The effect of interlayer spacing on the inhibitor release capability of layered double hydroxide based nanocontainers

Journal Pre-proof The effect of interlayer spacing on the inhibitor release capability of layered double hydroxide based nanocontainers

Eiman Alibakhshi, Ebrahim Ghasemi, Mohammad Mahdavian, Bahram Ramezanzadeh, Mana yasaei PII:

S0959-6526(19)34546-9

DOI:

https://doi.org/10.1016/j.jclepro.2019.119676

Reference:

JCLP 119676

To appear in:

Journal of Cleaner Production

Received Date:

11 September 2019

Accepted Date:

10 December 2019

Please cite this article as: Eiman Alibakhshi, Ebrahim Ghasemi, Mohammad Mahdavian, Bahram Ramezanzadeh, Mana yasaei, The effect of interlayer spacing on the inhibitor release capability of layered double hydroxide based nanocontainers, Journal of Cleaner Production (2019), https://doi. org/10.1016/j.jclepro.2019.119676

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The effect of interlayer spacing on the inhibitor release capability of layered double hydroxide based nanocontainers Eiman Alibakhshia,b*, Ebrahim Ghasemia,c**, Mohammad Mahdavianb, Bahram Ramezanzadehb, Mana yasaeid

a

Inorganic pigment and glazes department, Institute for Color Science and Technology, Tehran,

Iran b

Surface Coating and Corrosion Department, Institute for Color Science and Technology,

Tehran, Iran c

Center of Excellence for Color Science and Technology, Institute for Color Science and Technology,

Tehran, Iran d

Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran

* Corresponding authors at: Inorganic Pigment and Glazes Department, Institute for Color Science and Technology, Tehran, Iran E-mail: [email protected], [email protected] ** [email protected]

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Abstract: Layered double hydroxide (LDH) nanoparticles are utilized as nanocontainers for reserving corrosion inhibitors in order to construct a controlled release system with targeted delivery of inhibitive compounds on the corrosion sites of metal. In this way, the NO3intercalated Zn-Al LDH nanoparticles were synthesized through the co-precipitation route with different interlayer spacing by changing the pH of synthesis. The structure and interlayer spacing were characterized by X-ray diffraction, Fourier transforms infrared spectroscopy, Field emission scanning electron microscopy and high-resolution transmission electron microscopy. The outcomes showed that the greatest interlayer spacing belonged to the LDH particles synthesized at pH=9.5±0.5. The relation between the interlayer spacing and inhibitor release performance is discussed and conceptualized for the first time in this study. Results revealed that the structure and interlayer spacing strongly depends on the pH of the synthesis process. It was found that the synthesis pH is the most influential parameter affecting the inhibitor release by changing the interlayer spacing and ion charge density. Moreover, the LDH synthesized at pH=9.5±0.5 with a higher interlayer spacing exhibited the best corrosion inhibition performance. The results from surface characterizations also confirmed the formation of a protective film over the mild steel surface. The obtained material is believed has great potential being an environmentally friendly nanocontainer or nanopigment. It is thus recommended that the obtained material can be utilized to inhibit the metals from corrosion and provide ion-exchange via harsh ions of saline solution at the onset of corrosion. Keywords: Layered double hydroxide; release ability; synthesis; interlayer spacing; corrosion inhibition.

1. Introduction 2

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Layered double hydroxide (LDH), also known as hydrotalcite-type materials or anionic clay, is a class of layered compounds consisting of positively charged metal hydroxide sheets with exchangeable anions and water molecules in the interlayer zone. The general formula of LDHs 2 3 x n can be presented as [ M (1 x ) M x (OH ) 2 ] ( A ) x / n .mH 2 O (Hu et al., 2019; Lyu et al., 2019),

where M2+ and M3+ are the metal cations, An- is an anion with a valency of n, and x is defined as [M3+]/([M3+]+[M2+]) and is between 0.25 and 0.33 (Grover et al., 2019; Teixeira et al., 2018). LDHs have attracted great attention since its invention in 1960 (Auerbach et al., 2004) because of its unique properties and structure that made it a good candidate for the incorporation of different inorganic and organic ions relative to the other established layered structure materials (Guo et al., 2018a). Besides, a wide range of composition (e.g., Ni-Fe, Zn-Al, Ni-Al, and Mg-Al) and preparation techniques (e.g., co-precipitation, hydrothermal, microwave and electrochemical methods) have been introduced for the fabrication of LDH (Mishra et al., 2018; Monti et al., 2013). Guo et al. reviewed the preparation strategies of LDH coatings on magnesium with exclusive heed to self-cleaning, biocompatible and healing performance (Guo et al., 2018a). Wang et al. surveyed the unprecedented developments in the synthesis and application of LDHs (Wang and Hare, 2012) and divided them into two communal scenarios: top-down and bottomup. Various synthesis criterions like aging time, pH, M2+/M3+ molar ratio, stirring rate and calcination time have been proposed for controlling the structure, particle size distribution, morphology, anion release and composition of the resulting LDH nanoparticles in the coprecipitation method (Galvão et al., 2016; Li et al., 2018; Sun and Dey, 2015). It seems that pH is one of the main factors playing an effective role in the intrinsic and structural properties of LDHs in the co-precipitation process that has not been investigated in the previous researches (Seron and Delorme, 2008). Seron et al. (Seron and Delorme, 2008) reported the consequence of 3

