- Email: [email protected]
nanoSiC coatings
Materials and Design 30 (2009) 4450–4453
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
An investigation on corrosion resistance of as-applied and heat treated Ni–P/nanoSiC coatings Faryad Bigdeli, Saeed Reza Allahkaram * School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 11155-4563 Tehran, Iran
a r t i c l e
i n f o
Article history: Received 22 January 2009 Accepted 13 April 2009 Available online 22 April 2009
a b s t r a c t EN–SiC coatings are recognized for their hardness and wear resistance. In this work electroless Ni–P coatings containing nano SiC particles were co-deposited on St37 tool steel substrate. Scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), polarization and electrochemical impedance spectroscopy (EIS) were used to analyze morphology, structure and corrosion resistance of the coatings. The results showed that SiC nano-particles co-deposited homogeneously, and the structure of Ni–P–SiC nano-composite coatings as deposited was amorphous. Heat-treatment at 400 °C for 1 h induced crystallization of the electroless Ni–P coatings. Microhardness of electroless Ni–P–SiC composite coatings increased due to the existence of nano-particles, and reached to a maximum value after heat-treatment. Corrosion tests showed that both electroless nickel and electroless nickel composite coatings demonstrated significant improvement of corrosion resistance in salty atmosphere. Proper post heat-treatment significantly improved the coating density and structure, giving rise to enhanced corrosion resistance. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electroless nickel (EN) coatings have been widely used in the chemical, mechanical and electronic industries because of their inherently good properties such as high corrosion resistance, wear resistance and uniform coating thickness. These properties plus their good wear and corrosion resistance have been responsible for the development of autocatalytic nickel deposition over a quarter of the century [1,2]. Electroless Ni–P coatings are also widely used for corrosion protection application in a variety of environments. They are barrier coatings, protecting the substrate by sealing it off from the corrosive environments, rather than by sacrificial action. However, in this respect, only electroless Ni–highP coating is effective in offering an excellent protection, whereas electroless Ni–lowP and Ni–mediumP coatings are not recommended for severe environments [3]. Co-deposition of solid particles into coatings can further improve certain properties, thus enhancing their performance. Hard particle-containing deposits (NiP/X, X = SiC, WC, Al2O3, Si3N4. . .) have been developed when the main requirement for the composite coatings has been wear resistance [4]. Among these coatings, the combination NiP–SiC has proved to be the most cost-effective and best-performing combination. EN–SiC coatings are recognized for their hardness and wear resistance and are usually considered as a replacement for ‘‘hard chromium” in the aerospace industry [3,5]. These properties can be improved further by heat-treatment * Corresponding author. Tel./fax: +98 2161114108. E-mail address: [email protected] (S.R. Allahkaram). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.04.020
of the coated specimen. Heat-treatment is an important factor that affects the thickness, hardness, structure and morphology of deposit [1]. Most studies concerning NiP-hard particle systems have been performed using micron-sized particles and some commercial processes have been used to obtain such composites. However, the development of nanotechnologies has raised the interest on the metal matrix nano-composite coatings because of their unique mechanical, magnetic and optical properties [6]. In this study electroless nickel composite deposits with various amounts of SiC particles have been prepared. The objective of this paper is to investigate corrosion properties and effects of heattreatment on microstructure of these coatings. In this work, the polarization and electrochemical impedance tests have been used to analyze the corrosion behavior of the applied EN and EN composite coatings. These together with the effects of post-heat-treatment on corrosion resistance have been discussed.
2. Experimental EN and EN composite coatings were obtained on St37 steel substrates of 40 40 3.0 mm vertically positioned in a 250 ml bath. Different stages of sample preparations were as follows: (1) (2) (3) (4)
Magnetic grinding of the specimens. Washing samples with water and soap. Degreasing specimens. Washing samples with distilled water.
