Introduction

Conducting polymers, because of their unique combination of physical and chemical properties, possibility of both chemical and electrochemical synthesis, distinct electronic properties, diversity, processing advantages of conventional polymers and potentially low cost, have drawn the attention of scientists and engineers for various application possibilities like batteries, capacitors, transistors, photovoltaic cells, light-emitting diodes, aircraft fuselage, and biochemical analysis (Ref 1, 2). In recent years, the growing environmental concern has lead to their use as film-forming corrosion inhibitors or as anticorrosion coatings to protect the metallic substrates (Ref 3-9). Among the conducting polymers, polyaniline (PANi), polypyrrole (PPy), and their derivatives (Ref 10-17) are the most promising and are mainly considered for corrosion protection owing to their stability and synthesis advantages. The synthesis of conducting polymers could be realized either by chemical or electrochemical route. However, the film-forming electropolymerization at oxidizable metals has been hindered by several thermodynamic as well as kinetic problems. The metal's oxidation thermodynamic potentials are significantly lower than those of conducting monomers. As a consequence, the metallic electrode subjected to electropolymerization generally undergoes strong anodic dissolution before the oxidation potential of the monomer can be reached, thus preventing the occurrence of electropolymerization reaction.

One of the challenges in developing conducting polymer coatings, in general, has been to overcome the difficulty in processing these materials. The general lack of solubility and fusibility of these materials makes the formation of coating on active metals difficult and prohibit them as replacement for traditional coating systems. The charge stored in the polymer layer (used to oxidize the base metal and produce the passive layer) can be irreversibly consumed during the system’s redox reactions and hence the protective properties of the polymer coating may be lost with time. Further, the hydrophilic and porous nature of conducting polymer film may lead to serious drawbacks for anticorrosive applications under severe conditions. Also, the extent to which the conducting polymers can be used is limited due to the exclusivity of monomers that are essential for their synthesis. To overcome these limitations, different synthesis approaches have been attempted. These include the synthesis of substituted conducting polymer coatings, bi-layered composite coatings, and copolymer/terpolymer coatings. The copolymerization has long been utilized to improve various properties, for e.g., conductivity, stability, porosity, etc., of the polymer films (Ref 18). The addition of monomers with hydrophobic groups could lower the water up taking rate or another group may enhance the stability and adherence and thus help to prepare new polymers with inbuilt tailor-made properties suitable for the application. Another interesting alternative to conducting polymers is to consider conducting polymer/metal oxide nanocomposite systems (Ref 19-27). The conducting polymer/metal oxide nanocomposites perform better when compared with those of pure conducting polymer and metal oxide. A number of metals and metal oxide nanoparticles have been encapsulated into the shell of conducting PANi to produce a host of composite materials. These composite materials show better mechanical, physical, and chemical properties, due to the combination of the qualities of conducting PANi and inorganic particles (Ref 28-30).

In the previous work (Ref 31-34), we have reported the synthesis, characterizations, and anti corrosion properties of conducting homopolymers, copolymers, and terpolymer coatings on low-carbon steel surfaces in different corrosive environments. In continuation of our work, a copolymer from aniline (AN) and o-anisidine (OA), Poly(AN-co-OA) and its nanocomposite with ZnO nanoparticles Poly(AN-co-OA)/ZnO were synthesized by chemical oxidative polymerization. The resultant copolymer and nanocomposite were deposited on steel substrate by solution evaporation. The anticorrosive property of polymers was investigated in 0.1 M HCl, 5% NaCl solution, distilled water, and open atmosphere by subjecting them to different corrosion tests which include immersion test, open circuit potential (OCP), and potentiodynamic polarization measurements. The corrosion performance of nanocomposite was also compared with the copolymer.

Experimental Section

Materials and Methods

Chemicals

Aniline, o-anisidine, zinc acetate dihydrate, NaOH, HCl, NaCl, and NMP were purchased from Merck. Ammonium persulfate and all other organic reagents were of analytical grade and used without further purification. Deionized water was used in the synthesis and for all other purposes. The epoxy resin was synthesized by following previously described method (Ref 35).