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the pH of co-precipitation on the construction of Mg-Al LDH phases and demonstrated that LDH can be fabricated at pH varying from 10 to 13.2. Abderrazek et al. reported that the wellcrystallized LDHs can be obtained when the synthesis is implemented in the pH values range of 8-12 (Abderrazek et al., 2017). The uncontrollable consumption is the major bug of the direct addition of inhibitors into the aggressive environment (Samiee et al., 2019a). To overcome this bug, some researchers have enormously focused to follow the concept of using controlled release systems like mechanical triggering, ion exchange process, and capsules shell damage, etc (Li et al., 2018). Among these controlled release strategies, the anion exchange method has been tremendously exploited to eliminate the drawbacks because it can release the inhibitive ions while absorbing the chloride (Alibakhshi et al., 2017a). Alibakhshi et al. employed LDH as a nanocontainer of the phosphate and molybdate corrosion inhibitors in the hybrid coating to make a controlled release inhibitor system with an anion exchange mechanism (Alibakhshi et al., 2017c, 2017b). LDHs have been also introduced as environmental-friendly alternatives to classical chromate based inhibitors (Montemor, 2014). Although there has been a growing interest for LDH to be used as an efficient nanocontainer for reserving the corrosion inhibitive compounds (Guo et al., 2018b; Serdechnova et al., 2014; Shkirskiy et al., 2015), there is no paper on the relation between the interlayer spacing of LDH and its effect on the inhibitor release capability of LDH which can be altered by pH of synthesis. The interlayer spacing value for Zn-Al-NO3- LDH in our paper (E. Alibakhshi et al., 2016) is in contradiction with the previous findings reporting that this value was about 0.85-0.89 nm (Salak et al., 2012; Tedim et al., 2012). It seems that the interlayer spacing or gallery height of the LDHs is a critical parameter in active corrosion performance and anion-exchange capability.

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The present work was conducted to address the effect of pH values on the structure and release ability of Zn-Al-NO3- LDH prepared via co-precipitation route for the first time. For this purpose, the nitrate intercalated Zn-Al LDH was obtained at various pHs in the range of 7.5 to 12.5. The structure was studied by X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), and highresolution transmission electron microscopy (HR-TEM). Finally, the release and inhibition ability of the obtained LDHs have been examined from the extracted solution. Through this research, it is expected that a suitable nanocontainer is attainable in the harsh solution that consequently results in the increase of inhibitor release ability.

2. Experimental 2.1. Materials Zinc nitrate (99%), aluminum nitrate (98.5%), sodium nitrate (99.5%), and NaOH (98%) were obtained from Sigma-Aldrich and NaCl was purchased from the local Mojalali Co. Mild steel was prepared from locally Foolad Mobarake Co. Its elemental analysis results are presented in Table 1. Table 1 2.2. Preparation of Zn-Al LDHs Zn-Al-NO3- LDH particles were obtained by the co-precipitation route under a nitrogen atmosphere. The Zn:Al mole ratio chosen for the synthesis was 2:1 to obtain stable layered compounds. In the first step, a 20 mL mixture of 0.02 M zinc nitrate and 20 mL of 0.01 M aluminum nitrate solutions were slowly added to 70 mL solution of 0.02 M sodium nitrate under vigorous stirring for 1.5 h. During this step, 0.2 M NaOHsolution was simultaneously added to

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keep the pH constant. Various pH values (7.5, 8.5, 9.5, 10.5, 11.5, and 12.5) were used to prepare the LDHs. Finally, the obtained slurries were washed and centrifuged at a speed of 4500 rpm for 10 min (eight times) and then dried in a laboratory spray dryer MSD1.0 (Labmaq do Brasil) under the conditions presented in Table 2. Table 2 2.3. Characterization The X-ray diffraction (XRD) patterns of the synthesized LDHs were recorded by a PW 1800 Philips X-ray spectrometer with Cu-K radiation (=1.54060 Å). The XRD data were collected over the 2 range from 10-80 at a rate of 2.5 /min. Fourier transform infrared spectroscopy (FT-IR) was performed employing Bruker-Tensor 27 (Germany) using the KBr pellet system within the wavenumber range of 400-4000 cm-1. The morphology and structure of the LDHs were studied by a Zeiss field emission-scanning electron microscopy (FE-SEM) and an FIE Tecnai 20 HR-TEM. 2.4. Corrosion inhibitive performance of LDHs The corrosion inhibitive ability of the obtained LDHs in extract solution (1g/l) (Alibakhshi et al., 2017a) was probed by means of electrochemical impedance spectroscopy (EIS) on the mild steel panels. The EIS experiments were performed on an Ivium Compactstat power supply in NaCl electrolyte (3.5 wt.%) in a three-electrode configuration. Graphite and Ag/AgCl (3 M KCl) electrodes were chosen as counter and reference electrodes, respectively and the mild steel was selected as a working electrode. The EIS was conducted at open circuit potential (OCP) with an amplitude of ±10 mV across the frequency range of 10 kHz to 10 mHz.

The contents of the zinc and aluminum ions released form LDHs were assessed by an inductively coupled plasma-optical emission spectroscopy (ICP-OES) device (modeled as Varian Vista). The

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mild steel was immersed in the extract solutions for this evaluation. Further, the content of Cl- ions in solution was evaluated using ion chromatography (modeled as Shimadzu). Tescan FE-SEM/EDS was

chosen to observe the surface morphology of the samples after the EIS test. To further analyze the surface morphology of the samples, X-ray photoelectron spectroscopy (XPS) was employed using Specs EA 10 Plus Instrument.

