2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) 810–816 World Scientific Publishing Company DOI: 10.1142/S2010194512002784
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HIGH CORROSION RESISTANCE OF NI-P/NANO-SIO2 ELECTROLESS COMPOSITE COATINGS∗ S.R.Allahkaram1* School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, Tehran, Iran, P.O. Box: 11155-4563, Tel/Fax: +982161114108 *Corresponding Author,
[email protected] T. Rabizadeh 2 School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, Tehran, Iran,
[email protected]
The process of electroless plating Ni-P and Ni-P/nano-SiO2 on API-5L X65 carbon steels was improved. The Ni-P/nano-SiO2 composite coatings were prepared from the bathes containing different concentrations of nano-SiO2 particles. The coatings surface and morphologies were observed via scanning electron microscopy (SEM). The chemical compositions were analyzed by EDAX. The corrosion behaviors were evaluated by electrochemical impedance spectroscopy tests. The experimental results indicated that SiO2 nano-particles co-deposited but some agglomeration occurred. Micro-hardness of electroless Ni–P–SiO2 composite coatings increased due to the existence of nano-particles. Corrosion tests showed that both electroless Ni-P and Ni-P/nano-SiO2 composite coatings demonstrated significant improvement of corrosion resistance of substrate in salty atmosphere and latter coating type appeared to offer a better corrosion protection. Keywords: Electroless Ni–P/nano-SiO2 composite coating; micro-hardness; X-ray diffraction; Corrosion resistance; Electrochemical impedance spectroscopy.
1. Introduction Wear and corrosion are the main failure modes for many engineering components and result in high costs. Ni-P electroless plating is one of the most important surfaceengineering technologies which applied to industrial fields.1,2 Electroless composite plating refers to co-depositing the solid particles together with the metal coatings to improve certain properties.3,4 Some researches 5,6 about the coatings on steel substrates indicated that the corrosion resistance and wear resistance of composite coatings were better than that of electroless plating coatings owing to the dual properties of metal coatings and solid particles. Therefore, the electroless composite coatings can be more competent for protecting magnesium alloys in corrosive media.7 1.Associate Professor and Member of Center of Excellence in High Performance Ultra Fine Materials 2.MSc Student in Corrosion and Protection Engineering
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High Corrosion Resistance of Ni-P/Nano-SiO2 Electroless Composite Coatings 811
With the rapid development of nanometer technology, nanometer particles are deposited into the coatings, resulting in composite coatings owning the dual characteristics of metal coating and nanometer particles, Allahkaram at al.8,9 studied corrosion resistance of electroless Ni–P composite coatings by electrochemical impedance spectroscopy and confirmed that the degree of corrosion protection offered by them is the same or better than that provided by the electroless Ni–P coatings having similar thickness. In this context, the present work aims to study the effect of incorporation of hard ceramic nano-particles, nanoSiO2, on physical properties and corrosion protection behavior of electroless Ni–P matrix. The effects of bath particle concentration on the corrosion and mechanical properties of the NiP-SiO2 nano-composite coatings will be investigated. 2. Experimental methods Electroless coatings were obtained on API-5L X65 steel substrates (30×25×15 mm). The substrate surface was carefully polished with SiC emery papers upto #400. All the specimens were subjected to the following pre-treatment and plating procedure: Acetone cleaning → alkaline cleaning → acidic cleaning → electrocleaning. Water rinse was used after every step. 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 Ni-P 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. The SiO2 used for co-deposition was fumed type oxide by Degussa, with a mean diameter of 12-20 nm and concentrations in the bath equal to 1, 4, 7 and 10 g SiO2/l. Prior to the co-deposition, a Ni–P coating was deposited for 0.5 h. Then SiO2 nanoparticles were added to the plating bath were by ultrasonic dispersion for 1h, followed by deposition for 2 h under magnetic agitation at a speed of 300 rpm. For a comparison, pure Ni–P coating was prepared under the same conditions. The hardness of coatings was measured using an (AMSLER D-6700) Vickers diamond indenter at a load of 150 g for a loading time of 20s. The average of five repeated measurements is reported. The morphology and microstructure of the coatings were analyzed using scanning electron microscopy SEM, (CAMSCAN MV2300). X-ray diffraction (XRD) patterns were obtained using Philip's Xpert pro type X-ray diffractometer with a cobalt target and an incident beam mono-chromator. For corrosion measurements, a standard three-electrode electrochemical cell consisting of a working electrode, an Ag/AgCl reference electrode and a platinum counter electrode was used. The working electrodes referred to Ni-P and Ni-P/nano-SiO2 composite coatings. An area of 1cm2 was exposed to the corrosive electrolyte, and the left area was sealed. EIS measurements were carried out using a Solartron Model SI 1255 HF Frequency Response Analyzer (FRA) coupled to a Princeton Applied Research (PAR) Model 273A Potentiostat/Galvanostat. The EIS measurements were obtained at (OCP) in
812 S. R. Allahkaram & T. Rabizadeh
a frequency range from 0.01Hz to 100 kHz with an applied AC signal of 5mV (rms) using EIS software model 398 in 3.5 wt.% NaCl solution.
