THE INFLUENCE OF CO 2 + CONCENTRATION ON THE PROPERTIES OF THE ELECTRODEPOSITED ZINC – COBALT ALLOY COATINGS

The effects the concentration of cobalt on zinc-cobalt alloys obtained from sulphate baths under continuous current deposition are described. The deposit morphology was analyzed using Scanning Electron Microscopy (SEM) and an X-Ray Diffraction (XRD) was used to determine the preferred crystallographic orientations of the deposits. Protection against corrosion properties studied in a solution of 3 % wt NaCl in the potentiodynamic polarization measurements (Tafel), electrochemical impedance spectroscopy (EIS) to the potential of corrosion free. The parameters that characterize the corrosion behavior can be determined from the plots and Nyquist plots. It has been observed that the Zn–Co alloy is characterized by enhanced the resistance of corrosion compared to the Zn alloys and the addition of Co in the ZnCo increases the micro-hardness, XRD and SEM results an identify any coatings Zn-Co alloy composition reveals that. zinc rich ( ηphase) , cubic Co5Zn21 .


Introduction
Considerable research has been performed to improve the corrosion resistance of metallic surfaces.Zinc coatings are used widely to protect iron and steel substrates against corrosion [1].Significant research in electrodeposited zinc has been focused on the development of alloy coatings that contain small amounts of other elements, such as Mn, Fe, Co, Sn and Ni, Cr, the most common examples being Zn-Fe [2][3], Zn-Co [2,[4][5][6] and Zn-Ni [2,[7][8][9].Metals like iron, cobalt and nickel have been incorporated in zinc plating baths to obtain coatings with higher corrosion resistance [1] At high cobalt content (more than 10 wt.%), Zn-Co multilayer coatings have better corrosion resistance than zinc-cobalt single layer coatings as reported by Bahrololoom et al [1].The electrodeposition of Zn-Co alloys with a controlled morphology and composition has been the subject of many studies as a consequence of their observed improved corrosion resistance [1 ,10-11].The purpose of our work is the characterization of composites Zn-Co deposits on a mild steel substrate.These deposits are obtained using an acidic sulphate bath, which were introduced from the suspension (CoSO4.6H2O) with different concentrations.Technical XRD was used for the structure analysis; SEM was used for morphological analysis.Studies potentiodynamic polarization and electrochemical impedance in a solution of 3 % wt NaCl.DOI

Experimental 2.1 Coating processes
The deposition of Zn-Ni coatings was carried out onto steel substrates under galvanostatic conditions at operating current density of 30 mA/cm 2 and a temperature of 30 °С, The chemical composition of the basic electrolyte of pure Zn coating the plating bath alloys deposition was given in Table 1 [12,13,14].Electrodeposits Zn were obtained by varying the concentration of Co in the bath (0, 5 and 0.1, 0.15 M).

Coating characterization
XRD characterization of samples was carried out with a D8 Advance-Brucker using a Cu Kα radiation with the wavelength λ= 0.1540 nm and 0.02° step.Scherrer's formula is used for the determination of the Crystallite sizes of the coatings from the X-ray peak broadening of the (101) diffraction peak [15,16]: Where D is the grain size, is the X-ray wavelength (λ = 1.540 nm), β is the corrected peak full width at half maximum intensity (FWHM), and θ is Bragg angle position of peak.
The surface morphology of different Zn-Co deposits was observed by a JEOL JSM 5800 scanning electron microscope (SEM), Micro hardness of coatings was measured using a load of 100 g with a holding time of 15 s by using a Vickers hardness tester (HV) of deposits were performed in the surface by using a Wolpert Wilson Instruments (model 402UD) [12].

Electrochemical measurements
The corrosion behavior and the protection performance of Zn and Zn-Co alloy coatings were studied by using of electrochemical impedance spectroscopy (EIS) and electrochemical Tafel extrapolation in 3 % wt NaCl solution.The tests were performed using a potentiostat galvanostat (a Volta Lab 40 model), the electrode of work was a coated sample, the counter electrode was platinum with a surface of 1 cm 2 and the Hg/HgO/ 1 M KOH is used as reference electrode.
Whereas the impedance data were obtained at the open-circuit potential and the measurements were carriedout over a frequency range of 100 KHz-10 MHz using an amplitude of sinusoidal voltage (10 mV).The potentiodynamic polarization was carried out at a scan rate of 50 mV/s and the scanning potential ranged from −1500mV to +1500mV of open circuit potential.The corrosion current density and corrosion potential were determined based on Tafel's extrapolation 3 Results and discussions

