Production and characterization of a composite deposit on steel Nickel-Alumina
Mouna Rekaik Dept. Process Engineering
University of Jijel Algeria mrekaik@yahoo.fr
N. Azouz (a), M. Ferkhi (b)
dept.(a): Process Engineering and (b):chemistry University of Jijel
Algeria
Abstract— The need for improving the coatings with better corrosion resistance has been developed by the use of composite electrodeposits, which consists on adding solid particles in the structure of nickel, such as aluminium oxide (Al2O3). The objective of this research is the study of the resistance of nickel- alumina composite coatings on steel substrates (black plate), obtained from electroplating bath, at different levels of Al2O3 concentrations. The characterization is made by tests of corrosion in two different electrolytic solutions 0,5M K2SO4 and 0,5M NaCl, and physicochemical analyzes such as the X-rays diffraction (XRD) and scanning electron microscopy (SEM) with the microanalysis (EDAX). The electrochemical behavior of composite coatings in corrosive solutions was checked by methods of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).The various tests, carried out in theses conditions, revealed that the particles of Al2O3 added to the bath decrease the corrosion tendency of covered steels. They also determined the concentration of Al2O3, added to the bath, which provided the optimal protection against corrosion.
Keywords— Electrodeposition; Composite coating Ni-Al2O3; Electrochemical impedance spectroscopy (EIS); Potentiodynamic polarization
I. INTRODUCTION
Composite coatings have been the subject of intense research in recent years. They are produced by codeposition of the inert particles in a metal matrix from a contemplated electrolytic bath as a composite coatings metal matrix [1]. The presence of these particles changes the structure and composition of the coating, giving it to the map, new properties and features. The metal matrix may well see its hardness, corrosion resistance and many other properties increased by the appropriate choice of embedded particles. It is the choice of the particle size and composition that will have
a direct impact on the implementation of the process and lead to the quality of the finished coating [2]. The reasons for using the nickel in the corrosion protection of steel are, first, the ability of this to provide cathodic protection to steel and low nickel corrosion rate in non-aggressive environments, but also reasonable price and simple application on the steel surface or very hot baths or by electrodeposition. While nickel has a fairly high corrosion resistance, for which reason it is used in the protection of the steel, the average length of life of such coatings is limited, because of the aggressive nature of some media, in special those containing industrial pollutants. This is why we sought an improvement in corrosion resistance by the incorporation of inert particles in the metal matrix during electrodeposition. Thus obtaining composite materials with superior properties, such as resistance enlarged corrosion and wear, catalytic, magnetic and hardness. In addition, the electrolytic co-deposition process with oxide particles is at a reduced price and is versatile. In this regard, the most commonly used particles are oxides such as alumina, silica and titanium carbides such as chromium carbide and silicon carbide, metals and polymers as [3]. The evolution of the degree of solid particle co-deposition on an electrode depends on the main electrolysis parameters such as the electrolyte composition, current density, temperature, pH and the concentration of particles in the bath. In terms of keeping particles in suspension, a bath agitation is necessary when the particles tend to settle. These co-deposition techniques have been developed for a large number of materials including, alloy, semiconductor, superconductor oxides and conductive polymers. The formatter will need to create these components, incorporating the applicable criteria that follow.
II. EASE OF USE A. Experimental method
The material used in our work is ordinary steel (black plate). The samples were recovered in the form of plates of dimensions (25 X 40 X 1,5) mm of the same production in order to ensure the reproducibility and the validity of the results of our tests. The development of the nickel deposits and the composite deposits nickel-alumina was carried out on the level of the laboratory LIME (Laboratory of the Interaction Material-Environment) of the university of Jijel.
The composition, the concentrations and the conditions used for the various electrolytic deposits are shown in tables I and II respectively.
TABLE I. COMPOSITION OF THE BATH OF NI-AL2O3
constituents NiCl2.
