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Structural and physical properties of chromium layers obtained by pulse plating techniques

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Chromium plating has successfully been used since 1925, representing one of the most important processes in the galvanic industry due to its unique combination of wear, hardness and corrosion properties at low cost. Chromium electrodeposition is widely used in automotive, aerospace, industrial and general engineering applications for both decorative and functional coatings [1, 2]. Two types of chromium coatings can be classified according to their thickness: decorative chromium (0.80 μm) and functional or hard chromium (20÷500 μm). The most common hard chromium electrolyte is constituted by chromium oxide solutions containing a low concentration of one or more catalysts, usually sulphate. The current efficiency of chromium electrodeposition is low, using only 10÷15% of the applied current: the majority of the electric charge is consumed by the hydrogen evolution reaction (80÷90%) and 5% of the current is used in the reduction of hexavalent chromium to trivalent chromium species. However, and in spite of its low current efficiency and the carcinogenity and toxicity of hexavalent chromium, this process is not currently replaceable due to the excellent properties of the coatings obtained [3]. A key factor to modify the properties of a coating is to focus on its microstructure. Thus, it is well known that there is a relationship between the processing parameters and the microstructure, morphology and properties of the coatings. Different approaches can be undertaken to modify the microstructure of a coating. In this sense, pulse plating techniques are widely used in the electrodeposition of metals and alloys due to their reported influence into metallic distribution, efficiency, microstructure, morphology and coating properties [4÷6]. In pulse plating the current is interrupted or alternated on a short time scale during the metal electrodeposition, offering a higher number of variables comparing to direct current proces[...]

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