Improvement of surface properties of inconel 625 coatings by nitriding and electroless Ni-P

Nickel-based superalloys such as Inconel 625, are used in a wide range of applications including the marine and petroleum industries under where it is subjected to harsh conditions such as high temperatures and highly corrosive environments. However, its wear and corrosion resistance are poor. Accordingly, efforts are being made to improve both the anti-wear and the anti-corrosion properties in service. One plausible solution is to construct components entirely from specialty wear and corrosion-resistant materials, but this can be very expensive. Since wear and corrosion are responsible for causing surface material loss, an appropriate surface modification process can be effective for minimizing costs and prolonging the service life of nickel-based superalloys with a low cost and capability of mass production. In recent years, surface modification treatments seemed to repair these problems. When thermaly sprayed Inconel 625 coatings are modified by re-surface treatment, their service life may be longer and their working performance may be higher. In this study, structural, corrosion, mechanical and wear behaviour of Inconel 625 coating prepared by High-Velocity Oxy-Fuel (HVOF) thermal spraying on low carbon steel (St 37) alloy were investigated. The results were compared against HVOF-sprayed Inconel 625 coating + pulse plasma nitriding and HVOF-sprayed Inconel 625 coating + electroless Ni-P deposition which is a duplex surface treatment. Inconel 625 coating on low carbon alloy substrate by HVOF process was pulsed plasma nitrided at 520 C for 12 hours in a gas mixture of 75 % N2 and 25 % H2, while HVOF-sprayed Inconel 625 coating was used as the substrate for the preparation of electroless Ni-P coating. After these operations, microhardness measurements and reciprocating wear tests were determined in order to describe the mechanical and tribological properties. Structural analysis was performed using X-Ray Diffraction (XRD) analysis, Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), while corrosion performance of the coatings was analysed by using a potentiodynamic polarization scanning (PDS) technique. As a result, it was found that the pulse plasma nitriding and electroless Ni-P surface processes improved the corrosion and tribological properties of the HVOF-sprayed Inconel 625 coating.