Preparation and characterization of chromium deposits obtained from molten salts using pulsed currents

Abstract

The electroplating of chromium from fused chloride electrolytes was investigated. The experimental conditions were defined taking into account the mechanisms of the electrochemical reduction of CrCl₂ and of the chromium nucleation and electrocrystallization phenomena. Chromium was plated on various substrates from concentrated LiCl–KCl–CrCl₂ (600 to 800 mol m⁻³ CrCl₂) electrolyte. Direct or pulsed current electrolysis were carried out under a dry argon atmosphere in the 400 to 440 °C temperature range. The shape of the current signals was chosen, taking into account the chromium electrocrystallization phenomena onto a foreign substrate, so as to obtain well-defined structures for the chromium layers. The chromium deposits were characterized by SEM and EDX analysis, and by microhardness determination. Uniform chromium electroplates of high purity, high adherence with no cracks, were obtained by using pulsed current: signals with cathodic pulses and open-circuit periods preceded by cathodic pre-pulses. With this current shape, the mean rate of the chromium electroplating process remained lower than 10 μm h⁻¹. However, using a repeated of periodic cathodic pre-pulse/cathodic pulse/anodic pulse/open circuit sequences, the growth rate of compact chromium layers increased to 100 μm h⁻¹ or more.