HIGH TEMPERATURE CORROSION PROPERTIES OF IONIC LIQUIDS. Ilaria Perissi, Ugo Bardi, Stefano Caporali, Alessandro Lavacchi Dipartimento di Chimica, Consorzio Interuniversitario Nazionale di Scienza e Tecnologia dei Materiali (INSTM), Università di Firenze, Via della Lastruccia, 3 – Sesto Fiorentino (FI) – Italy Introduction. Due to their physicochemical properties Ionic Liquids experienced a rising interest as fluid for heat exchange and storage because they match most the requirements for thermal application fluids (negligible vapour pressure up to the decomposition temperature, high thermal capacity, high decomposition temperature [1]. Nevertheless just few studies have been reported on the corrosion of the materials commonly employed in the construction of thermal plants and data concerning corrosion rates at the operating temperature are still completely missing. The aim of this work is to fill this lack of knowledge reporting a characterization of the interaction of 1-butyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)- imide with several metals and alloys. Corrosion rates have been estimated both by electrochemical and gravimetric techniques in a temperature range from 25 up to 325 °C. The morphology of the corrosion process has also been studied by Scanning Electron Microscopy. Methods.  Electrochemistry Potentiodynamic Curves Tafel Plots Polarization resistance method (area exposed to solution 50 mm 2 ) Gravimetric method (48h immersion test)  Scanning Electron Microscopy Results and discussion Electrochemistry Tafel plots. All the metals and alloys investigated showed low corrosion current at room temperature (1-14 µm/yr, Table 1, fig. 2) resulting in a good resistance to general corrosion in [C 4 mim][Tf 2 N]. Room temperature potentiodynamic curves (Fig.1) show we can suppose a weak passivation only for copper while other metals and alloys show active regions followed by regions in which currents appear diffusion controlled. Gravimetric results and SEM analysis At 150 °C the copper surface showed the evidence of heavy general corrosion. In addition, we observed the crumbling of the specimens. Immersion tests also showed dark-brown coloration of the ILs [2]. Such coloration occurs at different temperature in dependence of the nature of the specimen examined. No correlation between the colour intensity and the metal corrosion rates has been observed. We suppose this is probably due to the occurrence of chemical transformation of the IL itself. 48 h immersion tests performed at 275 °C showed the occurrence of localized corrosion phenomena on the surface of AISI 1018, Nickel and Inconel. Brass still preserves. At 325°C pitting corrosion was also observed for brass. The corrosion features were examined by SEM. Table 3 refers to the undamaged surface. The values of the concentration of copper and zinc are very close to the nominal alloy composition. Table 4 shows the results of the analysis inside the pit. Here, the concentration of zinc has increased, nearly in a 3:2 ratio, which corresponds to the β phase of brass. Conclusion. All metals and alloys investigated show low corrosion current at room temperature (1-14 µm/yr) resulting in a good resistance to general corrosion in [C 4 mim][Tf 2 N]. 48 h immersion tests performed at 275 °C show the occurrence of localized corrosion on AISI 1018, Nickel and Inconel, while brass still preserves. The tests performed at 325 °C also evidence localized corrosion on brass surface. SEM and EDAX show that only the α-phase is corroded while Zn richer β-phases is not. During high temperature test, a different change in colour of the IL occurs according to the metal or alloy which the fluid is in contact with. This is probably related to the catalytical activity of the metals respect to the IL decomposition reactions. This aspect will be crucial to affirm ILs as diathermal fluids. In future we will examine the solutions and the surfaces by spectroscopic techniques in order to detect the species in the solutions and their relative concentration, providing a better insight of the interaction between metals an ILs. Electrochemical measurements performed on copper specimens at 70 °C showed (fig. 3) that temperature dramatically affects the corrosion rates for such metal (Table 2). ElemOFSCuZn Wt% ElemNOFSSnCuZn Wt% Table 3Table 4 Table 1 10 µm Fig.1 Potentiodynamic polarization curves Fig.2. Room temperature Tafel Plot Fig.3. Comparison between Tafel plots at room temperature and at 70°C for copper. SEM image showing localized corrosion on Inconel after 48h immersion test (600x). SEM image showing localized corrosion on AISI 1018 after 48h immersion test. SEM image showing the β phase of brass after 48h immersion test (2400x). Copper Inconel Nickel Brass Tested Metals and alloys Ionic Liquids T onset T start T onset other lit [bmim][Cl] , 234 [bmim][Br] [bmim][BF 4 ] , 360 [bmim][Tf 2 N] Ionic Liquids C p (25 °C) J mol -1 K -1 C p (50 °C) J mol -1 K -1 uncertainty [emim][Tf 2 N]  3.9% [bmim][Cl]  1% [bmim][Br]  2% [bmim][BF 4 ] .7% [bmim][PF 6 ]  4% [bmim][Tf 2 N]  3.6% Copper Brass Nickel Inconel Carbon steel 10 µm [1] Brennecke J.F. et al., J. Chem. Eng. Data 2004, 49, [2] Kosmulski M. et al., Thermochimica Acta, 412, 2004, Source: [1] Brennecke J.F. et al.. Table 1 [C 4 mim][Tf 2 N] Tested Metals and alloys Copper Brass Nickel Inconel Carbon steel Table 2