F. Bigdeli, S.R. Allahkaram / Materials and Design 30 (2009) 4450–4453
(5) Dipping in acetone solution. (6) Washing samples with distilled water. (7) Pickling with 10% Wt sulfuric acid at room temperature. A commercial electroless nickel bath (SLOTONIP 70 A from Schlotter) with sodium hypophosphite as reducing agent was used to obtain the coatings. This bath provided NiP deposits with a high phosphorous content, 9–10% P. Temperature changed within 88– 93 °C and pH changed within 4.5–4.7 range, during coating process. Nano-sized (average size 40 nm) b-silicon carbide particles were used for SiC co-deposition. Hexadecyltrimethyl ammonium bromide (HTAB) was added for particles dispersion and surface charge adjustment. Magnetic stirring was used to keep particles from sediment. Powders were used as received and ultrasonically dispersed in the bath for 15 min, before the deposition. The thickness of all coatings for evaluating the corrosion resistance is chosen as 20 ± 1 lm. The coated samples were subjected to heat-treatment at 400 °C for 1 h. The structures of the applied EN and EN composite coatings were analyzed by X-ray diffraction (XRD) technique. X-ray diffraction phase analysis was performed by a PHILIPS X’ Pert Pro. diffractometer, using Co Ka radiation. The morphologies of applied Ni–P and Ni–P composite coatings were examined using an Oxford Cam Scan MV 2300 scanning electron microscope (SEM). Phosphorus content and particle concentration in the NiP matrix surface across the coating thickness were determined via energy dispersive spec-
4451
trum (EDS). The volume rate of embedded particles in the deposits for the same deposition time was determined by using an image analysis technique. The microhardness of the coatings was measured at a load of 50 g for 15 s. The corrosion resistance of the coatings in 3% sodium chloride solution was assessed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The corrosion tests were conducted using an EG&G Potentiostat/Galvanostat Model 273A. A standard three-electrode configuration consisting of the sample as the working electrode, a conventional saturated calomel electrode (SCE) as a reference electrode and a platinum counter electrode was used to evaluate the polarization behaviors. The polarization curves were measured in the range of 0.8 to 0.8 V at a constant scan rate of 2 mV s 1. The Impedance was measured and plotted by Zview for Windows Electrochemical Impedance Software. The measurement frequency range was selected between 100,000 and 0.01 Hz. 3. Results and discussion Fig. 1 shows the surface morphologies of Ni–P and Ni–P composite coatings. A uniform distribution of particles on coating surfaces was observed. Fig. 2 shows diffractograms of the coatings, as-obtained and after heat-treatment. As it can be observed, the asdeposited electroless Ni–P and Ni–P–SiC composite coatings were amorphous. Particle incorporation did not affect the structure of
Fig. 1. Surface morphology of (a) Ni–P and (b) Ni–P–SiC coatings.
Fig. 2. XRD spectra of (a) as-deposited electroless coatings (b) heat treated electroless coatings.
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F. Bigdeli, S.R. Allahkaram / Materials and Design 30 (2009) 4450–4453
the electroless Ni–P matrix that consisted of a supersaturated solid solution of phosphorous in nickel. After heating to 400 °C for 1 h, the nickel phosphide (Ni3P) phase precipitated in the matrix and diffractograms showed well-defined peaks corresponding to crystalline Ni, Ni3P and embedded particles. Table 1 shows the volume rate and weight rate of nano-particles in the coatings in accordance to the amounts of SiC particles in the bath. Table 2 shows Vickers microhardness of NiP and NiP composite coatings for as-deposited and after heat-treatment. The microhardness values of the as-deposited electroless Ni–P– SiC composite coatings increased because of the presence of hard particles and dispersion strengthening. The microhardnesses of all four types of coatings were significantly increased after heattreatment, due to the formation of Ni3P alloy phase. The corrosion potential (Ecorr) and corrosion current density (Icorr) calculated using Tafel extrapolation method are given in Table 3. All the coatings exhibited more positive corrosion potentials (Ecorr) and lower corrosion currents (Icorr) than those of the substrate. These results further confirmed that these coatings could be used for corrosion protection application in salty environments. Heat treated coatings exhibited more positive corrosion potentials (Ecorr) and dramatically