Electrochemical behaviour of amorphous electrodeposited chromium coatings

Results obtained by studying the electrochemical characterisation of the passive dissolution and corrosion behaviour of amorphous chromium coatings are reported and discussed. An electrolyte based on trivalent chromium, complexed with glycine, was used in conjunction with a high speed flow electrodeposition cell. The structure, morphology and elemental analysis of chromium coatings deposited on copper substrates using various bath variables (bath temperatures of 30 and 60 degrees C; current densities of 0.3-1.8 A cm(-2); and flow rates of 0.2, 0.3 and 1.4 m s(-1)) were investigated by XRD and SEM/EDS. As a network of micro-cracks was found on all the amorphous chromium deposits running from the deposit surface to the copper substrate, mild electrochemical passivation conditions were applied to avoid the copper substrate being subjected to corrosive attack. Potentiodynamic and potential step chronoampero metric curves were recorded after vigorous cathodic activation in de-aerated sulphuric acid solution. The corrosion potentials of the Cr coatings, after a 2 h immersion time were established to be around -0.3 V vs. SCE. The evaluations of the impedance spectra measured at the quasi-stationary corrosion potential, E-corr were based on complex nonlinear least-squares (CNLS) fit to an equivalent circuit model of the corrosion kinetics in passive state. The polarisation resistance R-p of the overall corrosion process showed differences in the function of the deposition parameters, as the surface morphology and irregularities changed. The R-p values of amorphous chromium (a-Cr) deposits in passive state compared to the self-passivated crystalline chromium (c-Cr), revealed that the bright a-Cr coatings deposited at 60 degrees C using a medium flow rate and a high current outperform the corrosion protection ability of the c-Cr. (C) 2012 Elsevier B.V. All rights reserved.