Laser surface aluminizing of mild steel: Microstructural and corrosion characteristics

Laser surface alloying of mild steel AISI 1050 using aluminum alloy (AlFeSi) was attempted. Alloying material in powder form was preplaced on the surface of the substrate by oxyfuel flame spraying. The surface was then consolidated by scanning with a 2-kW CW Nd:YAG laser beam. The micro structures of the alloyed layers were studied by scanning electron microscopy, optical microscopy and X-ray diffractometry. The corrosion and cavitation-erosion characteristics in 3.5% NaCl solution at 23 °C were studied by potentiodynamic polarization technique and by means of a 20 kHz ultrasonic vibratory facility. The laser-alloyed specimens were found to be composed of ferrite and the intermetallic phases Fe3Al, FeAl and Fe 2Al5. The maximum hardness was significantly increased to 595 Hv. The corrosion behaviour was influenced by the degree of dilution of the alloying element. Although aluminizing led to an active shift in the corrosion potential from -510 mV to -890 mV, the specimens showed passivity with pitting and protection potentials of -550 mV and -710 mV respectively while the substrate did not. The corrosion resistance of the laser-alloyed specimens was significantly improved as evidenced by a much lower corrosion current density. In addition, the cavitation-erosion resistance of the laser-alloyed specimens was also significantly improved by 17.7 times that of the substrate due to the increase in both hardness and corrosion resistance of the laser-alloyed layer.