Abstract
The corrosion behaviour of 316L stainless steel (316L ss) in aqueous solutions of ionic liquid 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]) with different contents of water was investigated by scanning electron microscopy (SEM), potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). The results indicate that 316L in aqueous solutions of ionic liquid [EMIM][DEP] shows an obvious surface passivation, a broad passive region and low corrosion current densities. It also has better corrosion resistivity than in 0⋅177 M LiBr aqueous electrolytes. Furthermore, in order to explain the corrosion mechanism in two working fluids, the Van der Waals volume of ions and the interaction energies of ion pairs were calculated based on the density functional theory (DFT) in quantum chemistry combined with gradient corrected functional using Gaussian 09.
Introduction
With rapid economic development, environmental and energy issues have become more and more prominent. Using green chemicals and clean energy or improving the utilisation efficiency of energy resources are the main ways to solve these issues. Absorption heat pump or absorption refrigerator are important energy saving devices, which are driven by industrial waste heat, solar energy and so on. The conventional working pairs in absorption heat pump or absorption refrigerator are mainly ammonia-water and lithium bromide-water.1 However they have great disadvantages of toxicity, corrosion and crystallisation respectively.2 Therefore, it is necessary to seek new working pairs to overcome these problems in application.
Ionic liquids are widely researched and used in various fields such as organic synthesis,3 extraction,4 catalysis reaction,5 synthesis of inorganic nanomaterials6 and electrochemistry.7 The properties of ionic liquids make some of them excellent candidates for use as absorbents of refrigerant in absorption heat pump or absorption refrigerator.8 – 11 However the corrosion behaviours of ionic liquids on metal equipments is also an important factor to be considered in its real industrial applications. After the investigation of the corrosion of ionic liquid 1-butyl-3-methyl-imidazolium bis-(tri-fluoromethanesulfonyl) imide on the AZ91D alloy, copper and Inconel 600 at different temperatures, appreciable amounts of carbon, oxygen, nitrogen, fluorine and sulphur were detected on the alloy's surface due to thermal decomposition of the ionic liquid at high temperature, and the increase of corrosion rate showed its dependence on temperature.12 – 14 The corrosion behaviours of carbon steel, austenitic stainless steel, copper, brass and aluminium in seven ionic liquids showed that the corrosion of ionic liquid-containing media depends mainly on the chemical structure of the cation and the nature of the anion.15 The corrosion measurements of magnesium and AZ91 alloys in 1-butyl-3-methyl-imidazolium trifluoromethylsulfonate with variable water contents indicated that higher the content of water in the ionic liquid, the lower the corrosion potential and the higher the corrosion rate.16 Zhang and Hinton17 have found that both trimethyl(butyl)phosphonium diphenyl phosphate and trihexyl(tetradecyl)-phosphonium bis-2,4,4-trimethylpentyl-phosphinate pretreatments can increase the corrosion resistance of AZ31 in simulated body fluids. Currently 316L stainless steel is the main material for heat pumps, so it is important to investigate the corrosion behaviours of 316L in ionic liquids and its solutions.
In this work, the corrosion behaviour of 316L ss in the aqueous solutions of [EMIM][DEP] or LiBr were investigated by SEM analysis and electrochemical techniques. The results indicate that 316L ss has better corrosion resistivity in aqueous solutions of [EMIM][DEP] than in aqueous solution of LiBr. Furthermore, the results were also explained by the Van der Waals volume of ions and interaction energies of ion pairs based on density functional theory (DFT) combined with gradient corrected functional using Gaussian 09.
