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

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea
Department of Mechanical Engineering, Mehran University of Engineering and Technology, SZAB Campus, Khairpur Mir’s 66020, Pakistan
Author to whom correspondence should be addressed.
Academic Editor: Alessandro Lavacchi
Coatings 2017, 7(2), 31;
Received: 30 December 2016 / Revised: 6 February 2017 / Accepted: 13 February 2017 / Published: 17 February 2017
PDF [7871 KB, uploaded 17 February 2017]


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)→(α-Al2O3 scale)/(Al13Fe4–rich, Al5Fe2-containing layer)/(Al5Fe2 layer)/(AlFe interlayer)→(α-Al2O3 scale)/(AlFe–rich, Al5Fe2-containing layer)/(AlFe layer)/(AlFe3 layer))→((α-Al2O3, Fe2O3)-mixed scale)/(AlFe3 layer)/(Fe(Al) layer) from the surface. As the oxidation progressed, the scale changed from α-Al2O3 to the (α-Al2O3, Fe2O3)-mixture, which provided the necessary oxidation resistance. View Full-Text
Keywords: hot-dip aluminizing; P91 steel; oxidation; Al-Fe intermetallics; α-Al2O3 hot-dip aluminizing; P91 steel; oxidation; Al-Fe intermetallics; α-Al2O3

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Abro, M.A.; Lee, D.B. Microstructural Changes of Al Hot-Dipped P91 Steel during High-Temperature Oxidation. Coatings 2017, 7, 31.

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