One-Step In Situ Electrochemical Synthesis of Polyaniline/CeO₂ Composite Coating for Enhanced Corrosion Protection of Mild Steel

Abstract

The electrolytic composition significantly influences the structure and corrosion protection performance of polyaniline (PANI) coating. In the present work, oxalic acid and benzoic acid were employed to electropolymerize PANI coating on a mild steel substrate using the cyclic voltammetry (CV) technique. Then, cerium nitrate was introduced into the benzoic acid medium to electrochemically synthesize a PANI/CeO₂ composite coating in situ. Scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical measurements were used to characterize the coating structure and corrosion resistance. The results suggested that the PANI coating prepared from benzoic acid electrolyte possessed a neater structure and better anti-corrosive properties. The co-deposition of CeO₂ further increased the thickness and improved the compactness of the PANI coating. The synthesized PANI/CeO₂ composite coating possessed the smallest corrosion current density and the largest inhibition efficiency of 98.2%. The charge transfer resistance and coating resistance also increased significantly after the implantation of CeO₂ in the PANI coating. The enhanced corrosion protection performance of the PANI/CeO₂ hybrid was also elucidated.

1. Introduction

Due to their good physical blocking effect and inhibition performance, conducting polymers have been widely applied for the corrosion protection of various metal materials [1,2,3,4,5]. It is generally believed that a conducting polymer coating slows down the corrosion rate by shifting the open circuit potential to the passive region. Polyaniline, a typical conducting polymer, has been electropolymerized in multiple electrolytes, such as oxalic acid [6,7], metanilic acid [8], sulfuric acid [9,10], phosphoric acid [11], and so on. The composition of the electropolymerization electrolyte plays an important role in the PANI coating structure and corrosion resistance [12]. Pawar [13] electrochemically synthesized an anti-corrosive polyaniline coating on mild steel from the salicylate electrolyte. The prepared PANI coating was uniform, and the corrosion rate was considerably reduced. Ganash [14] electrodeposited polyaniline films from two acidic media and found that the growth rate and thickness of the PANI coating from sulphuric acid were much larger than that of phosphoric acid. Disappointingly, pure PANI coatings are not perfect alternatives for traditional chromatation and phosphatization coatings due to the weak adhesion and insufficient anti-corrosion capability since electrochemically synthesized PANI coating is apt to produce cracks and results in a loose structure [15]. To overcome these shortcomings, it is necessary to optimize the electropolymerization condition and further modify PANI coatings to improve their corrosion protection property.

The implantation of metal oxides in the PANI coating is one of the most effective strategies to enhance corrosion resistance [16,17,18]. Till now, various materials, including TiO₂ [19,20], CuO [21], Nb₂O₅ [22], CeO₂ [23], V₂O₅ [24], etc., have been explored. Among them, CeO₂ attracted the greatest attention due to its good cathodic inhibition effect and low toxicity [25,26]. Sasikumar [17] chemically prepared a PANI/CeO₂ nanocomposite, which acted as a mixed-type inhibitor for mild steel in HCl solution. PANI/CeO₂ nanocomposite has also been fabricated by Shetty [27] by the in situ polymerization of aniline and CeO₂ nanoparticles, which was applied to prevent the corrosion of 316 steel in HCl solution. However, the preparation of polyaniline/CeO₂ composite coating by an in situ co-electrodeposition from aniline and cerium nitrate electrolyte has rarely been reported, and the detailed investigation of its corrosion protection performance is still sparse.

In this paper, oxalic acid and benzoic acid were employed as two different electrolytes to electropolymerize PANI coating on a mild steel substrate using the cyclic voltammetry (CV) technique. The influence of electropolymerization electrolyte composition on the PANI structure and corrosion resistance was studied. After that, cerium nitrate was directly introduced into the benzoic acid electrolyte to electrochemically synthesize a PANI/CeO₂ composite coating in situ. Scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical measurements were used to characterize the coating structure and anti-corrosion property. The improved corrosion protection performance of the PANI/CeO₂ hybrid was also elucidated.

2. Experimental

2.1. Materials

Commercial grade mild steel, obtained from Shengtak New Materials Co. Ltd. (China), was cut into pieces with dimensions of 1 cm × 1 cm × 0.2 cm, and the chemical composition is shown in

Table 1. Chemical composition of mild steel.