6H2O
NH4Cl NaCl H3B O3
Al2O3
Content[
g.L-1] 11.885 12.303 4.091 6.183 0, 20 and 30
TABLE II. CONDITIONS OF BATH CO-DEPOSITION
operating Conditions
Temperature Voltage time deposit
40± 2°C 4 V 75 min
A cell with a capacity of 250 ml is used. This cell was filled with about 100 ml of solution to each bath. The cell contains three electrodes, two of which constitute the anodes are made of nickel in this case, while the third is that the cathode is steel with the distance between the electrodes is 2 cm. The cathode (working electrode) is connected to the negative pole of a current generator (or voltage), while the anodes are connected to the positive pole through a multimeter. The cell is placed on a heating magnetic stirrer with a stirring device, which rotates a magnetic bar inside the electrolyte solution. Before electrochemical investigations of corrosion, the surfaces of composite nickel-alumina deposits were examined with X-ray diffraction (XRD) and scanning electron microscope (SEM). For measurements of the electrochemical corrosion have been used a potentiostat- galvanostat VoltaLab 40 type Radio-Meter PGZ 301 with integrated transfer function, controlled by the software VoltaMaster 4 LIME laboratoire, a cell comprises a three- electrode assembly, the steel coated with the composite deposit Ni-Al2O3 as working electrode (WE), a cylindrical grid of large surface surrounding the working electrode as against electrode (AE) and saturated calomel electrode (SCE) as an reference electrode. As corrosion test potassium sulphate solutions (K2SO4 0.5M) and sodium chloride (NaCl 0.5M) were used at room temperature, ventilated and without agitation. For the polarization curves were potensiodynquamies measures the potential applied to the sample varies continuously, with a scanning speed equal to 0.25 mV · s-1, -800 mV to +600 mV / SCE. The potential of the working electrode (WE) reached stability after a 30 minute wait. The corrosion current density (icorr) for the individual specimens was determined by extrapolating the Tafel curves anode and cathode.
III. RESULTS AND DISCUSSING A. scanning electron microscope (SEM)
The scanning electron microscope allows having an idea about the morphology of the surface of our samples. Figure 1
shows the composite deposit microstructure Ni-Al2O3 alumina having concentrations 0, 20 and 30 g. L-1.
For 0 g. L-1 alumina, we see spherical and large grains of uniform size (Fig. 1.a). However, with the addition of Al2O3 particles, the shape of nickel grains became nodular (Fig. 1.b and c) and their size becomes smaller and smaller with increasing alumina content. However, increasing the concentration of alumina particles in the nickel plating baths below a certain limit results in agglomeration due to the high surface energy of the particles and the more intense interaction between the particles [4]. These agglomerates are incorporated and distributed in a nonuniform manner over the entire surface of the electrode.
Fig. 1. SEM micrographs of the surface of the deposit of pure Ni (a) and composite coatings of Ni-Al2O3, obtained with different concentrations of Al2O3 particles in the plating bath 20 g L-1 (b) and 30 g L-1 (c)
Some researchers studied the co-deposition of the inert particles by electroplating and found that there is a strong tendency among agglomeration of particles due to their high activity. If the particles are not well dispersed in the bath, they are likely to be incorporated as aggregates in the metal matrix [5].
B. X-ray diffraction (XRD)
The plates coated in baths with concentrations of 0, 20, 30 g. L-1 Al2O3 were analyzed by X-rays, the rays are of a length λ = 1.5406 A°. Figure 2 represents the diagrams of diffraction of our samples obtained by the composite deposit Ni-Al2O3.
Fig. 2. X-ray diffraction analysis of a sample coated in the bath in the presence of 0 g. L-1 (a), 20 g. L-1 (b) and 30 g L-1 (c) Al2O3
Whatever the alumina concentration added to the electroplating bath, the XRD diagrams obtained at different concentrations of Al2O3 clearly show the shape of the Ni phase which crystallizes in the space group Fm3m cubic system.
These diffractograms thus confirm the presence of a single phase structure.
C. polarization curves potensiodynamic
The polarization curves potensiodynamic coated steel of different concentrations of alumina in the two above mentioned settings, after 30 minutes immersion are reported in Figure 3.
The values of corrosion potentials (Ecorr) and corrosion current densities (icorr) are obtained from the processing of the polarization curves (extrapolation method straight Tafel). The corrosion rate values (Vcorr) is calculated from the following formula:
With:
Vcorr: Corrosion rate [mm. an-1], icorr: Current density [mA. cm-
2], M: Molecular weight [g. mol-1], ρ: Density [g. cm-3], n Valencia [1].
