Control of chloride ion corrosion by MgAlOx/MgAlFeOx in the process of chloride deicing

Adding a corrosion inhibitor to the chloride deicing salt can prevent the corrosion and pollution of Cl-, which is very important. Layered double hydroxide (LDHs), calcined at high temperature is used as adsorbents to remove various anionic contaminants, and it can reduce the freezing point of solution after adsorbing anions. Therefore, this paper reports the use of calcined LDHs as corrosion inhibitors in deicing salts, which are denoted as MgAlOx or MgAlFeOx depending on the preparation element. By analyzing the removal efficiency and the freezing point of MgAlOx and MgAlFeOx to Cl-, the feasibility of the study was determined. Resulted that the removal efficiency to Cl- of MgAlFeOx at low temperature (0 ± 2 °C) and room temperature (25 ± 2 °C) was higher than that of MgAlOx, reaching 39.4% and 85.60%, respectively. And the freezing point of MgAlFeOx was lower than that of MgAlOx, the value was −12.0 °C. At the same time, we also found that CaCl2-MgAlOx and CaCl2-MgAlFeOx significantly reduced the corrosion of carbon steel and concrete compared with chloride salts, and CaCl2-MgAlFeOx had the lowest corrosion degree. Hence, MgAlFeOx was chosen as the corrosion inhibitor in chloride deicing salt. The metal molar ratio, synthesis temperature, and calcination temperature for preparation of MgAl/MgAlFe-LDHs were determined by XRD and TG-DSC analysis that were 9/2/1, 120 °C, and 500 °C, respectively. Characterization methods such as Zeta, XRD, XPS, BET, and SEM were used to study in detail the characteristic changes of MgAlFe-LDHs and MgAlFeOx after Fe3+ was added, and the mechanism of corrosion inhibitors was further determined that was achieved by adsorption and neutralization.


