Effects of Laser Re-melting on the Corrosion Properties of HVOF Coatings B.S. Yilbas, I.H. Toor, F. Patel, and M.A. Baig (Submitted August 11, 2012; in revised form October 6, 2012) HVOF coating of Inconel 625 powder on carbon steel is carried out. Laser melting of the resulting coating is realized to improve coating structural integrity. Morphological and microstructural changes are examined in the coating prior and after laser treatment process using scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction (XRD). The residual stress developed is measured on the surface vicinity of the laser-treated coating using the XRD technique. The corrosion resistance of the laser-treated and untreated coating surfaces is measured, incorporating the potentiodynamic tests in 0.5 M NaCl aqueous solution. It is found that laser treatment reduces the pores and produces cellular structures with different sizes and orientations in the coating. Laser-controlled melting improves the corrosion resistance of the coating surface. Keywords corrosion resistance, HVOF, laser 1. Introduction High velocity oxy-fuel (HVOF) coating is one of the thermal spraying processes, which is widely used in industry to protect the surface from high temperature, corrosion, and erosion environments. HVOF coating has advantages over the other techniques and some of these advantages include easiness of operation, low cost, and achievement of relatively low porosity coatings. Although molten state of the sprayed particles depends on the spraying parameters, not all the particles remain in the molten state when impacting at the surface. This results in locally distributed small cavities in the coating, particularly at the coating-base material interface. This situation is more pro- nounced for the particles with high concentration of carbides, as the melting temperature of the carbides is considerably high. The cavitation and other coating structural defects can be eliminated through controlled re-melting of the resulting coating, which can be achieved using the high power laser source. The laser processing involves with local treatment, precision of operation, and low cost. Due to the mismatch among the thermal expansion coefficients of carbide particles, coating alloy, and the base material, high levels of residual stress could be formed in the coating. However, proper selection of laser melting parameters could minimize the residual stress levels while forming uniform re-melted coating at the substrate surface. Consequently, inves- tigation into laser re-melting of HVOF coating on Inconel 625 becomes essential. Considerable research studies were carried out to examine laser processing of metallic coatings. Laser surface re-melting of iron-based alloy coatings deposited by HVOF was carried out by Cui et al. (Ref 1). They showed that the re-melted thickness had a corrosion current density less than that of the untreated coatings. Corrosion performance of HVOF coating after laser treatment was studied by Rakhes et al. (Ref 2). Their findings revealed that the improvement of resistance to microgalvanic corrosion between WC and Co after laser treatment could be limited depending on the extended of melting occurred to the WC within the coatings. Laser surface modification of HVOF coating was investigated by Cho et al. (Ref 3). They demonstrated that the laser-treated coatings had superior wear properties as compared to untreated coating. Laser treatment of HVOF coating of WC-CoC-Ni powders was studied by Zhang et al. (Ref 4). They showed that fully dense and crack-free laser-treated HVOF coating was possible on steel workpiece. Investigation on microstructure and corrosion resistance of laser re-melted HVOF coating was carried out by Bakare et al. (Ref 5). They indicated that the localized formation of Ni(OH) 2 instead of Cr 2 O 3 on as-sprayed Inconel 625 coating contributed significantly to the performance deficit as compared to coatings subjected to laser treatment. Corrosion behavior of HVOF sprayed and Nd:YAG laser re-melted coatings was studied by Tuominen et al. (Ref 6). They showed that laser treatment improved the corrosion resistance of the coating partially due to surface asperities resulted during laser scanning of the surface. CO 2 laser treatment of WC blended HVOF coating and corrosion properties of the resulting surface were investigated by Zhang et al. (Ref 7). Their findings revealed that the pitting corrosion resistance was improved due to decreasing the size and the number of porosities at the laser- treated surface. The effect of heat treatment on the corrosion resistance of selective HVOF flame-sprayed alloy coatings was examined by Bolelli et al. (Ref 8). They showed that the precipitation of sub-micron-sized secondary phases after the treatment might produce galvanic microcells at intralamellar scale; however, the healing of the interlamellar defects occurred after the heat treatment. B.S. Yilbas, I.H. Toor, F. Patel, and M.A. Baig, Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Kingdom of Saudi Arabiae-mail: bsyilbas@kfupm.edu.sa . Contact e-mail: bsyilbas@kfupm.edu.sa. JMEPEG ÓASM International DOI: 10.1007/s11665-012-0428-4 1059-9495/$19.00 Journal of Materials Engineering and Performance