lower corrosion currents (Icorr) than those of as-plated coatings. Fig. 3 shows the Nyquist plots of EN and their composite coatings before and after heat-treatment. The equivalent circuit shown in Fig. 4 was used to fit the coating corrosion property parameters, where Rs is resistance of solution, Rc is resistance of coating and CPE is the constant phase element which is mainly used to explain the system inhomogeneous and some distribution of the value of physical property of the system. The fitted results listed in Table 4 for EN and EN–SiC coatings also demonstrated high corrosion resistance several times greater than that of the St37 steel substrate. According to one of the most convincing models proposed to explain the high corrosion resistance of electroless Ni–P coatings, preferential dissolution of nickel occurs even at the open circuit potential, leading to the enrichment of phosphorus on the surface layer [7,8]. The enriched phosphorus surface reacts with water to
Table 1 Volume rate and weight rate of nano-particles in coatings according to the concentration of particles in the bath. Amount of particles in the bath (g/l)
1
3
5
Weight rate of nano-particles in coatings Volume rate of nano-particles in coatings
2.39 6.44
2.83 10.66
4.01 15.20
Table 2 Microhardness of NiP and some NiP composite coatings for as-deposited and after heat-treatment. Amount of particles in the bath (g/l)
0
1
3
5
Microhardness (as-deposited) (Hv) Microhardness (heat-treated) (Hv)
550 818
584 866
643 983
721 1122
Table 3 Ecorr and Icorr of EN and EN composite coating in 3% NaCl electrolyte. Coating St37 steel Ni–P (as-deposited) Ni–P–SiC (as-deposited) Ni–P (heat-treated) Ni–P–SiC (heat-treated)
Ecorr (mV) 725 483 294 304 255
Fig. 3. Nyquist plots of EN and EN composite coating in 3% NaCl electrolyte.
Fig. 4. Equivalent circuit of EIS test of EN and EN composite coating.
Table 4 Fitted results of nyquist plot. Coating
Ni–P (as-deposited) Ni–P–SiC (as-deposited) Ni–P (heat-treated) Ni–P–SiC (heat-treated)
Rs (X cm2)
8.273 7.632 7.919 7.496
Rc (X cm2)
3280 15,566 18,427 39,846
CPE CPE-T (f cm
2
CPE-P
7.5158 10 3.5596 10 3.0105 10 5.6831 10
4
0.90621 0.85610 0.86227 0.89709
)
4 4 4
form a layer of adsorbed hypophosphite anions. This layer in turn will block the supply of water to the electrode surface, thereby preventing the hydration of nickel, which is considered to be the first step to form either soluble Ni2+ species or a passive nickel film. According to Table 4 all the fitted corrosion parameters of asdeposited and heat treated coatings varied with the same tendency. The coating corrosion resistances Rc increased and (CPE-T) the capacitive component of the constant phase elements (CPE) decreased in different range, meaning that the coating structures changed to a denser and more homogenous film after heat-treatment. The changes in the resistive component of the constant phase clement (CPE-P) also showed similar behavior to the CPE-T values. By comparing the corrosion resistance of electroless Ni–P and Ni–P–SiC coatings, the latter coating appeared to offer better corrosion protection. This effect can be ascribed to a reduction in the effective metallic area available for corrosion in Ni–P-nano SiC coating.
4. Conclusions Icorr (lA) 17.46 10.62 2.35 4.23 10 1.58 10
6 6
(1) The results showed that NiP and Ni–P–SiC composite coatings were amorphous as-deposited. The nano-particles did not change the structure of the Ni–P alloy during electroless plating.
F. Bigdeli, S.R. Allahkaram / Materials and Design 30 (2009) 4450–4453
(2) Heat-treatment of as-deposited electroless Ni–P and Ni–P– SiC composite coatings resulted in the formation of crystalline structure. The final products of the crystallization and reaction of composite coatings were crystalline Ni and Ni3P alloy phase. (3) The microhardness of electroless Ni–P–SiC composite coating was higher than that of Ni–P alloy coating, due to the presence of superfine SiC particles, and its microhardness increased to a maximum value after heat-treatment at 400 °C for 1 h. (4) Corrosion tests performed showed that both electroless nickel and electroless nickel composite coatings demonstrated significant improvement of corrosion resistance. (5) Higher corrosion resistance was obtained for Ni–P–SiC over EN coating, which can be ascribed to a reduction in the effective metallic area available for corrosion. (6) Heat-treatment at 400 °C for 1 h significantly improved the coating density and structure, giving rise to an enhanced corrosion resistance for the applied EN and EN Composite coatings.