Element	C	Si	Mn	Cr	Ni	Cu	Fe
%	0.17	0.15	0.39	0.11	0.13	0.32	balance

. Analytical grade aniline (purity > 99.5%) and anhydrous ethanol were bought from Shanghai Macklin Biochemical Technology Co. Ltd (Shang Hai, China). Analytical grade benzoic acid (purity > 99.5%), cerium nitrate (purity > 99.5%) and ammonium acetate (purity > 99.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shang Hai, China). Before electropolymerization, the mild steel pieces were sealed with epoxy resin and only one 1 cm × 1 cm face was exposed. Then, the exposed face was abraded with different grades of silicon carbide (SiC) paper. At last, the polished face was cleaned in acetone and ethanol for 15 min and dried with pure N₂.

2.2. Electropolymerization of Polyaniline and Polyaniline@CeO₂ Coatings

Traditional three-electrode systems were applied to fabricate polyaniline and polyaniline@CeO₂ coatings. Polished mild steel species served as the working electrode, whereas a saturated calomel electrode (SCE) and a large platinum sheet served as the reference electrode and counter electrode, respectively. Prior to electrochemical synthesis, mild steel species were immersed in electrolytes of open circuit potential for 30 min to ensure a steady-state condition. Three different electrolytes, as shown in

Table 2. Compositions of three different electrolytes for electropolymerization of polyaniline and polyaniline@CeO₂ coatings.

Sample	Aniline (mol/L)	Oxalic Acid (mol/L)	Benzoic Acid (mol/L)	Cerium Nitrate (mol/L)	Ammonium Acetate (mol/L)
1	0.1	0.3	-	-	-
2	0.1	-	0.04	-	-
3	0.1	-	0.04	0.1	0.1

, were explored to fabricate polyaniline and polyaniline@CeO₂ coatings. Samples 1 and 2 were prepared to investigate the influence of electrolyte composition on the polyaniline structure and corrosion protection property. Samples 2 and 3 were compared to study the impact of co-deposited CeO₂ on the structural and anti-corrosive performance of the composite coating. The electropolymerization was conducted by using a commercial CHI660A electrochemical workstation with the cyclic voltammetry (CV) (Shanghai Chenhua Instrument Company, China) technique between −0.5 and 1.6 V (vs. SCE) at a scan rate of 20 mV/s. After that, the working electrodes were washed with double-distilled water several times and subsequently dried at 50 °C for 2 h. The polyaniline and polyaniline@CeO₂ coatings in the following measurements were all prepared with 10 cycle numbers without otherwise specified.

2.3. Structure Characterization

The thicknesses of the electropolymerized polyaniline and polyaniline@CeO₂ coatings were measured using a Dektak 150 profilometer. The polyaniline from oxalic acid, benzoic acid and polyaniline@CeO₂ coatings possessed thicknesses of 28.6 ± 2.3 μm, 22 ± 1.9 μm and 34 ± 2.8 μm, respectively. The surface morphology of different coatings was characterized using a Hitachi SU-8010 scanning electron microscopy (SEM). The element composition of fabricated polyaniline@CeO₂ coating was detected using energy-dispersive X-ray spectroscopy (EDX). ESCALAB250 X-ray photoelectron spectroscopy (XPS) was applied to investigate the chemical composition of the prepared polyaniline@CeO₂ coating.

2.4. Electrochemical Measurements

The corrosion protection performance of constructed polyaniline and polyaniline@CeO₂ coatings on mild steel substrate were studied in a 3.5 wt.% NaCl solution using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). Prior to electrochemical tests, the coated and uncoated electrodes were immersed in 3.5 wt.% NaCl solution for 30 min to achieve a steady-state condition at the open circuit potential (E_ocp). Potentiodynamic polarization curves were recorded in the potential range from E_ocp −250 mV to E_ocp +250 mV with a scanning rate of 0.1 mV/s. Some informative electrochemical parameters were obtained from the extrapolation of Tafel curves. The inhibition efficiency ($IE\%$) was calculated as follows [28]:

$$IE=1-{\displaystyle \frac{j_{corr}^c}{j_{corr}^{bare}}}$$

(1)

where $j_{corr}^c$ and $j_{corr}^{bare}$ correspond to the corrosion current densities of the coated samples and bare mild steel sample, respectively. EIS data were measured in the frequency range from 10⁵ kHz to 10⁻² Hz with an AC amplitude of 20 mV. Z-view software (Z-view 3.1 software) was employed to fit the EIS result. The polarization resistance was derived from the following equation [29],

$$R_P=R_c+R_{ct}$$

(2)

where $R_c$ and $R_{ct}$ are the surface coating resistance and charge transfer resistance, respectively. Then, the following equation served to calculate the corrosion protection efficiency (PE%),

$$PE\%={\displaystyle \frac{R_p^c+R_p^b}{R_p^c}}$$

(3)

where $R_p^c$ and $R_p^b$ are the polarization resistances of coated and uncoated mild steel samples, respectively. Three parallel tests were replicated for each condition to guarantee reproducibility.

