Pulsed current with square waves are well-known to increase the properties of coatings obtained by electrodeposition. It was observed that either by periodically reversing the polarity of the current or by applying off times, deposits of improved properties can be observed [1]. This concerns for example a much finer grain size as well as reduction of hydrogen embrittlement. The strategy to be implemented for pulse parameters choice should be made carefully. If hydrogen discharge occurs only in case of limitation of metallic species diffusion, the study of transient curves (potential vs time as a response of a current pulse) highlighted the presence of a faradic range which allows the determination of the best cathodic parameters (pulse time and current density) as well as off-time duration. The situation where hydrogen discharge occurs at potentials less cathodic than the metallic species reduction makes it less straightforward. It is necessary to use reverse pulses, i.e. applying an anodic current during the pulses sequence, to obtain beneficial effects because they contribute to a rearrangement of the electrolyte layer close to the electrode [2]. For example, the reverse pulses lead to the double layer discharge, and counterbalance the effect of pH increase due to proton reduction during the electrolysis. This last phenomenon is detrimental for the electrochemical species present in complexed form, because it frequently results in a precipitation of hydroxides which will be included as contaminants in the coating. Thus, the measurement of local dependence of the pH at the electrode vicinity (around 100µm) is interesting, and was performed using micro-capillary pH electrodes (10µm inner diameter) [3]. After silanization, a capillary was filled with a buffer solution, except in its sharpened end, which was filled with an ionophore resin synthetized for this purpose and acting as a selective proton membrane. This pH electrode was fixed on the stage of a three-dimensional micromanipulator. Calibration of the pH electrode indicated a Nernstian response on a given pH range, systematically tested before and after each measurement.
Local pH measurements in the vicinity of a steel electrode surface in unstirred conditions were performed during the relaxation phase after a polarization period of 1 min, in order to avoid any perturbation of the polarization on the potential of the pH electrode. The local pH was measured until the bulk pH value was reached. First results obtained with high yield electrolytes such as nickel or gold solutions confirm the dependence of the pH value with the modulation of current in pulsed mode and additives presence.
[1] F. Lallemand, L. Ricq, E. Deschaseaux, L. De Vettor, P. Berçot, Electrodeposition of cobalt-iron alloys in pulsed current from electrolytes containing organic additives, Surf. Coat. Technol., 197 (2005) 10–17.
[2] J. Rolet, M.P. Gigandet, S. Lallemand1, P. Bercot1, J.Y. Hihn; Effect of pulsed current parameters on the microstructure and properties of deposits for hard chromium electroplating from trivalent electrolytes (to be submitted)
[3] A.Q. Vu, B. Vuillemin, R. Oltra, C. Allély; Cut-edge corrosion of a Zn–55Al-coated steel: A comparison between sulphate and chloride solutions, Corrosion Science 53 (2011) 3016–3025
