The Effect of 2-Mercaptobenzimidazole-Polyaniline- CeO2 Ternary Nanocomposite Addition as a Superior Pigment for Improvement of Corrosion Resistance in Epoxy Coatings


 The 2-mercaptobenzimidazole-polyaniline-ceria (MBI-PANI-CeO2) ternary nanocomposite synthesized and used as pigment into epoxy (EP) to improve protection properties of coating on mild steel. The MBI-PANI-CeO2 nanocomposite was prepared using layer by layer assembly. Initially, the PANI layer was polymerized on the surface of CeO2 nanoparticles. Then, the MBI inhibitor was adsorbed on PANI with opposite electrostatic charges. The anticorrosive performance of EP coatings was investigated using electrochemical impedance spectroscopy, salt spray test, and scanning electron microscopy by incorporating various amounts (0.5, 1, and 2 wt. %) of the MBI-PANI-CeO2 nanocomposite. The EP coating containing 1 wt. % of nanocomposite showed the highest corrosion resistance and minimum agglomeration. This coating indicated the coating resistance of 19.1 MΩ cm2, which is greater than EP and EP/CeO2 coatings. The EP/MBI-PANI-CeO2 (1 wt. %) coating showed water uptake percentage (2.09 %) about two times lower than EP/CeO2 coatings (4.56 wt. %), indicating appropriate barrier performance of inhibitor incorporated EP.


Introduction
Anodic protection, cathodic protection, and organic coating are well known as practical ways for protecting metallic substrates from corrosion. Over time, researchers considerably focused on organic coatings as a nancial protection technique [1][2][3][4][5][6][7]. Among organic coatings, the epoxy (EP) has been used extensively due to the superior features like good barrier properties against corrosive species, suitable adhesion, appropriate chemical, and mechanical resistances [8][9][10][11][12][13][14]. However, the lifetime of these coatings was decreased during immersion in corrosive media, which resulted in a diffusion of corrosive species at the metal/coating interface, initiating corrosion of the substrate beneath coating [15][16][17]. Therefore, improving the quality of the EP coating by organic and inorganic pigments has been attracting a lot of attention [13,[18][19][20][21][22]. In this regard, many kinds of research have been conducted to enhance the anticorrosive resistance of EP coatings by various compounds [23][24][25][26]. Embedding inorganic metal oxide nanoparticles (NPs) into EP resin improves the resistance of coating against corrosion and degradation due to decreasing of electrolyte diffusion pathways [27][28][29][30][31]. To uniform dispersion of nanoparticles without much agglomeration in EP and reduce the permeability of EP based coatings against oxygen, water, and ion transfer, the surface of nanoparticles was modi ed with a variety of organic and inorganic compounds [32][33][34][35]. Corrosion inhibitors can provide more corrosion protection, especially in active corrosion areas of coating. Direct embedding of inhibitors in the EP coating can decrease the interactions between the resin and inhibitor molecules, causing the reduction of inhibition performance [36-38]. One of the strategies for overcoming this problem is using the corrosion inhibitors to modify the metal oxides NPs [39,40]. Layer by layer electrostatic deposition is reported as an effective method for treatment of metal oxide nanoparticles, which is based on adsorption of layers with opposite electrostatic charges. In our previous work, the surface of cerium (IV) oxide NPs was successfully modi ed with imidazole [41]. It was found out that EP coating with modi ed CeO 2 NPs displayed higher coating resistance with 5.4 orders of magnitude compared to unmodi ed CeO 2 NPs.
In this research, 2-mercaptobenzimidazole (MBI) was used to modify the ceria (CeO 2 ) NPs. For electrostatic adsorption of MBI on the surface of CeO 2 NPs, oxidative polymerization of polyaniline (PANI) was rst performed to obtain a positive layer. The impact of MBI-PANI-CeO 2 nanocomposite on the barrier performance of the EP coating was investigated.

Preparation of coatings
The mild steel panels (1×1 cm 2 ) were polished and degreased with EtOH. Different amounts of MBI-PANI-CeO 2 nanocomposite were poured in 1g epoxy (NANYA EP resin, NPEL-127). Then, the 0.5 g ACR Hardener (H-3892, amine type) was added. The application of coatings on mild steel were carried out by brushing.

EIS measurements
The potentiostat-galvanostat (Origalys, 01A) was used for EIS test in the 3.5 wt. % NaCl at OCP. The frequency range was 10 5 Hz -10 −2 Hz with an AC voltage of ± 5 mV. The saturated calomel electrode, platinum plate, and mild steel coated samples were used as the reference, counter, and working electrodes, respectively.

Salt spray test
The salt spray evaluation was carried out as reported by the ASTM B117 standard using GT7004-M chamber. The EP, EP/CeO 2 and EP/MBI-PANI-CeO 2 (1 wt. %) coated substrates were put in the room and encountered to the sodium chloride 5 wt. % (1.5 ± 0.5 ml h −1 ) for 200 h at 35 ℃.

