Investigations on High Temperature Corrosion and Erosion Behaviour of Plasma Sprayed Co-Based Composite Coatings

Abstract

The components of the gas turbines such as combustor and transition duct, exit blades, vanes, and disks operate between the temperatures of 600-800ºC. These components also work typically under higher cyclic loads in association with oxidising, corrosive and erosive environment. Providing surface coating through thermal spray process is one of the promising techniques to protect against such surface degradations and maintain the mechanical stability of underlying components. Plasma spray coating process is one of the most versatile and widely used cost-effective technique to deposit metals and ceramic materials on materials used in structural applications and machine parts to improve their oxidation, hot corrosion, and tribological resistance. The MCrAlY are promising coatings (M=Ni, Co and Fe) for the protection of components such as in the hot sections of gas turbines, aero engines, land-based gas turbines and naval diesel engine manufactured out of superalloys, particularly while operating at high temperatures. In the present study, oxides and carbides reinforced CoCrAlY feedstock powders such as CoCrAlY+28%Al2O3+2%YSZ, CoCrAlY+2%CeO2, CoCrAlY+30%WC-Co and CoCrAlY+30%Cr3C2-NiCr are coated on MDN 321 and Superni 76 superalloys by plasma spray technique. The microstructure, composition, phases and mechanical properties of coatings are characterized to evaluate and investigate their potential under hightemperature erosion, corrosion and oxidation conditions. The high-temperature solid particle erosion experiments were carried out using air jet erosion test rig. Two impact angles; 90° and 30° of the jet are investigated and alumina sand is used as erodent at a temperature of 600°C. The thermo cyclic oxidation and hot corrosion behaviour of coated and uncoated alloys are investigated in both static air and molten salt (Na2SO4-60%V2O5) environment at 700°C for 50 cycles. Each cycle consisted of heating at 700°C for 1 hour, followed by 20 minutes of cooling in static air. The thermogravimetric technique is used to approximate the kinetics of oxidation. The corrosion products and eroded surface are analysed using X-ray diffraction (XRD) and Scanning electron microscopy/Energy dispersive X-ray analysis (SEM/EDAX). The characterization studies have revealed that CoCrAlY+WC-Co coating has achived comapratively higher hardness (384 Hv) and fracture toughness (14.3±3.2 MPa m1/2), whereas CoCrAlY+CeO2 coating exhibited the least porosity and higher bond strength (15.6±2.2 MPa). All the coatings have undergone adhesive fracture during bond strength test indicating better cohesive strength between the splats. CoCrAlY+WC-Co coating has shown superior erosion resistance at 600°C temperature among the coatings with a volume loss of 0.9 mm3 and 2.1 mm3 at 90° and 30° impact angle respectively for 5 cycles. This is mainly attributed to its higher hardness and fracture toughness with lower porosity. CoCrAlY+WC-Co coating has experienced both ductile and brittle mode of erosion mechanism with the coating showing features of indentations, ridges, cracks, ploughing marks, lips and minor carbide pull. WC splats resist the erodent impact with minimal deformation thereby providing the shielding effect to Co matrix. The better erosion resistance of CoCrAlY+CeO2 coatings is owing to its least porosity, higher fracture toughness and microstructure refinement by CeO2 reinforcement. The coating has experienced ductile erosion mechanism with the absence of cracks and crates. CoCrAlY+Al2O3+YSZ coating exhibited brittle fracture and hard phase pull out due to repetitive erodent impact. The CoCrAlY+Cr3C2-NiCr coating showed least erosion resistance and has undergone severe brittle fracture at both impact angle and this mainly due to carbide dissolution at elevated temperature. From the XRD studies of eroded surface and observation of crack formation and oxide fragments, it very strongly indicates that all the coatings have undergone oxidation modified erosion. The hot corrosion tests were conducted under molten salt environment and at 700°C. CoCrAlY+Cr3C2-NiCr coating showed at least of 23% higher hot corrosion resistance than other coatings and also the substrates. This coating has shown dense, thin, continuous Cr rich oxide layer on the surface which contributes to its better performance. The presence of dominant Cr2O3 and CoCr2O4 oxide layer having lower solubility for corrosive melts and oxygen. The presence of stable metal oxide reinforcement of α-Al2O3 with surface oxide scale of Cr2O3, CoCr2O4 and CoAl2O4 results in slow-scale growth kinetics duringhot corrosion of CoCrAlY+Al2O3+YSZ coating. In the case of CoCrAlY+WC-Co coating the formation of CoWO4, CoCr2O4 and Cr2O3 as strong phases with stable CoSO4 provides hot corrosion resistance by developing dense non porous well adhered oxide scale. The growth of CeVO4 as a result of acidic fluxing develops stresses on surrounding splats/oxide scale leads to the development of cracks. The infiltration of molten salt through the cracks results in higher corrosion rate and oxide scale delamination. Thus CoCrAlY+CeO2 coating has shown the least hot corrosion resistance. All the coatings have shown the parabolic weight gain kinetics during hot corrosion studies. The MDN 321 and Superni 76 have shown higher corrosion rate with linear weight gain nature. In oxidation condition, CoCrAlY+Al2O3+YSZ coating has exhibited higher oxidation resistance than other coatings and substrates by showing least weight gain. The coatings experienced parabolic weight gain nature indicating all the possibilities of the formation of protective oxide layer. The uncoated alloys showed para-linear weight gain with a change from steady state to linear condition. The coatings under study have been found to be successful in protecting the given substrate alloys tested under laboratory conditions against erosion, hot corrosion and oxidation. In addition to applications in gas turbine, these coatings can be applied to other applications like superheater zone of coal fired boilers, fluidized bed combustors, industrial waste incinerators and internal combustion engines etc.