3. Results and discussions 3.1. Characterization of LDHs 3.1.1. XRD In order to better assess the mechanism of Zn-Al LDHs intercalated with nitrate, the synthesis was performed in a wide range of pH. Fig. 1 displays the XRD results for the synthesized Zn-AlNO3- LDHs at different pHs. The XRD patterns given in Fig.1 depict the good crystallization of structure for all samples. The characteristic reflections of (003) and (006) at pHs=7.5-10.5 are strong proof on the formation of nitrate LDH. On the other hand, the basal reflections (0 0 3) and (0 0 6) at pH=11.5 and pH=12.5 are different from those at other pHs. The XRD peaks of the Zn-Al-NO3- at pH=11.5 and pH=12.5 clearly demonstrate the template of a conventional hydrotalcite (according to the JCPDS 00-048-1026), suggesting that at two mentioned pHs the carbonate anions has been intercalated into the LDHs gallery via anion exchange mechanism from nitrate to carbonate. The presence of reflection at 11.5 in other pHs is related to the carbonate too. In other words, two peaks were observed for the basal reflections of (003) and (006) at pHs= 7.5, 9.5, and 10.5, which can be assigned to the intercalation of nitrate and carbonate in the gallery (Salak et al., 2012). Although this is amazing regarding the synthesis procedure which has been under the

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nitrogen atmosphere, the intercalation of carbonate has been reported in other publications (Salak et al., 2012; Tedim et al., 2014) and has been attributed to the drying/storage in air. It has mentioned that the time of drying or storage is important and the long-term storage/drying can hence the carbonate intercalation into LDH structure (Salak et al., 2012). Fig. 1 It can be depicted that the pattern of the Zn-Al-NO3- at pH=8.5 differs from others. One peak can be seen for the basal reflections (003) and (006) at this pH; confirming that only nitrate can be intercalated in the LDH gallery. However, the sharp reflection observed at 29.6° in Fig. 1 might be indexed to the presence of Na(NO3) as a byproduct. The formation of Na(NO3) according to the presence of Na+ and NO3- ions in the synthesis procedure is reasonable which maybe formed after drying on the external surfaces of LDHs in spite of washing procedure. The unit cell frameworks and the gallery height (L) values estimated for the obtained LDHs are displayed in Table 3. As expected from the sheet-like shape of LDH crystals, the dimensions in the c-direction were smaller than those in the a-direction. Also, the obtained unit lattice parameters are in agreement with the literature values (Galvão et al., 2016; Seftel et al., 2008). The interlayer spacing value (cˊ) of Zn-Al-NO3- LDH at various pHs can be calculated using the basal reflection (003) according to the Eq. 1:

d



(1)

2 sin 

where, λ , d, and Ө are the X-ray wavelength, the basal spacing, and the diffraction angle, respectively.

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It is obvious that the interlayer spacing value of Zn-Al-NO3 LDH is increased up to a pH of 9.5, and thereafter is dropped continuously. The highest and lowest values were recorded at pH=9.5 and pH=12.5, respectively. The Zn-Al-NO3- LDH shows an interlayer spacing in the range of 8.6-8.9 Å. The abrupt change in the interlayer spacing could be assigned to an intrusive variation of the pillared nitrate anions in their structural arrangement (compact or stick-lying, flat-lying, tilt-lying) in the gallery space (Xu and Zeng, 2001a, 2001b). It has been also reported that the changes in the interlayer spacing of Zn-Al-NO3- LDH could be connected to the dehydration (Salak et al., 2012). Notably, the interlayer spacing values at pH = 11.5-12.5 are between 7.5-7.6 Å. These values are agreed with the interlayer spacing values reported for the carbonate loaded LDH (E Alibakhshi et al., 2016). This result is matched with the results given in Fig. 1 (presence of the only carbonate), and can be explained by an increase in the charge density on the brucitelike sheets when the pH increase, leading to a higher amount of carbonate anions reqired to maintain the electroneutrality of the final material (Seftel et al., 2008). In spite of LDH intercalation with carbonate ions, the effect of water in structural changes of LDH cannot be ignored as the OH- ions show good affinity to the LDH structure. The intercalation and orientation of the organic anions mainly depend on the LDH layers charge on the layers, anion concentration, and the amount of entrapped water in the interlayer space of LDH (Khan, et al., 2002). Regarding the general formula of LDHs (which has mentioned in the introduction), both of OH- and H2O are the structural species in the formula. The OH- ions exist in the LDH structure with a constant molar coefficient of 2 and H2O presents as intercalated species with variable content (y=0.66-0.8) which depend on the charge density and type of "A" ion. A simple computation reveals that the water (H2O) content nearly varies in range of 13-18 wt% which is depending on the magnitudes of X, n and y coefficients. This value is compatible with literatures

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in which the physical and chemical water content has evaluated via thermogravimetric analysis (TGA) (Marappa and Kamath, 2015; Stanimirova and Balek, 2008). Table 3 3.1.2. FT-IR The FT-IR analysis was carried out to determine the presence of nitrate and carbonate ions into the LDH gallery. Fig. 2 depicts the FT-IR spectra of the synthesized Zn-Al-NO3- LDHs at different pHs (7.5, 9.5 and 12.5). Fig. 2

