Resumen
The 9Cr-1Mo steel (ASTM P91) was hot-dip aluminized, and its microstructural changes during oxidation were studied. Before oxidation, the coating consisted of (Al-rich topcoat containing a small amount of Al5Fe2 and Al13Fe4)/(Al13Fe4-rich, Al13Fe4-containing alloy layer)/(Al5Fe2 alloy layer containing a small amount of Al9Cr4 precipitates), from the surface. During oxidation at 700?900 °C for 20?100 h, Al diffused inward and the substrate elements migrated outward to broaden and soften the coating, and also to transform (high Al)-Fe intermetallics to (low Al)-Fe intermetallics. The phases in the coating progressively transformed during oxidation as follows; (Al-rich topcoat)/(Al5Fe2?rich, Al13Fe4-containing alloy layer)/(Al5Fe2 alloy layer)?(a-Al2O3 scale)/(Al13Fe4?rich, Al5Fe2-containing layer)/(Al5Fe2 layer)/(AlFe interlayer)?(a-Al2O3 scale)/(AlFe?rich, Al5Fe2-containing layer)/(AlFe layer)/(AlFe3 layer))?((a-Al2O3, Fe2O3)-mixed scale)/(AlFe3 layer)/(Fe(Al) layer) from the surface. As the oxidation progressed, the scale changed from a-Al2O3 to the (a-Al2O3, Fe2O3)-mixture, which provided the necessary oxidation resistance.