Corrosion mitigation in desalination plants by ammonium-based ionic liquid

CuNi (90:10) alloy is widely used in desalination plants. CuNi alloy corrosion in sulfide-containing seawater is the fundamental problem in the desalination industry. Here we have confronted this difficulty by using ammonium-based ionic liquid (Diethyl (2-methoxyethyl)-methyl ammonium Bis(fluorosulfonyl)imide) [DEMEMA][FSI]. The results revealed that the [DEMEMA][FSI] can suppress Cu–Ni alloy corrosion in a solution of (3.5% NaCl + 10 ppm sulphide) with an efficiency of 98.4% at 120 ppm. This has been estimated by electrochemistry and gravimetry. Furthermore, [DEMEMA][FSI] inhibits the growth of sulfate-reducing bacteria SRB in saline water. Surface morphology testing confirmed [DEMEMA][FSI] adsorption on Cu–Ni surface alloys. In addition, quantum calculations have been used to theoretically predict inhibition efficiency [DEMEMA][FSI].

Desalination from sea water is one of our time's main challenges 1,2 . The main task of desalination plants is to remove salts from marine water to produce drinking water 3 . During the desalination processes, the saline water pass though various tubes and units contain different metal and alloys 4 . The CuNi (90:10) alloy is the best alloy for constructing condensers and heat exchangers in desalination plants 5,6 . The corrosion resistance of CuNi (90:10) is very high due to the formation Cu 2 O passive film on the alloy surface 7,8 . However, this passive film can be eliminated by sulfide containing seawater 9 . Where the sulfide ions convert the strong Cu 2 O passive film to a tenuous black layer 10 . The source of the presence of sulfide ions in the seawater comes from industrial waste and/or bacteriological processes (Sulfate reducing bacteria SRB) 11 . This condition causes significant corrosion issues in desalination plant units and cooling systems 12,13 .
Organic inhibitors, such as azoles, Schiff bases and amino acids are widely used to protect CuNi alloy from corrosion [14][15][16] . However, these organic inhibitors are hazardous materials 17 .
Alternatively, toxic corrosion inhibitors are replaced with environmentally benign materials such as ionic liquids (ILs) [18][19][20] . ILs have recently been successfully used in different environments as inhibitors of corrosion [21][22][23] . Most of the ILs contain organic cation and inorganic anions 24 . They also have very low toxicity and low cost 25 .
Guo et al. produced ureido substituted imidazolium bromides and investigated their anti-corrosion effectiveness on steel in HCl solution 26 . They show that the imidazolium inhibitors are effective mixed-type corrosion inhibitors, with higher inhibition efficiencies as concentration and alkyl chain length increase. The inhibition mechanism of three quaternary-ammonium-derived ionic liquids was clearly discussed by Olivares-Xometl et al. 27 . The inhibition efficiency in this study ranged from 55 to 80 percent. Likhanova et al. 28 looked into the impact of organic anions on ionic liquids as inhibitors for steel corrosion in sulfuric acid solution. They found that the ethyl sulphate anion was found to have better inhibitory properties of corrosion. Shetty et al. 29 investigated the use of an environmentally friendly benzimidazolium-based ionic liquid as an inhibitor for aluminum alloy corrosion in acidic solution. The inhibition efficiency of ionic fluid based on benzimidazium was 98.7% in Hydrochloric acid and 98.8% in sulfuric solutions.
In this work, for the first time, ammonium based ionic liquid (Diethyl (2-methoxyethyl)-methyl ammonium Bis(fluorosulfonyl)imide) [DEMEMA][FSI] was used to protect CuNi alloy from corrosion in sulfide containing seawater medium. Our strategy here depends on the decrease of the corrosive action of seawater and the inhibition of the production of sulfide ions from biological activities of SRB.

Materials and methods
CuNi (90:10) alloy was obtained from desalination unit in Egypt with composition: Ni = 10%, Fe = 1.2%, Mn = 0.8%, Cu = Remaining. The preparation of alloy surface before the experiments was conducted according to ASTM G1-03(2017)e1 30  The analar sodium chloride and sodium sulfide with distilled water were used to prepare the corrosive solutions.
All the electrochemical tests were conducted in a standard cell fitted with three-electrode (CuNi alloy, Pt, Ag/AgCl) and recorded by Gamry 3000 electrochemical workstation. The polarization experiments conditions are scan rate = 1.0 mV s −1 , potential range = ± 250 mV vs. OCP, solution temperature = 298 K. Electrochemical impedance measurements (EIS) experiments conditions are frequency range = 30 kHz-0.01 Hz and amplitude = 10 mV at OCP.
Weighing the cleaned CuNi alloy electrodes (dimension 1.5 × 2.0 × 0.2 cm) before and after immersion in tested solutions for 48 h at 298 K was used to calculate gravimetric analysis. The following formula is used to calculate the corrosion rate (C R ): (W = mass loss (mg), S = electrode surface area (cm 2 ), t = immersion time (h)).
Desulfovibrio desulfuricans (SRB stains) was isolated from EAST BAHARIA oil fields (Egypt). Postgate's C (PGC) medium was used to inoculate SRB cultures. After 30 min of purging with high-purity nitrogen, the medium was degassed and autoclaved at 120 °C. Seven-day-old bacteria were injected into the testing system. Total SRB count (CFU) was calculated according to ASTM D4455-85 31 .
The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) was used for surface characterization (model: JEOL /JSM6510) equipped with EDX unit.
VAMP module (materials-studio, Accelrys) was used to calculate the quantum chemical parameters for [DEMEMA][FSI] in gas phase.