X-ray diffraction
Fig. 1 Shows the X-ray diffraction patterns obtained for several Zn and Zn-Co alloy deposits.In the absence of cobalt, the peaks associated with the characteristic crystallographic planes for zinc were observed, The diffraction lines corresponding to zinc-rich ηphase, cubic Co5Zn21 (101) and the peak associated with plane (002) having the highest intensity, the presence of cobalt (0.05, 0.1 and 0,15 M) in the alloy coatings slightly decreases the intensities of the diffracted planes as compared with those of the pure zinc.With an increase in cobalt content in Zn-Co alloy deposits, the intensity of peaks for planes (002), (100), and (101) decreases further.This decrease of intensity is also observed in other peaks related to phase of zinc-rich; The Zn-Co alloy coatings have the same structure as zinc.single phase structure (η-phase , cubic Co5Zn21) which is a solid solution of cobalt in zinc [17,18].
Table 2 shows the relationship between cobalt content and grain size of Zn-Co alloy coatings.Crystallite sizes of the coatings were calculated from the X-ray peak broadening of the (101) diffraction peak using Scherrer's formula.Zn pure alloy, deposited in Basic solution, exhibited a crystallite size of 72 nm, whereas the crystallite size of Zn alloy coating deposited in electrolyte Basic solution+0,15 M Co 2+ decreased to 51,08 nm.where the grain sizes diminish in the presence of cobalt ions, This finer structure explained probably the higher hardness of this deposit mentioned previously [19].

Effect of Cobalt content of the bath on coatings micro hardness
The variation of microhardness vs cobalt content in the deposited Zn-Co alloy coatings is shown in Fig. 2 and Table 3.This figure indicates that the micro hardness of Zn-Co alloy coatings increases with increasing cobalt content.This is because that the hardness of Co is greater than Zn, The hardness increased from 225 Hv for Zn, alloy to 250 and 310 Hv for 0.05 and 0.15M Co alloy coating respectively, Hardness values enhance with Co content, it can be correlated to the (i) formation of solid solution, (ii) formation of two phase structure, (iii) grain size effects [19][20][21][22].Basic solution + 0.15 M Cobalt 310

Surface morphology
Scanning electron micrographs of the electrodeposits obtained from the acidic sulphate Zn and Zn-Co baths are shown in Fig. 3.The surface of Pure Zn alloy coating (Fig. 3a) is uniform, homogenous and consists of irregular crystals particles, have a clear grey colour, the Zn-Co alloy (Fig. 3b, 3c and 3d) appear sufficiently well distributed, compact and continuous, homogeneity, having a clear brown colour, It is clear that the grains size is decreased with the increase of cobalt content in the Zn-Co deposits, The presence of cobalt modifies the growth of the zinc nuclei, leading to fine-grained deposits.Consequently, the compactness, adherence and hardness increase with elevation of the Cobalt content [1,15,20,21,23].Fig. 5 shows cyclic voltammograms obtained from the electrolyte containing 0.1 M CoSO4 at five different scan rates of 5, 10 and 20, 50,100 mV s -1 .It is observed that when scan rate increases from 5 to 100 mV s -1 , cathodic peak potential moves toward more negative values, and anodic peak potential moves toward more positive values.Therefore, with the increase of scan rate, the distance between anodic and cathodic peaks increases.This increase is related to the loss of ohmic drop [18].

Conclusion
The following conclusions were drawn:  XRD spectres illustrate the phase structure of composites coating was single zinc-rich (ηphase), cubic Co5Zn21 and incorporation of cobalt in the Zn-Co alloy coating decrease the grain size.


From the SEM images of the compsites coating morphologies the grains size decreases with the increase of cobalt content in the Zn-Co deposits. The test of the micro-hardness on the various electro deposited coatings has a maximum value (0.15 M) Co, because the increase of the Co concentration in the plating bath increases of micro-hardness. Also the surface morphology of the deposits was improved with increasing Co concentration, gives a more uniform surface morphology and more compact deposits. The height of the first dissolution anodic, peak decreases with increasing cobalt concentration in the plating bath. The study by the method from potentiodynamic polarization curves showed that the values of the corrosion potential (Ecorr) decreases, the corrosion current density (Icorr) and the polarization resistance (Rp) increases with increasing the concentration of Cobalt in the electrolyte bath. EIS analysis performed on the developed coatings that have increased resistors Rs.The increase in the size of the capacitive loop with the addition of Cobalt.

Fig. 2 3
Fig.2The effect of cobalt contents in the composite coatings on the hardness of deposits.Zn-Co alloy coating

Fig. 4
Fig. 4 Cyclic voltammetric study for deposition Zn-Co alloy in Co ions at different molar concentrations at 25°C and scan rate of 50 mV s -1

Fig 5 Fig. 5
Fig.5shows cyclic voltammograms obtained from the electrolyte containing 0.1 M CoSO4 at five different scan rates of 5, 10 and 20, 50,100 mV s -1 .It is observed that when scan rate increases from 5 to 100 mV s -1 , cathodic peak potential moves toward more negative values, and anodic peak potential moves toward more positive values.Therefore, with the increase of scan rate, the distance between anodic and cathodic peaks increases.This increase is related to the loss of ohmic drop[18].Fig 5 also indicates that the reduction-oxidation (redox) reactions of in Zn-Co alloy are quasi-reversible, because the ratio of cathodic to anodic peak current is almost close to one

Table 2
Values of the FWHM and crystallite size obtained from the strongest diffraction line of the metallic phases

Table 5
Extracted fitted data from the equivalent circuit of Zn-Co alloy coatings in a 3 % wt NaCl solution