Fig. 3. Polarization curves for a steel plate coated in a bath without alumina (a), 20 g. L-1 (b) and 30 g. L-1 (c) Al2O3 in 0.5M K2SO4 and 0.5 NaCl
All these values are given in Tables III, IV and V.
TABLE III. VALUES ELECTROCHEMICAL CHARACTERISTICS IDENTIFIED FROM POLARIZATION CURVES, IN THE CASE OF COATED METAL IN A BATH WITHOUT ALUMINA (IN BOTH SOLUTIONS)
Solution Valeurs
Ecorr
[mV]
icorr V[mA.cm-2] corr[mm.an-1]
K2SO4 -687.87 0.244 2.86
NaCl -752.19 0.632 7.398
TABLE IV. VALUES ELECTROCHEMICAL CHARACTERISTICS IDENTIFIED FROM POLARIZATION CURVES IN THE CASE OF COATED METAL IN A BATH OF 20
G.L-1AL2O3(IN BOTH SOLUTIONS)
Solution Valeurs
Ecorr
[mV]
icorr V[mA.cm-2] corr[mm.an-1]
K2SO4 -864.86 0.024 0.277
NaCl -886.18 0.632 3.676
TABLE V. VALUES ELECTROCHEMICAL CHARACTERISTICS IDENTIFIED FROM POLARIZATION CURVES IN THE CASE OF COATED METAL IN A BATH OF 30
G.L-1AL2O3(IN BOTH SOLUTIONS)
Solution Valeurs
Ecorr
[mV]
icorr V[mA.cm-2] corr[mm.an-1]
K2SO4 -623.92 0.469 5.491
NaCl -704.09 1.062 12.433
Following the observations with the naked eye samples after immersion in solutions of 0.5M NaCl and 0.5M K2SO4, and view the nature of nickel coatings on steel (cathode coatings), the form of corrosion encountered in case where the electrolyte is NaCl is localized corrosion. It consists of local attacks of passive film due to the presence of pores on the deposits; against by, in the case where the electrolyte is K2SO4, the form of corrosion is uniform (the attack is spread over the entire surface).
The polarization curves plotted for the electrodes covered with the composite deposit Ni-Al2O3, shows that there is a change of the corrosion potential of the material thus treated.
The composite deposition Ni-Al2O3 moves the corrosion potential to values less noble (cathodic displacement) when compared to that of the steel in the two solutions (NaCl and K2SO4). However, the offset of this potential is remarkably more negative for the submerged slides in NaCl than for those immersed in K2SO4 [1].
From the polarization curves obtained on the coated slides immersed in K2SO4 we see a decrease in the density of the corrosion current as a function of the added concentration of alumina in the nickel bath until 20 g. L-1 after this value, we see a further increase in the current density. According to (Tables III, IV and V) we see a progressive decrease in velocity over corrosion of the added alumina concentration in bath until an optimal value corresponding to the concentration
of 20 g. L-1. Beyond this optimum, there is a new gradually increasing corrosion rate. This is explained by the presence of alumina particles in the nickel matrix, which fill the pores existing therein and, in the interval [0, 20] g. L-1. Beyond 20 g.
L-1, there is an increase in corrosion rates that can be interpreted by the excessive presence of particles of Al2O3 in the electrodeposited nickel structure, which disrupts the efficiency of the film thus constituted by a recovery rate of Al2O3 more porous than the case of Ni less than alumina.
This brings us to complete the study by the physical techniques. Same results observed for the polarization curves coated slides (Tables III, IV and V) immersed in NaCl. We observe a gradual decrease in velocity over corrosion of the added alumina concentration in bath until an optimal value corresponding to the concentration of 20 g. L-1. Beyond this optimum, we see a further gradual increase in the corrosion rate can be explained by the presence of defects and dislocations or chemical heterogeneities in the metal matrix and / or in non-uniform incorporation of the particles due to the formation of agglomerates.
