Effects of substrate roughness on the surface morphology and corrosion properties of Fe- and Ni-aluminide coatings on martensitic stainless steel

Abstract

Surface morphology affects several important characteristics of airfoils such as corrosion resistance, fouling and aerodynamic properties. In the current investigation different morphologies were developed on the surface of a martensitic stainless steel used for gas turbine compressor blade applications. A low temperature aluminide diffusion coating was applied on the steel substrates having low, intermediate and high average roughness values. The aluminizing treatment was carried out at 470 °C for 24 h in a powder mixture of Al, AlCl3 and Al2O3. Some of the substrates with intermediate roughness were received a Ni electrodeposition with 5, 10, and 20 μm thicknesses before aluminizing to investigate the resultant influence on the final morphology. Surface morphology and microstructure of the specimens were studied via FE-SEM, XRD and EDS analysis. The surface roughness of the specimens was studied via a stylus profilometer and their corrosion behavior was examined by the potentiodynamic polarization test. The results showed that the surface roughness of the substrate increases after aluminizing treatment but the final roughness can be reduced by applying an appropriate thickness of a Ni interlayer. The raising of the surface roughness reduced the corrosion rate of the uncoated steel specimens but revealed a weak beneficial effect on the aluminized specimens. Thicker Ni interlayers resulted in higher corrosion rates of nickel aluminide coatings.