Reliance of Corrosion Characteristics for Two Iron-Based Alloys on the Water Content in 1-Butyl-3-Methylimidazolium Tetrafluoroborate

Although the corrosion of iron-based alloys by ionic liquids (ILs) has been reported, the influence of trace water in ILs on its corrosion mechanism is often ignored. In this work, we investigated the corrosion behavior of Q235 carbon steel (Q235 CS) and 304 stainless steel (304 SS) exposed to 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) with trace water (0.5 wt% to 4.0 wt%) at 323 K. Electrochemical tests and surface analysis manifested that the increasing water content accelerated corrosion of the two iron-based alloys in [BMIM]BF4. A nontypical passivation zone was observed for Q235 CS, while 304 SS exhibited completely active dissolution and its corrosion situation was not as serious as Q235 CS. The occurrence of pitting corrosion is responsible for 304 SS behaviors in [BMIM]BF4. Some corrosion products accumulated on the surface of both iron-based alloys were similar, including FeF2, FeF3, FeO, Fe2O3, and/or FeOOH. Gas products during corrosion were also monitored to avoid the complicated cathodic depolarization process, and it was found to be composed of BF3, HF, and H2. Finally, the corrosion mechanism of iron-based alloys in ILs with trace water was proposed. The illustrated mechanism would be meaningful for understanding the similar corrosiveness to iron-based alloys.