D. Electrochemical impedance spectroscopy (EIS) Impedance measurements are performed after 30 minutes, 2, 4, 6 and 24 hours of immersion in two different media K2SO4 (0.5M) and NaCl (0.5M) at room temperature, ventilated and without agitation. The parameters are:
amplitude of signals imposed: 10 mV compared to the corrosion potential; frequency range 100 kHz to 10 mHz;
number of points per decade: 10 ppd and the integration time:
4 s. All registered impedance spectra were analyzed as Nyquist plots. The electrochemical impedance diagrams determined at different immersion times and at various concentrations of alumina in K2SO4 and NaCl solutions are shown in Figures 4 and 5.
For samples covered by the composite Ni-Al2O3 deposition with different alumina concentrations: 0, 30 (g. L-1), the electrochemical impedance spectra obtained after various immersion time in both electrolytic media exhibit two semicircles; the first loop at high frequencies characterized the coating layer and the second low frequency indicates the diffusion impedance called Warburg impedance.
For the sample coated with the composite deposition, with the concentration of Al2O3 of 20 g. L-1, we see the presence of a single capacitive loop, reflecting the coating layer which has a resistance value (Rrev) very large compared to that of other samples. This reflects the more protective effect of this coating concentration of Al2O3.
For the simulation of the experimental impedance diagrams we adopted an RC equivalent circuit (Fig. 6) and we realized the simulation by ZSimpWin Version 3.21 software.
The parameters obtained after the simulation are shown in Tables VI, VII, VIII and IX.
Fig. 4. Nyquist plot of the impedance spectrum of a steel plate coated in a bath of 0 g. L-1 (a), 20 g. L-1 (b) and 30 g. L-1 (c) of Al2O3 in 0.5M K2SO4 after various immersion times
Fig. 5. Nyquist plot of the impedance spectrum of a steel plate coated in a bath of 0 g. L-1 (a), 20 g. L-1 (b) and 30 g. L-1 (c) of Al2O3 in 0.5M NaCl after various immersion times
Fig. 6. Equivalent circuit diagram in system Ni-Al2O3 / electrolyte to different concentrations of Al2O3 particles in the deposition bath 20 (a) and 0, 30 g. L-1 (b)
a b
TABLE VI. VALUE CHARACTERISTIC OF METAL COATED RESISTORS RECORDED AS A FUNCTION OF IMMERSION TIME IN K2SO4
[] Al2O3 Time (h)
0.5 2 4 6 24
0 g.L-1
Al2O3 1.70 1.357 1.075 0.984 0.567
20 g.L-1
Al2O3 198.1 158.2 155 149.8 60.1
30 g.L-1
Al2O3 15.36 0.926 0.631 0.623 0.504
TABLE VII. VALUE CHARACTERISTIC OF METAL COATED RESISTORS RECORDED AS A FUNCTION OF IMMERSION TIME IN NACL
[] Al2O3 Time (h)
0.5 2 4 6 24
0 g.L-1
Al2O3 8.796 5.458 4.039 0.613 0.304
20 g.L-1
Al2O3 81.28 36.31 18.84 13.70 12.16
30 g.L-1
Al2O3 19.41 1.065 0.653 0.522 0.427
TABLE VIII. VALUE OF THE CAPACITANCE OF THE COATING OBTAINED METAL COATED RECORDED AS A FUNCTION OF IMMERSION TIME IN K2SO4
[] Al2O3 Time (h)
0.5 2 4 6 24
0 g.L-1
Al2O3 2.023 2.549 4.108 6.276 8.07
20 g.L-1
Al2O3 0.102 0.106 0.117 0.502 1.467
30 g.L-1
Al2O3 1.312 3.42 7.003 7.47 8.514
TABLE IX. VALUE OF THE CAPACITANCE OF THE COATING OBTAINED METAL COATED RECORDED AS A FUNCTION OF IMMERSION TIME IN NACL
[] Al2O3 Time (h)
0.5 2 4 6 24
0 g.L-1
Al2O3 2.66 3.169 4.94 6.525 7.914
20 g.L-1
Al2O3 0.414 0.775 0.882 1.461 2.241
30 g.L-1
Al2O3 5.486 16.42 19.28 22.47 27.76
On examining the values in Tables VI to IX, we note a decrease overall coating of the resistance values (Rrev) and higher values of the capacity (Crev) as a function of immersion time. Indeed, by considering the relationship Stern-Geary, the Rrev and the corrosion current are inversely related.