The absorption bands of less than 700 cm−1 were metal-OH deformation and translation modes of LDH, which are typical of LDH nanoparticles (Feng et al., 2006; Persson et al., 2013). These patterns were similar for the three LDHs. Other similar absorption bands corresponding to the O–H vibration of hydroxyl group and water deformation were recorded at around 3500 and 1620 cm-1 (E Alibakhshi et al., 2016; Nemati et al., 2018). The spectrum of the LDH prepared at pH= 12.5 exhibited a intense band at around 1370 cm− 1 corresponded to the carbonate ion, whereas that of LDH at other pHs (7.5 and 9.5) showed two peaks at 1362 and 1384 cm-1 linked to the carbonate and nitrate ions, respectively (Rodrigues et al., 2012; Scavetta et al., 2009). Also, an absorption band close to 850 cm-1 is assigned to the modes of the carbonate group (Feng et al., 2006). This peak was stronger and wider for LDH at pH= 12.5. These results indicate that the intercalation of the carbonate and nitrate anion into the LDH gallery strongly depends on the pH synthesis. Thus, it can be inferred that the tendency of the carbonate adsorption into the LDH gallery increases with the increase in pH. This result is in agreement with the XRD analysis results.

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3.2. Microstructure evaluation The morphology and microstructure of the LDHs were studied by SEM and HR-TEM analyses. Fig. 3 displays the SEM results of the samples at various pHs. It can be seen that there is a clear difference between the microstructure of particles at each pH. In other words, the morphology of the LDHs changes due to the pH alteration. LDH particles at pHs=7.5 and 8.5 show a high quantity of agglomerated stacked particles. The morphology of these two pHs can be attributed to this fact that the sheets are in their initial stages of growth. As pH rises, the structures can be better crystallized, causing to the growth of a planar structure with sharp flake morphology. The difference in particle shape and morphology of the samples could possibly be attributed to pH effect on the nucleation and crystal growth action concurrently with the precipitation of LDH. Fascinatingly, Panda et al. reported that the pH and concentration of NaOH had an influence on the growth rate of the LDH nucleus because of the capping mechanism (Panda et al., 2011). Thus, it can be stated that more layers of Na+ might be formed around the LDH nucleus at high pHs, leading to specific particle shapes and various morphology as illustrated in Fig. 3. The LDH plates situated vertically on the stub were acclimated to calculate the thickness and distance of the layers. The distance between the layers was about 25-60 nm for various samples. As a result, the thickness of the interlayer spacing increases with the increase in pH and more layers are formed. The thickness of the interlayer can be affected by the number, size, internal arrangement, and fortification of the bonds between the hydroxyl groups of the brucite-like sheets and the anions (Cavani et al., 1991; Seftel et al., 2008). Fig. 3

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The morphological structure of the Zn-Al-NO3- LDHs at various pHs (9.5 and 12.5) was further determined by HR-TEM analysis. Fig. 4 shows a typical bright-field TEM image of Zn-Al LDH obtained in two pHs of 9.5 and 11.5. The particles are plate-like and some agglomerated in the form of rippled and crumpled silk-like structures can be seen (Figs 4a and 4b). The observed inter-planar spacing of 0.284 nm and 0.266 nm (Figs 4c and 4d) matches superbly with the (009) and (015) lattice planes of Zn-Al LDH, suggesting that the sheets consist of nanoparticles with Zn-Al LDH structure. The presented inter-planar spacing in both pHs of 9.5 and 11 are similar, showing that the LDH structure is the same for both pHs. The lattice fringes also reveal the high crystallinity of the layers. Fig. 4 3.3. Corrosion inhibitive performance of LDHs As discussed above, the interlayer spacing values of LDH decreased with increasing the pH with the highest value obtained at pH=9.5. Therefore, it can be concluded that the interlayer spacing largely depends on pH. In this section, the corrosion inhibition performance is conceptualized and discussed in terms of the interlayer spacing. The corrosion inhibition action of the prepared LDHs in 3.5 wt.% NaCl solution was studied through the EIS test. Fig. 5 illustrates the EIS plots of mild steel samples during 24 h exposure to NaCl solutions in the presence and absence of synthesized LDHs extracts. The radius of the capacitive arc in the Nyquist plots reveals the general performance of the LDHs in the solution phase. It is widely acknowledged that a higher radius of the capacitive arc in the Nyquist plot is responsible for an ameliorated corrosion inhibition performance (Alibakhshi et al., 2013; Mahdavian et al., 2018). From Fig. 5 it is seen that the LDH synthesized at pH=9.5 has the largest semicircle. Moreover, the high-frequency phase angle (θ10 12

kHz)

and low-frequency

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impedance (|Z|0.01Hz) are two famous features that can be obtained from Bode plots, indicating the corrosion inhibition performance of the inhibitors (Alibakhshi et al., 2014a, 2014b). Considering the obtained θ10 kHz parameter, the mild steel exposure to the solution without LDHs showed the lowest values. Therefore, a lower phase angle can be an indication of poor corrosion inhibition performance. The higher θ10 kHz was obtained for the LDH synthesized at pH=9.5. In agreement with the θ10 kHz, the highest |Z|0.01Hz after 24 h immersion was observed for this sample (pH=9.5), showing the most effective corrosion inhibition performance (Palimi et al., 2018, 2017). Fig. 5

