First order evaluation of the corrosiveness of ionic liquids : literature review and screening tests with imidazolium and phosphonium based ionic liquids

Abstract : Ionic liquids (ILs) are chemical salts, with a melting point below 100°C, often already liquid at room temperature (RTIL). ILs are obtained by combination of a large (organic) cation weekly coordinated with a smaller (organic and inorganic anion). Although they were discovered in 1914, they have only regain interest of researchers and the industry as alternative solvents, electrolytes and reactive media since the early 2000’s, promoted by the emergence of green chemistry, due to their “adjustable” functional properties (solvent property, inherent basicity/acidity, ionic conductivity, large liquid temperature range and their advocated safety related characteristics like operational thermal/chemical stability, low volatility and weak flammability). INERIS, Parc technologique Alata, BP2, F-60550 Verneuil-en-Halatte (France) Key potential applications in biorefineries of ILs have been recently discussed by Stark [1] most known of which are biomass pretreatment, cellulose dissolution. However and by contrast to still persisting messages on the matter, ionic liquids safety is not an intrinsic property of ILs. Not surprisingly, safety profile including (eco)toxicity, thermal and chemical stability or flammability may highly vary according to extraordinary variety of concerned chemical structures and therefore needs to be carefully revisited as shown by Seddon et al [2] or Diallo et al [3] to access actual physico-chemical, health and environmental hazard profiles of ionic liquids. With regard to corrosion behavior, limited information has been made so far available, and most studies have focused essentially on imidazolium based ILs. While some of early published papers often concluded that studied ILs are non corrosive to metals, more recent reviews are more cautious in their conclusions, as far as knowledge progresses. From extreme viewpoints, it can be concluded that some ionic liquids can be envisaged as corrosion inhibitors in as additives for some reactive media systems (e.g. for CO2 capture in replacement of corrosive amines)… while others may corrode noble metals like platinum under specific circumstances. Among recurrent trends featuring corrosive properties of ILs stated in the literature, we may outline the following points [4-9]: (i) corrosion process with water free ILs clearly differs from aqueous systems where [H]+ ions concentration and dissolved oxygen are the dominating factors; (ii) the length of alkyl chain length of the cationic structure seems to affect corrosiveness of ILs in opposite way as for toxicity (the shorter the alkyl chain length, the stronger the corrosiveness of the ILs); iii) addition of water way significantly increase intrinsic corrosion behavior of ILs [4/9-10]; iv) in acidic media hydrolysable halogenated anions (e.g. [BF4]-, [PF6]-) In a second part, the experimental approach performed is presented: a screening procedure inspired of a technique developed by IO-LI-TEC [5] has been developed and adjusted (7 days exposure in liquid and vapor phase of target IL at 100°C in small glass containers) and experienced with a representative set of common ILs based on imidazolium and phosphonium cations. Thin carbon steel, 316L stainless as well as aluminum coupons have been used to screen their corrosion behavior potential. Responses obtained were quite variable (from no effect to complete dissolution in the IL). Influence of water is confirmed from some imidazolium and phosphonium tested ILs, depending on the nature of the metal exposed. Influence of impurity mocked up by addition of 1% NaCl has also been examined. will induce corrosion from relating forming halide; v) the corrosion potential of F containing anions can reduced by integrating F as C-F bounds; vi) impurities may play a significant role in corrosion behavior of ILs, vii) owing to complexity of corrosion mechanisms, an experimental approach is desirable to work out in depth the potential of metallic corrosion induced by ILs in target application for a given metallic alloy.
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Bruno Narcy, Guy Marlair. First order evaluation of the corrosiveness of ionic liquids : literature review and screening tests with imidazolium and phosphonium based ionic liquids. 1. International Workshop on Corrosion in Advanced Biorefineries (CORABIO 2016), Mar 2016, Compiègne, France. ⟨ineris-01854727⟩

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