Physical, mechanical and corrosion properties of Zn-15Al alloy coating deposited at different voltage using thermal spray process

Abstract

Steel is the most widely used metal on earth, cheap and found everywhere. It has good properties such as machinability, strength, ductility, and compatibility with other materials. However, it has weakness such as high corrosion rate which need to improve to achieve satisfying properties. The coating is one of the enhanced methods to improve the quality of steel coating methods are dipping and thermal deposition processes. Both processes have their own advantages and disadvantages. However, the thermal deposition method can produce a coating with better characteristics and performance. Zinc and aluminium are common metals to be used in the thermal spray process; however, a few parameters need to be concerned in the thermal spraying process. They include spraying voltage, spraying distance, deposition rate and applied current. The basic concept is that it melts the material and burst it to the substrate with high velocity. The purpose of this research is to evaluate the physical and corrosion properties of thermal sprayed pure zinc, aluminum and Zn-15Al alloy coating on a mild steel plate. The experimental procedures include roughness, thickness, adhesion, SEM with EDX, and EIS tests. The results show that at 30V, the surface roughness becomes lower, zinc’s decreases from 7.30um to 6.55um, Zn-15Al’s is from 8.52um to 7.93um. For aluminium, its surface roughness increases at higher voltage, from 13.10um to 18.25um. For coating thickness, the value of zinc, aluminium and Zn-15Al of 20V and 30V spraying voltage decreased from 629um to 601um, 486um to 452um, 541um to 536um respectively. The reason for sprays the material at higher velocity to the substrate; these findings depicted smooth and dense coating. Based on SEM/EDX analysis on the cross-sectioned samples, there is no gap between the coating and substrate and demonstrates good coating adhesion. The Daimler-Benz adhesion test shows all coating samples demonstrate good adhesion as no failure at the indentation boundary. In the EIS corrosion test, the frequency range 0.1Hz to 100kHz in 3.5wt.% NaCl solution, the highest impedance shown by zinc, aluminium and Zn-15Al at 20V are 21, 2122 and 49ohms; at 30V, they possess 11, 1347 and 20ohms. It illustrates those coatings with lower spraying voltages demonstrate better corrosion resistance due to the higher coating thickness. Hence, the coating which owns the best properties in all aspects is pure aluminum coating.