Microstructure and Properties of Aluminizing Coatings on 0Cr21Al6Nb Stainless Steel Prepared by Pack Cementation

Abstract

Fe-Al intermetallic coatings were fabricated on 0Cr21Al6Nb stainless steel by pack cementation to systematically investigate the effects of temperature (650–800 °C) and holding time (2–16 h) on the coating microstructure, phase constitution, and hardness. The coatings primarily consisted of Fe2Al5, with a minor amount of FeAl3 confined to the outermost surface. The coating/substrate interface exhibited a characteristic serrated or tongue-like morphology, which became increasingly pronounced with increasing temperature and holding time, indicating enhanced interdiffusion across the interface. Both increasing temperature and prolonging holding time markedly promoted coating growth and increased the coating thickness, while simultaneously facilitating the enrichment of FeAl3 in the near-surface region. Despite these microstructural variations, the coating hardness remained relatively stable at approximately 960 HV, which was substantially higher than that of the substrate; meanwhile, the substrate exhibited a slight reduction in hardness after aluminizing. Thermodynamic analysis revealed that Fe2Al5 was preferentially formed owing to its relatively lower Gibbs free energy, and its formation remained thermodynamically favored during subsequent coating growth, whereas FeAl3 was restricted to the coating surface. Kinetic analysis demonstrated that coating growth followed a parabolic law, indicating a diffusion-controlled growth mechanism, with an apparent activation energy of 107.5 kJ·mol−1 for Al diffusion. Furthermore, temperature exerted a more pronounced influence on coating growth than holding time, highlighting temperature as the dominant kinetic parameter governing the formation and thickening of the Fe-Al intermetallic coating.