Introduction
In order to prevent or mitigate the traffic influence and hazards caused by snowfall in winter, it is necessary to quickly remove snow and ice from the road surface (Yuan et al. 2019). At present, the methods of deicing mainly include human and mechanical deicing (Laforte et al. 1998), thermal deicing (Yan et al. 2020;Lan et al. 2019), chemical deicing (Ganjyal et al. 2007;Zheng et al. 2018), and active anti-icing technology (Fn et al. 2019;Yong et al. 2014), etc. Because chemical deicing is the fastest way to remove ice and snow, it is the most widely used. And chemical deicing is achieved by using deicing salts (Hao et al. 2019). Among them, the mixed deicing salt has a low freezing point and has little impact on the environment, so it has attracted the attention of deicing departments in various countries (Warner and Ayotte 2015). However, with the extensive use of deicing salts, it has been corroded concrete and steel in infrastructure, such as roads, airports, and bridges . The cause of corrosion is attributed to Cl -. Due to the small ionic radius, Clcan penetrate into the concrete with the solution and achieve the purpose of breaking the internal structure and corroding the metal surface of the concrete; this is a longterm cumulative process (Farnam et al. 2015a). It is necessary to reduce the corrosion of infrastructure while ensuring the rapid deicing of the mixed deicing salt. Therefore, many researchers have done a lot of research on the corrosion inhibitors in deicing salt, and it is concluded that the addition of corrosion inhibitors greatly reduces the erosion of Cl - (Farnam et al. 2015b;. In general, corrosion inhibitors are classified into three categories according to their chemical composition. Inorganic corrosion inhibitor mainly composed of chromate, nitrite, silicate, etc., whose main function is to passivate the surface of the metal. Although it has remarkable anticorrosion effect, its biodegradability is low. So, it can cause secondary environmental problems (Ruza et al. 2019). The anticorrosion method of organic corrosion inhibitors is to prevent corrosive substances from approaching the surface of infrastructure through physical adsorption and chemical adsorption, which mainly including some heterocyclic compounds, such as polyvinyl amide, phosphonic acid, and polyaspartic acid (Tiu and Advincula 2015). And the anticorrosion mechanism of these substances is achieved through the combination of polar groups and nonpolar groups with the metal on the surface of the infrastructure in the form of bonds (Amin et al. 2010). Although organic compounds have fine anticorrosion and do not pollute the environment, they will be combined with the surface of the infrastructure, and the phenomenon of "salt dirty" will appear and affect the aesthetics of roads (Silva et al. 2006). In addition, corrosion inhibitors also include polymer corrosion inhibitors, such as polyethylene and POCA, which have fine anticorrosion and biodegradation, but they can also be adsorbed on the surface of objects (Benchikh et al. 2009). In summary, the current problem with adding corrosion inhibitors is secondary pollution to the environment and "salt dirty" phenomenon. Hence, the choice of corrosion inhibitors must not only consider the issue of environmental quality but also pay attention to the aesthetics of the roads. If the corrosion inhibitor has a freezing point after absorbing Cl -, this is a major breakthrough on the original basis. Compared with others, the corrosion inhibitor with a freezing point not only has a fine anticorrosion effect but also can melt snow to reduce the amount of deicing salt. As the continuous improvement of environmental protection awareness, it is imperative to research high-efficiency, nontoxic, and nonpolluting corrosion inhibitors.
The general chemical formula of the layered double hydroxide is [M 1-x 2+ M x 3+ (OH) 2 ] (A n− ) x/n ·mH 2 O. Because of adjustable main metal elements and the interlayer structure, LDHs have a very wide application field (Ingram and Taylor 1967;Allmann 2010). Among them, the high-temperature calcined LDHs (CLDH) can be used as adsorbents to adsorb numerous anions, which has the characteristics of "memory effect," physical adsorption, and high specific surface area (Huang et al. 2017;Dan et al. 2012). Wei et al. found that the Mg-Al-Fe hydrotalcite-like compound after 500°C calcination (HTlc500) had a good removal effect on fluorine, and the adsorption data could be well fitted by the Langmuir isotherm model and the pseudo-first-order kinetic model (Wang 2011). The anticorrosion performance of various forms of MgAl-LDHs synthesized on AA6082 was investigated, and it is confirmed that LDHs have a good inhibitory effect on the corrosivity of metals (Ahsan and Michele 2018). Numerous scholars have been studied the adsorption mechanism of CLDH. It is confirmed that the adsorption process of CLDH includes not only physical adsorption but also chemical adsorption (Zheng et al. 2019). At the same time, Chao et al. found that Mg-Al-LDHs can reduce the solution's freezing point (Peng et al. 2015). In summary, CLDH can be used as a corrosion inhibitor at low temperatures.
Hence, the main purpose of this article is to discuss the corrosion inhibition properties of MgAlO x and MgAlFeO x . Then, according to the corrosion inhibition performance, we chose one of them as corrosion inhibitor. Finally, used XRD, TG-DSC, Zeta, XPS, BET, SEM, etc. characterization methods detailedly analyze the structure and morphology changes of LDHs and CLDH doped with Fe 3+ , as well as the relationship with the corrosion inhibition performance.

Preparation of the MgAlOx/ MgAlFeOx corrosion inhibitor
Adding Mg (NO 3 ) 2 .6H 2 O, Al (NO 3 ) 3 ·9H 2 O, and Fe (NO 3 ) 3 · 9H 2 O with different molar ratios (Table S1) to the beaker to form A solution with a concentration of 0.55 mol·L -1 . The NaOH and Na 2 CO 3 were added in the beaker as the molar ratio of 3:1 to form 2.25 mol·L -1 as B solution. Then, the A and B solutions were simultaneously dropped into the deionized water to form C solution, controlling the drip rate (about 1 drop/s) and using the PHSJ-3F/4F model pH meter to detect the pH of 9.5 in real time. The mixture was stirred vigorously for 30 min, placed in a stainless steel autoclave of Teflon, and then put in an oven at reaction temperature which was 80°C/ 100°C/120°C for 12 h. After the completion of the reaction, the mixture was centrifuged and washed to neutrality, dried in an oven at 80°C for 10 h, and ground into a powder to finally obtain MgAl-layered double hydroxide (MgAl-LDHs) or MgAlFe-layered double hydroxide (MgAlFe-LDHs). The newly prepared LDHs were placed in a muffle furnace and calcined at different temperatures for 4 h, and the heating rate was 2°C·min -1 to obtain composite metal oxide MgAlO x or MgAlFeO x . The preparation process is shown in Fig. 1.

