For the base tube of SUS304, it is clear that the apparent contact angle θ′ in air condition was consistent with that in simulated flue gas condition. However, the surface coatings of Fusible Ni-SA-b, Fusible Ni-SA-c and NiCr-TS caused quite different θ′ in air and simulated flue gas conditions. Obviously, the apparent contact angle in simulated flue gas condition was higher than that in air. The presence of aggressive gases of HCl, SO2 in flue gas played an important role in ash wettability.