Corrosion behaviour of Cr coatings deposited from Cr(III) formate-urea electrolytes

Abstract

Chromium coatings were electrodeposited from trivalent chromium electrolytes with optimised additions of complexing agents to produce a good decorative appearance. The effect of hydrazine and hydroxylaminophosphate on the corrosion behaviour of Cr deposited from Cr(III) formate-urea electrolyte has been studied. The corrosion behaviour of the coatings was studied through the modelling of EIS spectra, recorded at different times of exposure to an acidic sulphate solution (pH 2·6), using various equivalent circuit models. The parameters obtained for circuit elements reflecting the corrosion behaviour of the samples at the solution/electrode interface were interpreted and correlated with the microstructure of coatings. The data obtained suggest the presence of open pores in the coatings from the onset of immersion. The samples plated in the basic formate-urea bath showed the highest corrosion rate, while the samples plated in the formate-urea bath containing hydrazine showed the lowest one. The enhanced corrosion resistance of Cr coating deposited in the presence of hydrazine may result from the change in microstructure, namely the size and number of pores (i.e. the porosity).