Passivation treatments for metals using group via metal oxy-anions
thesisposted on 19.12.2012 by Dogara Bijimi
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The literature on the surface treatment of zinc and tin in aqueous solutions containing sodium chromate, sodium molybdate and sodium tungstate has been reviewed. A search for suitable alternatives to chromate due to its toxicity level has been established. A. review on methods of film analysis has also been completed. Zinc was anodically passivated in aqueous solutions containing 0.01 - lM sodium chromate, sodium molybdate and sodium tungstate maintained between pH 5 - 11 and operated at temperatures in the range of 20 - 600C by the application of an external current for 3V overpotential. The effects of solution concentration, process pH and temperature variation on passivity have been established. It has been found that passivity is favoured between 0.1 - lM oxy-an.ion concentration and pH 7 - 11. Chromate treatment favours room temperatures whereas molybdate and tungstate treatment are best at 60oC. Coulometric analysis was used to estimate the chromium content and film composition of chromate films on zinc in which it was found that the chromium content increases with the corresponding increase in concentration, process pH, process potential or a fall in the treatment temperature...
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