铭酸溶液中金属铬电沉积的机理

从铬酸溶液中电沉积金属铬的机理曾经有过不少探索.许多研究者认为,阴极表面形成难溶的膜层金属铬的电沉积起着关键性的作用.本文应用交流阻抗谱研究金属铬电沉积过程中阴极表面膜的形成,利用X光电子能谱(XPS)和俄歇电子能谱(AES)分析了表面膜的组成,并且对SO_4~(2-)的作用进行了初步的探讨。

THE MECHANISM OF CHROMIUM ELECTRODEPOSITION FROM CHROMIC ACID SOLUTION

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 solutioncontaining sulfuric acid at potentials at which Cr can be deposited all show phenomenacharacterizing the formation of a surface film. AES and XPS tests also prove theexistance of a surface film and reveal the composition of the film being composedof oxide and hydroxide of trivalent chromium, chromate and a small amount ofsulfate. It is suggested that the surface film is essential for the deposition of Cr andthat sulfuric acid plays a key role in the formation of the surface film. It is likelythat sulfuric acid catalyses the reduction of Cr_2O_7~(2-) to Cr~(3+), causing the Cr~(3+) concen-tration and pH value of solution in the vicinity of the cathode surface to increaseand leading to the formation of a film on the cathode surface. This film changesthe 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.