Adsorption experiment of Clby the corrosion inhibitor
Determine the corrosion resistance of MgAlO x and MgAlFeO x to Claccording to batch adsorption experiments under different conditions. CaCl 2 was dissolved in deionized water to prepare a 100 mg·L -1 chloride solution. Approximately, 0.2 g MgAlO x or MgAlFeO x was added to the Erlenmeyer flask containing 100 mL CaCl 2 solution and then put on the stirrer rotating at 150 rpm to carry out adsorption experiments at low temperature and room temperature, respectively. The low-temperature was 0 ± 2°C and room temperature was 25 ± 2°C. Since deicing salt's residence time should not be too long, we chose 200 min for the longest reaction time. After the experiment was completed, the solution was separated using a 0.45 μm filter, and the amount of residual Clwas measured by titration.

Determination of Cl -
The concentration of Clwas determined by Chinese Standard GB/T 15453-2008. Firstly, pipetting 50 mL of chloride solution into a 250-mL Erlenmeyer flask. Secondly, adding 2 drops of phenolphthalein indicator, adjusting the pH value of the water sample with a weak base (0.1 mol·L -1 NaOH) or weak acid (0.1 mol·L -1 HNO 3 ) to make a solution change from red to transparent. Then, adding 1-mL 50-g·L -1 K 2 CrO 4 as reaction solution. Lastly, titrating with mol·L -1 AgNO 3 standard solution until brick red appeared, which is regarded as the end point, simultaneously, preparing a blank test for comparison. The formula for calculating the Clconcentration is as follows: where P 1 is concentration of Cl -, mg·L -1 ; V 1 is the value of AgNO 3 's volume consumed by the titrating chlorine solution, mL; V 0 is blank test consumes the volume of AgNO 3 solution, mL; V is the sample's volume, mL; c is the concentration of AgNO 3 solution, mol·L -1 ; M is the molar mass value of chlorine, g·mol -1 (M=35.45).

Corrosion inhibiting experiment
The anticorrosion performance of the corrosion inhibitor was studied through the dry-wet cycle of concrete and the rotary suspension of carbon steel. And the concrete and the carbon steel sheet were put into 5% deicing salt solution, respectively. After the experiment, it could be determined the corrosion strength of deicing salt by analyzing the quality loss of concrete and the corrosion rate of carbon steel. Deicing salt was obtained by uniformly mixing the corrosion inhibitor and anhydrous calcium chloride at a mass of 1:1 and was denoted as CaCl 2 -MgAlO x or CaCl 2 -MgAlFeO x , which was obtained in our previous research . The experimental methods of anticorrosion research are in the supplementary information.

Test of corrosion inhibitor freezing point
Placed the corrosion inhibitor in a CaCl 2 solution with a concentration of 100 mg·L -1 , and determined its freezing point at different concentrations (5 wt%, 10 wt%, 20 wt%, 30 wt%, and 40 wt%). This analysis method was implemented in accordance with the Chinese standard GB/T23851-2009 (consistent with ASTM D 1177-94), and the measured value was accurate to 0.1°C The determination of pH The 1.00 g ± 0.01 g deicing salt was added into a 250-mL beaker, and it was moistened with 5-mL ethanol. The 100-mL carbon dioxide-free water was added into the beaker to dissolve it, and a 100-mL solution was taken out of it for use. A standard buffer with a pH value close to the sample solution was used for positioning. The electrode was rinsed with deionized water and then with the sample solution. Both the sample solution temperature and the temperature compensation of the pH meter were adjusted to 25°C. The pH of the deicing salt solution was measured by the PHSJ-3F/4F acidity meter. In order to measure accurate results, the pH reading was stable for at least 1 min.

