Influence of laser remelting on the microstructure and corrosion behavior of HVOF-sprayed Fe-based coatings on magnesium alloy

An HVOF-sprayed Fe-based (Diamalloy 1010) coating deposited on AZ91 magnesium alloy was remelted using 650 W and 1000 W laser power. The microstructure of as-sprayed and laser remelted coatings and their phase composition were analyzed using SEM (Scanning Electron Microscopy) and XRD (X-Ray Diffraction) spectroscopy. The microstructure of the as-sprayed coating included crystalline and amorphous areas, which were shown by TEM (Transmission Electron Microscopy). According to XRD analysis the microstructure consisted of austenite and M7C3 carbides. After laser remelting, the structure of as-sprayed Diamalloy 1010 recrystallized and well-developed dendritic grains were formed. XRD analysis showed that laser remelting did not lead to the formation of new phases. On the other hand, the grain recrystallization and microstructure arrangement in the laser remelted layer led to increased peak intensities of individual phases in the XRD pattern. This combined with a reduction in porosity from 0.7% to 0.0% resulted in an increase in microhardness from 554 ± 8 HV0.1 up to 633 ± 8 HV0.1 after remelting. Potentiodynamic polarization tests and EIS (Electrochemical Impedance Spectroscopy) showed that laser remelting under a power of 650 W can improve the corrosion behavior in 3.5% NaCl. The remelting of the coating using 650 W laser power resulted in an increase in corrosion potential from −425 ± 13 mV to −210 ± 21 mV and a decrease in corrosion current density from 23.57 ± 4.59 μA·cm−2 to 0.44 ± 0.05 μA·cm−2 when compared to the as-sprayed coating.