Balakrishnan, K. (1975) Studies on the Kinetics of oxygen reduction and metal dissolution – A Ring-Disc Study of Copper and Brass Systems. PhD thesis, Madurai-Kamaraj University.

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The mechanism of anodic dissolution of copper and brass and also that of cathodic reduction of oxygen at copper and brass have been examined using the rotating ring-disc electrode under potentiodynamic conditions in solutions of sodium chloride, sodium sulfate, sodium nitrate, ammonium chloride and ammonium sulfate. Even though the dissolution of copper and brass in electrolytes such as sodium sulfate and sodium chloride have been studied by a number of workers, many of these investigations were carried out either potentio dynamically under stationary electrode condition or with rotating electrodes under galvanostatic conditions. Hence, in the present work, the dissolution of copper and brass in the solution mentioned above has been studied with rotating ring-disc electrodes under potentiodynamic conditions with very slow sweep rates to understand the influence of anions and ammonium ion as well as that of oxygen on the kinetics of anodic dissolution of copper and brass (with particular reference to dezincification). Copper exhibits an active-passive-active behavior in chloride solutions both in the presence and absence of ammonium ion. The potential characteristics of this region depend on the speed of rotation, concentration of chloride and also the pH of the solution in the range 1.0 to 2.7 in the case of sodium chloride and also on the concentration of ammonium ion in the case of ammonium chloride. It is proposed that, in sodium chloride solutions, a film consisting of basic cupric chloride is formed lending to passivation of the surface. At constant chloride ion activity, the increase in concentration of ammonium ion increases the dissolution rate and the possible mechanism of the same has been discussed both interms of complex formation and/or changes in the stability domain of basic cupric chloride. At high anodic potentials, current oscillations are observed in sodium chloride solution due to formation and dissolution of passive film, probably consisting of a basic cupric chloride. In the active region which is diffusion in character near about the limiting current region, copper essentially dissolves as monovalent copper in neutral and acidic chloride solutions. In ammonium chloride, ammonium sulfate and acidic sulfate solutions, the amount of Cu+ formed is very small. In the active region, while the reaction order with respect to chloride ion is 1, in the case of ammonium ion, no simple relationship is obtained between log I and log NH4+ indicating a change in the mechanism both with respect to the change in concentration of NH4+ ion and potential. Tha anodic dissolution of copper is independent of pH in the range 1.0 to 3.0 in the case of sulfate solution over the entire range whereas it is so only upto a potential of – 40 mv in the case of acid chloride solution. However in alkaline chloride and sulfates, copper gets passivity through formation of Cu2O and CuO or Cu(OH2). The behaviour of copper in mixtures of sodium chloride and sodium sulfate is quite different from that in individual salts. The studies on brass indicate that the mechanism of dissolution depends on the potential range as well as on the composition of the medium. The regions of preferential dissolution of zinc and simultaneous dissolution of copper and zinc have been identified from a comparison of the potential at which brass starts dissolving, and potential at which pure copper dissolves and potential at which copper is detected at the ring in the case of brass dissolution. The region of preferential dissolution is succeed by a region of a limiting current, at the end of which copper also starts dissolving. The limiting current is ascribed to the limiting diffusion of zinc through a barrier layer consisting of porous copper formed as a result of dezincification. In alkaline chloride solution, the preferential dissolution takes place both in the region of more negative potentials, (-510 to -220 mV) and in the region of positive potentials (+100 to +580 mv). In neutral sodium sulfate solution, the region of preferential dissolution lies between -380 and -10 mV. In chloride solutions, it lies between -400 and -190 mV in neutral and -400 and -230 mV in acidic sodium chloride solutions and between -480 and -370 mV in the case of ammonium chloride solution. In the region of simultaneous dissolution, copper and zinc dissolve in the atomic ratio in Na2So4 and acidic sodium chloride solutions. In neutral sodium chloride and ammonium chloride, it is not in the atomic ratio and it is observed that the surface itself is covered with copper. In the region before the limiting current, the dissolution of brass is under diffusion control in sodium chloride, whereas it is significantly so only at potentials greater than +100 mV in sodium sulfate solution. The increase in concentration of ammonium ion, chloride ion and sulfate ion increases the rate of dissolution. However, the effect of the change in the hydrogen ion concentration in the region pH 1.0 to 2.8 depends on the potential region. In alkaline chloride and alkaline sulfate solution, brass exhibits three maxima corresponding to dezincification, Cu2O and CuO formation and undergoes essentially dezincification. Oxygen inhibits dissolution of copper and brass and also affects the characteristics of the region of the limiting current. Dezincification also is suppressed in presence of oxygen supporting the fact that only a restricted supply of oxygen favours dezincification. Oxygen reduction at copper and brass electrodes proceeds in two steps in neutral and alkaline solutions where H2O2 is detected at the platinum ring and in a single step in the case of acid solutions. The reduction current is higher on an oxide-free surface than on an oxide-covered surface. The tafel slopes obtained by plotting log i/i2-I vS. E for different solutions range from 65 mV to 240 mV depending upon the medium. The diagnostic plots of Id/Ir Vs. w-1/2 indicate that, on copper, in sodium sulfate solution. H2O2 is produced during oxygen reduction in parallel path and does not react further, whereas in ammonium sulfate, it is produced as an intermediate. The rate of oxygen reduction is the highest in ammonium sulfate solution both on copper and brass, whereas it is the lowest in NH4CL in the case of copper. In sodium sulfate and sodium chloride solutions, the rate of oxygen reduction is higher on copper than on brass, though in ammonium sulfate and ammonium chloride solutions, the difference is not much. In the case of sodium nitrate solution, the total reduction of nitrate and oxygen is much higher on copper than on brass indicating increased corrosion of copper when compared to brass in nitrate solution. This was also confirmed by direct corrosion tests.

Item Type: Thesis (PhD)
Uncontrolled Keywords: metal dissolution; copper and brass; oxygen reduction
Subjects: Corrosion Science and Engineering
Depositing User: Dr. N Meyyappan
Date Deposited: 11 Jun 2012 09:20
Last Modified: 11 Jun 2012 09:40

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