NiCr–Cr3C2 coatings are widely used for high temperature and tribological applications due to their high hardness, oxidation, and wear resistance properties. In the present investigation, an attempt is made to further enhance the hardness and adhesion strength of NiCr–Cr3C2 coatings by reinforcing them with multi-walled carbon nanotubes. The carbon nanotubes (3–7 wt%) with varying weight percentages were mixed with NiCr–Cr3C2 using a planetary ball rolling mill and sprayed on SA213 T12 (T12 alloy steel tube) using a high-velocity oxy-fuel spraying process. The microstructures of mixed powder, coating cross-section, and fractured coating surface were characterized using a scanning electron microscope while X-ray diffraction was used for phase identification in the fractured coating surface. The coated samples were subjected to microhardness and adhesion strength tests according to ASTM E384 and ASTM D4541-09 standards. Out of all coatings, NiCr–Cr3C2/7% carbon nanotube composite coating showed the lowest porosity of 1.17%, highest microhardness, and adhesion strength of 563.8 HV and 55.8 MPa, respectively. A fracture analysis after a pull-off adhesion test revealed adhesion failure for NiCr–Cr3C2 coating and combined adhesion/cohesion failure for NiCr–Cr3C2/7% carbon nanotube composite coating.

The thermal power plant boiler tubes experience degradation of their surfaces due to oxidation and molten salt corrosion while working at elevated temperatures. The formation of protective coating layers can overcome such surface degradation. In the current research, protective coated layers composed of Cr3C2-25% NiCr/Carbon nanotube coatings are deposited on the ASTM SA-213T12 boiler tube substrate by the High-Velocity Oxy-Fuel (HVOF) thermal method. The coating powder Cr3C2-25%NiCr is reinforced with 3, 5 and 7 wt% of Multi-Wall Carbon Nano-Tube (MWCNT) prior to deposition. The uncoated and coated specimens are evaluated for hot corrosion behaviour in the molten salt environment (Na2SO4 + 60%V2O5) for 50 cycles, constituting 1 h of heating at 600 °C. Thermogravimetric analysis is used to investigate the kinetics of hot corrosion behaviour in both coated and uncoated samples. Coatings are characterized with the help of X-ray diffraction and Scanning Electron Microscope/Energy Dispersive Spectroscopy (SEM/EDS) techniques. Microstructural investigations with the help of SEM/EDS and X-ray diffraction of coated and uncoated samples revealed that 7 wt% of CNT coated samples minimized the voids and pores of the coatings, leading to better interlocking with the substrate surface. Furthermore, it is observed in the studies that the appreciable corrosion resistance of 7 wt% of CNT composite coatings is higher than 3 and 5 wt% of CNT-coated and uncoated samples.