To more profoundly explore the mechanism of the corrosion inhibition behavior of the LDHs, two suitable equivalent circuits are employed to simulate the EIS outcomes (Samiee et al., 2019b, 2018) as shown in Fig. 5. In these circuits, the Rs, and Rct are the solution and charge transfer resistances, respectively. The constant phase element of the electrical double layer exhibiting the non-ideal capacity is accordingly demonstrated by CPEdl. The double-layer capacitance can be estimated by Eq. 2 (Haddadi et al., 2019; Hirschorn et al., 2010) as follow: 1

𝐶𝑑𝑙 = (𝑄𝑑𝑙 . 𝑅𝑐𝑡1 ― 𝑛)𝑛

(2)

where n and Q represent the exponent and admittance of the constant phase element, respectively. The elements extracted from EIS data within various test periods are illustrated in Fig. 6 and Table 4. Fig. 6 Table 4 It can be recognized from this figure that both Rct and Cdl values altered continuously with increasing the exposure period. The Rct values of the blank and the LDHs synthesized at higher 13

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pH (11.5 and 12.5) decreased with the passage of time, while an increasing trend was observed for other samples (pH=7.5-10.5). In the case of the LDH synthesized at pH=11.5 and 12.5, the high charge transfer value at primal exposure and the depreciatory tendency with rising the immersion might be linked to the immediate zinc ions adsorption (slightly released from LDH scaffold) on the metal surface during the early immersion stage (Alibakhshi et al., 2017a). The Zn and Al release from the LDH extracts after 24 h immersion was confirmed by ICP-OES. As presented in Table 5, the content of zinc is nearly equal at pHs=7.5-10.5. This amount (Zn) at higher pHs is lower compared to other pHs. Also, the Zn ions concentrations in the extract solution of the LDHs synthesized at pH=11.5 and pH=12.5 after 1 h immersion were 1.2 and 1.1 mg/L, respectively. Table 5 The decrease in Zn ions content for these extracts (pHs=11.5-12.5) demonstrates the adsorption of Zn ions on the metal surface. The XRD results implied that the carbonate as a non-inhibiting anion is intercalated into the LDH gallery. It is reported that the carbonate-intercalated LDH provides the corrosion protection effect due to the entrapment of harmful chloride ion from the harsh environment (Williams and McMurray, 2004; Zheludkevich et al., 2010). Furthermore, the presence of carbonate in the LDH gallery may cause the increase in the pH of the solution. Therefore, the preferential dissolution of

Al is higher than Zn at alkaline pH. This could be related to the solubility constant of individual hydroxides depending on the pH range (Rives, 2001) and higher solubility of Al than Zn ions (Shkirskiy et al., 2015). This result is in agreement with the ICP result (see Table 5). At pHs=7.5-10.5, the enhancement in Rct with immersion time might be connected to the growth of a protective layer on the active zones of the metal surface . Different trend was obtained from the Cdl values for these samples. The evolution of the Cdl values of the samples represents that 14

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the blank solution had the highest double-layer capacitance among the examined solutions. It can be recognized from Fig. 6b that the inclusion of LDHs into solution led to the decrease of the double layer capacitance compared to the sample without LDH nanoparticles. The decrease in Cdl values could be linked to the increase in the electrical double layer thickness or the formation of a protective film (Khamseh et al., 2018, 2017). However, lower values were seen for the solution including nitrate compared to carbonate, probably because of the inhibition activity of the nitrate (Meng et al., 2002; Newman and Ajjawi, 1986). Furthermore, it seems that the time constant ascribed to the double layer is quite high and maybe overlapped with mass transport controlled processes as a consequence of the accumulation of corrosion products on the metal. To analyze the surface of the specimens after 24 h immersion in the extract solution, FE-SEM, EDS, and XPS were employed. The FE-SEM image of the samples exposed to the synthesized LDH extract (pH=9.5, pH12.5) and the blank solution is depicted in Fig. 7. The sample exposed to the solution including LDH synthesized at pH=9.5 extract illustrates a protecting film over the surface. Such a layer cannot be seen for the sample exposed to the LDH synthesized at pH=12.5 and blank solution. Detailed information about the surface elemental analysis (EDS) of the samples is summed up in Table 6. In the case of LDHs, the oxygen concentration is diminished which shows minor corrosion over the surface in comparison with the blank solution. This diminution is more conspicuous at pH=9.5 which reverberates the minimum corrosion for this sample. The presence of zinc and nitrogen elements on the mild steel surface exposed to the extract solution of LDH synthesized at pH=9.5 illustrates the development of corrosion inhibitive film. XPS analysis was further considered to probe the presence of the protective layer on the metal surface. The overall XPS spectrum of the surface of the sample exposed to the LDH synthesized 15