Results and discussion
Electrochemical measurements. The impacts of [DEMEMA][FSI] on the corrosion rate of Cu-Ni alloy in (3.5% NaCl + 10 ppm sulfide) solution was analyzed using polarization tests (see Fig. 1). As evident from  Fig. 1 were used to extract the corrosion current density (i corr ), corrosion potential (E corr ) and anodic/cathodic Tafel slopes (B a , B c) (see Table 1) 32  (1)  EIS measurements were performed in (3.5% NaCl + 10 ppm sulfide) and in inhibited solutions. Typical EIS are presented in Fig. 2 in the form of Nyquist plots. All Nyquist plots display two depressed semicircle. The 1st one at high frequency zone and it is related to film layer 35,36 . The 2nd one at low frequency zone and it is related to corrosion reaction on the alloy surface 37,38 . The Nyquist plots exhibit Warburg impedance at low frequency zone due to diffusion process. The Warburg impedance seems to as a diagonal line with a 45° slope on a Nyquist plot. There is two phase maximum in all cases (Fig. 2) Table 2 39 .
Surface coverage (θ) values were determined using impedance data, Eq. (3).   www.nature.com/scientificreports/ R CT and R 0 CT represent the electron transfer resistance in inhibited and blank solutions, respectively. The Nyquist plots can be well simulated using the equivalent circuit sketched in Fig. 3. Table 2    [FSI] concentration. At 120 ppm, the greatest inhibitory efficiency (94.8%) was achieved using gravimetric measurements. It's worth noting that the gravimetric data in Table 3 corroborate the EIS and polarisation data in Tables 1 and 2. Surface characterization. In order to know some information on the composition of layer formed on the surface of Cu-Ni alloy in (3.5% NaCl + 10 ppm sulfide) solution in the absence and presence of [DEMEMA] [FSI], the SEM and EDX analysis were performed and presented in Fig. 4. After Cu-Ni alloy immersion in (3.5% NaCl + 10 ppm sulfide) solution (Fig. 4a), the alloy surface was completely covered by black corrosion product and the surface suffered from severe crevices. According to the EDX analysis (recorded in Fig. 4a), the corrosion products are mainly copper oxide and copper sulfide. Furthermore, the image in Fig. 4b    Biological activities of SRB. The biological activities of SRB lead to sulfide ions generation in the seawater and this represents the major problem in the corrosion of Cu-Ni alloy in desalination plant 43 Fig. 5. The charger distribution in the HOMO and LUMO is concentrated on the N, O and S atoms. This indicates that these atoms represent the adsorption centers 45 . The quantum calculations in Table 5 indicate that the [DEMEMA]

Gravimetric analysis.
[FSI] has the high HOMO energy (i.e. E HOMO = − 7.33 eV), which reflects the high ability of ionic liquid molecules to pay the electrons to the unoccupied orbital of Cu-Ni alloy 46 . On other hands, the low LUMO energy (i.e. E LUMO = -2.63 eV), reflects the high ability of ionic liquid molecules to receive the electrons from the Cu-Ni alloy. Moreover, the small energy band gap (i.e. ΔE = 4.7 eV) confirms that the interaction between [DEMEMA] [FSI] molecules and Cu-Ni alloy surface is strong 47  A high χ value suggests a high chance to acquire electrons and, consequently, a high adsorption performance. The number of electrons transfer from the molecule to the metal alloy (ΔN) is given by 49 : The value of ΔN (see Table 5) demonstrated an inhibition effect caused by transferring electrons, which is consistent with the findings of Lukovits et al. 50 . This finding confirmed the hypothesis that [DEMEMA][FSI]   Fig. 6).
The adsorption processes can be summarized in the following steps 51 : [DEMEMA] + ) on the SRB cell wall and followed by penetration inside the cell 56,57 .This leads to the degradation of proteins and nucleic acids inside the SRB cell.

Conclusions
In this study, for the first time, ammonium based ionic liquid (Diethyl