Regeneration experiment of corrosion inhibitor
The 0.2 g used corrosion inhibitor was put in 100-mL 0.28 mol·L -1 Na 2 CO 3 solution, and the mixed solution was placed on a stirrer for 12 h. The filtered product was washed with deionized water and dried in an oven at 80°C for 10 h. Finally, the dried powder was placed in a muffle furnace and calcined at 500°C for 4 h (2°C·min -1 ) to obtain the product which was again used as a corrosion inhibitor. Anticorrosive resistance after corrosion inhibitor regeneration is determined by measuring its removal rate of Cl -.

Results and discussion
The effect of preparation conditions on MgAlFe-LDHs Different temperatures of the reaction process Figure 2b shows the XRD analysis of MgAlFe-LDHs prepared at different temperatures (80°C, 100°C, and 120°C), respectively. All of them exhibited characteristic diffraction peak of double-layer hydroxide (Lv et al. 2019). It can be seen from Fig. 2b that when the reaction temperature was 80°C, only the (003) and (006) crystal planes of LDHs (PDF#52-1625) appeared, and the corresponding peaks were not sharp and contained impurities Mg(OH) 2 and Al(OH) 3 . This indicated that the chemical process was not completely reactive and the crystallinity of the sample was low (Jian et al. 2019). When the temperature was 100°C, the characteristic diffraction peaks of LDHs were (003), (006), (012), (110), and (113) crystal planes (PDF#52-1625) (Tao et al. 2018), but the symmetry peak was not obvious and also contained impurities. At a temperature of 120°C, the characteristic diffraction peaks of LDHs had a sharp peak shape and a perfect symmetrical peak, and no other phases. It shows that as the synthesis temperature increased, the purity of the sample was higher, the crystallinity was more ideal, and the content of impurity phases was less (Xu and Lu 2005). Therefore, 120°C was the optimum preparation temperature.
The TG-DSC and calcination temperature of MgAlFe-LDHs The thermal behavior of MgAlFe-LDHs was analyzed by TG-DSC spectroscopy in Fig. 2c. It can be seen from the curve that the thermal decomposition of MgAlFe-LDHs was mainly divided into three parts ( Ji et al. 2017). The first absorption peak rose from 30 to about 258°C, which was mainly caused by the decomposition of water molecules adsorbed on the surface and crystal water between layers. The endothermic peak between 274 and 445°C was attributed to the decomposition of the interlayered crystal water and the -OH layer and part of the CO 3 2- . The third absorption peak was in the range of 466 to 600°C. Because the interlayer anions were completely removed in the form of CO 2 and H 2 O in this temperature range, there was a significant mass loss . Table S2 is the detailed TG-DSC results for MgAlFe-LDHs. Figure 2d shows the columnar distribution of the Clremoval efficiency of MgAlFeO x at different calcination temperatures. It can be seen from the figure that the removal efficiency after calcination at 500°C was the highest, reaching 85.5%, which was significantly higher than the removal efficiency of Clat other temperatures (300°C~30.70%, 400°C 63.23%, 600°C~24.32%). Combined with the analysis of TG-DSC, when calcination temperature was lower than 572°C , MgAlFe-LDHs sequentially lost interlayer water and water molecules on the hydroxyl layer and CO 3 2-, and until they formed metal oxides after complete dehydration (Ji et al. 2017;Yang et al. 2012). This calcined product could restore the original layered structure after the re-adsorption of anions. And the Clwas fixed by surface charge adsorption and chemical adsorption. Therefore, the removal efficiency was increased along with increasing calcination temperature between 100 and 500°C. When the temperature exceeded 572°C , a calcined product with the spinel structure was formed, whose structure was unrecoverable by absorbing anions, and Clwas fixed by physical adsorption on the product surface, so the corresponding Cl-removal rate was low (Jian et al. 2019).
Hence, when the temperature was 600°C, the removal efficiency decreased. After the above analysis, it is confirmed that the optimum calcination temperature of MgAlFe-LDHs was 500°C.