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at pH=9.5 extract after 24 h is depicted in Fig. 8. The XPS analysis confirms the presence of Zn, N, Fe, and O as the major ingredients on the metal surface. The existence of Zn over the surface shows that the protecting layer deposited on the metal surface. Therefore, the inhibitive action of the LDHs over the metal surface could be related to the zinc oxide/hydroxide compounds and nitrate anions. Evidently, the best corrosion inhibition was detected for the sample synthesized at pH=9.5 that also shows the lowest Cdl and the highest Rct. It seems that the growth of an inhibiting film deposited on the mild steel surface via the reaction of Zn2+ with OH- ions and inhibitory action of nitrate anions might explain the superior inhibition performance of this sample (pH=9.5±0.5) compared to another sample. Moreover, the interlayer spacing of LDH may play a beneficial designation in the improvement of the inhibition performance. According to the XRD results, the highest and lowest interlayer spacing was obtained at pH=9.5 and pH=12.5, respectively. Fig. 7 Fig. 8 Table 6 The relation between the interlayer spacing values versus charge transfer resistance is demonstrated in Fig. 9. The decline in inhibition performance was recorded when the interlayer spacing decreased. This could be related to the change of the interlayered ions of NO3- to CO32/OH-. This means that the higher nitrate anions released at pH=9.5 show an inhibitive behavior compared with the carbonate anions. As was said before, the OH- ions play a determinant role in the LDH structure and can affect the gallery height due to the bonding state and molecular shape (as OH- ion or H2O molecules). Then the changes in the structural parameter of LDH are the relevant incident in which the OH- ions have dominant impress. This increase in interlayer

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spacing could be also accompanied by the higher entrapment of the chlorides from an aggressive electrolyte. As confirmed by ion chromatography (See Fig .10), when the interlayer is bigger a decrease in the rate of corrosion processes occurs. This observation can be interpreted as the influence of the interlayer spacing on the inhibition action of the system. The percentage of Clions in solution detected by ion chromatography after 24 h immersion of samples and the obtained concentration is illustrated in Fig. 10. The maximum amount of the Cl- ions was detected for the LDH synthesized at higher pH while a higher entrapment of Cl- ions (lower content) was detected for other pHs. Also, the lowest content was obtained for the LDH synthesized at pH=9.5. Fig. 9 Fig. 10 From the above discussions, these findings suggest the following conclusions: 1) At higher pHs (11.5 and 12.5), because of the interlayer spacing decrement and the presence of the carbonate, the corrosion inhibition performance declined, and 2) At other pHs, a higher content of nitrate and low content of carbonate were intercalated into the LDH gallery. As a result, the increase in the interlayer spacing occurs, which can effectively delay the initiation of the corrosion due to higher adsorption of chloride. Also, better corrosion inhibition is attributed to nitrate inhibition. As a result, an increase in the interlayer spacing is accompanied by further improvement of the corrosion inhibition performance. Therefore, the LDH synthesized at pH=9.5±0.5 showed a better corrosion inhibition action with respect to other samples due to both nitrate inhibition and chloride entrapment. To accurately grasp the concept of this behavior, a schematic outline for the corrosion inhibition mechanism of the LDHs with various pHs is sketched in Fig. 11.

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Fig. 11 4. Conclusion The nitrate intercalated Zn-Al-LDHs were obtained by the co-precipitation route in different pHs. The consequence of pH variation of co-precipitation on the crystallinity, structure, and interlayer spacing of Zn-Al LDH nanoparticles was investigated as a perspective for the improvement of corrosion inhibition performance. The results indicated that the interlayer spacing of the LDHs depends on the pH of synthesis. The highest and lowest interlayer spacing values (which plays a key role in corrosion inhibition performance) were recorded at pH=9.5 and pH=12.5, respectively. The SEM and TEM results demonstrated a planar structure with the flake-like morphology by increasing the pH. Finally, the corrosion inhibition action of the obtained LDHs at different pHs was discussed and conceptualized using EIS in terms of interlayer spacing. The variation of interlayer spacing was attributed to the CO32-/OH- ion exchanging and charge density. The corrosion inhibition performance largely depends on the interlayer spacing, moreover the LDH synthesized at pH=9.5±0.5 with a higher interlayer spacing provided the highest corrosion inhibition performance in the mild steel exposed to a saline solution.

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lithium zinc phosphate pigment on corrosion inhibition of mild steel and adhesion strength of epoxy coating. J. Sol-Gel Sci. Technol. 72, 359–368. https://doi.org/10.1007/s10971014-3441-2 Alibakhshi, E., Ghasemi, E., Mahdavian, M., 2013. Corrosion inhibition by lithium zinc phosphate pigment. Corros. Sci. 77, 222–229. https://doi.org/10.1016/j.corsci.2013.08.005 Alibakhshi, E., Ghasemi, E., Mahdavian, M., Ramezanzadeh, B., 2017a. A comparative study on corrosion inhibitive effect of nitrate and phosphate intercalated Zn-Al- layered double hydroxides (LDHs) nanocontainers incorporated into a hybrid silane layer and their effect on cathodic delamination of epoxy topcoat. Corros. Sci. 115, 159–174. https://doi.org/10.1016/j.corsci.2016.12.001 Alibakhshi, E., Ghasemi, E., Mahdavian, M., Ramezanzadeh, B., 2017b. Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel. J. Taiwan Inst. Chem. Eng. 80, 924–934. https://doi.org/10.1016/j.jtice.2017.08.015 Alibakhshi, E, Ghasemi, E., Mahdavian, M., Ramezanzadeh, B., 2016. Corrosion inhibitor release from Zn-Al- [ PO 43- ] - [ CO 32- ] layered double hydroxides nanoparticles. Prog. Color. Color. Coatings J. 9, 231–246. Alibakhshi, E., Ghasemi, E., Mahdavian, M., Ramezanzadeh, B., Farashi, S., 2017c. Active corrosion protection of Mg-Al-PO43- LDH nanoparticle in silane primer coated with epoxy on mild steel. J. Taiwan Inst. Chem. Eng. 75, 248–262. https://doi.org/10.1016/j.jtice.2017.03.010 Alibakhshi, E., Ghasemi, E., Mahdavian, M., Ramezanzadeh, B., Farashi, S., 2016. Fabrication and characterization of PO4 3− intercalated Zn-Al- layered double hydroxide nanocontainer. J. Electrochem. Soc. 163, C495–C505. https://doi.org/10.1149/2.1231608jes Auerbach, S.M., Carrado, K.A., Dutta, P.K., 2004. Handbook of layered materials: layered double hydroxides, P. K. Dutt. ed. CRC Press, New York. Cavani, F., Trifiro, F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 11, 173–301. https://doi.org/10.1016/0920-5861(91)80068-K Feng, Y., Li, D., Wang, Y., Evans, D.G., Duan, X., 2006. Synthesis and characterization of a UV absorbent-intercalated Zn–Al layered double hydroxide. Polym. Degrad. Stab. 91, 789–794. https://doi.org/10.1016/j.polymdegradstab.2005.06.006 19

