Cast iron corrosion protection with chemically modified Mg Al layered double hydroxides synthesized using a novel approach

Abstract Layered double hydroxides (LDHs) intercalated with corrosion inhibitive species are considered as promising additives to protection coatings. However, the conventional method of LDH preparation via co-precipitation followed by anion exchange is a water consuming and slow process hardly applicable to industrial use. In this work, a novel approach to LDH synthesis via hydration of sol-gel prepared mixed metal oxides and two-step anion exchange, all assisted by high-power sonication, was applied. Mg Al and Mg-Al-Ce LDH with cations ratios 2:1 and 2:0.9:0.1, respectively, intercalated with corrosion inhibitive dihydrogen phosphate anion were successfully prepared. The obtained LDH were characterized by X-ray diffraction and scanning transmission electron microscopy. Anion release from these LDH in NaCl solutions and their corrosion inhibitive action on cast iron samples were monitored by electrochemical impedance spectroscopy. The results show that the dihydrogen-phosphate-intercalated LDHs produced using the novel technique are efficient in corrosion protection.

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Introduction
A wide variety of protective coatings from one-layer coatings to complex combinations are used nowadays on metallic structures owing to the easy application and cost efficiency. [1] Coating performance is dependent on intrinsic properties of the film (barrier properties), substrate/coating interface and the inhibitive pigments used to prevent corrosion in aggressive environments.
A great attention is paid to the development of new protective materials that can replace toxic chromium (VI) based compounds as effective corrosion inhibitors. A number of organic and inorganic species intercalated into layered structures has been proposed and optimized for application in protective coatings [1]. LDH crystallites act as smart nanocontainers that release the inhibitive species only when corrosion conditions occur [2].  Rm ) [7]. Parameter a is a function of both size and ratio of cations M II and M III [8,9]. The c parameter depends mainly on size, charge and orientation of the intercalated species [8,10,11].
The main method of LDH production is co-precipitation followed by anion-exchange (Miyata approach [12]). This method is rather direct and reproducible; however, it is slow and water-consuming.
In this work, we report on the layered double hydroxides of the [Mg 1-x same procedures. Cerium-substituted LDH was suggested to exhibit additional protection functionality owing to corrosion-inhibitive properties of Ce III -containing species: the products of decomposition of the nanocontainer itself may additionally provide useful effects.
Obtained LDHs were characterized by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The inhibition efficiency and the protective properties of the released phosphate on the cast iron substrate were evaluated using electrochemical impedance spectroscopy.
The final objective of this work was to study the corrosion protection functionality of the layered double hydroxides synthesized using a novel approach, including LDHs whose metal hydroxide layers were modified with cerium. The cast iron (GG20) samples were provided by SEW-EURODRIVE Portugal LDA.

Synthesis of magnesium-aluminium and magnesium-aluminium-cerium LDH via sol-gel method
Sol-gel method in LDH preparation [13] emerged as an alternative to the conventional coprecipitation method. The final powder LDH samples were obtained by vacuum filtration followed by drying at 60°C for 24 hours.

Anion exchange and formation of chlorine-and phosphate-intercalated LDH
At the hydration stage of MMO in deionised and decarbonised water, the only available anion for intercalation was OH -. Obtained LDH were Mg(2)Al-OH and Mg(2)Al-10%Ce-OH. It was recently shown that direct hydroxide-to-phosphate anion exchange is unlikely but can be implemented via formation of the intermediate chlorine-intercalated phase [14].
For a OH -→ Clanion-exchange, 1 g of Mg(2)Al-OH was added to 250 ml of 1 M NaCl solution at room temperature. The mixture was sonicated for 4 min. The obtained slurry was put in vacuum filtration without any additional washing and dried for 24 hours at 60°C.
Chloride-to-phosphate anion exchange was carried out in a 0.1 M Na 2 HPO 4 solution at room temperature. 1 g of the Mg(2)Al-Cl powder was immersed in the solution followed by addition of NaH 2 PO 4 to adjust the pH value to 7.5. At such conditions, the intercalated anion is dihydrogen phosphate, H 2 PO 4 - [14]. The reaction was sonication-assisted and took 8 min.
The obtained slurry was filtered and dried at the same conditions as mentioned above.
Similar process was used for the hydroxide-to-chloride and chloride-to-phosphate exchange reactions in Mg(2)Al-10%Ce LDHs.

Analytical characterization
X-ray diffraction (XRD) characterization of all LDH compositions was performed using a PANalytical X'Pert Powder diffractometer (Ni-filtered Cu Kα radiation, Theta-Theta goniometer in the reflection mode, PIXcel 1D detector, step 0.02°, exposition time ~1.5 s per step) at room temperature.
Observations of particle shapes and agglomerations were carried out using a Hitachi

Electrochemical characterization
For the electrochemical measurements, a three-electrode cell with a large platinum auxiliary electrode, a saturated calomel reference electrode (SCE) and a working electrode with an exposed area of 1 cm 2 bare cast iron was used.

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The electrochemical impedance measurements were performed using the Autolab PGSTAT30 over a frequency range of 100 kHz-10 mHz with seven points per decade.
A 50 mM NaCl solution prepared by adding NaCl (reagent grade) to distilled water served as corrosive medium. To analyse the inhibitor efficiency in aqueous media, three different conditions were applied. First, the cast iron samples were immersed in a 50 mM NaCl solution with pH 6.73, to evaluate the resistance of the metal in the aggressive environment. In the second case, 0.071 g of NaPO 4 was added to 50 mM NaCl solution (100 ml) to form a solution of 5 mM NaPO 4 and 50 mM NaCl with pH 8.55. In the third case, 0.25 g of phosphate-intercalated LDH, with molar mass of 50 g / mol, was added to the 50 mM NaCl solution (100 ml) to obtain a solution of 5 mM phosphate-intercalated LDH + 50 mM NaCl with pH 8.65. Three samples were tested for each the above-mentioned solutions to ensure reproducibility of the measurements.