Experimental
Ionic liquid [EMIM][DEP] used in this work was synthesised in the laboratory according to the following method.18 After being purified by a rotary evaporator, an appropriate amount of N-methylimidazole was placed in a 1000 ml flask fitted with a return condenser and mixed with an equimolar amount of triethyl phosphate. The reaction lasted 10 h at 423 K. When the resultant was cooled to room temperature, the unreacted reagents were extracted from it with ether. The raffinate was treated with the rotary evaporator to remove volatile impurities. The structure of the synthetic product [EMIM][DEP] was characterised with a nuclear magnetic resonance spectrometer (1H NMR). The 1H NMR spectra showed the same structure as the literature.19
The surface of 316L ss specimens with an exposed area of 0⋅5 cm2 for electrochemical tests and 1 cm2 for immersion tests were mechanically polished to mirror-like smoothness by 1200 grade wet SiC abrasive papers. After polishing, the samples were rinsed with distilled water, degreased with ethanol and finally dried under air.
All experiments were carried out under the protection of a nitrogen atmosphere. The morphology of samples after 336 h of immersion in different corrosive liquids was studied by SEM operating at 20 kV (JEOL-6300, JAPAN).
Electrochemical measurements were performed using an electrochemical workstation (CS310 from Wuhan Corrtest Instrument Co., Ltd) with a three-electrode system. The 316L sample was used as a working electrode (WE), the counter electrode (CE) was a platinum electrode, while the reference electrode (RE) was a saturated calomel electrode (SCE). As pointed out by Zhang,20 the sweep rate has an effect on the corrosion current density and the suitable range is 0⋅5-1 mVs−1. The potentiodynamic polarisation curves were obtained by using a sweep rate of 1 mVs−1, which was the same as literature,20 in the potential range from −0⋅5 V to 1⋅5 V versus the open circuit potential (OCP). In the electrochemical impedance spectroscopy (EIS) experiments, the range of frequency was 105–10−2 Hz with a 10 mV amplitude perturbation at the corrosion potential. Each experiment was repeated at least three times to check the reproducibility. The impedance data was analysed using the Zview software and the fitting error was less than 5%.
Results and Discussion
Scanning electron microscopy
Figure 1 shows the corrosion morphology of 316L ss in solutions of [EMIM][DEP](1)+H2O(2) or LiBr(1)+H2O(2), the samples exposed to aqueous solutions of [EMIM][DEP] (x 1 = 0⋅5) at 416 K and LiBr (x 1 = 0⋅177)) at 408 K for 336 h. The sample was seriously corroded in the LiBr solution and a rough surface with deep split cracks was seen (Fig. 1c and d). However, the sample was almost undamaged in the ionic liquid solution and exhibited complete integrity (Fig. 1b) even at higher temperature compared to the bare sample (Fig. 1a). The result confirms that 316L ss has good resistivity in the ionic liquid solution.
Scanning electron microscopy (SEM) micrographs of 316L ss a bare sample, ×10 000; b, ×10 000 in [EMIM][DEP] (x 1 = 0⋅5) aqueous solution at 416 K for 336 h; c, ×10 000 and d, ×1000 in LiBr aqueous solution at 408 K for 336 h
Electrochemical techniques
The OCP curve of 316L stainless steel that immerged in the solution [EMIM][DEP](1)+ H2O(2) (x 1 = 0⋅2) at 293 K is shown in Fig. 2. The OCP increases constantly and reaches to a steady value finally. Similarly it has the same tendency in LiBr aqueous solution containing Li2CrO4 as the corrosion inhibitor. The main reason may be that the imidazolium cation acts as an inhibitor and forms a passive film on the substrate in the solution of [EMIM][DEP]. As the imidazolium cation [EMIM] contains an aromatic ring that has π electrons and a nitrogen atom with a free electron pair available to be donated on the metallic surface, the chemisorption occurs by a retro-donation mechanism and [EMIM] is deposited on metallic surface and forms a passive film to protect the substrate from corrosion.21
Open circuit potential curve of 316L stainless steel that immerged in the aqueous solution of [EMIM][DEP] (x 1 = 0⋅2) and 0⋅177M LiBr solution at 293 K
