Characterization of molybdenum-modified aluminide coating on 304 stainless steel via slurry

Abstract

This study investigates the microstructural changes and growth kinetics of intermetallic compounds in Mo-modified aluminide coatings on 304 stainless steels with varying temperatures and times. Molybdenum and alumina were introduced via slurry aluminizing. Heat treatments were conducted at 750 °C, 800 °C, and 850 °C for varying times (6, 8, and 10 h). Aluminide coating was characterized with SEM, EDX, and XRD, revealed the presence of multilayer phases comprising of Fe-Al and Mo-Al intermetallic compounds, along with an alumina scale on the coating surface. Samples heated at 750 °C showed uniform coatings, while those at 800 °C and 850 °C exhibited voids and cracks. The growth kinetics of the coating obey a parabolic law conforming to diffusion-controlled growth. The activation energies calculated for FeAl and Fe3Al were 360 kJ/mol and 237 kJ/mol, respectively, which is higher than that of coatings containing aluminium (FeAl layer: 180 kJ/mol and and Fe3Al layer: 260 kJ/mol). The total activation energy (Q) was 26 kJ/mol for coatings containing molybdenum and 53 kJ/mol for coatings with only aluminium. Hardness of the intermetallic layers varied from 378 to 966 HV for Mo-modified coatings as compared to aluminide coating (380 to 1030 Hv). The corrosion behaviour of Mo-aluminide coatings was investigated in 3.5% NaCl using polarization and electrochemical impedance spectroscopy (EIS) tests. Results show improved performance of corrosion resistance due to Mo-rich intermetallic phases, with the 6 h sample exhibiting the best corrosion resistance.