The electrochemical study of a chromium plating bath. II. Chromium metal and surface film formation

The investigation of the electrode reactions occurring at metal and vitreous carbon cathodes in the standard chromium electroplating solution, 200 g dm^?3 CrO₃ and 2 g dm^?3 H₂SO₄, has been continued and this paper considers the mechanism of deposition of metallic chromium and the role of surface films in this process. It is confirmed that chromium deposition occurs at potentials negative to ?1.6 V versus Hg/Hg₂SO₄ and under galvanostatic or potentiostatic conditions, the current efficiency is 30–45%. Moreover at a vitreous carbon electrode, potential step experiments lead to risingI-t transients characteristic of instantaneous nucleation and three dimensional phase growth. At potentials just prior to metal deposition the potential sweep and step experiments show clear evidence for the formation of a strongly passivating film (this may be in addition to an existing less passivating layer). The important of the film varies with H₂SO₄ concentration and at intermediate concentrations, potential step experiments lead to unusual oscillatingI-t transients.