The potentiodynamic polarisation curves of 316L ss in [EMIM][DEP] (with 2400 ppm H2O), solutions of [EMIM][DEP](1)+H2O(2) (x 1 = 0⋅6 and 0⋅2) and 0⋅177M LiBr solution are shown in Fig. 3. All potentiodynamic polarisation curves of the samples have the same shape, and show obvious passivation phenomenon and a broad range of passive region. Beyond the anodic activation area, the corrosion current density becomes basically stable in the passive region. When the potential reaches a certain value (namely breakdown potential), the increasing of current density causes pitting gradually. The slow rather than a sudden increasing of the polarisation current density near the pitting potential indicates that the occurrence of spot corrosion is due to the partial dissolution of the passive layer caused by adsorption of aggressive anions.22
Potentiodynamic polarisation curves of 316L ss in a [EMIM][DEP] (with 2400 ppm H2O), b aqueous solution of [EMIM][DEP] x 1 = 0⋅6 c, x 1 = 0⋅2 and in d 0⋅177M LiBr aqueous solution
The potentiodynamic polarisation curves in Fig. 3 reveal that higher the temperature, the higher the passive current density and the corrosion rate, which is consistent with the reports in literature.15 However, the passive region still remains quite broad with the rise of temperature. The corrosion current densities of the sample in different solutions and at different temperature are listed in Table 1, obtained from the polarisation plots by extrapolating the linear portion of the curve at the corrosion potential. The error of temperature is ±0⋅5 K and ±0⋅2 μA cm−2 for corrosion current density.
Corrosion current density (I c) of the samples in different testing electrolytes at various temperatures
Figure 4 shows the potentiodynamic polarisation curves of 316L ss in four aqueous solutions and at 313 K, 343 K and 373 K, respectively. It can be seen from the figures that the higher the water content in ionic liquid, the greater the corrosion current density. The corrosion current densities of the sample in ionic liquid aqueous solutions are lower than those in the industrial LiBr solution (containing corrosion inhibitor) at the same temperature. Moreover, the passive region of the specimen in the [EMIM][DEP] solution is much wider than that in the LiBr solution. 316L ss is more anticorrosive in the aqueous solution of [EMIM][DEP] as a result of the passivation effect.
Potentiodynamic polarisation curves of 316L ss in four aqueous solutions at a 313 K, b 343 K and c 373 K
Figure 5 shows the Nyquist plots of 316L ss immerged in the aqueous solution of [EMIM][DEP] (x 1 = 0⋅2) at 298 K for increasing time intervals. The obvious increase in the real axis intercept with time, based on the extrapolation of the semicircle, indicates the increase in the conversion coating resistivity, which was consistent with the result of the open circuit potential test. After 87 h the curves were overlapping, which implies the complete formation of the passivation coating. Indeed, the corrosion products on the surface can enhance the corrosion resistivity of 316L ss.
Nyquist plots of 316L ss in aqueous solution of [EMIM][DEP] (x 1 = 0⋅2) at 298 K for increasing time intervals
In Fig. 6a, Nyquist impedance spectroscopy of 316L ss electrodes in the ionic liquid (with 2400 ppm H2O) shows a capacitive arc in high frequency and a straight line region called ‘Warburg impedance’ in low frequency. The small semicircle in the high frequency region is attributed to the time constant, which is related to both charge transfer resistant R ct and the double-layer capacitance (C dl). The Warburg impedance reflects the anodic diffusion process of soluble iron species from the surface of the electrode to the bulk solution and the cathodic diffusion process of dissolved oxygen from the bulk solution to the surface of the electrode. It can be noted that the corrosion process is controlled by electrochemical reactions and the diffusion of the reactants. At 298 K, the Warburg resistance (in Fig. 5) is not visible, while it appears at higher temperatures (in Fig. 6). This may be that the reaction rate is slow and the diffusion process is weak at low temperature.23
Nyquist plots of 316L in a [EMIM][DEP] (with 2400 ppm H2O) and b 0⋅177M LiBr aqueous solution
Table 2 lists the element values of the equivalent circuit shown in Fig. 7. In the circuits, the constant phase elements (CPE) were substituted for the double-layer capacitors for more accurate results. Intuitively, there is a reduction of radius of the capacitive reactance arc in high frequency with increasing temperature. Hence the charge transfer resistance Rct in Table 2 and the erosion resistance of 316L ss decline with a rise of temperature. The large diffusion resistance in low frequency slowing down the corrosion rate effectively is due to the corrosion resistant protective film on electrode and the high viscosity of ionic liquid.