3. Results and discussion
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3.1. SEM and Elemental composition of the composite coatings Fig.1(a,b) shows the SEM images of the surface morphologies of electroless Ni-P and NiP/nano-SiO2 prepared from the bath containing 4g/l nano-particles. It is seen that SiO2 nano-particles has been embedded in the Ni-P matrix via co-deposition, and some of the SiO2 nano-particles have been agglomerated to a certain degree even in the presence of ultrasonic and magnetic agitation. It can be seen that surface of Ni–P coating is very smooth, but the surfaces of the composite coatings are coarser. There are many nodular protrusions over the surfaces. These nodules are spherical in shape and compositional analysis carried out on nodules showed the presence of SiO2 particles apart from nickel and phosphorus. It is also evident that the SiO2 particles are distributed throughout the surface although agglomeration is seen in some places.
a
b
Fig.1. SEM morphologies of electroless coatings (a) Ni-P (b) Ni-P/nano-SiO2
To confirm the presence of the nano-SiO2 particles in the coatings, EDX analysis of the surface has been carried out for Ni-P/nano-SiO2 coating at 4g/l nano-particles prior and post addition of nano-particles and the corresponding spectra is shown in the Fig.2. Silicon peak is observed other than nickel and phosphorous peaks. This confirms the presence of second phase silicon oxide particles in the Ni-P matrix. Lower incorporation of nano-particles in composite coatings can be ascribed to the smaller size of the particles, which are (believed to be) swept away from the electrode surface.
High Corrosion Resistance of Ni-P/Nano-SiO2 Electroless Composite Coatings 813
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a
b
Fig.2. EDAX parrens of (a) Ni-P and (b) Ni-P/nano-SiO2 composite coatings
3.2. XRD analyses of Ni-P/nano-SiO2 composite coatings Fig. 3 shows the XRD patterns of Ni-P and Ni-P/nano-SiO2 composite coatings. It is seen that both types of coating have amorphous structures.
Fig.3. XRD patterns of electroless coatings with and without nano-SiO2 particles
814 S. R. Allahkaram & T. Rabizadeh
Micro-hardness(HV150)
760 740 720 700 680 660 640 620
735.66 698.89 683.39 661.89
1 gr/l
4 gr/l
7 gr/l
10 gr/l
Nano-SiO2 concentration
Fig.4. Microhardness of electroless coatings prepared with various concentrations of SiO2 particles.
3.4. Electrochemical corrosion performance Fig.5 shows the Nyquist plots obtained for Ni–P and Ni-P/nano-SiO2 composite coatings in 3.5% sodium chloride solution at their respective open circuit potentials. All the curves appear to be similar (Nyquist plots), consisting of a single semicircle in the high frequency region signifying the charge controlled reaction. However, it should be noted that though these curves appear to be similar with respect to their shape, they differ considerably in their size. This indicates that the same fundamental processes must be occurring on all these coatings but over a different effective area in each case.9 To account for corrosion behavior of these coatings at their respective open circuit potentials, an equivalent electrical circuit model given in Fig.6 has been utilized to simulate the metal/solution interface and to analyze the Nyquist plot. 12000
Z"(Ohm.sqcm)
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3.3 Micro-hardness of Ni-P/nano-SiO2 composite coating The influences of SiO2 nano-particles on the hardness of the composite coatings are illustrated in Fig. 4. It should be mentioned that micro-hardness of Ni-P coating was 608.3 HV150. In general, Ni-P/nano-SiO2 composite coatings have higher micro-hardness than Ni-P coatings. It can be supposed that SiO2 nano-particles co-deposited in the composite coating could act as barriers to retard the plastic deformation of Ni-P matrix and hence increase the micro-hardness.