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Journal Pre-proof Declaration of interest statement Manuscript title: The effect of interlayer spacing on the inhibitor release capability of layered double hydroxide based nanocontainers

The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patentlicensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript. Author names: Eiman Alibakhshi Ebrahim Ghasemi Mohammad Mahdavian Bahram Ramezanzadeh Mana Yasaei

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All authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. First, 2nd, 3rd and 4th authors conceived and planned the experiments. 1st and 5th carried out the experiments. 1st, 2nd, 3rd and 4th contributed to the interpretation of the results. 1st and 2nd took the lead in writing the manuscript.

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(003) Carbonate

Intensity (a.u)

(006) Carbonate

(003) Nitrate

(009) (015) (012)

(113) (110)

pH=12.5

pH= 11.5 pH= 10.5

(006) Nitrate

pH= 9.5 pH= 8.5

0

10

20

30

40

50

2ϴ (degree)

60

pH= 7.5 70

Fig. 1. XRD patterns of the prepared Zn-Al-NO3- LDHs at different pHs.

80

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Transmittance (a.u)

pH=12.5

pH=9.5

pH=7.5

4000

3600

3200

2800

2400

2000

Wavenumber

1600 (cm-1)

1200

800

Fig. 2. FT-IR spectra of the prepared Zn-Al-NO3- LDHs at different pHs.

400

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pH=7.5

pH=8.5

pH=9.5

pH=10.5

pH=11.5

pH=12.5

Fig.3. SEM micrographs showing the microstructure of the Zn-Al-NO3- LDH at various pHs.

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Fig. 4. (a) HR-TEM images of the Zn-Al-NO3 at (a,c) pH=9.5±0.5 (b, d) and pH=12.5±0.5 (c, d).

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log |Z| (Ω.cm²)

(b)

-Z'' (kΩ.cm²)

3.00E+00

2.00E+00

1.00E+00

3.5

6.00E+0

3

5.00E+0

2.5

4.00E+0

2

3.00E+0

1.5

2.00E+0

1 0.5

1.00E+0

0

0.00E+0

0

-2

00

-1

0

1

2

log f (Hz)

3

4

4.

00

3.

2.

00

Z' (kΩ.cm²)

7.00E+0

E+ 0

0 E+ 0

0 E+ 0

0 E+ 0 00 1.

0.

00

E+ 0

0

0.00E+00

4

-Phase angle (°)

4.00E+00 (a)

(d)

5.00E+00

(c)

4

6.00E+0

-Z'' (kΩ.cm²)

4.00E+00 3.00E+00 2.00E+00 1.00E+00

5.00E+0

-Phase angle (°)

log |Z| (Ω.cm²)

3.5 3

4.00E+0

2.5 2

3.00E+0

1.5

2.00E+0

1

1.00E+0

0.5 0

0.00E+00 0.00E+00

0.00E+0 -2

2.00E+00

4.00E+00

Z' (kΩ.cm²)

0

2

log f (Hz)

Fig. 5. (a) Nyquist and (b) Bode plots for the mild steel samples after 24 h immersion in 3.5 wt.% NaCl solution with and without synthesized LDHs extracts. Typical fitted (c) Nyquist and (d) Bode plots.

4

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(a)

(b) 1h 4h 24 h

4 3 2 1 0

1200

1h 4h 24 h

1000

Cdl (μF/cm2)

Rct (kΩ.cm2)

5

800 600 400 200

Blank

pH=7.5 pH=8.5 pH=9.5 pH=10.5 pH=11.5 pH=12.5

0

Blank

pH=7.5 pH=8.5 pH=9.5 pH=10.5 pH=11.5 pH=12.5

Fig. 6. (a) Charge transfer resistance and (b) double layer capacitance parameters extracted from EIS data for the mild steel samples after 1, 4 and 24 h immersion in 3.5 wt.% NaCl solution with and without synthesized LDHs extracts.

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Fig. 7. FE-SEM micrographs of the mild steel panels immersed in the extract solutions after 24 h immersion, (a) blank sample, (b) LDH synthesized at pH=12.5 and (c) LDH synthesized at pH=9.5.

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Fig. 8. XPS survey spectrum of the mild steel surface immersed in 3.5% NaCl solution containing LDH synthesized at pH= 9.5 extract after 24 h.

