Effect of Welding Thermal Cycle on Corrosion Behavior of Q315NS Steel in H₂SO₄ Solution

As the main corrosion form of coal-or heavy oil-fired boilers, dew point corrosion occurs when corrosive gases (SO₃, HCl, NO₂, et al) are cooled and converted to condensed acids. The condensed acids (H₂SO₄, HCl and HNO₃) are much corrosive to steel, causing corrosion damage to plant materials. The service temperature is designed lower and lower to improve energy efficiency recently, which makes dew point corrosion more and more serious. Q315NS steel produced by appropriate alloy design is much suitable for those parts vulnerable to dew point corrosion in power and petrochemical industry due to its excellent corrosion resistance in H₂SO₄ solution. As an efficient and low-cost process, welding is an essential process in the utilization of Q315NS. The corrosion mechanism of the heat affected zone is much complex due to the presence of microstructure gradients, which is largely determined by the welding thermal cycle. However, there is little research elucidating the effect of welding thermal cycle on corrosion behavior of Q315NS steel in H₂SO₄ solution. In this work, the microstructure evolution and corrosion behaviour in the 50% H₂SO₄ (mass fraction) solution of welding heat affected zones of Q315NS was investigated by comparison with base metal using welding thermal simulation technique, scanning electron microscope and electrochemical measurements. The results show that the microstructures of ferrite and pearlite are observed in base metal, fine-grained region and incomplete recrystallization region, while coarse-grained region consists of granular bainite. All the equivalent circuits of Q315NS with or without welding thermal cycle contain a resistor of corrosion product and a capacitor of electric double layer, and all specimens have passivation behavior. The base metal and the incomplete recrystallization region have the lowest corrosion current density and the largest charge-transfer resistance, which means the best corrosion resistance, while the coarse-grained region has the highest corrosion current density and the least charge-transfer resistance. Rod-like shaped corrosion product was formed by deposition on the surface of the coarse-grained region specimen while a porous-structured corrosion product was formed on the surface of other specimens.