Adsorption of chloride ions by corrosion inhibitors
Since the corrosion of the chloride deicing salt is caused by Cl - (Farnam et al. 2015b;, the anticorrosion capability of corrosion inhibitor is quantitatively expressed by the removal efficiency of Cl -. Figure 3a shows the removal efficiency of Clby MgAlO x and MgAlFeO x at low temperature (0 ± 2°C) and room temperature (25 ± 2°C). It can be seen from the figure that the adsorption capacity of MgAlO x and MgAlFeO x to Clwas increased as time growth. When the adsorption time was 60 min, the removal efficiency of Clby MgAlO x and MgAlFeO x at low temperature was 7.1% and 10.7%, respectively, and that at room temperature was 19.71% and 39.34%. When the time was 200 min, the removal efficiency of Clby MgAlO x and MgAlFeO x was respective 25.6% and 39.6%, respectively, at low temperature, and it also increased to 58.74 and 85.60% at room temperature. The removal efficiency of Clat room temperature was higher than that at low temperatures (the lowest was −2°C). The fluidity and activity of the solution were reduced at low temperature. So, the chemical adsorption was affected, resulting in a decrease in the amount of Cladsorbed . But it could still be seen that MgAlO x and MgAlFeO x had adsorption effect on Clat low temperature. In addition, the Clremoval efficiency of MgAlFeO x was much larger than MgAlO x 's. This indicated that the doping of Fe 3+ in MgAlO x enhanced its ability to remove Cl -. It may be that the addition of Fe 3+ changed the structure of LDHs, which affected the adsorption capacity of MgAlFeO x to Cl - . Therefore, MgAlFeO x was selected as a corrosion inhibitor in the chloride deicing salt.

The anticorrosion capability of corrosion inhibitor
Dry and wet corrosion of concrete It can be found from Fig. 3b that when the concrete was subjected to the wet-dry cycle test in the deicing salts solution, the mass loss of the concrete test block increased with an increase of the soaking time. Among them, the most loss of quality was the cement block in the CaCl 2 solution, which was 0.051 g. Compared with the other two deicing salts, the concrete test block immersed in the CaCl 2 -MgAlFeO x solution had no quality damage before 100 h, and the mass loss was 0.008 g at 168 h, and there was almost no falloff. During the wet and dry cycle test, because the concrete was repeatedly immersed in the deicing salt solution, the chloride salt solution entered the concrete to cause supersaturation and crystallization. Crystallization pressure exacerbated the destruction of concrete (Thaulow and Sahu 2004;Farnam et al. 2015a). Therefore, the control of Clis a key factor for corrosion protection. The analysis shows that the addition of corrosion inhibitors could reduce the corrosion effect of chloride deicing salt on concrete, and MgAlFeO x had a greater anticorrosion effect on concrete. The reason was that MgAlO x and MgAlFeO x could fix Clin the solution of chloride deicing salt by adsorption. In addition, by measuring the pH of solution CaCl 2 -MgAlFeO x (Table S3), it could be known that the addition of corrosion inhibitors adjusted the pH value of the concrete surface solution, which protected the Ca(OH) 2 in the internal structure of the concrete and avoided the destruction of the concrete structure (Wang et al. 2006). Therefore, CaCl 2 -MgAlFeOx exhibited low corrosivity.

