Cavitation erosion-corrosion behaviour of laser surface alloyed AISI 1050 mild steel using NiCrSiB

A Ni-based hardfacing alloy NiCrSiB (Ni-16.5%Cr-15.5%Fe-3.5%Si-3.8%B-1%C) was surface alloyed on AISI 1050 mild steel (Fe-0.2%Cr-0.4%Mn-0.5%C) specimens using a two-step process. NiCrSiB powder was preplaced on the substrate by flame spraying and then remelted by a 2 kW continuous wave Nd:YAG laser to achieve surface alloying. The cavitation erosion-corrosion characteristics of the surface alloyed specimens in 3.5% NaCl solution at 23°C were determined by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm and a potentiostat. At a dilution ratio of 12% (corresponding to a laser scanning speed of 25 mm s-1), the overall cavitation erosion-corrosion resistance of the alloyed surface was 8.9 times that of the as-received AISI 1050 specimen. The corrosion resistance was also improved as reflected by a reduction in the current density of least one order of magnitude as compared with the as-received specimen at the same potential. The improvement in cavitation erosion resistance could be attributed to the superior mechanical properties of the NiCr-alloyed matrix formed and the presence of borides and boro-carbides which increased the hardness. The improvement in corrosion resistance was due to the increase in Cr and Ni content in the alloyed layer. The relative contributions of pure mechanical erosion, electrochemical corrosion, and synergism between erosion and corrosion to the overall cavitation erosion-corrosion were also determined.