Cathodic Film formation during Chromium Electrolysis on Low-carbon Steel using short duration current steps

Abstract

Short duration galvanostatic electrolyses of chromium plating solutions (1.0 M CrO₃ and 0.01 M H₂SO₄) were performed on rotating disk electrode of low-carbon steel. The influence of sulphate concentration (0.01 and 0.02 M), cathodic current density (30, 60 and 90 A dm⁻²), temperature (25, 40 and 55 °C), electrode rotation rate (25 to 1000 rpm) and Na₂SiF₆ addition (0.01 M) on E/t curves and cathodic current efficiency was investigated. Electrical charge was always consumed to form a cathodic film and effective metallic chromium deposition only occurred when the potential reached a low and stable value. The determination of cathodic current efficiency after short duration electrolyses corroborated these results. The time required to form the cathodic film decreased with current density and addition of Na₂SiF₆ and increased with sulphate concentration, temperature and electrode rotation rate, suggesting that this film is not completely compact. A passive film is formed at the onset of the electrolysis process and a viscous layer is formed on the surface of the first film.