1g/l
10000
4g/l 7g/l
8000
10g/l 6000
Ni-P
4000 2000 0 0
5000
10000
15000
20000
25000
30000
35000
Z'(Ohm.sqcm)
Fig.5. Nyquist plot for electroless coatings prepared with various concentrations of SiO2 nano-particles
High Corrosion Resistance of Ni-P/Nano-SiO2 Electroless Composite Coatings 815
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The charge transfer resistance Rct and double layer capacitance Cdl obtained for Ni–P and Ni-P/nano-SiO2 composite coatings are compiled in Table1. The occurrence of a single semicircle in the Nyquist plots indicates that the corrosion process of these coatings involves a single time constant.9
Fig. 6. Equivalent electrical circuit model used to analyze the EIS data .
The high values of charge transfer resistance (Rct), in the range 14806-21824 Ω cm2, obtained for the coatings of present study imply a better corrosion protective ability of Ni-P/nano-SiO2 composite coatings than Ni-P electroless coating. The capacitance value obtained for electroless composite coating is in the order of 9-21µF/cm2. The Cdl value is related to the porosity of the coating. The low Cdl value confirms that the electroless composite coating of present study is relatively less porous in its nature.9 Table 1. Electrochemical parameters from EIS data of Ni-P electroless coatings with and without nano-particles in 3.5% sodium chloride solution. Type of coating
OCP(mV vs. Ag/AgCl)
Rct (Ω cm2)
Cdl (µF/cm2)
Ni-P Ni-P/ nano-SiO2(1g/l) Ni-P/ nano-SiO2(4g/l) Ni-P/ nano-SiO2(7g/l) Ni-P/ nano-SiO2(10g/l)
-343 -266.3 -282 -288 -310
13934 24335 21047 18342 16739
38.334 2. 9299 14.841 20.743 26.375
It is evident from literature reports on Ni-P coatings that preferential dissolution of nickel occurs at open circuit potential, leading to the enrichment of phosphorus on the surface layer. The enriched phosphorus surface reacts with water to form a layer of adsorbed hypophosphite anions (H2PO2-). 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.9 As it can be seen from Fig.7(a,b) which shows corrosion microstructure of electroless coatings, that Ni-P coatings suffered severe corrosion attack than nano-composite coating and by comparing the corrosion resistance of electroless Ni–P and Ni-P/nano-SiO2 composite coatings, the latter coating type appeared to offer better corrosion protection. This seems to be logical, because even though the apparent area remains the same, the effective metallic area prone to corrosion is decreased considerably in the case of
816 S. R. Allahkaram & T. Rabizadeh
electroless composite coating. Ni-P/nano-SiO2 coatings prepared with addition of 1g/l nano-particles to the electroless bath shows the best corrosion protective behavior among other nano-composite coatings with more nano-particle concentrations. This is because by increasing nano-SiO2 concentrations more agglomeration occurs and this may result in less porosity coverage of nano-composite coatings.
Int. J. Mod. Phys. Conf. Ser. 2012.05:810-816. Downloaded from www.worldscientific.com by UNIVERSITY OF LEEDS on 07/26/14. For personal use only.
a
b
Fig.7. SEM morphologies of corrosion microstructure of electroless coatings (a) Ni-P (b) Ni-P/nano-SiO2
4. Conclusions 1. SiO2 nano-particles have been successfully co-deposited with Ni-P matrix to generate Ni-P/nano-SiO2 composite coatings in the absence of any surfactants. 2. Plain electroless Ni-P deposits seem to be smooth compared to the slightly nodular deposits obtained due to the particle reinforcement. 3. XRD results obtained for the both coatings revealed that particles addition did not influence the structure of the coatings. 4. Higher corrosion resistance was obtained for Ni-P/nano-SiO2 over Ni-P coating, which can be ascribed to a reduction in the effective metallic area available for corrosion.
References 1. 2. 3. 4. 5. 6. 7. 8. 9.
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