3. Results and Discussion

Figure 1 shows the cyclic voltammograms of mild steel in three different electropolymerization electrolytes. In the case of oxalic acid medium, during the anodic polarization process, the current density increases at approximately 0.22 V, which indicates the initial oxidation of the aniline monomer. Increasing the anodic deposition potential, the aniline oligomers are coupled with each other to form aniline cation radicals. A large anodic peak located at around 0.82 V can be observed, which may be related to the generation of a long-chain polyaniline polymer. It is worth noting that the appearance of a large, broad peak in the CV graph generally means a diffusion-controlled reaction, which will produce a relatively loose structure of deposits [30]. During the cathodic polarization scan, the oxidized form of polyaniline will turn into the reduced form of leucoemeraldine at around −0.03 V [31,32]. Meanwhile, by increasing the cycle numbers, the anodic current density increases while the cathodic current density decreases, suggesting the formation of a thicker polyaniline coating. Whereas, in the benzoic acid electrolyte, a rapid increase in anodic current density appears at about 0.78 V. Noticeably, the large, broad anodic peak at 0.82 V disappeared, and no cathodic peak can be detected during the cathodic sweep, which verifies the electrochemical-controlled process of electropolymerization of aniline from the benzoic acid electrolyte. Moreover, a lower anodic current density is obtained in the benzoic acid solution compared to that of the oxalic acid electrolyte at the same cycle number, which agrees well with the measured coating thickness. The above results infer that the benzoic acid electrolyte supplies improved coating structure and better stability [7].

As shown in Figure 1c, the characters of this CV plots of mild steel in the electropolymerization electrolyte containing aniline, benzoic acid and cerium nitrate are similar to that of the electrolyte without cerium nitrate. The significant difference between these two electrolytes lies in the larger current density in the electrolyte containing cerium nitrate than in the absence. The anodic oxidation current density increased from about 3 mA/cm² to 12 mA/cm² after introducing cerium nitrate, which demonstrated that the added cerium nitrate possibly facilitated the electropolymerization rate of aniline and may produce a thicker and more compact coating. Meanwhile, the cerium ions can be transferred to cerium oxide to form a polyaniline/CeO₂ composite coating through the following mechanism [33,34]:

$$C{e}^{3+}+2H_{2}O\to {Ce\left(OH\right)}_2^{2+}+2H^{+}+e^{-}$$

(4)

$${Ce\left(OH\right)}_2^{2+}\to {CeO}_2+2H^{+}$$

(5)

The surface morphology of prepared polyaniline coatings from oxalic acid and benzoic acid electrolytes and polyaniline/CeO₂ coating are presented in Figure 2a–c, respectively. In the case of the oxalic acid medium, the synthesized polyaniline coating exhibits some pores and aggregates over the surface. The high magnification SEM image reveals a filamentary structure that makes the polyaniline coating loose and porous, which was in accordance with the CV result. When electropolymerized in the benzoic acid, the coating surface looks more compact. The high magnification graph suggests the whole mild steel substrate was covered by the prepared polyaniline coating. Consequently, the dense structure of electropolymerized polyaniline from the benzoic acid electrolyte will cause a better corrosion protection property. Moreover, the whole mild steel surface was covered by a compact polyaniline/CeO₂ composite coating without any visible holes (Figure 2c). The high-magnification SEM picture also reveals a perfect film on the mild steel substrate. As expected, C, N and Ce elements were all present according to the EDX result (Figure 2d), which indicates the successful doping of cerium in the polyaniline coating.

Figure 3 depicts the XPS result of prepared polyaniline/CeO₂ coating on a mild steel substrate to analyze the chemical composition. The elements of C, N, O and Ce are present. The N element can be divided into three main peaks, located at around 398.3 eV, 399.5 eV and 401.2 eV, which were ascribed to the imine (=N=), amine (-NH-) and protonated amine (-NH⁺) structures, respectively [35,36]. The high resolution of the Ce 3d spectrum includes eight characteristic peaks [37]. The peaks labeled U1, U2, U3 and U4 are assigned to Ce 3d_3/2, whereas the remainder are related to Ce 3d_5/2 [38,39]. Additionally, the typical peaks signed by V2 and U2 were associated with Ce³⁺, and the other peaks were attributed to Ce⁴⁺ [40]. The XPS result suggested the successful modification of the polyaniline coating with CeO₂ and the coexistence of Ce³⁺ and Ce⁴⁺ in this composite coating.

