Influence of Pulse-Current Parameters on pH Measured By Local Method

Abstract

The electrodeposition process often comes with the reduction of protons at the cathode interface. In addition to the creation of dihydrogen bubbles, this reaction leads to an increasing of the pH at the interface vicinity, which can induce problems such as metallic hydroxides generation and precipitation. Several electrolytes are concerned by this phenomenon, but this is particularly crucial for trivalent chromium electrolytes for which faradic efficiency remains very low. Moreover, trivalent chromium ions must be complexed to be reduced and the pH value influences complexation equilibrium. To limit the local pH variations, pulse current sequences with scare waves can be used. In fact, periodically reversing the polarity of the current and/or applying off times lead to the double layer discharge and counterbalance the effect of pH increase. To have a better comprehension of the current parameters influence on the surface pH variations, it is necessary to measure this value. Local pH increasing have already been measured in several electrolytes with direct current using a rotating pH electrode and metallic meshes as cathode [1]. The method used in this study is totally different, based on micro-capillary pH electrodes (10 µm inner diameter) [2]. The micro-capillaries composing electrodes, are firstly silanized, then filled with a buffer solution, except in its sharpened end, which is filled with an ionophore resin synthesized for this purpose and acting as a selective proton membrane. Electrode position is managed by a three-dimensional micromanipulator and by two cameras. Its calibration indicates a Nernstian response on a given pH range, systematically tested before and after each measurement. The local pH measurements are realized during the relaxing phases, just after turning off the current, to avoid perturbations of the electrode potential induced by the electromagnetic field existing during the polarization. The local pH is measured until the bulk pH value is reached. Measurements have been done on simple copper sulfate and on trivalent chromium electrolytes. The results show that turning on the current, pH close to the interface grown of several units. It has been also shown that using current modulation, compare to direct current, have an impact on the pH increase. To support the results found using micro-capillary pH electrodes, a micro-picking system has been developed. The process consists on picking samples at the cathode vicinity surface and to measure its pH thanks to pH paper observed with a microscope. [1] H. Deligianni et L. T. Romankiw, « In situ surface pH measurement during electrolysis using a rotating pH electrode », IBM Journal of Research and Development, vol. 37, no 2, p. 85-95, mars 1993. [2] A. Q. Vu, B. Vuillemin, R. Oltra, et C. Allély, « Cut-edge corrosion of a Zn–55Al-coated steel: A comparison between sulphate and chloride solutions », Corrosion Science, vol. 53, no 9, p. 3016-3025, sept. 2011. Acknowledgments : the authors would like to thanks IRT M2P, project CRONOS 2024 for its financial support Figure 1