Acknowledgements We hereby express our sincere thanks to Iran National Science Foundation (INSF) for providing the funding of this Project.
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References [1] Ashassi-Sorkhabi H, Rafizadeh SH. Effect of coating time and heat treatment on structures and corrosion characteristics of electroless Ni–P alloy deposits. Surf Coat Technol 2004;176:318–26. [2] Huang YS, Zeng XT, Annergren I, Liu FM. Development of electroless NiP–PTFE– SiC composite coating. Surf Coat Technol 2003;167:207–11. [3] Taheri R, Oguocha INA, Yannacopoulos S. The tribological characteristics of electroless NiP coatings. Wear 2001;249:389–96. [4] Grosjean A, Rezrazi M, Takadoum J, Bercot P. Friction and wear characteristics of nickel-SiC electroless composite deposits. Surf Coat Technol 2001;137:92–6. [5] Apachitei I, Duszczyk J, Katgerman L, Overkamp PJB. Electroless Ni–P composite coatings: the effect of heat treatment on the microhardness of substrate and coatings. Scripta Mater 1998;38:1347–53. [6] Sarret M, Müller C, Amell A. Electroless NiP micro- and nano-composite coatings. Surf Coat Technol 2006;201:389–95. [7] Diegle RB, Sorensen NR, Clayton CR, Helfand MA, Yu YC. An XPS investigation into the passivity of an amorphous Ni–20P alloy. J Electrochem Soc 1988;135:1085–92. [8] Carbajal J, White E. Electrochemical production and corrosion testing of amorphous Ni–P. J Electrochem Soc 1988;135:2952–7.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
An investigation on corrosion resistance of as-applied and heat treated Ni–P/nanoSiC coatings Faryad Bigdeli, Saeed Reza Allahkaram * School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 11155-4563 Tehran, Iran
a r t i c l e
i n f o
Article history: Received 22 January 2009 Accepted 13 April 2009 Available online 22 April 2009
a b s t r a c t EN–SiC coatings are recognized for their hardness and wear resistance. In this work electroless Ni–P coatings containing nano SiC particles were co-deposited on St37 tool steel substrate. Scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), polarization and electrochemical impedance spectroscopy (EIS) were used to analyze morphology, structure and corrosion resistance of the coatings. The results showed that SiC nano-particles co-deposited homogeneously, and the structure of Ni–P–SiC nano-composite coatings as deposited was amorphous. Heat-treatment at 400 °C for 1 h induced crystallization of the electroless Ni–P coatings. Microhardness of electroless Ni–P–SiC composite coatings increased due to the existence of nano-particles, and reached to a maximum value after heat-treatment. Corrosion tests showed that both electroless nickel and electroless nickel composite coatings demonstrated significant improvement of corrosion resistance in salty atmosphere. Proper post heat-treatment significantly improved the coating density and structure, giving rise to enhanced corrosion resistance. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electroless nickel (EN) coatings have been widely used in the chemical, mechanical and electronic industries because of their inherently good properties such as high corrosion resistance, wear resistance and uniform coating thickness. These properties plus their good wear and corrosion resistance have been responsible for the development of autocatalytic nickel deposition over a quarter of the century [1,2]. Electroless Ni–P coatings are also widely used for corrosion protection application in a variety of environments. They are barrier coatings, protecting the substrate by sealing it off from the corrosive environments, rather than by sacrificial action. However, in this respect, only electroless Ni–highP coating is effective in offering an excellent protection, whereas electroless Ni–lowP and Ni–mediumP coatings are not recommended for severe environments [3]. Co-deposition of solid particles into coatings can further improve certain properties, thus enhancing their performance. Hard particle-containing deposits (NiP/X, X = SiC, WC, Al2O3, Si3N4. . .) have been developed when the main requirement for the composite coatings has been wear resistance [4]. Among these coatings, the combination NiP–SiC has proved to be the most cost-effective and best-performing combination. EN–SiC coatings are recognized for their hardness and wear resistance and are usually considered as a replacement for ‘‘hard chromium” in the aerospace industry [3,5]. These properties can be improved further by heat-treatment * Corresponding author. Tel./fax: +98 2161114108. E-mail address: [email protected] (S.R. Allahkaram). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.04.020