Preparation of Specimen

The steel coupons of dimension 40.0 × 15.0 × 1.3 mm were used in corrosion studies. The chemical composition (weight %) of the low-carbon steel as obtained from optical emission spectrophotometer is 0.049% C, 0.0289% P, 0.081% Mo, 0.723% Mn, 0.051% Cr, 0.013 Al, 0.033 V, and balance iron. In order to remove any existing passive film, the surface of the steel coupons was mechanically polished using a series of emery papers and then rinsed with double-distilled water and acetone and finally dried in hot air.

Synthesis of ZnO Nanoparticles

ZnO nanoparticles were synthesized by the procedure reported elsewhere (Ref 36). Aqueous zinc acetate dehydrate (0.02 M) was dissolved in 50 mL distilled water under continuous stirring. At room temperature, aqueous NaOH (2.0 M) solution was added drop wise to attain pH 12. The solution was vigorously stirred for 2 h. After completion of the reaction, the white precipitate formed was washed thoroughly with distilled water followed by ethanol to remove the impurities. The precipitate was dried in a hot air oven for overnight at 60 °C. Complete conversion of Zn (OH)2 into ZnO nanoparticles took place during drying.

Synthesis of Poly(AN-co-OA) Copolymer and Poly(AN-co-OA)/ZnO Nanocomposite

Poly(aniline-co-o-anisidine) copolymer was synthesized by chemical oxidative copolymerization following previously described method (Ref 37). A typical procedure of the synthesis of copolymer with a 50:50 monomer ratio is as follows: A mixture consisting of 6.6 mL of aniline (0.1 M) and 8.3 mL of o-anisidine (0.1 M) were dissolved in 150 mL of 1 M HC1 taken in a 250-mL two-necked glass flask. This solution was maintained at 0-5 °C and constantly stirred for about 1 h. Another solution prepared by dissolving 15 g of ammonium persulfate in 50 mL of 1 M HCl was added drop by drop to this solution. The reaction was continued for 24 h, after which a green precipitate was formed which was filtered and first washed with 1 M HC1 and then distilled water until the disappearance of the color of the filtrate. The copolymer hydrochloride salt was subsequently neutralized in 0.1 M ammonium hydroxide for 24 h to obtain the copolymer base. The copolymer base was washed with excess water. The powder was obtained which was left to dry in ambient air for one day. Poly(AN-co-OA)/ZnO nanocomposite was synthesized by dispersing ZnO nanoparticles (10% w/w based on the comonomer content) to the mixture of aniline (6.6 mL, 0.1 M) and o-anisidine (8.3 ml, 0.1 M) dissolved in 150 mL of 1 M HCl taken in a 250-mL two-necked glass flask. The synthesis of the nanocomposite was further continued following the identical procedure as prescribed for the synthesis of the copolymer. The nanocomposite containing 10% ZnO was chosen for this study because of its significantly higher conductivity.

Characterization of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO Nanocomposite

Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO nanocomposites were characterized by FTIR, XRD, electrical conductivity technique, SEM-EDS, and TEM. XRD studies were carried out in the 2θ range of 20-80° using x-ray diffractometer (Model: Shimadzu 6100 X). FTIR spectroscopy (Model: Perkin Elmer) of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO nanocomposite was studied in the frequency range of 500-4000 cm−1. The spectra of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO nanocomposite were taken as KBr disks. The electrical conductivity of the copolymer and nanocomposite was measured for the palletized pigment using a four-probe resistance meter. The morphological and compositional analyses of the copolymer and nanocomposite were carried out using SEM (Model: JEOL JSM-6510LV) with an EDS (INCA, Oxford) attachment and TEM (Model: JEOL JEM-2100).

Preparation of Coatings of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO Nanocomposite on Low-Carbon Steel

The saturated solutions of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO were separately obtained in NMP; this was followed by mixing of 10% epoxy resin to each solution. The resultant solutions were filtered and spread on the steel substrate with the help of a dropper. This was followed by evaporation of solvent at temperature 85-90 °C. The solution was poured on the steel surface till a thick and near uniform coating was obtained. The coating thickness and uniformity were maintained by continuously monitoring the weight of the deposited coating per unit area. The coating was applied on only one side of steel samples. The other side and edges were covered with clear nail polish. A strongly adherent dark blue (copolymer) or dark black (nanocomposite) coating was obtained on the steel substrate. More coated samples were obtained following identical procedure. The coating thickness was measured using Elcometer (Model: 456) and found to be in the range of 10.26-12.92 μm.