Equivalent circuit proposed for simulating the electrochemical response in [EMIM][DEP] aqueous solutions
Values of elements in equivalent circuit to fit the impedance spectra for 316L ss in [EMIM][DEP] aqueous solutions
Corrosion resistance of electrodes can be quantitatively evaluated by the low frequency impedance values (Zlf). From Fig. 8 it can be seen that for all the samples in the ionic liquid, the Zlf values were higher compared to the ones in the LiBr solution, which agree with the results obtained from the polarisation techniques. On the other hand, the Zlf values for all samples will decrease with increase in the water content. Dilution with water can lead to the hydrolysis of the anion of the ionic liquid, and then produce acid (e.g. phosphoric acid) causing acid corrosion15 and make the charge transfer resistant Rct decline.
Bode plots of 316L ss in four aqueous solutions at a 313 K, b 343 K and c 373 K
Explanation based on quantum chemistry
Usually, pitting corrosion occurs under the condition of an existing protective film. Bianchi et al.24 described some possible mechanisms for pitting nucleation, one of which is aggressive ions penetrating into the oxide film from where defects exist and reacting with the substrate to form soluble compounds (such as FeBr2). Pitting corrosion mainly depends on the ion penetration rate and dissolved ability of the corrosion product in solution. Ion penetration rate mainly relies on its volume and solution viscosity. The bigger the ion volume or higher the viscosity of the solution, the more difficult it is for ions to penetrate into the film.
The Van der Waals volume of ions, and interaction energies of ion pairs were calculated based on Gaussian 09. In calculation, LACVP** basis set was used, which corresponds to the double-f basis set with polarisation functions 6-31G(d,p) for C, H and N, whereas for Fe the pseudo potential LanL2DZ was used.
The calculated regions of Br− and DEP− encompassed by the isovalue of 0⋅002 e/bohr3 are shown in Fig. 9.The calculated Van der Waals volume of DEP− and Br− are 157⋅21 Å3 and 42⋅28 Å3, respectively, in this research.
Regions of Br− and DEP− encompassed by the isovalue of 0⋅002 e/bohr3
The interaction energy of ion pairs is defined as follows25
All interaction energies were counterpoise(CP) corrected with the procedure of Boys and Bernardi26 in order to account for the basis set superposition errors (BSSEs), from which we got ΔECP. In addition, the zero-point vibration energy (ZPE) correction was considered to obtain ΔEZPE.
For the ion pairs of FeBr2 and Fe[DEP]2, their optimised geometries are shown in Fig. 10 and interaction energies are listed in Table 3. The absolute value of the interaction energy of Fe[DEP]2 is much larger than FeBr2, namely the energy for hydrolysis Fe[DEP]2 is much larger than FeBr2. Therefore, FeBr2 is easier to dissolve in water, which accelerates the breakage of the protective film. These calculation results based on quantum chemistry are well in agreement with the above experiment results.
Optimised geometries of FeBr2 and Fe[DEP]2
Interaction energies of ion pairs
Conclusion
A protective film can be formed on 316L stainless steel in aqueous solutions of [EMIM][DEP], which leads to a broader passive region and better corrosion resistance than those in LiBr. Increasing temperature and water content makes the corrosion rate of 316L ss in aqueous solutions of [EMIM][DEP] rise and its resistance decline. According to quantum chemical calculation, 316L ss has better erosion resistance in the [EMIM][DEP] aqueous solutions than in the LiBr aqueous solution because the DEP− anion has the larger Van der Waals volume, larger interaction energy with Fe2+ and lower transfer rate than Br−.
Footnotes
Acknowledgements
The authors are grateful for the financial support from National Natural Science Foundation of China (No: 51076021, 50876014) to this research project.