Carbon steel corrosion
The experimental results of the rotating coupon show that the corrosion rates of CaCl 2 , CaCl 2 -MgAlO x , and CaCl 2 -MgAlFeO x on carbon steel were 0.31 mm·a -1 , 0.17 mm· a -1 and 0.10 mm·a -1 , respectively (Table S4). Compared with CaCl 2 and CaCl 2 -MgAlO x , CaCl 2 -MgFeAlO x reduced the corrosion of carbon steel by 67.74% and 41.18%, respectively. Zheng et al. researched that the corrosion rate of carbon steel by the mixed deicing salt was 34.78% lower than that of the chloride salt under the same conditions ). Jang et al. found that the corrosion of steel bars caused by salt substitutes and deicing salt added with corrosion inhibitors was 50% lower than that caused by ordinary chloride solutions (Jang et al. 1995). Compared with the above studies, the CaCl 2 -MgAlFeO x deicing salt had a lower corrosion rate on carbon steel. The main reason for the corrosion of carbon steel was the long-term accumulation of chloride on the surface, which continuously reduces the pH value of the surface solution of the steel bar and making the corrosion more and more serious ). Secondly, due to the contact between chloride deicing salt and carbon steel, chloride salt consumed the iron on the surface of carbon steel . Compared with other deicing salts and chloride salts, the advantage of MgAlO x and MgAlFeO x corrosion inhibitors was that they could adsorb and fix Clin the deicing salt solution and reduce the content of Clin contact with the surface of carbon steel. In addition, they existed in the form of CO 3 2--LDHs and Cl --LDHs after Cladsorption . This made the deicing salt solution weakly alkaline, with a pH of 8.2 (Table S3), which increased the acidic solution on the surface of the steel bar, and made it appear neutral or weakly alkaline. Therefore, the corrosion rate of mixed deicing salt to carbon steel was less than other deicing salt. A comprehensive analysis of 3.2 shows that the removal efficiency of Clby MgAlFeO x was higher than that of MgAlO x , so the corrosion rate of CaCl 2 -MgAlFeO x on carbon steel was less than that of calcium chloride and CaCl 2 -MgAlO x .  , and LDHs will be ionized in the solution, so the freezing point of the solution will be reduced (Peng et al. 2015). It can be seen from the figure that as the concentration increased, the freezing point of MgAlO x and MgAlFeO x was decreased. When the mass concentration was 5 wt%, the freezing point of MgAlO x and MgAlFeO x were −3.3°C and −6.2°C, respectively. And when the concentration increased to 25 wt%, the freezing point of MgAlO x and MgAlFeO x , respectively, decreased to −10.0°C and −12.0°C. Combining the conclusion of 3.2, it can be seen that both MgAlO x and MgAlFeO x could be used as corrosion inhibitors. They could not only adsorb Clbut also melt ice. However, MgAlFeO x had higher Clremoval efficiency than MgAlO x at room temperature and low temperature, and MgAlFeO x also had a lower freezing point. Therefore, MgAlFeO x was chosen as a corrosion inhibitor.

The freezing point of MgAlO x and MgAlFeO x
The effect of surface charge on chloride ion adsorption Figure 3d shows the zeta potentials MgAlO x and MgAlFeO x in an aqueous solution of various pH values. Since the pH of the deicing salt solution in the Chinese GB/T23851-2009 standard must be in the range of 6.0-10, this range was selected as the interval for measuring zeta. It can be seen from the figure that as the pH value increased, the zeta potentials of MgAlO x and MgAlFeO x decreased in the range of 25.30-−7.94 mV and 30.30-−1.04 mV, respectively. The pH value of the isoelectric point of MgAlO x was 8.8, and that of MgAlFeO x was 9.8. Therefore, both materials were positively charged at pH=7 (Zheng et al. 2019). From the above analysis, it is known that due to the addition of Fe 3+ , the zeta potential of MgAlFeO x was higher than that of MgAlO x , and the isoelectric point was increased, too.
This phenomenon indicates that the positive charge and physical adsorption capacity of the MgAlFeO x surface were increased, and it had a stronger adsorption capacity for negatively charged anions in the solution (Lei et al. 2016). Hence, the removal ability of MgAlFeO x to Clwas higher than that of MgAlO x .

