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...
A Master's Thesis submitted in partial fulfilment of the requirements for the award of Master of Philosophy of Loughborough University.