Cr(VI) ion reduction reaction on nickel and stainless steel electrodes in acid medium

Abstract

The reduction of Cr(VI) ion in 0.5 M H2SO4 deaerated solutions with different concentrations of K2CrO4 on nickel and 18Cr-10Ni stainless steel electrodes have been studied by potentiostatic polarisation technique depending on the chromate concentration in solution. The kinetic parameters -Tafel slope, charge transfer coefficient and limiting current density -were evaluated comparatively. For the solutions containing ≤5 mM K2CrO4 the Tafel slopes are higher on stainless steel than on nickel (around -135 mV·dec-1 for nickel and -165 mV·dec-1 for stainless steel). Moreover, by increasing the chromate concentration the Tafel slopes increase on nickel and decrease on stainless steel. The low Tafel slope values of -75 mV·dec-1 obtained on stainless steel electrode in the potential region -0.5÷ -0.75V vs. SCE could be due to a decrease of the charge transfer process rate because of the presence of a physical barrier. The values for the reaction rates are slightly higher for nickel than for stainless steel electrodes on all the studied potential range. The reaction product is soluble trivalent chromium, excepting the solution with 500 mM K2CrO4, when a Cr(OH)3 film may cover the electrode surface that determines the decrease of the reaction rate.