Elctrochemical Behaiviour Of Some Copper Nikel Alloys Aqueous Medi :

Ali Yousry Mohamed Ali Aletr

(1)A literature survey on the previous studies on electrochemical behaviours of copper, nickel and copper-nickel alloys in aqueous solutions is given, in the first part of the thesis, with particular emphasis on their behaviours in alkaline solutions.(2)Cyclic voltammograms were constructed for copper, nickel and the three copper-nickel alloys (70%, 50% and 30% Cu) in sodium hydroxide solutions. The effects ofcyclization, reversal potential, sodium hydroxide concentration and voltage scan rate on the obtained cyclic voltammograms were studied. It was found that:(a)For copper electrode, the cyclization gives essentially similar voltammograms except that the peak current increases and the peak potential shifted to more negative. The cyclic voltammogram of copper in sodium hydroxide solution is characterized by four anodic steps before oxygen evolution and three cathodic ones. The successive oxidation steps correspond to the formation of CuzO; Cu(OH)z; HCuCi and Cuz0m. The three cathodic steps correspond to successive reduction of the formed oxides to metallic copper.(b)For nickel electrode, the cyclic voltammogram is characterized by four anodic steps before oxygen evolution1and only one cathodic step. The successive oxidation steps correspond to the formation of oc-Ni(OH)z, p-Ni(OH)z and NiOOH. The reduction step is attributed to the reduction of NiOOH to irreducible p-Ni(OH)z. The cyclization of electrode results in different voltammograms due to the presence of p-Ni(OH)z on the electrode surface at the end of the firstcycle.(c) For copper-nickel electrodes; the voltammograms of the three alloys have the same general features (namely, four anodic steps before the oxygen evolution and three cathodic ones). The cyclization of electrodes gives the same voltammograms except that the peaks currents increase slightly. The oxidation steps corresponding to the formation of oc-Ni(OH)z together with CuzO; p-Ni(OH)z together with Cu(OH)z; thickening of j3-Ni(OH)z layer and NiOOH together with CuzOs. The cathodic steps correspond to the reduction of the formed oxides to metallic copper and p-Ni(OH)z. Generally, it was found that, alloy (I)(70% Cu) and alloy (II) (50% Cu) behave upon anodic oxidation and cathodic reduction in a manner similar that of pure copper. On the other hand, alloy (III) (30% Cu) behaves upon oxidation and reduction in sodium hydroxide solution, in the same manner of pure nickel electrode.(3) Cyclic voltammograms for copper, nickel and the three copper-nickel alloys were constructed in sodium sulfidesolutions. The effects of cyclization, reversal potential, sodium sulfide concentration and voltage scan rate on the cyclic voltammograms were studied. from the abtained resultsit was concluded that:(a) For copper electrode, the voltammogram ischaracterized by four anodic and two cathodic steps. The second and third sweeps gives essentially similar voltammograms which differ from that of the first sweep in some significant characteristics, namely, the disappearance of the last anodic peak as well as the two cathodic peaks with the appearance of another two new cathodic peaks instead of the original ones. It was concluded that, the four oxidation steps correspond to the formation of CuzS together with CuzO; CuS; thickining of Cuz0 layer and transformation of CuS to CuO. The two cathodic peaks of the first sweep correspond to the reduction of formed CuO. On the other hand the electrode was covered by CuS at the end of the subsequentsweeps.(b) For nickel electrode, the voltammogram was characterized by five oxidation steps correspond to the formation of m-Ni(OH)z; 0-Ni(OH)2; NisSz; NiS and Nin ions together with sulfer. Two reduction peaks corresponding to reduction of Ni2+ ions are observed in the cathodic half-cycle. No oxygen evolution was noticed during the anodic half-cycle. The major characterestic of nickel-voltammogramis the presence of an isopotential point (y) under all experimental conditions. This point is the intercept of the forward and backword branches of the voltammogram indicating same surface structure.(c) The copper-nickel alloys voltammograms are also characterized by the isopotential point (y). Its potential shifts to more noble direction as the copper content increased. There are five oxidation steps in the anodic half-cycle with no oxygen evolution. These steps correspond to formation of CuzS together with m-Ni(OH)z; p-Ni(OH)z; NiaSz; CuS together with NiS; and CuO together with Ni2+ ions and sulfer. These compounds are reduced under two cathodic peaks during the cathodic half-cycle giving copper and0-Ni(OH)z.(4) The corrosion rates of copper, nickel andcopper-nickel alloys were measured in sodium hydroxide in presence and absence of non-ionic surfactants as corrosion inhibitors, using potentiodynamic polarization and polarization resistance techiques. The corrosion rate of the tested electrodes increases in the order, Ni < Ni70 < Ni50 < Ni30< Cu. The presence of nickel reduces the rate of corrosion of copper through the improvement of its passive film characters. On the other hand, the presence of copper enhances the dissolution of nickel, due to their positions in the electromotove series.In the presence of non-inoic surfactants, the corrosion rates of all electrodes were reduced due to the blocking and hyDROPhobic effects of surfactants. The effeciency of the inhibitors increases as the number of ethoxylated groups increases. The degree of inhibition of the five electrodes was found to be decreased in the following order:Ni > Ni70 > Ni50 > Ni30 > Cu.Pitting corrosion was studied for all electrodes in sodium hydroxide solution as a function of chloride ion concentration and addition of organic inhibitors (aniline and p-hydroxyaniline), using potentiodynamic anodic polarization. It was found that pitting potential for the five samples decreases according to the following order:Cu > Ni > 30% Cu> 50% Cu > 70% Cu.This sequence reflects the high resistivity of pure metals to pitting attack than the alloys composed of them. In addition, the pitting potential varies linearly with the logarithm of chloride ion concentration according to:E. = a - b log CCL-The addition of aniline and P-hydroxyaniline as pitting corrosion inhibitors results in shifting of the pitting potentials for all electrodes to more noble direction. It wasfound that the effeciency of aniline is more than that of P-hydroxyaniline for all electrodes. In addition the inhibition effeciency of the two inhibitors at concentration 5X109 M, for the different electrodes are decreased in thefollowing sequance:Cu > Ni > Cu70 > Cu50> Cu30