Effects of iron ion contents on composition, morphology, structure and properties of chromium coatings electrodeposited from novel trivalent chromium sulphate electrolyte

Abstract

Chromium coatings were electrodeposited from a novel trivalent chromium sulphate electrolyte using oxalate as the complexing agent. Energy dispersive spectroscopy, Scanning electron microscopy, X-ray diffraction, Tafel curve, and X-ray photoelectron spectroscopy techniques were used to characterise the compositions, surface morphologies, structures, corrosion resistances, and element chemical states of the coatings. The effects of Fe²⁺ ions were investigated in detail. Results show that the small amount of iron ions generated coatings containing iron. The coating obtained is actually a Cr–Fe alloy of solid solution structure and microcrack surface morphology. X-ray photoelectron spectroscopy study revealed that a thin Cr(III) oxide film was formed on the surface of the coatings, and Cr(s) and Fe(s) states were observed within the coatings. As Fe²⁺ ion contents were increased in the electrolyte, the iron content of the coatings significantly increased. The electrodeposited coatings exhibited a crack-free surface morphology when Fe weight content exceeded 30·1%. The coating with 26·3 wt-%Fe had an amorphous structure and the lowest electrochemical activity.

Keywords:

• Trivalent chromium
• Chromium plating
• Iron ion
• Sulphate
• Coating
• Characterisation

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant no. 20873114, no. 20833005 and no. 21021002), the National Basic Research Program of China (grant no. 2009CB930703), and the Project of Fujian Province, China (development and reform commission document, 2010, no. 299).