Evaluation of Corrosion Protection Performance of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO Coatings

In order to evaluate the corrosion protection performance of copolymer and nanocomposite coatings in different corrosive media, e.g., 0.1 M HCl, 5% NaCl solution, distilled water, and open atmosphere, the coated samples were subjected to immersion test, OCP measurements, and potentiodynamic polarization measurements. The corrosion tests were done on uncoated, coated, and coated scribed samples at room temperature under static condition. The micrographs of corroded steel samples were obtained by SEM.

Immersion Test

After taking the initial weight and dimension, the uncoated, coated, and coated scribed specimens were hanged in the test solution with the help of nylon thread. The immersion tests were performed on triplicate samples for the duration of 30 days under the static condition at room temperature. The corrosion rate in mills per year (mpy) was calculated by using the following equation:

$${\text{Corrosion}}\;{\text{rate}}\; ( {\text{CR)}}\; ( {\text{mpy) = }}\frac{ 5 3 4W}{\rho AT},$$
(1)

where W is the weight loss in mg, ρ is the density of specimen in g/cm3, A is the area of the specimen in square inch, and T is the exposure time in hours. After 30 days of immersion, the specimens were withdrawn from the test solution, washed thoroughly with distilled water and dried with air, and then weighed again. The integrity of the coating was visually examined. The %IE was calculated using the following equation:

$$(\% {\text{ IE}}) \, = \, \frac{{{\text{CR}}_{0} - {\text{CR}}_{i} }}{{{\text{CR}}_{0} }} \times 100,$$
(2)

where CR0 is the corrosion rate of low-carbon steel in the absence of coating and CR i is the corrosion rate of low-carbon steel in the presence of coating.

Free Corrosion Potential Measurements

The free corrosion potential measurement of uncoated, coated, and coated scribed specimens was carried out in 0.1 M HCl, 5% NaCl solution, and distilled water. The steel specimen was connected to a wire having an alligator clip on both the ends. One end of the alligator clip was connected to a multimeter, whereas the other end was connected to the steel specimen and placed into the test solution. The change in voltage against saturated calomel electrode (SCE) used as reference electrode was plotted versus time. The potential measurement in a particular medium was continued till a steady state was obtained or it went down to the potential of bare steel.

Potentiodynamic Polarization Measurements

The potentiodynamic polarization measurements were performed on a conventional three-electrode cell assembly, using an Autolab Potentiostat/Galvanostat Model 128 N. The experiments were carried out using Ag/AgCl electrode (saturated KCl) as a reference electrode, Pt wire as the counter electrode, and low-carbon steel specimens as working electrode. Uncoated, coated, and coated scribed steel specimens with exposed surface area of 1 cm2 were used as working electrode. The polarization studies were carried out by sweeping the potential between −250 and 250 mV with respect to the steady-state potential at a scan rate of 0.001 V/s. Before starting the measurements, the specimen was left to attain a steady state which was indicated by a constant potential. All the experiments were carried out at room temperature (301 °C) under static condition. All the measurements were repeated at least three times and good reproducibility of the results was observed.

Surface Morphological Studies

SEM was used to evaluate the surface morphology of the Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO coatings on low-carbon steel before and after one-month immersion in 0.1 M HCl solution. After completion of immersion, the test specimens were taken out, thoroughly washed with DDW and dried, and then subjected to SEM studies.

