Mixi) Fig. 7. Auger depth profile from the Ni—Fe—W—P brush-plated film: (a) before corrosion treatment; (b) after corrosion treatment.
W.-H. Hui et al.
/
Brush-plated Ni—Fe—W—Pfilms
551
4. Discussion
free oxygen ions on the surface would inhibit the ionization of the metals in the compound film and increase its
The broad diffraction peaks for Ni3Fe shown in the XRD pattern of the compound film result from the size effect of the crystalline structure. The grain size of Ni3 Fe can be estimated by Laue’s formula
corrosion resistance.
0B
B = R2 Lhfk cos where R is the instrument constant, 2=1.1845 A is the X-ray wavelength, Lhjk is the grain size, 0B is the diffraction angle and B is the width of the diffraction peak. The calculated grain size of Ni 3Fe for the (111) plane is approximately 8.0 nm. This estimation is in agreement with the result of TEM analysis which shows that the distributed particle-like precipitations are Ni3Fe with crystal structure of f.c.c. and average grain size of 8 nm. The results of XRD, TEM and STM analyses consistently show that the compound film is an amorphous base with dispersed intermetallic compound of Ni3Fe at grain size of less than 10 nm. In a corrosive environment, the preferential migration of W in a compound film toward its surface increases the W content on the surface from 6 wt.% to 56 wt.%. XPS analysis reveals that the migrated W tends to form an oxide of low valence, i.e. WO2, on a passivated surface. Consider the following exchange reactions of W02: W02 + OH- -*WO3 + H’~’+2e 2W02 + H2O—+W203+2H’~+2e’ During the corrosion process, it is apparent that the presence of WO2 on the surface would advance the formation of a passivated layer and resist the extended corrosion reactions on the surface. The result of the AES analysis indicates that some free oxygen ions are physically absorbed on a surface of corrosion-treated compound film. The existence of the
5. Conclusion The corrosion resistance of brush-plated Ni—Fe—W--P compound film is excellent, compared with that of Cr plated film and Ni plated film. Microstructure analyses of the compound film reveal that the compound film is an amorphous base with dispersed intermetallic cornpound of Ni3Fe. During the corrosion process, the W preferentially migrates toward the surface and forms oxides. The excellent corrosion resistance of the cornpound film is attributed to its specific microstructure and surface compositions.
References [1] J.E. Sundgren, Thin Solid Films, 128 (1985) 21.
mt., 26(1993)115. [3] M. 551.Uemura, K. Saito and K. Nakao, Tribol. Trans., 33 (1990) [4] G. Abbas and D.R.F. West, Wear, 143 (1991) 353. [5] M. Boas, M. Bamberger and G. Revez, Surf. Coat. Technol., 42 [2] E. Vancoille, J.P. Celis and JR. Roos, Tribol.
(1990)175. [6] M.J. Hsu and PA. Molian, Wear, 132 (1989) 123. [7] DL. Williams, L. Wang, R. Wei and P.J. Wilbur, Mater. Lett., 9
(1990) 302. [8] 5. Aggarwal, AK. Goel, R.K. Mokindra, P.K. Ghosh and A. Chaud, Thin Solid Films, 196 (1991) 223. [9]M. Rubinstein, Met. Finish., 7 (1981) 21. [10] M. Rubinstein and R.M. Penrose, in R. Krutenat (ed), Metallurgical Coatings 1988, Vol. 2, Elsevier, New York, 1988, p. 847. [11] W.H. Hui, Ph.D. Thesis, Southeast University, China, 1992. [12]W.H. Hui, J.P. Zhang, Y.M. Zhang, J.J. Liu and B.L. Zhu, Wear, 167 (1993) 127. [13]JR. Scully, in R. Baboian and SW. Dean (eds.), Corrosion Testing and Evaluation, ASTM, Philadelphia, PA, 1990, p. 351.
Surface and Coatings Technology, 68/69 (1994) 546—551
A study of the corrosion resistance of brush-plated Ni—Fe—W—P films Wen-Hua Huia, Jia-Jun Liu”, Yi-Shung Chauge aDepartment of Mechanical Engineering, Southeast University, Nanjing, China 1’Department of Mechanical Engineering, Tsinghua University, Beijing, China cSummit Scientific Company, 1486 Lodge Court, Boulder, CO 80303, USA
Abstract Electrochemical metallization by means of brush plating has been developed for coating of Ni—Fe—W—P compound films. The hardness of this compound film is about 800—910 HV, which is similar to that of plated chromium film. The corrosion resistance of this compound film in various corrosive environments was studied. Results showed that the corrosion resistance of this compound film was 1.7 times and 1.4 times that of plated chromium film in the corrosive environments of NaCl (1S03768) and H 2S04 respectively. This indicates that Ni—Fe—W—P brush plating is one of the alternatives to substitute chromium plating. X-ray diffraction, transmission electron microscopy and scanning tunneling microscopy were applied to study the microstructure and the fine surface morphology of this compound film. It was found that the film is composed of an Ni-base amorphous and dispersive precipitated intermetallic compound of Ni3Fe. The surface characteristics of this compound film, analyzed by X-ray photoelectron spectroscopy and Auger electron spectroscopy, revealed that a preferential diffusion of W to the surface would be induced in the corrosive environments. The excellent corrosion resistance of brush-plated Ni—Fe—W—P film attributed to its microstructure and surface characteristics will be discussed.