of the coated specimen. Heat-treatment is an important factor that affects the thickness, hardness, structure and morphology of deposit [1]. Most studies concerning NiP-hard particle systems have been performed using micron-sized particles and some commercial processes have been used to obtain such composites. However, the development of nanotechnologies has raised the interest on the metal matrix nano-composite coatings because of their unique mechanical, magnetic and optical properties [6]. In this study electroless nickel composite deposits with various amounts of SiC particles have been prepared. The objective of this paper is to investigate corrosion properties and effects of heattreatment on microstructure of these coatings. In this work, the polarization and electrochemical impedance tests have been used to analyze the corrosion behavior of the applied EN and EN composite coatings. These together with the effects of post-heat-treatment on corrosion resistance have been discussed.
2. Experimental EN and EN composite coatings were obtained on St37 steel substrates of 40 40 3.0 mm vertically positioned in a 250 ml bath. Different stages of sample preparations were as follows: (1) (2) (3) (4)
Magnetic grinding of the specimens. Washing samples with water and soap. Degreasing specimens. Washing samples with distilled water.
F. Bigdeli, S.R. Allahkaram / Materials and Design 30 (2009) 4450–4453
(5) Dipping in acetone solution. (6) Washing samples with distilled water. (7) Pickling with 10% Wt sulfuric acid at room temperature. A commercial electroless nickel bath (SLOTONIP 70 A from Schlotter) with sodium hypophosphite as reducing agent was used to obtain the coatings. This bath provided NiP deposits with a high phosphorous content, 9–10% P. Temperature changed within 88– 93 °C and pH changed within 4.5–4.7 range, during coating process. Nano-sized (average size 40 nm) b-silicon carbide particles were used for SiC co-deposition. Hexadecyltrimethyl ammonium bromide (HTAB) was added for particles dispersion and surface charge adjustment. Magnetic stirring was used to keep particles from sediment. Powders were used as received and ultrasonically dispersed in the bath for 15 min, before the deposition. The thickness of all coatings for evaluating the corrosion resistance is chosen as 20 ± 1 lm. The coated samples were subjected to heat-treatment at 400 °C for 1 h. The structures of the applied EN and EN composite coatings were analyzed by X-ray diffraction (XRD) technique. X-ray diffraction phase analysis was performed by a PHILIPS X’ Pert Pro. diffractometer, using Co Ka radiation. The morphologies of applied Ni–P and Ni–P composite coatings were examined using an Oxford Cam Scan MV 2300 scanning electron microscope (SEM). Phosphorus content and particle concentration in the NiP matrix surface across the coating thickness were determined via energy dispersive spec-
4451
trum (EDS). The volume rate of embedded particles in the deposits for the same deposition time was determined by using an image analysis technique. The microhardness of the coatings was measured at a load of 50 g for 15 s. The corrosion resistance of the coatings in 3% sodium chloride solution was assessed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The corrosion tests were conducted using an EG&G Potentiostat/Galvanostat Model 273A. A standard three-electrode configuration consisting of the sample as the working electrode, a conventional saturated calomel electrode (SCE) as a reference electrode and a platinum counter electrode was used to evaluate the polarization behaviors. The polarization curves were measured in the range of 0.8 to 0.8 V at a constant scan rate of 2 mV s 1. The Impedance was measured and plotted by Zview for Windows Electrochemical Impedance Software. The measurement frequency range was selected between 100,000 and 0.01 Hz. 3. Results and discussion Fig. 1 shows the surface morphologies of Ni–P and Ni–P composite coatings. A uniform distribution of particles on coating surfaces was observed. Fig. 2 shows diffractograms of the coatings, as-obtained and after heat-treatment. As it can be observed, the asdeposited electroless Ni–P and Ni–P–SiC composite coatings were amorphous. Particle incorporation did not affect the structure of
Fig. 1. Surface morphology of (a) Ni–P and (b) Ni–P–SiC coatings.
