The Electrochemical behavior of Zn-Ni alloys was studied in borate solutions using cyclic voltammetry, potentiodynamic anodic polarization, and current transient techniques under the effect of alloy composition, stepwise increasing potential, scan rate, and electrolyte concentration. The voltammogram consists of two potential regions separated by the critical potential
The anodic behavior of zinc in Na2B4O7 solutions has been investigated by the galvanostatic polarization technique. The polarization curves are characterized by one distinct arrest corresponding to Zn(OH)2 or ZnO, after which the potential increases linearly with time before reaching the oxygen evolution region [
In spite of the earlier studies, there remains considerable disagreement in the literature on the interpretation of the nature and the mechanism of the anodic layers on nickel [
The aim of this study was to investigate the effect of alloying zinc with nickel on increasing the life time of sacrificial anodic protection of zinc for automobile body against corrosion. The electrochemical behavior of Zn-Ni alloy in 0.15 N of borate solution. The voltammetric profiles of pure Zn and pure nickel were included for comparison. The microstructure and composition of the passive film formed during the anodic sweep were characterized by X-ray diffraction analysis.
Three zinc-nickel alloys (Table
Composition of the studied alloys.
Alloy | Zn% | Ni% |
---|---|---|
I | 95.0 | 5.0 |
II | 90.0 | 10.0 |
III | 85.0 | 15.0 |
All of the electrodes were mounted in Teflon so that only the cross-sectional area of 0.126 cm² was in contact with solution. Before each experiment, the working electrode was polished with successively finer grades of emery paper and then with alumina paste to obtain a mirror-like surface finish then degreased with ethyl alcohol and rinsed with doubly distilled water. Then, the electrodes were transferred to the solution where they were left at −2000 mV for 1 min, to remove any oxide present, before running the experiments.
The solutions used were prepared from Analar grade chemicals. All experiments were performed using freshly prepared solutions and freshly polished electrodes. The electrolytic cell used was of 100 cm³ capacity and consisted of three separate compartments, which were used for the working, counter, and reference electrodes. the counter electrode was a rod of graphite while the reference electrode was a saturated calomel electrode (SCE). The cyclic voltammetric polarization was applied by means of EG&G potentiostat/Galvanostat Model 273A using the 352 SoftCorr III software on a Pentium II computer. The morphology of the alloy surface in the potential range beyond the critical potential
The electrochemical behavior of Zn-Ni alloys was studied in 0.15 N boric acid and 0.15 N borax (borate buffer solution) using cyclic voltammetric technique, and the data are given in Figure
Cyclic voltammograms of (1) Zinc, (2) Nickel, (3) alloy I, (4) alloy II, and (5) alloy III in 0.15 N of borate buffer solution at 25°C and 50 mVs−1.
Curve 2 represents the cyclic voltammetric behavior of pure nickel in 0.15 N of borate buffer solution. The forward sweep is characterized by the appearance of three anodic peaks
Visscher and Barendrecht [
The cyclic voltammetric behavior of alloys I, II, and III is represented in Figure
The potential region II was characterized by the appearance of three anodic peaks
X-ray diffraction analyses of Zn-Ni alloys at potential values noble to the potential region of the anodic peak
X-ray diffraction pattern of alloy I surface potentiodynamically polarized to −600 mV at 25°C and 50 mVs−1.
X-ray diffraction pattern of alloy III surface potentiodynamically polarized to 1300 mV with at 25°C and 50 mVs−1.
The complementary relationship between the anodic and the cathodic peaks was obtained by reversing the potential at different step potentials
Cyclic voltammograms of alloy I in 0.15 N of borate buffer solution at 25°C, scan rate 50 mVs−1, and various reversing anodic potentials: (1) −900 mV, (2) −300 mV, (3) 350 mV, (4) 900 mV, and (5) 1300 mV.
The effect of increasing sweep rate was studied for the three alloys in 0.15 N of borate buffer solution at 25°C. Figure
Cyclic voltammograms of alloy I in 0.15 N of borate buffer solution at 25°C and various scan rates; (1) 25 mVs−1, (2) 50 mVs−1, (3) 100 mVs−1, (4) 125 mVs−1, and (5) 150 mVs−1.
Relation between the peak current density,
where
The cyclic voltammetric behavior of Zn-Ni alloys at 25°C and at scan rate 50 mVs−1 was examined in different concentration of borate buffer solution, and the results of alloy I is given in Figure
Cyclic voltammograms of alloy I at 25°C scan rate 50 mVs−1 and in various concentrations of borate buffer solutions; (1) 0.1 N, (2) 0.15 N, (3) 0.2 N (4) 0.25 N and (5) 0.4 N.
Relation between the peak current density of the anodic peaks,
In order to get more information about the electrochemical behavior of Zn-Ni alloys in borate buffer solution, potentiostatic current/time were performed at different anodic steps
Current transients versus time recorded for alloy I in 0.15 N of Borate buffer solution at 25°C and constant anodic step potentials; (1) −900 mV, (2) −600 mV, (3) 200 mV and (4) 1000 mV.
Dependence of the current density on
The cyclic voltammetric behavior of zinc-nickel alloys was studied in 0.15 N of borate buffer solution using cyclic voltammetry, anodic polarization, and current time/transient techniques.
The anodic voltammetric profiles of the alloys lie below those of the pure metals indicating decreased rates of dissolution of the two metals, zinc and nickel, from the alloys. On alloying with nickel, the rate of zinc dissolution was decreased which increases its protective life as sacrificial anode for protecting automobile body against corrosion.
The forward sweep was characterized by the appearance of five anodic peaks corresponding to the formation of Zn(OH)2, ZnO, Ni(OH)2, NiOOH, and Ni2O3 before oxygen evolution takes place.
The backward sweep shows two cathodic peaks in pure nickel corresponding to the reduction of Ni2O3 and NiOOH to Ni(OH)2, the backward sweep in alloys shows only one cathodic peak due to the reduction of Zn+2 species to Zn.
A study of the effect of scan rate has shown that the dissolution processes in the regions of the anodic peaks in the alloys are under diffusion control.
Potentiostatic current/time transient measurements reveal that the formation of Zn(OH)2, ZnO, Ni(OH)2, NiOOH, and Ni2O3 layers involves a nucleation and growth mechanism under diffusion control.