Hardness and corrosion resistance of Zn−Mn/Al2O3 composite coatings produced by electrochemical deposition

Abstract

The aim of this study is development and characterization of the novel Zn-Mn/Al2O3 composite coatings. The coatings were electrodeposited with constant current density on steel, from chloride solution, without any commercial additives. The Zn−Mn alloy coatings that show high corrosion resistance, may be used in future as an alternative to cadmium coatings. However, the pure Zn and Zn alloy coatings are characterized with poor mechanical properties. Therefore, in this work the alumina particles were incorporated into the Zn-Mn matrix, and the hardness and corrosion resistance of the obtained composite coatings were tested. The mechanical and ultrasound agitation were used to achieve good dispersion of plating solution and homogeneous co-deposition of second phase. The goal was to examine the effect of the agitation type in bath, on the attributes of the deposited composite coatings. The incorporation of Al2O3 particles was enhanced when the mechanical agitation of the solution was applied. However, in case that this agitation method was used, the agglomeration of the alumina particles occurred. In contrast, when an ultrasonic agitation of the plating solution was applied, the uniform distribution of the alumina particles could be achieved. The presence of particles in the matrix, along with applied ultrasound, resulted in grain refinement and homogeneous microstructure. The Al2O3 nanoparticles incorporated in Zn-Mn alloy matrix, resulted in a significant increment in the indentation hardness and a modest increase in the coating corrosion resistance. However, the coating hardness increased with alumina addition, only in case when an ultrasonic agitation of the electrodeposition solution was used.