| Abstract |
| ASTM P91 steel (viz. 9Cr-1Mo steel) was hot-dip aluminized to form a ~70 μm thick coating, and was then corroded in N2/0.1%H2S gas for 50 h at 800 and 900 ℃, to study the effect of aluminizing on the corrosion resistance of P91 steel in highly corrosive H2S environments. Before corrosion, the aluminized coating consisted of an Al-rich topcoat, an Al13Fe4 upper layer, and an Al5Fe2 lower layer from the surface. During corrosion, Al, oxygen, and sulfur diffused inwardly, while substrate elements diffused outwardly. Impurity oxygen in the gas reacted preferentially with Al to form α-Al2O3 at the surface, which increased the corrosion resistance significantly. Bare P91 steel corroded quickly, to form bi-layered, fragile, nonprotective FeS scales. The coating transformed into either a (Al13Fe4, Al5Fe2)-mixed layer, AlFe layer, AlFe3 layer, and α-Fe(Al) layer when corroded at 800 ℃/50 h, or into a AlFe3 layer and α-Fe(Al) layer when corroded at 900 ℃/ 50 h. Interdiffusion that occurred during corrosion increased the total coating thickness, and transformed (high Al)-Fe phases to (low Al)-Fe phases. The corrosion accompanied the formation of voids and microcracks in the coating.
(Received November 30, 2018; Accepted March 5, 2019) |
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| Key Words |
| aluminizing, P91 steel, H2S gas corrosion, hot dipping |
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