1. Introduction Various coating deposition techniques have been developed and used for tribological applications. In many instances, chromium coatings have been extensively used for many years because of its high hardness level, low friction, and good wear and corrosion resistance. However, the finding that the hexavalent form of chromium is a carcinogen makes chromium compounds hazardous as vapors in coating operations and as waste products. The regulations for chromium coating processes and their waste controls, therefore, have become tightened because of the environmental concerns. This has stimulated an extended search for alternative materials and coating processes to replace chromium coatings, such as ceramics, polymers, physical vapor deposition, laser chemical vapor deposition plasma nitriding and ion-assisted coating [1—8]. Although each alternative has advantages for specific applications, no one process or material can completely replace chromium coatings in terms of wear resistance, corrosion resistance, the complexity of the process and the cost of the process. Electrochemical metallization by means of brush plating is used yearly for the application of metallurgical coatings [9]. Traditionally, brush plating is a tool for repair and salvage. Its applications for the prevention of fretting and seizing, the improvement of brazeability of braze metals on selected areas of a tool, machine parts and electrical or electronic components are being
0257—8972/94/87.00 SSDI 0257-8972(94)08107-A
exploited [10]. Recently, the coating of Ni—Fe—W—P compound films by means of brush plating has been developed [11]. The hardness of this compound film is about 800—9 10 HV, which is similar to that of the electrochemically deposited chromium film [12]. Wear tests on this film also showed excellent wear resistance [12], and thus the applications of brush-plated Ni—Fe—W—P compound film in tribology have been presumed. We present here the study of the corrosion resistance of brush-plated Ni—Fe—W—P compound film in the corrosive environments of NaC1 (1S03768) and H2S04. The attribution of the film’s microstructure and surface characteristics to the corrosion resistance are also studied. The microstructure and the fine surface morphology of the film were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to study the changes in the surface characteristics of the film in a corrosive environment.
2. Experiment The samples for the corrosion tests were Ni—Fe—W—P compound film, chromium plated film and nickel plated film where the chromium plated film and the nickel plated film were used for the comparison of corrosion
© 1994
—
Elsevier Science S.A. All rights reserved
W.-H. Hui et al.
/
Brush-plated Ni—Fe—W—P films
resistance. The Ni—Fe—W—P compound films, approximately 30 pm thick, were brush plated on aluminium substrates. The chemical composition of the compound film was 65 wt.% Ni, 28 wt.% Fe, 6 wt.% W and 1 wt.% P. The chromium plated film and the nickel plated film were coated on aluminum substrates by a conventional bath plating technique. Prior to this study, all the samples were sanded using emery paper (no. 800) to remove the surface oxidic layer and washed by distilled water and underwent acetic elimination by CH2COCH3. An electrochemical method of Tafel line extrapolation was employed for the corrosion test [13]. It was performed in an electrochemical polarization cell where the test sample was immersed in a corrosive solution for 30 mm. The corrosive solutions used in this study were (1) NaC1, 50 ±1 g l’, pH 6.0—7.0 (ISO 3768) and (2) 1 N H2S04. During the test, the polarization current was measured by the potentiostat at each setting of the cathode polarization, i.e. L~E= 0.1, 0.2, 0.5 mY. The data were then treated by the linear regression method and the corrosion current was acquired by extrapolating cathode polarization curve. The corrosion rate was calculated by 2 h’)=~-~x io~ V(gm nF where V is the corrosion rate, A (g mol is the atomic mass, i (A cm 2) is the corrosion current density, n is the ionic valence, and F=26.8 A h mol’ is the Faraday constant. —‘
The crystal structure of the brush-plated compound film was examined by XRD. The XRD measurements were made with a conventional diffractometer (D/MAX-YA) using Cu Ktx radiation. A transmission electron microscope (JEOL JEM-200EX) was employed to study the microstructure of the compound film. The fine surface morphology and the composition of the compound film before and after the corrosive treatment were also examined by STM and energy-dispersive analysis of X-rays (EDAX) respectively. The corrosive treatment was performed by immersing the sample in a corrosive solution in the electrochemical polarization cell until the anode polarization was finished. A passivated layer on the surface of the compound film was prepared by cathode polarization of the sample with + 4 V for 45 mm in the electrochernical polarization cell after the anode polarization was finished. The surface characteristics of the compound film and the passivated layer were examined by XPS. XPS analysis was performed in a Perkin—Elmer model PHI-550 electron spectrometer with a monochromatic Al Kcc X-ray source and a spherical analyzer. During the XPS measurements, an electron flood gun was used to reduce the sample charging effect. The binding energies of the XPS peaks were determined by using the carbon peak at a binding
547
energy of 284.5 eV as the reference position. The cornposition depth profiles of the compound film and the corrosive-treated film were also analyzed by AES. The AES analysis was carried out with a Perkin—Elmer model 600 Auger electron spectrometer equipped with a single-pass cylindrical mirror analyzer and a sputter gun. Based on the predetermined sputter etching rates for the compound film under selected conditions, the AES depth profile analysis was performed in the Auger chamber.
