| Abstract: |
The behaviour of nickel in 1M HCl solutions in the presence of aniline and some of its derivatives (o-, m- and p-anisidine) has been studied using weight loss and polarization techniques.A- From the weight loss measurements, it was found that:i- Aniline, o-, m- and p-anisidine decrease the corrosion rate of nickel in 1M HCl solution.ii- At constant temperature, the percent inhibition efficiency, %IE, increases with increasing the inhibitor concentration. On the other hand, at the same concentration, % IE decreases with increasing temperature.iii- The activation energy, ∆Ea, the equilibrium constant, k, as well as the other thermodynamic parameters (∆G°ads, ∆H°ads and ∆S°ads) for the dissolution and inhibition processes were calculated and analyzed.B- From the anodic polarization measurements, it was found that:i- Increasing the acid concentration causes an increase in the anodic dissolution of nickel.ii- Raising of temperature enhances the anodic dissolution of nickel. The activation energy for the dissolution process was found to be 12 k cal.mol-1.iii- Stirring the solution causes a shift in the polarization curves to the more active direction accounts for increased dissolution. On the other hand, presence of oxygen gas shifts the polarization curves to more positive direction indicating increased iv- Addition of aniline and its derivatives as inhibitors shifts the anodic polarization curves in the positive direction. The anodic Tafel slope is always the same in the absence and presence of the inhibitors. This indicates that the presence of such coC- From the cathodic polarization measurements, it was found that:i- Increasing of acid concentration causes an increase in the rate of hydrogen evolution reaction.ii- The reaction order of the hydrogen evolution reaction with respect to hydrogen ion concentration was found to be one.iii- Raising the temperature shifts the corrosion potential into more positive values and increases the cathodic current density.iv- The activation energy for hydrogen evolution reaction was found to be 10 k cal.mol-1.v- Addition of aniline and its derivatives, as inhibitors, shifts the cathodic polarization curves in the negative direction. The cathodic Tafel slope is always the same in the absence and presence of inhibitors. This indicates that the presence of such cvi - Inhibition occurs by adsorption according to the Langmuir isotherm.vii- The inhibition efficiency increases in the order: p-anisidine > o-anisidine > m-anisidine > aniline.viii- The relative inhibitive efficiencies of these compounds depend on the relative position of the – OCH3 group in the aniline ring and the effective electron densities which affect the adsorption characteristics on metal surface.D- From the electrochemical behaviour of Ni in Na2B4O7 solutions, using the cyclic voltammetric and anodic polarization techniques, one can draw the following conclusions:i- The shape of the cyclic voltammograms depends on the sweep number.ii- The anodic portions of the CVs are characterized by a pronounced peak and a passive region due to the formation of NiO and other nickel oxides before oxygen evolution.iii- The cathodic portions show one reduction peak corresponding to the reduction of β-NiOOH to β-Ni(OH)2. The latter cannot be reduced further to Ni even in the hydrogen evolution region.iv- As the concentration of the borate anions increases, the current density of the anodic peak increases while its potential is shifted towards more negative values.v- An increase in the scan rate increases the current density of both the anodic and cathodic branches.vi- Addition of halide anions up to a certain concentration has no effect on the mechanism of nickel passivity.vii- Further increase in the concentration of the aggressive anions causes the destruction of the passive film and the initiation of pitting corrosion. The breakdown potential, Eb, varies with the concentration of the aggressive anions, Cagg, according toEb = a - b log Caggviii- At a constant aggressive anion concentration, the aggressive action decreases in the order: Cl- > Br- > I-.ix-The breakdown potential is shifted to more positive values with a rise of pH of the solution, according to equation:Eb = a + b log pHx- Addition of increasing concentrations of MoO42-, CrO42- and WO42- causes a decrease in the current density along the active region and also a shift in the breakdown potential toward noble direction. The efficiencies of these anions as pitting corrosion
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