Fig. 2. XRD spectra of (a) as-deposited electroless coatings (b) heat treated electroless coatings.
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F. Bigdeli, S.R. Allahkaram / Materials and Design 30 (2009) 4450–4453
the electroless Ni–P matrix that consisted of a supersaturated solid solution of phosphorous in nickel. After heating to 400 °C for 1 h, the nickel phosphide (Ni3P) phase precipitated in the matrix and diffractograms showed well-defined peaks corresponding to crystalline Ni, Ni3P and embedded particles. Table 1 shows the volume rate and weight rate of nano-particles in the coatings in accordance to the amounts of SiC particles in the bath. Table 2 shows Vickers microhardness of NiP and NiP composite coatings for as-deposited and after heat-treatment. The microhardness values of the as-deposited electroless Ni–P– SiC composite coatings increased because of the presence of hard particles and dispersion strengthening. The microhardnesses of all four types of coatings were significantly increased after heattreatment, due to the formation of Ni3P alloy phase. The corrosion potential (Ecorr) and corrosion current density (Icorr) calculated using Tafel extrapolation method are given in Table 3. All the coatings exhibited more positive corrosion potentials (Ecorr) and lower corrosion currents (Icorr) than those of the substrate. These results further confirmed that these coatings could be used for corrosion protection application in salty environments. Heat treated coatings exhibited more positive corrosion potentials (Ecorr) and dramatically lower corrosion currents (Icorr) than those of as-plated coatings. Fig. 3 shows the Nyquist plots of EN and their composite coatings before and after heat-treatment. The equivalent circuit shown in Fig. 4 was used to fit the coating corrosion property parameters, where Rs is resistance of solution, Rc is resistance of coating and CPE is the constant phase element which is mainly used to explain the system inhomogeneous and some distribution of the value of physical property of the system. The fitted results listed in Table 4 for EN and EN–SiC coatings also demonstrated high corrosion resistance several times greater than that of the St37 steel substrate. According to one of the most convincing models proposed to explain the high corrosion resistance of electroless Ni–P coatings, preferential dissolution of nickel occurs even at the open circuit potential, leading to the enrichment of phosphorus on the surface layer [7,8]. The enriched phosphorus surface reacts with water to
Table 1 Volume rate and weight rate of nano-particles in coatings according to the concentration of particles in the bath. Amount of particles in the bath (g/l)
1
3
5
Weight rate of nano-particles in coatings Volume rate of nano-particles in coatings
2.39 6.44
2.83 10.66
4.01 15.20
Table 2 Microhardness of NiP and some NiP composite coatings for as-deposited and after heat-treatment. Amount of particles in the bath (g/l)
0
1
3
5
Microhardness (as-deposited) (Hv) Microhardness (heat-treated) (Hv)
550 818
584 866
643 983
721 1122
Table 3 Ecorr and Icorr of EN and EN composite coating in 3% NaCl electrolyte. Coating St37 steel Ni–P (as-deposited) Ni–P–SiC (as-deposited) Ni–P (heat-treated) Ni–P–SiC (heat-treated)
Ecorr (mV) 725 483 294 304 255
Fig. 3. Nyquist plots of EN and EN composite coating in 3% NaCl electrolyte.
Fig. 4. Equivalent circuit of EIS test of EN and EN composite coating.