Atmospheric Test

The atmospheric test was performed as per ASTM designation G-7-05 (standard practice for atmosphere testing of non-metallic materials). The Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO-coated steel samples along with coated scribed and uncoated steel samples were weighed and subsequently fixed on a panel which stood on a heavy metallic base and placed at the roof of the department. The exposure time was 60 days. The humidity and temperature were monitored during the entire period of atmospheric exposure. The temperature during the period varied between 18 and 32 °C, whereas average relative humidity was 20%. After the completion of the exposure test, the samples were taken off from the panel and then physically examined for any coating deterioration. To further examine the effect of atmosphere on the corrosion performance of the polymer coatings, the samples obtained after exposure to open atmosphere were immediately immersed in distilled water and were subjected to potentiodynamic polarization measurements.

Results and Discussion

Characterization of Poly(AN-co-OA) Copolymer and Poly(AN-co-OA)/ZnO Nanocomposite

FTIR Spectroscopy Results

Figure 1(a) and (b) shows the FTIR spectra of the pure copolymer and its nanocomposite with 10% ZnO nanoparticles. The copolymer and nanocomposite showed almost identical spectra. Considering the FTIR spectrum of copolymer (Fig. 1a), the broad band centered at 3230 cm−1 is attributed to the characteristic free N-H stretching vibration of a secondary amine (-NH) group (Ref 38). The bands at 1575 and 1493 cm−1 are assigned to C-N and C=C stretching vibrations of quinoid and benzenoid rings, respectively (Ref 39). The peak at 1284 cm−1 has been attributed to the C-N stretching vibration in the quinoid imine units. The band at 1172 cm−1 is considered as a measure of the degree of the delocalization of electrons (Ref 40). The band corresponding to out of plane bending vibration of C-H bond of p-disubstituted rings appeared at 825 cm−1. The appearance of these IR bands verified the formation of copolymer. In the nanocomposite (Fig. 1b), we observed the respective vibrational bands of the both copolymer and ZnO (the Zn-O band appearing at 458 cm−1). However, the corresponding bands of the pure copolymer have been shifted to 1585, 1509, 1286, 1174, and 830 cm−1, respectively, in the nanocomposite. In addition, the intensity ratio of the quinonoid band has changed. These results indicated the existence of hydrogen bonding interaction between the copolymer and ZnO.

Fig. 1
figure 1

FTIR absorption spectra of (a) Poly(AN-co-OA) copolymer and (b) Poly(AN-co-OA)/ZnO nanocomposite

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Electrical Conductivity Results

The conductivity of the copolymer Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO nanocomposites was recorded at room temperature and found to be 7.20 × 10−4 and 6.8 × 10−3 Scm−1, respectively. The conductivity of nanocomposite was increased by the addition of 10% ZnO nanoparticle by one order of magnitude. The increase in the conductivity is attributed to the interaction between copolymer chain and ZnO which effectively improved the degree of electron delocalization between the two components (Ref 41).

XRD Results

The XRD patterns of the pure copolymer and nanocomposite are shown in Fig. 2(a) and (b). In the XRD patterns of copolymer/nanocomposite, the peaks centered at 2θ = 20-30° may be ascribed to periodicity parallel to the copolymer chain (Ref 42). XRD patterns of copolymer have a broad peak at about 2θ = 25.2°, which is the characteristic peak of copolymer (Ref 43). In the XRD patterns of nanocomposite (Fig. 2b), the interaction of Poly(AN-co-OA) with ZnO nanoparticles leads to the highly ordered structure, which can be clearly seen by the pattern in the high-angle region (Ref 44). The sharp peaks observed at 2θ = 43.9, 64.2, and 77.4° which correspond to the crystal planes (102), (103), and (202) imply the presence of ZnO nanoparticles in Poly(AN-co-OA)/ZnO nanocomposite and ordered structure which results in crystallinity (Ref 45-47). The addition of ZnO nanoparticles caused an increase in the intensity of copolymer peaks. This confirmed the formation of a conducting organic-inorganic nanocomposite.

