The work described i n this thesis deals mainly with a fundamental study of two cementation reactions, i.e., Cu(II)/Zn and Co(II)/Zn, using the electrochemical techniques of linear potential sweep voltammetry, cyclic vol tammetry, chronopotentiometry (stripping), capacitance measurements,ring current measurements and impedance spectroscopy. In addition, solution analysis, x-ray diffraction and scanning electron microscopy were
employed to achieve a better understanding of the kinetics and mechanisms of the two cementations. The half reactions of the two systems were also investigated as an essential part of the whole study. Of the two systems investigated, the Cu(II)/Zn system, was mainly used as a reference system
on which some of the new techniques could be proved.
The Cu(II)/Zn reaction has been shown to be diffusion controlled. Five methods, including solution analysis, have been used to determine the rate of reaction. The values of the rate constants thus determined were in good agreement provided allowance was made for the nonstoichiometry of this reaction due to proton displacement by zinc.
The Co(II)/Zn cementation reaction, though thermodynamically highly
favourable, is kinetically very sluggish. Of the factors affecting this reaction, the solution pH was found to play a substantial role. For example, while the reaction is largely under chemical control, at pH 5 the reaction rate becomes limited by the speed at which cobalt(I1) ions diffuse to the reacting metal surface. The favourable influence of a higher pH on the reduction of cobalt(II) carried out either electrochemically or chemically using zinc powder was ascribed to the increasing formation of the electrochemically active species, CoOH+ . Increasing the temperature also favours the shift from chemical control to diffusion control.
Values for the rate constants of the Co(II)/Zn reaction in the absence of added zn2+ ions were determined from Evans' diagram, chronopotentiometry and impedance spectroscopy, The reasons for the differences in the values of rate thus obtained were discussed.
The presence of zinc ions in the reaction solution greatly suppressed the rate of Co(II)/Zn cementation reaction. Several factors have been identified as contributing to the reduced reaction rate. These are the double layer effect, zinc ion and hydrogen atom adsorption, and precipitation of basic zinc salts which blanket the reacting metal surface.
The action of compounds such as Sb(III) and As(III), which are commonly used to activate the Co(II)/Zn reaction in the presence of large amounts of zn2+, was investigated mainly by impedance spectroscopy. From the similarity of the impedance spectra for the reaction i n the presence of
benzoqui none, it was inferred that one role of the activators was to suppress the formation of adsorbed hydrogen on the reacting surface and to allow a higher concentration of the active species, COOH'.
| Identifer | oai:union.ndltd.org:ADTP/221740 |
| Date | January 1987 |
| Creators | Xiong Jiang |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Xiong Jiang |
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