3. Results 3.1. Corrosion tests The corrosion rates of the compound film, the chromium film and the nickel film in the corrosive solutions of NaCl and H2SO4 are shown in Table 1. The cornpound film has excellent corrosion resistance in these corrosive solutions. The corrosion resistance of the cornpound film in the corrosive solution of NaCl is 1.7 times and 5.2 times that of chromium film and nickel film respectively; that in corrosive solution of H2S04 is 1.4 timesrespectively. and 2.7 times that of chromium film and nickel film 3.2. Crystallinity and microstructure analysis Fig. 1 shows the XRD pattern from an Ni—Fe—W—P compound film. The diffraction peaks at 20=43.8° and 51°corresponding to (111) and (200) reflection for Ni 3Fe Table 1 Corrosion rates of the plated films in NaCl (1S03768) and H2S04 Plated film
Corrosion ri~tei1~ Corrosion rat~ein1
Ni—Fe-W—P brush-plated film Cr plated film Ni plated film
0.025
0.084
0.044 0.131
0.116 0.288
2
4
~ ~ ~
Ni~Fe(200)
I 40
50
60
2 Theta Fig. 1. The XRD pattern from Ni—Fe—W—P brush-plated film.
70
W.-H. Hui et al.
548
__~
~_i~
/
Brush-plated Ni—Fe—W—P films
-~—-
a Y(’4LF1i~ii
—
(a)
Fig. 2. TEM image of Ni—Fe—W—P brush-plated film.
zI~~.IwI1rLz~ Fig. 3. Electron diffraction pattern from Ni—Fe—W—P brush-plated film,
(b) Fig. 4. STM images of Ni—Fe—W—P brush-plated film: (b) is at 20 x the magnification of (a).
W-H. Hui et al.
(a) Before Corrosion
/
549
Brush-plated Ni—Fe—W—P films
Ni
(a)
Ni(2p~)
Nic
-I
Fe
Ni203
w
>1
“-I
.11 III -~ ____________________________ -
_j
____________
__________________________________________________
a, SI
A
H
B
I
(b) After Corrosion 893
873
W
‘-1
w(4~~
woa
Ni
Fe
J_~I
833
Binding Energy (eV) (b)
—
853
L
~1 .~
arebefore this indicates some possible effect of film: the Fig. (a) 5.broad; EDAX corrosion spectra treatment; from (b) the after Ni—Fe—W—P corrosion size treatment. brush-plated crystalline structure. The diffraction peak at 20 = 51°is not symmetrical because the right-hand side of the peak is much broader. The diffraction peak of (200) reflection for Ni is at 51.8°. In the case of amorphous Ni, this peak should be very broad. It is suggested that the asymmetry of the diffraction peak at 20 = 51° is correlated with the result from the scattering reflection of an Ni-base amorphous material. XRD result indicates that the crystal structure of the compound film is composed of an intermetallic compound of Ni3Fe and Ni-base solid solution. A TEM image from a prepared compound film is shown in Fig. 2. It clearly shows that the particle-like precipitations, having an average dimension of 8 nrn, are distributed in the film. An example of the electron diffraction pattern observed in the prepared film is illustrated in Fig. 3. It consists of some multicrystal circles which are attributed to an ordered f.c.c. substance in the film. The lattice parameter of this ordered f.c.c. substance is determined to be 0.356 nm which is same as that of Ni3Fe. Fig. 4(a) is the STM surface morphology of the corn-
w02
—
AB
H SIw >1 SI ~,
39
I
I
37
35
33
Binding Energy
I
I
31.