Fig. 2
figure 2

XRD patterns of (a) Poly(AN-co-OA) copolymer and (b) Poly(AN-co-OA)/ZnO nanocomposite

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SEM-EDS and TEM Results

Figure 3(a) and (b) shows the SEM images and EDS profile of the Poly(AN-co-OA) copolymer and Poly(AN-co-OA)/ZnO nanocomposite. The ZnO nanoparticles have a strong effect on the morphology of the copolymer. SEM micrograph of the pure copolymer (Fig. 3a) shows significant difference in its morphology compared to the morphology of its nanocomposite (Fig. 3b). EDS analysis of copolymer shows the presence of characteristic peaks of the elements constituting the copolymer, whereas, in copolymer nanocomposite, additional peaks of Zn are observed. The copolymer showed a typical amorphous morphology (which is also confirmed by XRD, Fig. 2a), whereas the nanocomposite showed growth of a chain pattern of the copolymer with the ZnO nanoparticles presents between the junctions of the copolymer chain network. In nanocomposite, the ZnO nanoparticles are fairly dispersed in the copolymer matrix. The nanoparticles are almost uniform, global, and slightly agglomerated. EDS mapping of the nanocomposite indicated that the ZnO nanoparticles were well dispersed in the copolymer (Fig. 4).

Fig. 3
figure 3

SEM images and EDS profile of (a) Poly(AN-co-OA) copolymer (b) Poly(AN-co-OA)/ZnO nanocomposite

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Fig. 4
figure 4

EDS mapping of Poly(AN-co-OA)/ZnO nanocomposite

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The TEM image of Poly(AN-co-OA)/ZnO nanocomposite is depicted in Fig. 5. TEM micrograph clearly reveals that the ZnO nanoparticles in the range of 25-30 nm are homogeneously dispersed and embedded in the copolymer matrix. This suggests that the ZnO interacts with copolymer by the formation of H-bonding between the proton on N-H and the oxygen atom on ZnO surface.

Fig. 5
figure 5

TEM image of the Poly(AN-co-OA)/ZnO nanocomposite

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Corrosion Protection Performance of Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO Coatings

Immersion Test

Table 1 shows the immersion tests results of uncoated, coated, and coated scribed low-carbon steel specimens in different corrosive solutions, e.g., 0.1 M HCl, 5% NaCl solution, and distilled water. Comparing the results of the immersion tests in three corrosive media, 0.1 M HCl is found to be most corrosive; this is followed by 5% NaCl solution and distilled water. The corrosiveness of NaCl solution and distilled water is almost same and comparable. The corrosion performance of Poly(AN-co-OA)/ZnO nanocomposite coatings was found much better (PE being highest) than Poly(AN-co-OA) coatings in all corrosive media. The efficiency of nanocomposite coating in 0.1 M HCl, 5% NaCl solution and distilled water being 82.04, 72.33, and 93.24%, respectively, whereas that of copolymer coating in the respective medium being 73.97, 64.50, and 87.08%, respectively. The presence of scribed marks on the copolymer and nanocomposite coatings only slightly affected their performance and caused a slight decrease in the PE. This confirms the self-passivating nature of the copolymer and nanocomposite coatings. The self-passivating nature of the coatings is attributed to the incorporated PANi homopolymer which has the ability to repair the artificial defects in the coating system.

Table 1 Results of immersion tests
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The better performance of nanocomposite coating is because of its superior barrier property. The presence of ZnO nanoparticles in the Poly(AN-co-OA) copolymer film restricted the penetration and diffusion path of electrolytes and other corrosive species and caused an improvement in the PE of the nanocomposite coating (Ref 48). Further, the typical flaky microstructure of Poly(AN-co-OA)/ZnO nanocomposite may limit the diffusion path of corrosive species to the underlying metal. In addition to the above, in the nanocomposite coating, the PANi as p-type semiconductor and ZnO as n-type semiconductor may form a p-n junction which may further limit the passage of electrolyte to the base metal (Ref 26, 49, 50).

