ELECTRODEPOSITION KINETICS AND THE SURFACE MORPHOLOGY OF CADMIUM AND CADMIUM-ORGANIC COATINGS CONTAINING ε-CAPROLACTAM
Abstract
Metal polymeric coverings are of special interest as a new class of materials that have the properties of both metals and polymers. Unfortunately, information about the similar coverings electrodeposition and the mechanism of transformation taking place in a solution or at electrode surface is quite limited. We investigated the influence of caprolactam (CPL), tetrahydrofuran (THF) and propylene carbonate (PC) additives on the kinetics of cathodic processes in the cadmium sulfate electrolyte, microstructure and the grain size of metal and metal–polymeric deposits. The addition of cyclic lactam changes adsorption and coordination properties of the electrolyte components, the type of cathodic process and leads to a considerable improvement in their structure and physical-mechanical characteristics.
Using chronopotentiometry analysis with impedance data, it was found that the real electroactive species formed in aqueous–organic mixtures determine the electrodeposition kinetics due to complexation of discharging metal ions with adsorbed molecules of additives. It was established the e-caprolactam addition resulted in the growth of cathodic polarization DE the value of which depends on the volume concentration coL
According to the polarization and impedance measurements, an increase in coL intensifies the participation of organic molecules, first in the adsorption process at the Cd-cathode (coL ≤ 0.1 M) and then in the complexion with cadmium ions in the solution volume (coL > 0.1 M). In the latter case, elecroactive particles that are formed in the solution take place in electrodeposion reactons which leads to increased CPL content in the Cd-covering. The influence of THF and PC results in a slight increase of exchange current and coefficient bc and a decrease of diffusion coefficient D and transfer coefficient α.
The codeposition of metal with polymer leads to a significant decrease in the grain volume and size of metal inclusions. A comparison of the 3D scans of the surface of the obtained coatings verified the formation of the cadmium based metal-polymer coatings in presence of CPL. Varying the electrolyte composition and technological conditions of plating makes it possible to control both the metal–polymer ratio in the coating and the grain sizes of the metallic phase in the formed metal–polymeric films.
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