The effect of Fe 3+ on Clremoval performance
The change of structural characteristics Figure 4 shows the XRD diffraction spectra of uncalcined and calcined LDHs, respectively. From Fig. 4a, it can be seen that the typical diffraction peaks at around 11.6°, 23.3°, 34.5°, 38.7°, 46.0°, 60.2°, and 61.6°correspond to characteristic features of LDHs' planes, which are (003), (006), (012), (015), (018), (110), and (113) (PDF#52-1625), respectively (Tao et al. 2018). It is indicating that both MgAl-LDHs and MgAlFe-LDHs were hydrotalcite-like compound with a typical layered structure, and the peak shape was sharp, and they had symmetrical peaks (Lv et al. 2019). Figure 4b shows samples after calcination at 500°C, and characteristic peak of Fe 2 O 3 appeared in MgAlFeO x (PDF#01-1053). From the above analysis, Fe 3+ doped on the basis of MgAl-LDHs entered the metal main laminate of LDHs through chemical reaction, without destroying the original layered structure. After the incorporation of Fe 3+ , the XRD structure parameters of LDHs were changed, and the crystallite size was increased from 182 to 249 A (Scherrer equation). Meanwhile, as can be seen from Table S5, the interplanar spacing of each crystal face was increased ). Since Fe 3+ replaced Al 3+ on a partial layer, the atomic density on the crystal plane was reduced, and the distance between metal ions in adjacent hexagonal unit cells was increased . So, the lattice parameter was increased, too. In addition, the incorporation of Fe 3+ increased the positive charge density on the main layer, enhanced the bonding force between main laminate and anion, and increased the number of anions filled between layers, resulting in a larger unit cell parameter c (Rong et al. 2016). It is known from Bragg's law a = 2d (110) and c = 3d (003) that the interplanar spacing was also increased (Pourfaraj et al. 2017). From the above analysis, it can be seen that the chemical adsorption and physical adsorption capacity of MgAlFeO x were enhanced, so more Clcould be adsorbed. Figure 5 shows the XPS survey spectra of MgAlO x and MgAlFeO x . It can be seen from Fig. 5a that the MgAlO x 's peaks of Mg 1s, Al 2p, O 1s, and C1s of were observed at 1302.05, 74.10, 531.2, and 284.6 eV, respectively. In the MgAlFeO x total spectrum, Mg 1s, Al 2p, O 1s, C1s, and Fe 2p were around 1303.20, 74.15, 530.70, 284.6, and 712.20 eV, respectively. Compared with MgAlO x , MgAlFeO x also exhibited the analytical characteristics of double-layer hydroxide (Shan et al. 2015); the relative content of metal elements was increased (Table S6), as shown by the fact that the peak positions of Mg 1s and Al 2p were converted to high binding energy ( Fig. 5c and d) (Zou et al. 2017) and the appearance of Fe 2p at 712.20 eV as shown in Fig. 5b, the result accorded to XRD' (Zhou et al. 2011). The increase of metal elements indicated that the bonding strength of MgAlFeO x was greater, so the adsorption capacity for anion was stronger (Gao et al. 2018).  (Zou et al. 2017;Chen et al. 2018Chen et al. , 2009). The XPS analysis shows that the incorporation of Fe 3+ increased the bonding force of MgAlFeO x , and more Clwas adsorbed by it (Rong et al. 2016). Figure S1 shows the BET spectrum and pore size distribution curve of the MgAlO x and MgAlFeO x . According to the classification of IUPAC, the adsorption isotherms of two substances behaved as typical type IV adsorption behavior and H3 hysteresis loop, with the existence of mesoporous structure . Table S7 shows the specific surface area, pore size, and pore volume of samples. Compared with   indicates that MgAlFeO x had a better ability to capture Cl - (Lei et al. 2016).
The change of surface topography Figure 6 shows SEM images of uncalcination MgAl-LDHs (a), MgAlFe-LDHs (c), and after calcination MgAlO x (b) and MgAlFeO x (d). It can be seen from (a) and (c) that uncalcined LDHs were aggregated by many lamellar structures (Jian et al. 2019). Among them, the MgAlFe-LDHs formed by the incorporation of Fe 3+ into the original layer didn't destroy the original layered structure. However, the interlayer gap increased, and the crystalline morphology changed from an octahedral-like layer to an elliptical layer. Figure 6(b) and figure 6(d) are respectively SEM images of MgAlO x and MgAlFeO x after calcination, and their layers were thinned and also exhibited a layered structure. Compared with MgAlO x , the layer space of MgAlFeO x was significantly increased, and this feature had been illustrated by XRD and BET. Figure 6e and f are the energy dispersion spectra (EDS) and mapping of the two calcined materials. The analysis showed that the characteristic distribution of iron appeared in the EDS and mapping diagrams of MgAlFeO x , indicating the existence of Fe 3+ . In addition, it can be seen from Table S8 that the metal content of MgAlFeO x was increased. This indicated that the positive charge density on the sample surface was increased and the physical adsorption capacity had also been enhanced, this analysis corresponds to the results of XRD, Zeta, and XPS results Gao et al. 2018). Therefore, MgAlFeO x had better adsorption capacity for Cl -.