Table 4 Fitted results of nyquist plot. Coating
Ni–P (as-deposited) Ni–P–SiC (as-deposited) Ni–P (heat-treated) Ni–P–SiC (heat-treated)
Rs (X cm2)
8.273 7.632 7.919 7.496
Rc (X cm2)
3280 15,566 18,427 39,846
CPE CPE-T (f cm
2
CPE-P
7.5158 10 3.5596 10 3.0105 10 5.6831 10
4
0.90621 0.85610 0.86227 0.89709
)
4 4 4
form a layer of adsorbed hypophosphite anions. This layer in turn will block the supply of water to the electrode surface, thereby preventing the hydration of nickel, which is considered to be the first step to form either soluble Ni2+ species or a passive nickel film. According to Table 4 all the fitted corrosion parameters of asdeposited and heat treated coatings varied with the same tendency. The coating corrosion resistances Rc increased and (CPE-T) the capacitive component of the constant phase elements (CPE) decreased in different range, meaning that the coating structures changed to a denser and more homogenous film after heat-treatment. The changes in the resistive component of the constant phase clement (CPE-P) also showed similar behavior to the CPE-T values. By comparing the corrosion resistance of electroless Ni–P and Ni–P–SiC coatings, the latter coating appeared to offer better corrosion protection. This effect can be ascribed to a reduction in the effective metallic area available for corrosion in Ni–P-nano SiC coating.
4. Conclusions Icorr (lA) 17.46 10.62 2.35 4.23 10 1.58 10
6 6
(1) The results showed that NiP and Ni–P–SiC composite coatings were amorphous as-deposited. The nano-particles did not change the structure of the Ni–P alloy during electroless plating.
F. Bigdeli, S.R. Allahkaram / Materials and Design 30 (2009) 4450–4453
(2) Heat-treatment of as-deposited electroless Ni–P and Ni–P– SiC composite coatings resulted in the formation of crystalline structure. The final products of the crystallization and reaction of composite coatings were crystalline Ni and Ni3P alloy phase. (3) The microhardness of electroless Ni–P–SiC composite coating was higher than that of Ni–P alloy coating, due to the presence of superfine SiC particles, and its microhardness increased to a maximum value after heat-treatment at 400 °C for 1 h. (4) Corrosion tests performed showed that both electroless nickel and electroless nickel composite coatings demonstrated significant improvement of corrosion resistance. (5) Higher corrosion resistance was obtained for Ni–P–SiC over EN coating, which can be ascribed to a reduction in the effective metallic area available for corrosion. (6) Heat-treatment at 400 °C for 1 h significantly improved the coating density and structure, giving rise to an enhanced corrosion resistance for the applied EN and EN Composite coatings.
Acknowledgements We hereby express our sincere thanks to Iran National Science Foundation (INSF) for providing the funding of this Project.
4453
References [1] Ashassi-Sorkhabi H, Rafizadeh SH. Effect of coating time and heat treatment on structures and corrosion characteristics of electroless Ni–P alloy deposits. Surf Coat Technol 2004;176:318–26. [2] Huang YS, Zeng XT, Annergren I, Liu FM. Development of electroless NiP–PTFE– SiC composite coating. Surf Coat Technol 2003;167:207–11. [3] Taheri R, Oguocha INA, Yannacopoulos S. The tribological characteristics of electroless NiP coatings. Wear 2001;249:389–96. [4] Grosjean A, Rezrazi M, Takadoum J, Bercot P. Friction and wear characteristics of nickel-SiC electroless composite deposits. Surf Coat Technol 2001;137:92–6. [5] Apachitei I, Duszczyk J, Katgerman L, Overkamp PJB. Electroless Ni–P composite coatings: the effect of heat treatment on the microhardness of substrate and coatings. Scripta Mater 1998;38:1347–53. [6] Sarret M, Müller C, Amell A. Electroless NiP micro- and nano-composite coatings. Surf Coat Technol 2006;201:389–95. [7] Diegle RB, Sorensen NR, Clayton CR, Helfand MA, Yu YC. An XPS investigation into the passivity of an amorphous Ni–20P alloy. J Electrochem Soc 1988;135:1085–92. [8] Carbajal J, White E. Electrochemical production and corrosion testing of amorphous Ni–P. J Electrochem Soc 1988;135:2952–7.