<p>Understanding the influence of substrate heterogeneities, particular porosity and oxide content, on the formation mechanisms of aluminized coatings is key to improving engineering material performance. Pack aluminizing was carried out at 850&#xa0;°C for up to 6&#xa0;h using a pack mixture consisting of 10&#xa0;wt.% Al, 3&#xa0;wt.% NH₄Cl, and 87&#xa0;wt.% Al₂O₃. Heterogeneous substrates were produced using high-velocity oxygen fuel (HVOF), flame spray (FS), and wire arc spray (WAS). Characterization of aluminized coatings was done to establish a formation mechanism model for aluminizing of heterogeneous substrates. Results revealed aluminized coating formation is ruled by multiple and simultaneous diffusion mechanisms, including volume diffusion from the surface, rapid diffusion along splat boundaries, volume diffusion into the splats, and diffusion through oxide phases. High porosity facilitates aluminum diffusion from the surface, leading to a thinner outer aluminide (Fe₄Al₁₃/Fe₂Al₅/(Fe,Ni)₂Al₅) zone and thicker interdiffusion zone (FeAl/Fe₃Al/α-Fe(Al)), compared to identical coatings processed on homogeneous substrates. In contrast, higher oxide content hinders aluminum diffusion, accounting for thinner coatings and limiting the growth of the interdiffusion zone. Findings highlight the critical role of substrate characteristics in controlling aluminized coating growth and emphasize the need to consider these factors when designing aluminizing treatments in non-homogeneous materials.</p> Graphical Abstract <p></p>