Anticorrosion mechanism of corrosion inhibitor
The anticorrosive mechanism of corrosion inhibitors could be explained from two aspects, as shown in Fig. 7. First, the anticorrosive effect was to fix the Clin the deicing salt solution through the corrosion inhibitor. It existed in the form of compound Cl --LDHs by the chemical and physical adsorption of corrosion inhibitor. The harm of chloride deicing salt to the environment and infrastructure is caused by Cl - (Farnam et al. 2015b;. In this study, we found that MgAlO x and MgAlFeO x could fix Clat room temperature and low temperature by the physical adsorption and chemical adsorption. After Fe 3+ was incorporated; the number of positive charges on the surface of the material, BET area, interplanar spacing, interlayer spacing, metal content, and the bonding force between the main laminate and anion was all increased; and these characteristics were obtained through the above zeta potential and characterization analysis. The addition of Fe 3+ enhanced the physical adsorption and chemical adsorption of corrosion inhibitor. Hence, MgAlFeO x had a stronger adsorption capacity of Cl -. Second, the deicing salt solution is weakly alkaline (pH=8.2, as shown in Table S3), which can protect the internal structure of the concrete and the surface of the steel bar to prevent damage from chloride salts. Fig. 7 The anticorrosion mechanism of corrosion inhibitor The CaCl 2 solution is weakly acidic. After it penetrated into the concrete, it not only reacted with pyrite (Ca(OH) 2 ) and consumed the main composition of concrete but also corroded the passive film on the surface of the steel bar and consumed iron (Kim 2015;Zheng et al. 2018). After adding the corrosion inhibitor MgAlFeO x , the CaCl 2 -MgAlFeO x solution is weakly alkaline, which increased the pH value of the chloride salt solution, because it could absorb Cland generated LDHs and adjusted the pH value of the deicing salts solution. Therefore, MgAlFeO x had a good anticorrosion effect.

Conclusion
This paper discusses the use of MgAlO x /MgAlFeO x as a corrosion inhibitor in chloride deicing salts. The results show that when the molar ratio of Mg/Al/Fe was 9:2:1, the synthesis temperature was 120°C, and the calcination temperature was 500°C, the removal efficiency of MgAlFeO x was the highest, reaching 85.50%. At the same time, by comparing the Clremoval efficiency of MgAlO x and MgAlFeO x at low temperature and room temperature, MgAlFeO x was chosen to be a corrosion inhibitor in chloride deicing salts. According to the experimental results, compared with CaCl 2 and CaCl 2 -MgAlO x , CaCl 2 -MgAlFeO x had the lowest corrosion to carbon steel and concrete, being 0.008 g and 0.10 mm· a -1 , respectively. After a series of characterization and analysis, it can be seen that after Fe 3+ was added to the original LDHs main laminate, the physical adsorption and chemical adsorption of MgAlFeO x were enhanced. MgAlFeO x could reduce the freezing point of the solution after adsorbing Clat low temperature. And its freezing point decreased from −6.2 to −12.0°C as the MgAlFeO x concentration increased. In short, based on the results of this study, it can be concluded that MgAlFeO x is a corrosion inhibitor with a freezing point and high corrosion resistance, which provides a new research idea for deicing and anticorrosion.
Funding This work was supported by a special fund project of Harbin science and technology innovation talents research (2016RQQXJ109), Heilongjiang provincial institutions of higher learning basic research funds basic research projects (KJCX201812).
Data Availability All data generated or analyzed during this study are included in this published article and its supplementary information files.

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Competing interests The authors declare that they have no competing interests.