Open Circuit Potential

The OCP values (Ecorr) of uncoated, and Poly(AN-co-OA) copolymer and Poly(AN-co-OA)/ZnO nanocomposite (both coated and scribed) steel samples were measured against time in three different media, namely, 0.1 M HCl, 5% NaCl solution, and distilled water and the results are produced in Fig. 6, 7, and 8. Considering the results of OCP measurements in different corrosive media, when steel is covered with copolymer or nanocomposite coatings, the potentials are shifted toward more noble values compared with the uncoated steel. The noble shift in potential is more pronounced for nanocomposite coating than copolymer coating in all corrosive media subjected to investigation. A noble potential for coated steel indicates that it has greater resistance to corrosion (Ref 51), which is attributed to both barrier effect and formation of a passive oxide due to redox reaction at the coating/steel interface (Ref 52). The barrier effect remains operative till the coatings are undamaged and intact and isolated the steel from the corrosive solutions. With continuation in immersion, the initial OCP started to increase (become less noble) as a result of initiation of corrosion process under the polymer coatings due to the ingress of electrolyte via the pores in coatings. When sufficient amount of electrolyte reaches to the steel surface, the corrosion processes are initiated at the coating/steel interface leading to the anodic dissolutions of steel. In this context, the porosity of coatings has considerable importance in the initiating and progression of corrosion at the coating/steel interface. In coated steel, the initial increase in potential is interrupted and a subsequent positive (noble) shift in potential is observed; this is again followed by an increase in potential till a steady potential is observed. The subsequent positive shift in the potential is attributed to the formation of passive film on the steel substrate due to the presence of PANi in the polymer coatings (Ref 24, 27, 53, 54). The better performance of nanocomposite coating (more noble shift) than copolymer coating is attributed to the superior barrier behavior of nanocomposite coating owing to the presence of a more uniform and dense film on the steel substrate. The Zn present in the nanocomposite coating may convert to Zn+2 ions, which may interact with the nitrogen atom of the PANi and may change the morphology of the copolymer into compact cluster and decrease the corrosion of underlying steel (Ref 53, 55, 56). The small percentage of Zn+2 ions may also inhibit the corrosion of steel substrate (Ref 26, 57). The presence of ZnO nanoparticles can also improve the redox behavior of PANi significantly. In the case of coated scribed sample, though the initial OCP was higher (less noble) than the respective coated steel owing to the break in the coating, the coating repassivated as a result of redox reaction and remained nobler than the potential of uncoated steel.

Fig. 6
figure 6

OCP variations of uncoated, coated, and scribed steel during 200-h immersion in 0.1 M HCl

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Fig. 7
figure 7

OCP variations of uncoated, coated, and scribed steel during 200-h immersion in 5% NaCl solution

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Fig. 8
figure 8

OCP variations of uncoated, coated and scribed steel during 200-h immersion in distilled water

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Potentiodynamic Polarization

The potentiodynamic polarization curves for uncoated, Poly(AN-co-OA) and Poly(AN-co-OA)/ZnO (both scribed and unscribed) coated steel recorded in 0.1 M HCl, 5% NaCl solution and distilled water, respectively, are shown in Fig. 9, 10, 11. The corrosion kinetics parameters derived from these curves, e.g., corrosion potential (E corr), corrosion current density (I corr), cathodic Tafel slope (b c), anodic Tafel slope (b a), and polarization resistance R p are listed in Table 2. The analysis of potentiodynamic polarization curves shows noble shift (positive shift) in E corr, substantial reduction in I corr, and increase of R p values of the low-carbon steel in the presence of both copolymer and copolymer/nanocomposite coatings in all the three medium subjected to investigation. This confirms the corrosion-resistant characteristics of the coatings in different corrosive media. In general, the shift in Ecorr is higher for nanocomposite coatings as compared to the copolymer coatings implying that the nanocomposite coating provides more effective protection to the low-carbon steel corrosion in all three medium by depressing the anodic current of the corrosion reaction (Ref 51). There is a change in the values of both the Tafel slopes implying that corrosion of low-carbon steel in the presence of nanocomposite and copolymer coatings is under both anodic and cathodic control. In case of coated scribed samples, the damage inflected on the coatings has some deteriorating effect on the protective properties of the coatings due to the activation of corrosion process at the coating/metal interface. However, the values of I corr and corrosion rates are still lower than the bare steel indicating that protection other than barrier is operating. Again the performance of scribed nanocomposite coating is better than the scribed copolymer coating.

