The petrochemical industry is implementing practices to recover flue gases to reduce energy waste. H2S in the recovered gas makes the gaseous environment acidic during reuse. This work evaluates the corrosion of 9Cr-1Mo steel under an acidic gas environment in the temperature range of 450–750 ºC and 200 h exposure time. We used X-ray photoelectron spectroscopy to understand the role of H2S in the corrosion process. Results suggest that the gas recovered and employed as flue gas in the refinery should be restricted to facilities where the components are not vulnerable to sulfidation.