Charlotte BORRILL1,2, Jason HALLETT1, Kyra L S CAMPBELL2
1Imperial College London, London, United Kingdom
2University of Sheffield , Sheffield, United Kingdom
Ionic Liquids (ILs) have been identified as sustainable chemicals which could replace more volatile organic compounds as solvents in industry. (1) As we approach commercialisation of IL technologies employing IL-aqueous solutions, used in separation and recycling applications, we must address inevitable scale-up challenges. (2) One hurdle is understanding the implications for using metal materials of construction in IL process, transport, and storage. Employing a fundamental understanding the relationship between aqueous IL solutions and metals will allow for translation to important industrial implications including selection of construction materials and hazard/safety risk assessments. Thus far studies have focussed on the use of ILs at low concentrations as corrosion inhibitors, but little research has addressed the interaction between metals and ILs at bulk concentrations in aqueous solution. Critically, the behaviour of ILs in very low concentrations cannot be extrapolated to higher concentrations. (3) This highlights a critical gap in research that will be essential to large scale deployment.
Three ILs were selected 1-Methylimidazolium chloride ([HC1im]Cl), 1-Butyl-3-methylimidazolium chloride ([C4C1im]Cl) and 1-Octyl-3-methylimidazolium chloride ([C8C1im]Cl) for investigation to explore the effects of alkyl chain length and cation protonation on corrosion rate. The IL aqueous solutions have demonstrated a range of clearly distinct behaviours between 70-100 mol% water concentrations; evidenced through with large changes in conductivity, density, and viscosity measurements. Hydrogen bonding networks have been identified within these systems, with shifting physical properties been used as a framework to evaluate the corrosion, employing electrochemistry, of Cu, Zn, and Brass under the range of concentrations. The use of Linear Polarisation, Electrochemical Impedance Spectroscopy, and Cyclic Voltammetry (each at 75 mol%, 85 mol%, 97 mol% and 99.5 mol% water in IL) support both the delivery of a corrosion rate metric and providing an understanding of the underpinning chemical and electrochemical phenomena that contribute to the corrosion rate. Contributing of this knowledge was the additional ex situ analysis (SEM, XRD, ICP-MS) to understand both the electrode surface and electrolyte.
The overarching finding of this study was that corrosion rate is non-linear with respect to IL concentration, importantly, the maxima for corrosion rate is coincidental with the maxima in solution conductivity. This highlights that corrosion rate is reliant on both concentration of Cl- and ion migration. Cu substrates undergo similar corrosion mechanisms for all three aqueous ILs; In aqueous [C4C1im]Cl solutions demonstrate three distinct corrosion regimes, whilst in [HC1im]Cl and [C8C1im]Cl solutions only exhibit two behaviours. XRD results also demonstrate significantly different surface damage depending on IL concentration, with higher IL concentrations (75 and 85 mol% water in IL) leading to pitting and etching of Cu surfaces and lower concentrations resulting in the deposition of copper oxides and chlorides. The chemical structural differences of the selected ILs have significant implications on corrosion rate, with [C8C1im]Cl demonstrating corrosion rates an order of magnitude lower than those of [C4C1im]Cl and [HC1im]Cl.
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3. Feng, L. et al., Appl Surf Sci 483, 901–911 (2019).