Pull-off test
The ASTM D4541 procedure was used for pull-off test. The analysis was done using Digital Tester BGD500 for coated substrates (EP, EP/CeO 2 and EP/MBI-PANI-CeO 2 (1 wt. %)).

Characterization
The TENSOR27 spectrometer was used for FTIR analysis. The Raman spectroscopy was carried out by Teksan, Takram P50C0R10 spectrophotometer. The thermogravimetric evaluation (TGA) was done with Linseis STA PT-1000 analyzer (heating rate: 10°Cmin −1 ). The DLS and Zeta potential measurements were carried out with Microtrace, Nanotrace Wave. The morphology was investigated by eld emission scanning electron microscopy (FE-SEM, MIRA3).

Characterization of MBI-PANI-CeO 2 nanocomposite
The FTIR spectra of CeO 2 , PANI-CeO 2 , and MBI-PANI-CeO 2 nanocomposite were depicted in Fig. 1. The peaks at 545cm −1 and 722 cm −1 are related to the vibrations of Ce-O, which are seen at all spectra [42,43]. Fig. 1 (pattern b) approves the synthesis of PANI on the CeO 2 NPs. The characteristic peaks of PANI were observed at 3258 cm −1 for the N-H of aromatic amines, 1579 cm −1 for the C=N, 1503 cm −1 for C=C, and 1336, 1290 cm −1 for C-N stretching. The N-H stretching and bending vibrations of MBI can be observed at 3155 and 1456, respectively ( Fig. 1 pattern c). Also, the stretching vibrations of C-H, C=N, C=C, C-N and C-S were perceived at 2800-3000 cm −1 , 1697 cm −1 , 1510 cm −1 , 1350 cm −1 and 1167 cm −1 , respectively [44].
The Raman spectra were illustrated in Fig. 2. The peak at 459 cm −1 is related to the F 2 g mode of CeO 2 NPs The TGA results of PANI-CeO 2 and MBI-PANI-CeO 2 nanocomposite were shown in Fig. 3.
In both curves, the weight loss at 50-200°C was obtained from removing adsorbed water. Removing of water molecules in MBI-PANI-CeO 2 nanocomposite is gentle than the PANI-CeO 2 . The hydrophobicity of the MBI decreased the amount of adsorbed water. The weight loss at 200 -600°C is due to the deterioration of PANI in Fig. 3 (a). The sharp weight loss at 200-600°C is due to the degradation of MBI and PANI [47].

Morphological studies of coatings
The morphological properties of the EP coatings containing CeO 2 and MBI-PANI-CeO 2 nanocomposite were investigated by SEM (Fig. 4).

EIS test
The barrier properties of EP coatings containing CeO 2 and MBI-PANI-CeO 2 nanocomposite were evaluated by EIS in NaCl 3.5 wt. % at 65 ℃. The Bode-phase curves of the EP, EP/CeO 2 , and EP/MBI-PANI-CeO 2 coatings at 2, 24, 72, and 200 hours were depicted in Fig. 5. The EIS data were tted with a more complex equivalent circuit as shown in Fig. 6  The protection properties of EP/MBI-PANI-CeO 2 coatings depend on the MBI-PANI-CeO 2 content. By increasing decrease was observed. A similar trend was also observed in the R ct values of coatings, con rming less surface corrosion of these coatings.
CPE is one of the essential parameters which was calculated according to Eq. 1 for the coating and electrical double layer.

CPE = (1)
Where R is the resistance (Ωcm 2 ), Y 0 is the admittance (Ω −1 cm −2 s n ), and n is the CPE exponent, which is between 0 and 1. The CPE coat values of all samples were increased with diffusion of aqueous electrolyte into the coating (Fig. 7). A smaller increase of CPE coat was observed for EP/MBI-PANI-CeO 2 . The CPE coat of EP coating was higher than that of EP/CeO 2 and EP/MBI-PANI-CeO 2 coatings, respectively. These results indicate that incorporating CeO 2 nanoparticles in the EP coating had a positive effect on the corrosion protection, while the tremendous improvement was observed in the case of MBI-PANI-CeO 2 nanoparticles. The same behavior was also observed for CPE of double layer ( Fig. 7(b)).
The amount of water uptake was calculated by Eq. 2 [49] and shown in Fig. 7 (c) for all samples.
Water uptake (%) = {[log (CPE coat , t /CPE coat , 0 )] / log (86)} × 100 (2) The water uptake values for EP are 1.86 and 4.05 times higher than that of EP/CeO 2 and EP/MBI-PANI-CeO 2 (1 wt. %), respectively. This was due to the promotion of the protective behavior of the EP coating in the presence of the MBI modi ed NPs.

Salt spray tests
The visual perspective of the neat EP, EP/CeO 2 , and EP/MBI-PANI-CeO 2 (1 wt. %) coatings were illustrated in     The equivalent circuit model for tting EIS data.  The pull-off results for the EP, EP/CeO2, and EP/MBI-PANI-CeO2 (1 wt. %).