The potentiodynamic polarization curves of bare mild steel prepared polyaniline and polyaniline/CeO₂ coatings in 3.5 wt.% NaCl solution are shown in Figure 4. It is obvious that both the cathodic and anodic current densities decreased when coated with electropolymerized polyaniline and polyaniline/CeO₂ coatings, which demonstrates that both the anodic dissolution process ($Fe-2e^{-}={Fe}^{2+}$) and cathodic reaction rate ($O_2+4e^{-}+2H_{2}O=4{OH}^{-}$) were significantly inhibited for the coated samples. Additionally, the corrosion potential of polyaniline and polyaniline/CeO₂-coated samples moved in a positive direction compared to the bare mild steel sample, which indicated that the prepared coatings acted as an anodic-type inhibitor [41]. The obtained values of electrochemical parameters and IE% are listed in

Table 3. Potentiodynamic polarization parameter for bare mild steel, prepared polyaniline and polyaniline/CeO₂ coatings in 3.5 wt.% NaCl solution.

Condition	Ecorr. (V)	jcorr. (μA.cm−2)	IE%
bare	−0.923	34.62	-
polyaniline (oxalic acid)	−0.836	8.185	76.4
polyaniline (benzoic acid)	−0.675	1.096	96.8
polyaniline/CeO2	−0.543	0.632	98.2

. Obviously, the corrosion potential (E_corr.) followed the order of polyaniline/CeO₂ composite coating (−0.543 V) > polyaniline (benzoic acid) (−0.675 V) > polyaniline (oxalic acid) (−0.836 V) > bare mild steel substrate (−0.923 V). The fabricated polyaniline/CeO₂ composite coating possessed the largest positive shift of 380 mV, suggesting the most chemical stability [42], which confirmed a synergistic passivation effect of conducting polyaniline coating and introduced CeO₂. It is notable that the polyaniline coating from the benzoic acid medium exhibited a smaller value of j_corr. And a larger value of IE% than that from oxalic acid. The better corrosion protection performance may be related to the more compact structure, which will consequently produce a better physical shielding of polyaniline coating from the benzoic acid electrolyte. Meanwhile, the synthesized polyaniline/CeO₂ composite coating had the largest inhibition efficiency of 98.2%, which indicated the slowest corrosion rate of mild steel after being coated with polyaniline/CeO₂ hybrid.

Figure 5 depicts the Bode and Nyquist plots of bare mild steel, prepared polyaniline and polyaniline/CeO₂ coatings in 3.5 wt.% NaCl solution. It is interesting that the fabricated polyaniline from benzoic acid electrolyte and polyaniline/CeO₂ hybrid coatings present significant influence on the EIS plots, while no obvious changes can be observed for the polyaniline coating from oxalic acid solution. Both the impedance and arc diameters considerably increased for the prepared polyaniline coating from the benzoic acid electrolyte and polyaniline/CeO₂ composite coating, revealing good anti-corrosion properties of these two coatings coated with mild steel samples in chloride-containing solution [43]. Generally, the impedance modulus at a typical low frequency (Z_{0.01 Hz}) can be employed as a qualitative criterion to evaluate the corrosion protection ability of a coating [44]. In the present work, the Z_0.01Hz of the fabricated polyaniline/CeO₂-coated mild steel presented a larger value than that of the prepared polyaniline-coated samples from both oxalic acid and benzoic acid medium and over 20 times higher than that of the bare mild steel substrate, which suggests the best anti-corrosion performance of polyaniline/CeO₂ among these synthesized coatings [45]. The equivalent electric circuits, as shown in Figure 5c, were applied to analyze the experimental EIS data [46]. The equivalent electric circuit with one time constant was used to fit the EIS plot of bare mild steel, whereas two time constants were employed to analyze the EIS plots of electropolymerized polyaniline and polyaniline/CeO₂ hybrid coatings. In the equivalent electric circuits, R_s, R_c and R_ct represent the solution resistance, coating resistance and charge transfer resistance, respectively. CPE_dl and CPE_c correspond to a double-layer capacitance and coating capacitance, respectively. The obtained values of EIS parameters are summarized in

Table 4. EIS parameters for bare mild steel, prepared polyaniline and polyaniline/CeO₂ coatings in 3.5 wt.% NaCl solution.