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A C C E P T E D M A N U S C R I P T 6 H 2 PO 4 suggests that the phosphate anion is arranged in a such way that a line along its maximum dimension is perpendicular to the layer plane [14].
The lattice parameters values of cerium-substituted LDH compositions were found to be regularly higher than the corresponding values of Mg(2)Al LDH intercalated with the same anions (see Table I). This reflects the increase of the hydroxide layer thickness caused by a partial substitution of Al 3+ by a larger sized Ce 3+ . The observed values of a-parameter of Cesubstituted LDHs were found to be in good with the calculated using the expression proposed by Richardson (Ref [16]) for the case of two different trivalent cations:

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7  (2)Al-10%Ce-OH LDH obtained with or without the ultrasound treatment are shown in Figure 2.
In all the above-mentioned cases, hexagonal flake-shaped crystallites inherent to the layered double hydroxides were observed. The characteristic size (diameter) of the flakes was estimated to range from 50 to 150 nm. It should be noticed here that the typical size of Mg(2)Al LDH prepared using the conventional methods via co-precipitation from nitrates is in the range of 50-200 nm [6].
Based on comparative analysis of series of the STEM images, we concluded that the high-power sonication did not impact either size or shape of the LDH crystallites. But, the crystallites of LDH produced with application of ultrasounds were found to be less agglomerated.

Electrochemical characterization and corrosion protection
EIS was used to evaluate the corrosion progress in the solutions containing corrosioninducing and corrosion inhibitive species.
Initially, the cast iron substrates were immersed in two test solutions (50 mM NaCl and 50 mM NaCl + 5 mM Na 2 HPO 4 ) as described in Experimental to reveal the effect of

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9 phosphate anions on the sample surface in a corrosive media. Figure 3 shows the EIS spectra acquired after 2 to 168 h of immersion in the solutions. Total impedance (|Z|) at relevant frequencies is the main parameter used for corrosion activity assessment in all studied substrate-solution systems.
It has been observed that the impedance decreased immediately after the first minute of immersion of the iron substrate in the 50 mM NaCl solution. The values of impedance observed in the spectra in Figure 3a suggest the formation of corrosion products on the surface of the sample, justifying the slow decrease of low frequency impedance through the 168 h of measurement.
When the sample is immersed in the solution containing Na 2 HPO 4 ( Figure 3b) the impedance values is stable at least during 24 h. After this period of time, due to oxide formation and deposition of phosphate on the substrate, the values of |Z| increase again; however, a part of the sample was kept protected even after one week of immersion.

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10 The images of the cast iron substrates immersed for one week in a NaCl solution either without or with the corrosion inhibitive species are shown in Figure 4. The next step of the study was an immersion test using the inhibitive species intercalated into the LDH produced using the novel approach, Mg(2)Al-H 2 PO 4 . The same test was also performed in the solution containing Mg(2)Al-OH for comparison. Figure 5 shows the EIS spectra acquired after 2 to 168 h of immersion in the respective solutions.
A C C E P T E D M A N U S C R I P T  To support the results obtained from the immersion test, an XRD study of the corrosion products collected from the surface of the samples after 1-week immersion was performed.  Mg(2)Al-Cl LDH. However, as its pattern overlaps that of sodium phosphate hydrate a distinction cannot be done. Also, peaks for iron phosphate hydrate can be found in the same sample.
The corrosion protection functionality of a Ce-substituted layered double hydroxide intercalated with dihydrogen phosphate was also studied. The EIS results obtained for the samples immersed in the NaCl solution containing Mg(2)Al-10%Ce-H 2 PO 4 for 2-168 h were found to be essentially similar to those presented above for the case of Mg(2)Al-H 2 PO 4 LDH.
As mentioned in Introduction (lines 69-70), Ce III -containing species in the products of decomposition of the LDH nanocontainer itself may provide additional protection effect.
However, to estimate such an effect, a much longer immersion time is needed alongside with the continuous exposition of the Mg(2)Al-10%Ce LDH to UV radiation to promote the LDH degradation. This study is in progress.

Conclusions
High-power sonication assisted synthesis and anion-exchange were successfully applied to produce LDH intercalated with dihydrogen phosphate (H 2 PO 4 -), demonstrating this novel approach as an efficient alternative to the conventional co-precipitation synthesis technique.
Parent OH --intercalated LDH with cation compositions Mg(2)Al and Mg (2)Al-10%Ce were synthesized by hydration of sol-gel prepared mixed metal oxides. Mg(2)Al-H 2 PO 4 and Mg(2)Al-10%Ce-H 2 PO 4 LDH were obtained from the parent compositions via sequential anion exchanges from OHto Cland from Clto H 2 PO 4 -, respectively. Application of highpower sonication at the stages of synthesis and anion exchange turns the processes to be much faster than when the conventional LDH preparation technique is used. Crystallites of the LDH produced using the novel approach are of dimensions and shape similar to those (hexagonal flake shaped, 50-150 nm sized) typical of Mg(2)Al LDH prepared using the conventional coprecipitation route.
Efficiency of the layered double hydroxides produced using the novel approach as nanocontainers was demonstrated via corrosion protection effect of dihydrogen phosphate released from these LDH on cast iron substrate in a NaCl solution.
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