29
27
(eV)
2P3
Fig. 6. (a) Ni 12 XPS spectra of the Ni—Fe—W—P brush-plated film before corrosion treatment (spectrum A) and the passivated surface of Ni—Fe—W—P brush-plated film (spectrum B). (b) W 4f712 XPS spectra of the Ni—Fe—W—P brush-plated film before corrosion treatment (spectrum A) and the passivated surface of Ni—Fe—W—P brush-plated film.
pound film. The image shows the roughness pattern of the film where irregular partially curved streaks are displayed across the surface. At 20 x the magnification of Fig. 4(a), some equidistant straight lines are observed, as shown in Fig. 4(b). This indicates that some degree of ordering exists on the surface. It is consistent with the results of XRD and TEM that the compound film is composed of dispersed microcrystals. 3.3. Composition and depth profile analyses The EDAX spectra of the compound film before and after the corrosion treatment are shown in Fig. 5(a) and
W.-H. Hui et al.
550
/
Brush-plated Ni—Fe—W—P films
Fig. 5(b) respectively. It is clear that the corrosion treatment of the compound film enriches its W content, which increases from 6.0 wt.% to approximately
passivated layer, as shown in Fig. 6(b). The result reveals that the W on a passivated surface tends to form an oxide of low valence, i.e. WO2.
56 wt.%. It is evident that the W in the compound film is preferentially induced to migrate toward the surface when the film is exposed to a corrosive environment, Fig. 2P3/2 6(a) and and Fig. W 4f6(b) are comparisons of the spectra of Ni 712 XPS spectra from a surface of the compound film and a surface of a passivated layer respectively. It shows that nickel oxides, i.e. NiO and Ni203, are formed on both surfaces. No significant change is observed in the chemical state of the Ni on the surface of a passivated layer; see Fig. 6(a). However, a pronounced increase in the W02 component in the W 2p XPS spectrum is observed on the surface of a
The composition depth profiles of the compound film and the corrosion-treated sample are shown in Figs. 7(a) and 7(b) respectively. A great amount of oxygen is found on the corrosion-treated sample where the relative Auger element peak intensities of Ni, Fe and W are quite small compared with that of 0, as shown in Fig. 7(b). By calculating from the relative Auger element peak intensities corrected for sensitivity factors, it is suggested that the oxygen on the corrosion-treated surface would be chemically absorbed by Ni, Fe and W to form oxides, but would also be physical adsorbed on the surface in the form of free oxygen.
.2
~
Sputter Time (Mm) .04 (‘j
~
(b)
0
.04
0
I
I
I
20
40
60
Fe 80
100
Sputter Time (Mixi) Fig. 7. Auger depth profile from the Ni—Fe—W—P brush-plated film: (a) before corrosion treatment; (b) after corrosion treatment.
W.-H. Hui et al.
/
Brush-plated Ni—Fe—W—Pfilms
551
4. Discussion
free oxygen ions on the surface would inhibit the ionization of the metals in the compound film and increase its
The broad diffraction peaks for Ni3Fe shown in the XRD pattern of the compound film result from the size effect of the crystalline structure. The grain size of Ni3 Fe can be estimated by Laue’s formula
corrosion resistance.
0B
B = R2 Lhfk cos where R is the instrument constant, 2=1.1845 A is the X-ray wavelength, Lhjk is the grain size, 0B is the diffraction angle and B is the width of the diffraction peak. The calculated grain size of Ni 3Fe for the (111) plane is approximately 8.0 nm. This estimation is in agreement with the result of TEM analysis which shows that the distributed particle-like precipitations are Ni3Fe with crystal structure of f.c.c. and average grain size of 8 nm. The results of XRD, TEM and STM analyses consistently show that the compound film is an amorphous base with dispersed intermetallic compound of Ni3Fe at grain size of less than 10 nm. In a corrosive environment, the preferential migration of W in a compound film toward its surface increases the W content on the surface from 6 wt.% to 56 wt.%. XPS analysis reveals that the migrated W tends to form an oxide of low valence, i.e. WO2, on a passivated surface. Consider the following exchange reactions of W02: W02 + OH- -*WO3 + H’~’+2e 2W02 + H2O—+W203+2H’~+2e’ During the corrosion process, it is apparent that the presence of WO2 on the surface would advance the formation of a passivated layer and resist the extended corrosion reactions on the surface. The result of the AES analysis indicates that some free oxygen ions are physically absorbed on a surface of corrosion-treated compound film. The existence of the
5. Conclusion The corrosion resistance of brush-plated Ni—Fe—W--P compound film is excellent, compared with that of Cr plated film and Ni plated film. Microstructure analyses of the compound film reveal that the compound film is an amorphous base with dispersed intermetallic cornpound of Ni3Fe. During the corrosion process, the W preferentially migrates toward the surface and forms oxides. The excellent corrosion resistance of the cornpound film is attributed to its specific microstructure and surface compositions.
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