Condition	Rct kΩ.cm2	CPEdl Ω−1·cm−2·sn	nc	Rc Ω.cm2	CPEc Ω−1·cm−2·sn	ndl	Rp kΩ.cm2	PE%
bare	1.06	3.81 × 10−5	0.83	-	-	-	1.06	-
polyaniline (oxalic acid)	1.14	3.17 × 10−5	0.83	27.7	9.57 × 10−5	0.73	1.17	9.4
polyaniline (benzoic acid)	11.3	6.59 × 10−6	0.84	990	5.38 × 10−5	0.77	12.3	91.4
polyaniline/ CeO2	23.6	3.26 × 10−6	0.86	4754	2.93 × 10−5	0.81	28.4	96.3

. The value of R_ct for prepared polyaniline from the oxalic acid electrolyte increased slightly in comparison with the bare carbon steel sample, and the value of R_c was small. However, for the electropolymerized polyaniline from benzoic acid, the value of R_ct was enlarged over one order of magnitude compared to the bare mild steel, and R_c also showed a large value. Zhang [47] synthesized a polyaniline coating on the copper substrate from the sodium oxalate electrolyte. EIS results suggested that the impedance diagram of the copper electrode covered by the fabricated polyaniline film was similar to that of the uncoated sample, and the prepared polyaniline coating exhibited no corrosion protection capability. This phenomenon has also been reported by other researchers [48,49]. Notably, the synthesized polyaniline/CeO₂ coating exhibited the largest R_p and PE% values, which agreed well with the results of potentiodynamic polarization curves. The improved corrosion protection performance of the prepared polyaniline/CeO₂ coating may be correlated with the following factors. Firstly, the introduced CeO₂ promotes the oxidation process to fabricate a dense polyaniline coating. Secondly, the implanted CeO₂ nanoparticles act as an inorganic filler in the polyaniline/CeO₂ hybrid coating (Figure 2c), which presents excellent physical barrier effects and efficiently inhibits the penetration of chloride ions to the mild steel substrate. Moreover, the presence of Ce³⁺ in the composite coating will convert to a stable cerium oxide coating through the following mechanism [50], which subsequently blocks the penetration paths and suppresses the propagation of corrosion.

$${4Ce}^{3+}+O_2+2H_{2}O+{4OH}^{-}\to {4Ce\left(OH\right)}_2^{2+}$$

(6)

$${Ce\left(OH\right)}_2^{2+}+{2OH}^{-}\to {CeO}_2+2H_{2}O$$

(7)

In comparison with previous works, the polyaniline/CeO₂ composite coating prepared through the electrolyte containing benzoic acid and cerium nitrate in the present study exhibited excellent anti-corrosion properties (

Table 5. Comparison of anti-corrosion performance of some polyaniline-based coatings prepared from electropolymerization.

Sample	Electrolyte	IE%	Ref.
polyaniline	benzoic acid	96.8	this work
polyaniline/CeO2	benzoic acid + cerium nitrate	98.2	this work
polyaniline	oxalic acid	81.4	[51]
polyaniline	sodium potassium tartrate	82.5	[7]
polyaniline/graphene	sulfuric acid + grahpene	97.0	[52]
polyaniline	sulfuric acid	94.4	[53]
polyaniline-GO	sodium dodecyl sulfate+GO	98.4	[54]

), which implied significant prospects for industrial applications.

4. Conclusions

In the present work, PANI coatings were electropolymerized on a mild steel substrate using oxalic acid and benzoic acid as two different electrolytes. The coating structure and corrosion resistance are significantly influenced by the electrolyte composition. The PANI coating that was electropolymerized in benzoic acid exhibited a compact structure, while a porous and loose morphology was observed for the oxalic acid electrolyte. Potentiodynamic polarization curves revealed a more noble corrosion potential and lower corrosion current density for the benzoic acid than the oxalic acid electrolyte. EIS results further confirmed a better corrosion protection performance of electropolymerized PANI coating in the benzoic acid electrolyte. In addition, the introduction of cerium nitrate in the benzoic acid electrolyte increased the polymerization reaction rate and consequently produced a denser polyaniline/CeO₂ composite coating. This co-electrodeposited polyaniline/CeO₂ hybrid significantly enhanced the corrosion resistance of mild steel in NaCl solution, which is significantly promising to be scaled up for practical applications.