High temperature oxidation behavior of aluminide coatings applied on HP-MA heat resistant steel using a gas-phase aluminizing process

Abstract

Aluminide diffusion coatings were applied on a high-performance micro-alloyed (HP-MA) heat resistance steel, using a single step gas-phase aluminizing process. Powder mixtures with different compositions were employed for the coating process to achieve low-Al and high-Al coatings. This paper investigates the phase composition, growth mechanism, as well as high temperature oxidation behavior of aluminide-coated HP-MA steels. X-ray diffraction (XRD), scanning electron microscope (SEM), and field emission scanning electron microscopy (FESEM) were used to investigate characteristics of coated specimens. To evaluate the oxidation behavior, specimens were heated to 1100 °C for 100 h in the air atmosphere. Weight measurements were conducted at regular ten-hour time intervals. Furthermore, the parabolic rate constant (kp) values were obtained from the (ΔW/A) vs. oxidation time graphs. Moreover, stresses generated in oxide scale, formed on the aluminide coatings, were theoretically calculated following the first cycle of oxidation. Results showed that the stresses created in the oxide scale exceed the compressive strength of alumina, resulting in its spallation. After 100 h of cyclic oxidation test, the thickness of the oxide scale for the HP-MA steel, low-Al, and high-Al aluminide coated specimens was 22, 14, and 6 μm, respectively. The oxidation behavior of all specimens followed the parabolic rate law. The value of kp was the lowest at all oxidation time intervals for the coatings with higher Al content. This was due to the presence of more Al and Cr elements in the high-Al aluminide coating, improving the ability to form an outer dense alumina layer.