The Effect of Pulse-Currents on Zinc Coatings: Determination of Operating Parameters By Transient Study in Zincate Electrolytes

Abstract

A crucial economic problem resides in the long-term protection of metallic parts from atmospheric exposure. A simple way to ensure their corrosion resistance is the application of sacrificial zinc coatings, which may be obtained by galvanization or electrodeposition. Several zinc electrolytes may be used, depending on the substrate properties and the geometrical constraints of the piece to plate. To avoid to large gaps in thickness distribution, alkaline baths are favored with zincate electrolytes and organics additives. The use of additives is a necessity for brilliance, cover ratio or even hardness properties of the coatings. The zincate baths are also an often-used base for numerous alloys electrolytes. In this configuration, pulse currents appear to be an appealing way to restrain the use of organics additives while keeping the required functional properties. Linear sweep voltammetry in pure zincate electrolyte shows that the hydrogen reduction starts at high potentials, before the zinc reduction peaks take place. This means that in plating conditions, competition between hydrogen generation and metallic reduction will be always presents. After studying the main electrochemical properties of the system (reversibility, diffusion coefficients, heterogeneous kinetic constant…), it has been possible to calculate estimating times by Sand equation for hydrogen discharge at the end of the Faradic plateau [1]. This allows the choice of a time line for simple pulses parameters (Time-on, Time-off, cathodic peak current). Then, transient curves have been plotted between 2 and 500A/dm², and it is possible to observe the transition between Faradic plateau and the step of potential showing a depletion of the electrochemical active species close to the interface. This study is also realized at different cathodic peak current densities, while keeping the same average current density by adjusting Time-off (see fig.1). This is also repeated for different Time-on, again at the same average current density. A parallel was also made with the results of the early stages of deposit growth, by the determination of germination parameters in the Scharifker and Mostany equation [2]. This helps in the comprehension of pulse process as the repetition of germination steps. Coatings were elaborated for various pulse sequences deduced from this study, corresponding to extreme situations: -short pulses at low current densities -long pulses at low current densities -short pulses at strong current densities -long pulses at strong current densities Coatings were fully characterized (hardness, microstructure, EDS…) [1] B. Pollet, J. P. Lorimer, J.-Y. Hihn, F. Touyeras, T. J. Mason, et D. J. Walton, « Electrochemical study of silver thiosulphate reduction in the absence and presence of ultrasound », Ultrasonics Sonochemistry, vol. 12, no 1-2, p. 7-11, janv. 2005. [2] B. R. Scharifker et J. Mostany, « NUCLEATION WITH DIFFUSION CONTROLLED », p. 11. Acknowledgments : the authors would like to thank the IRT M2P project ATLAS for its financial contribution Figure 1