Electrochemical behaviour of chromium–graphene composite coating

Abstract

This work illustrates the role of graphene in enhancing the corrosion resistant property of chromium-graphene composite coating when compared to the corrosion resistant property of pure chromium coating containing ZnO nanoparticles. Chromium based coatings have been used extensively due to their various useful attributes such as corrosion and wear resistance and enhanced surface finish. In this work, Cr and Cr-graphene composite coatings were electrodeposited over mild steel substrate using Cr(III) plating bath. To deposit the coatings, electrodeposition method was also adopted due to its attributes such as low cost, less equipment intensive methodology, accuracy of reproduction and suitability for large scale surface modification. Three different coatings were produced: (a) Cr coating containing ZnO nanoparticles (C), (b) Cr coating containing ZnO nanoparticles deposited using formic acid (C F ) and (c) Cr coating containing ZnO nanoparticles and graphene deposited using formic acid (C FG ). Graphene used in the deposition process was produced from the electrochemical exfoliation of graphite in sodium lauryl sulphate (SLS) electroactive media. ZnO nanoparticles were synthesized using the precipitation method followed by calcination. Microstructural charaterization of the coatings revealed that the coating 'C' contained large cracks. Addition of formic acid (in the coating C F ) noticeably reduced the cracks in the coating which now also contained hillock structures. Addition of graphene (in coating C FG ) further enhanced the coating 2 morphology which now contained negligible cracks and increased hillock structures completely covering its surface. Diffraction analysis revealed that the addition of graphene also altered the texture of the chromium coatings. Potentiodynamic polarization and electrochemical impedance spectroscopic analysis revealed that the change in the morphology and microstructure of the deposit due to the addition of graphene substantially enhanced the corrosion resistance of the C FG coating when compared to both C and C F coatings.Hard chromium coatings are extensively applied due to attributes such as mechanical strength, 1 wear and corrosion resistance, 2 and ability to provide decorative appearance to tools and machineries etc. 3 Generally, chromium plating bath containing Cr(VI)/hexavalent chromium ions is used to deposit chromium. 4 Carcinogenicity and serious environmental problems posed by the hexavalent chromium ions has encouraged development of eco-friendly trivalent chromium/Cr(III) plating baths 5 for depositing chromium coatings. It was however observed that the chromium coatings produced from trivalent chromium bath contained cracks which deteriorated the ability of the coating towards protecting the degradation of the underlying substrate. 2,6 Cracks can be reduced by modifying the morphology of the deposit by altering the deposition parameters or using additives. 7 Danilov et al 8 and Surviliene et al 9 produced crack free chromium coating using trivalent chromium plating bat...