The interpretation of the variation of the resistance of the coating Rrev a function of time can be explained by the progressive formation of the hydration products of the above coating layer leads to a slower decreasing of Rrev. From VI - IX the simulation parameters, we see an increase in resistance values of Rrev coating and a decrease in coating Crev the capacitance values in the presence of particles of Al2O3, which can be explained by: the layer of corrosion products formed on the surface of the composite deposit is thicker and
less permeable than in their absence; the decrease of the active surface into direct contact with the corrosive environment, as a result of incorporation of the particles, or by the growth of the thickness of the layer of corrosion products.
The impedance results are consistent with the extracted results of the polarization curves, the best values being observed in the case of Ni-Al2O3 composite coatings, to a concentration of 20 g. L-1.
IV. CONCLUTION
Aluminum oxide (Al2O3) stable, dense and adherent, is a protective oxide which, together with nickel plating baths, tends to reduce corrosion of the resulting deposits.
The incorporation of particles of Al2O3 in the metal structure of nickel, as evidenced by the SEM analysis leads to a composite material, Ni-Al2O3, which provides the steel substrate the more interesting corrosion resistance (higher).
The decrease in corrosion rate is due to the incorporation of particles of Al2O3 in the pores of the coating (clogging of the porosity of the coating).
The concentration of Al2O3 (0 and 30 g. L-1) in the nickel plating bath also influences the properties of the composite coatings. These alumina concentrations were used, the concentration of 20 g. L-1 has been determined as the optimum concentration of particles. The deposits obtained at this concentration exhibited the corrosion resistance concerning improvements which is larger in comparison with the other concentrations.
The presence of Al2O3 in the nickel deposits, and this up to a concentration of 20 g. L-1 for the electrodeposition baths, is inversely proportional to the corrosion rate.
It should be noted that a concentration of Al2O3 exceeds 20 g. L-1 for the electrodeposition baths, induces an increase in corrosion rate. Also, corrosion has a less uniform appearance can be attributed to the random distribution of Al2O3 in the repository, the result of the agglomeration of the particles in the bath.
The Al2O3 particles embedded in the nickel matrix disrupt nickel increase during electroplating.
The methods of electrochemical impedance spectroscopy and potentiodynamic polarization methods are powerful techniques to verify quality of corrosion protection composite deposits Ni-Al2O3.
The composite deposition Ni-Al2O3 moves the corrosion potential to values less noble (cathodic displacement) when compared to that of the steel in the two solutions (NaCl and K2SO4). However, the offset of this potential is remarkably more negative for the submerged slides in NaCl than for those immersed in K2SO4.
The corrosion current density is larger in the middle than 0,5M sodium chloride medium 0.5M potassium sulfate. This shows the aggressiveness of chlorides whatever the nature of the coatings (composite coating in our case considered new type of coating).
References
[1] A. C. Ciubotariu, L. Benea, M. Lakatos Varsanyi, and V. Dragan ,
“Electrochemical impedance spectroscopy and corrosion behaviour of Al2O3–Ni nanocomposite coatings”, International Journal for Electrochemical Acta, 53, 4557–4563, 2008.
[2] Research funded by the Division of Research and Scientific Cooperation of the DGTRE 2008.
[3] R.Q. Fratari, and A. Robin, Production and characterization of electrolytic nickel–niobium composite coatings, International Journal for Surface and coatings technology, 200, 4082 – 4090, 2006.
[4] B.M. Praveen, and T.V. Venkatesha, Electrodeposition and properties of Zn-nanosized TiO2 composite coatings, International Journal for Applied surface science, 254, 2418 –2424, 2008.
[5] A. Sohrabi, A. Dolati, M. Ghorbani, A. Monfared, and P. Stroeve.
Nanomechanical properties of functionally graded composite coatings:
electrodeposited nickel dispersions containing silicon carbide nanoparticles, International Journal for Materials Chemistry and Physics, 121, 497-505, 2010.
