Acta Phys. -Chim. Sin. ›› 1992, Vol. 8 ›› Issue (04): 571-574.doi: 10.3866/PKU.WHXB19920430

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The Mechanism of Chromium Electrodeposition From Chromic Acid Solution

Tan Qi-Xian; Liu Shu-Lan; Cheng Dan-Hong; Guo He-Tong   

  1. Department of Applide Chemistry, Tianjin University, Tianjin 300072
  • Received:1991-02-28 Revised:1992-03-20 Published:1992-08-15
  • Contact: Tan Qi-Xian

Abstract: Electrodeposition process of metallic Cr from chromic acid solution was investigated with techniques of AC impedance, polarization curve, AES and XPS. The Nyquist plots and polarization curves of Fe electrode in the chromic acid solution containing sulfuric acid at potentials at which Cr can be deposited all show phenomena characterizing the formation of a surface film. AES and XPS tests also prove the existance of a surface film and reveal the composition of the film being composed of oxide and hydroxide of trivalent chromium, chromate and a small amount of sulfate. It is suggested that the surface film is essential for the depositon of Cr and that sulfuric acid plays a key role in the formation of the surface film. It is likely that sulfuric acid catalyses the reduction of Cr_2O_7~(2-) to Cr~(3+), causing the Cr~(3+) concentration and pH value of solution in the vicinity of the cathode surface to increase and leading to the formation of a film on the cathode surface. This film changes the state and nature of the cathode surface, forces the potential of the cathode to shift to more negative values, and ultimately promotes the reduction reaction of chromate ions to metallic chromium.

Key words: Metallic chromium, Electrodeposition, Chromic acid solution