Electrochemical Studies on the Cementation of Copper and Cobalt with Zinc

The work described i n t h i s thes i s deals mainly w i t h a fundamental study of two cementation react ions , i . e . , Cu(II)/Zn and Co(II)/Zn, using the electrochemical techniques of l inear potential sweep voltammetry, cycl i c vol tammetry, chronopotentiometry ( s t r ipp ing) , capacitance measurements, r ing current measurements and impedance spectroscopy. In addi t ion, solution analysis , x-ray d i f f rac t ion and scanning electron microscopy were employed to achieve a be t te r understanding of the kinet ics and mechanisms of the two cementations. The half reactions of the two systems were a l so investigated as an essent ia l par t 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 control led. Five methods, including solution analysis , have been used to determine the r a t e of reaction. The values of the r a t e constants thus determined were i n good agreement provided allowance was made f o r the nonstoichiometry of t h i s reaction due t o proton displacement by z inc , The Co(II)/Zn cementation reaction, though thermodynamically highly favourabl e , i s k ine t ica l ly very sluggish. Of the fac tors affect ing t h i s react ion, the solution pH was found to play a substantial ro le . For example, while the reaction i s largely under chemical control, a t pH 5 the reaction r a t e becomes limited by the speed a t which coba l t ( I1 ) ions d i f fuse to the reacting metal surface. The favourable influence of a higher pH on the reduction of cobal t ( I 1 ) carr ied out e i t he r electrochemically or chemically using zinc powder was ascribed t o the increasing formation of the e lect rot chemically act ive species , CoOH . Increasing the temperature a lso favours the s h i f t from chemical control to diffusion control . Values f o r the r a t e constants of the Co(II)/Zn reaction i n the absence of added zn2+ ions were determined from Evans' diagram, chronopotentiometry and impedance spectroscopy, The reasons f o r the differences in the values of r a t e thus obtained were discussed. The presence of zinc ions in the reaction solution great ly suppressed the r a t e of Co(1 I)/Zn cementation react ion. Several fac to rs have been ident i f ied as contributing to the reduced reaction r a t e . These a re the doubl e 1 ayer e f f e c t , zinc ion and hydrogen atom adsorption, and precipi ta t ion of basic zinc s a l t s which blanket the reacting metal surface. The action of compounds such as Sb(II1) and A s ( I I I ) , which a re commonly used t o ac t i va t e the Co(II)/Zn reaction i n the presence of large amounts of zn2+ , was investigated mainly by impedance spectroscopy. From the s imi la r i ty of the impedance spectra f o r the reaction i n the presence of benzoqui none, i t was inferred t h a t one ro le of the ac t iva tors was to suppress the formation of adsorbed hydrogen on the reacting surface and to allow a higher concentration of the act ive species, COOH'.