采用焊接热模拟技术和电化学测试技术,研究了Q315NS钢焊接热影响区组织转变规律及焊接热循环对其腐蚀行为的影响。结果表明,Q315NS钢母材、细晶区和混晶区微观组织均由铁素体和珠光体组成,粗晶区主要由粗大的粒状贝氏体组成。在质量分数为50%的H2SO4溶液中,母材和热影响区的等效电路中均包含一个电荷转移电阻和一个由双电层产生的常相位角元件,且均产生了钝化行为。母材和混晶区的电荷转移电阻最大,腐蚀电流密度最小,耐腐蚀性能最好;粗晶区电荷转移电阻最小,腐蚀电流密度最大,耐腐蚀性能最差。腐蚀72 h后,母材、细晶区和混晶区表面生成的腐蚀产物均呈多孔状结构,而粗晶区表面生成的腐蚀产物为短棒状结构。2种结构的腐蚀产物均主要为Fe的硫酸盐,并含有Cu、Sb的氧化物及少量Si。
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.
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