9.5

5000

9

4000

8.5

3000

8

2000

7.5

1000

Rct (KΩ.cm2)

d-spacing (Å)

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7 7.5

8.5

pH of synthesis 9.5

10.5

0 11.5

12.5

pH of synthesis Fig. 9. Variation of the interlayer spacing (d-spacing) values versus charge transfer resistance of the Zn-Al-NO3- LDH at various pHs.

Concentration of Chloride (%)

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80

70

60

50

40 7.5

8.5

9.5

10.5

pH of synthesis

11.5

12.5

Fig. 10. The concentration of the chlorine in the solution after 24 h immersion of mild steel.

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Fig. 11. Mechanistic description of corrosion inhibition performance of LDHs synthesized at pH=9.5±0.5 (highest interlayer spacing) and pH=12.5±0.5 (lowest) assigned to good and poor performance.

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

Construction of controlled release layered double hydroxide based nanocontainers.



Relation between pH and interlayer spacing in layered double hydroxide synthesis.



Role of pH on structure and morphology of layered double hydroxide (LDH).



Interlayer spacing played significant role in the corrosion inhibition performance.

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Table 1. Composition of the mild steel samples Elements

Fe

C

Si

Mn

P

S

Cr

Mo

Co

Cu

wt%

balance

0.19

0.415

1.39

<0.005

< 0.005

0.026

0.018

0.0559

0.0429

Table 2. Spray drier process condition. Parameters

Values

Air temperature at the entry

130 °C

Air temperature at the exit

40 °C

Atomizer nozzle diameter

1.2 mm

Slurry flow rate

1.05 L/h

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Table 3. The calculated unit cell parameters, the interlayer spacing for the synthesized LDHs at different pHs from XRD data. Synthesis pH a (Å)* c (Å)* 7.5±0.5 3.06 25.86 8.5±0.5 3.06 26.10 9.5±0.5 3.07 26.67 10.5±0.5 3.06 25.50 11.5±0.5 3.02 22.89 12.5±0.5 3.01 22.77 * a = 2d110, *c = 3d003, c = 3cˊ. ** L = cˊ - thickness of layers (4.8 Å)

cˊ (Å)* 8.62 8.70 8.89 8.50 7.63 7.59

L (Å)** 3.82 3.90 4.09 3.70 2.83 2.79

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Table 4. Electrochemical parameters extracted from the impedance plots for the mild steel samples immersed in different test solutions at various times; the values are the mean of three replicates and (±) corresponds to the standard deviations.

Extract of synthesized

Time

Rs

Rct

Y0,dl

LDH at

(h)

(Ω.cm2)

(Ω.cm2)

(μsn.Ω-1.cm-2)

Blank

1

4.6±0.1

1602±57.5

591.6±61.7

0.81±0.01

4

4.8±0.2

1230±79

753.4±141.8

0.8±0.01

24

5.2±0.1

1021.5±71.6

1034.6±104

0.79±0.01

1

5.5±0.1

1700±63

571.6±66.2

0.78±0.01

4

5.8±0.2

2218±74.3

493.4±41.7

0.78±0.01

24

6.2±0.1

2765±142

424.5.6±36

0.77±0.01

1

5.6±0.1

1732±81

532.6±24.9

0.75±0.01

4

7.8±0.2

2480±

462.1±18.4

0.74±0.02

24

10.2±0.1

3274±

421±34.2

0.74±0.01

pH= 9.5

1 4 24

8.5±0.8 10.1±0.5 13.3±2.3

1826±295 2117.5±111 4465±355

449.8±24.8 405±57.6 336.1±127.7

0.72±0.02 0.74±0.02 0.71±0.01

pH= 10.5

1

7.6±0.0.6

1670±96.3

478±51.9

0.76±0.01

4

9.8±0.9

1840±128.7

462±41.8

0.76±0.02

24

10.9±1.1

2595±164

432.1±25.4

0.74±0.01

1

7.6±0.4

2700±136.1

436.2±52.2

0.75±0.01

4

7.8±0.5

2430±142

458±27.4

0.76±0.01

24

8.2±0.7

2168±166.4

485±26.9

0.77±0.01

1 4 24

7.7±0.2 7.9±1.7 8.1±1.3

2200±123 1890±96.7 1538±67.2

495±41.1 514.2±31.2 564.3±1.3

0.75±0.02 0.79±0.01 0.80±0.02

pH= 7.5

pH= 8.5

pH= 11.5

pH= 12.5

ndl

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Table 5. Concentration of Zinc and aluminum released from the LDH extracts after 24 h.

Extract of synthesized

Zn (mg/l)

Al (mg/l)

7.5

1.05

0.1

8.5

1.07

0.1

9.5

1.1

0.1

10.5

1.07

0.1

11.5

0.5

3.1

12.5

0.2

3.4

LDH at pH

Journal Pre-proof

Table 6. Elemental weight percentage of the film formed on the mild steel specimens immersed in the test solutions without and with LDHs extracts. Extract of synthesized LDH at

Fe

O

Na

Cl

Zn

Al

N

C

Blank

balance

5.5

0.2

0.3

-

-

-

0.2

pH= 9.5

balance

2.4

0.1

0.1

1.7

0.1

0.2

0.2

pH= 12.5

balance

3.8

0.2

0.2

0.1

0.1

-

1.4