Studies On Technology And Mechanism Of Non-Cyanide Electrodeposition Silver

Silver has good chemical stability, electric conductivity and solderability. In view of their excellent properties, silver is a metal electroplated for a wide variety of applications both decorative and functional. Industrial electroplating of silver is conventionally carried out from cyanide solutions containing silver as its cyanide complex metal. However, cyanide compounds have strong toxicity and a large amount of cost is required for securing safe working conditions and waste treatment. Thus, an extensive search has been made for satisfactory alternative electrolytes.In the present work, 5,5-dimethyl hydantoin non-cyanide silver electrodeposition bath was choosed as the research bath. To improve the disadvantage of the process, conductive salt and additive of the bath were studied. Based on the single factors study, the optimal composition formula was obtained by an orthogonal method. And with the optimal formula, the influence of bath composition and operating conditions on the properties of solution and coatings was studied. And the result showed the optimal electrolyte composition and operating conditions were : silver nitrate 30g/L , 5,5-dimethyl hydantoin 100g/L , potassium carbonate 80g/L , potassium pyrophosphate 40g/L ,Temperature 40℃,pH value11,current density 0.4A/dm2.To improve silver coatings, pulse plating process was investigated. Based on the optimal composition and technique conditions by the orthogonal test, the effect of pulse on time, pulse off time and pulse current density on silver coatings was studied particularly and the preferred pulse plating parameters is:the pulse frequency 40%,the duty cycle 3ms,the average current density 0.6A/dm2。It has been found by examinations on dispersing power, throwing power of bath and surface morphology , tarnish resistance power and binding energy of silver films that this technics has the same or better effects than cyanide techics, and fitted for industry。The electrodeposition process from DMH non-cyanide silver electrodeposition bath was investigated by CV, Cathodic polarization curves, Tafel curves and chronoamperometric methods, and the influence of additive on the silver deposition mechanism was also discussed preliminary. The results indicated that the DMH is stabile in the range of measurement region. The results of CV curves indicated that the deposition of metallic silver in DMH electrolyte is a diffusion-controlled and reversiable growth process. It also can be observed from CV curves that a typical hysteresis loop existed and the additives have obvious inhibiting effect on the silver discharge process. The results of Tafel curves showed the additive decrease the exchange current density values and electrode reaction rate constant. Chronoamperometric results indicated that the electrodeposition process of silver at GC electrode belongs to a three-dimensional progressive nucleation mechanism and the additives didn't change the nucleation mechanism, but inhibits the crystal epitaxial growth.