Microstructural Changes of Al Hot-Dipped P91 Steel during High-Temperature Oxidation

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 degrees 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)->(alpha-Al2O3 scale)/(Al13Fe4-rich, Al5Fe2-containing layer)/(Al5Fe2 layer)/(AlFe interlayer)->(alpha-Al2O3 scale)/(AlFe-rich, Al5Fe2-containing layer)/(AlFe layer)/(AlFe3 layer)) -> ((alpha-Al2O3, Fe2O3)-mixed scale)/(AlFe3 layer)/(Fe(Al) layer) from the surface. As the oxidation progressed, the scale changed from alpha-Al2O3 to the (alpha-Al2O3, Fe2O3)-mixture, which provided the necessary oxidation resistance.