EROSION-CORROSION BEHAVIOUR OF HVOF SPRAYED N1CrBSi COATINGS. ON BOILER STEEL AND Ni-BASED SUPERALLOYS

Abstract

Erosion corrosion has always been serious topic. So many materials and techniques are devised to meet this corrosion but still this has always remained a mystery. It is really a sad thing to say that not even a single alloy has been able to combat it. For a long time, it was considered to be neglected topic but again it has come into picture with new features. Basically, erosion corrosion is extended, part of hot corrosion. Micro structural uniformity has long been considered necessary to improve the thermal, chemical and mechanical properties of materials. Metallurgists are continuously searching various processes with the intent to produce a uniform microstructure throughout the material. This study reveals the successful high-velocity oxy-fuel (HVOF) deposition of near-uniform NiCrBSi wire coatings, approximately 200-250 nm thick. The coatings were deposited onto on boiler steel and Ni- based superalloys for hot corrosion applications. The micro-structural characteristics of the deposits have been studied using the combined techniques of X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX). The coatings exhibited characteristic splat-like, layered morphologies due to the deposition and re-solidification of successive molten or semi-molten droplets. Under the employed HVOF spray conditions, a high proportion of the feedstock wire appeared to have been fully melted prior to impact on the substrate. The developed coatings have a dense and nearly uniform microstructure, with almost uniform hardness values in the range of 800-930 VHN and porosity less than 6%. XRD and EDAX results revealed that the main constituent outstanding phase of NiCrBSi coating is Ni3B, NiSi and Cr3C2. A very small fraction of Cr7C3,Cr2O3 oxide phase has formed in the . structure of the coating in the form of either inter splat lamellae or globules. Some partially melted particles are also observed in the structure of the coatings.