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In this paper, we discuss the formation of ceramic coatings by a combined processing of low-temperature pack aluminizing and oxidation treatment on the surface of X80 pipeline steel substrates in order to improve the corrosion resistance ability of X80 pipeline steel. First, Fe-Al coating consisting of FeAl₃ and Fe₂Al₅ was prepared by a low-temperature pack aluminizing at 803 K which was fulfilled by adding zinc in the pack powder. Pre-treatment of X80 pipeline steel was carried out through surface mechanical attrition treatment (SMAT). Further oxidation treatment of as-aluminized sample was carried out in the CVD reactor at 833 K under oxygen containing atmosphere. After 1 h duration in these conditions, ceramic coating consisting of α-Al₂O₃ was formed by in situ oxidation reaction of Fe-Al coating. Those coatings have been characterized by different techniques including X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscope (EDS), respectively. Ceramic coating shows a dense and uniform microstructure, and exhibits good coherences with X80 pipeline steel substrates. By electrochemical corrosion test, the self-corrosion current density of X80 pipeline steel with as-obtained ceramics coating in 3.5% NaCl solution shows an obvious decrease. The formation of α-Al₂O₃ ceramic coating is considered as the main reason for the corrosion resistance improvement of X80 pipeline steel.

ASTM P23 steel (Fe-2.25Cr-1.6W-0.1Mo in wt.%) was hot-dip aluminized and oxidized at 800^∘C and 1000^∘C for 20 h in air in order to determine the effect of aluminizing on the microstructure, hardness, and oxidation resistance of P23 steel. Aluminizing effectively increased the oxidation resistance of P23 steel by forming protective $\alpha$-Al₂O₃ scales. During oxidation, outward diffusion of substrate elements and inward transport of Al and oxygen occurred simultaneously. The oxidation and interdiffusion formed voids in the coating, lowered the microhardness, and transformed the original (Al-rich topcoat)/(Al${}_{13}$Fe₄ layer) to either (thin $\alpha$-Al₂O₃ scale)/(Al₅Fe₂ layer)/(AlFe layer)/(AlFe₃ layer)/($\alpha$-Fe(Al) layer) at 800^∘C or (thick $\alpha$-Al₂O₃ scale)/(AlFe₃ layer)/($\alpha$-Fe(Al) layer) at 1000^∘C. At 1000^∘C, Fe₂O₃ was also formed in addition to $\alpha$-Al₂O₃ scale, due to the enhanced outward diffusion of Fe, thus suppressing the formation of cracks in the coating.