Fig. 9
figure 9

Potentiodynamic polarization curves in 0.1 M HCl for (a) Uncoated steel; (b) Poly(AN-co-OA) coated steel; (c) Poly(AN-co-OA) coated scribed steel; (d) Poly(AN-co-OA)/ZnO coated steel and (e) Poly(AN-co-OA)/ZnO coated scribed steel

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Fig. 10
figure 10

Potentiodynamic polarization curves in 5% NaCl for (a) Uncoated steel; (b) Poly(AN-co-OA) coated steel; (c) Poly(AN-co-OA) coated scribed steel; (d) Poly(AN-co-OA)/ZnO coated steel and (e) Poly(AN-co-OA)/ZnO coated scribed steel

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Fig. 11
figure 11

Potentiodynamic polarization curves in distilled water for (a) Uncoated steel; (b) Poly(AN-co-OA) coated steel; (c) Poly(AN-co-OA) coated scribed steel; (d) Poly(AN-co-OA)/ZnO coated steel and (e) Poly(AN-co-OA)/ZnO coated scribed steel

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Table 2 Results of potentiodynamic polarization measurements
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The porosity in the coating is also an important parameter as it decides its suitability to protect the underneath metal against corrosion. The porosity of the copolymer and nanocomposite coatings on low-carbon steel in different corrosive media was determined from potentiodynamic polarization measurements by following the relationship (Ref 58):

$$P\text{ } = \text{ }\frac{{R_{\text{p (uncoated)}} }}{{R_{\text{p (coated)}} }}10^{{ - \left( {\frac{{\Delta E_{\text{corr}} }}{ba}} \right)}},$$
(3)

where P is the total porosity, R p(uncoated) and R p(coated) are the polarization resistances of uncoated and coated low-carbon steel, respectively, ΔE is the difference between the corrosion potential, and ba is the anodic Tafel slope for uncoated low-carbon steel. The porosity values are listed in Table 2. The porosity in the nanocomposite coating was found to be significantly lower compared to the porosity in copolymer coating in all the corrosive media under investigation. This again suggests the improvement in the corrosion resistance of nanocomposite coating which greatly hindered the access of the electrolyte to the low-carbon steel substrate.

Atmospheric Test

After the completion of the atmospheric test, the samples were physically examined for color change and any coating deterioration. In the test, both copolymer and nanocomposite coatings did not show any color change. But the coatings were found to be detached from the substrate at some places. However, the performance of nanocomposite coating was better than copolymer coating as it showed insignificant detachment from the substrate. The potentiodynamic polarization curves for uncoated, coated, and coated scribed steel samples recorded in distilled water after 60 days exposure to open atmosphere are shown in Fig. 12. The values of corrosion kinetic parameters were obtained from these curves and are listed in Table 3. The Tafel plots show a positive shift in corrosion potential and lowering in corrosion current density for the coated steel with respect to bare steel for the same condition. The results of the potentiodynamic polarization studies show that the corrosion performance of both nanocomposite and copolymer coatings is only slightly affected during the atmospheric exposure test and corrosion rates of coated samples are only slightly higher than those polarized before atmospheric exposure. This suggests that after 60 days of exposure to atmosphere, though the adherence of the coatings was affected, it still maintained the protective properties giving good protection to underneath metal. This again implies that a protection mechanism other than barrier protection is operating.

Fig. 12
figure 12

Potentiodynamic polarization curves in distilled water for (a) Uncoated steel; (b) Poly(AN-co-OA) coated steel; (c) Poly(AN-co-OA) coated scribed steel; (d) Poly(AN-co-OA)/ZnO coated steel and (e) Poly(AN-co-OA)/ZnO coated scribed steel after 60 days exposure to open atmosphere

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Table 3 Results of potentiodynamic polarization measurements after 60 days exposure to open atmosphere
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Surface Morphological Studies

Figure 13(a) to (d) shows the surface morphology of the copolymer and nanocomposite coatings on low-carbon steel before and after one-month immersion in 0.1 M HCl. Before immersion in 0.1 M HCl, the copolymer and nanocomposite coatings did not show any cracks or defect (Fig. 13a, c). The nanocomposite coating appeared more dense and uniform than copolymer coating, hence providing higher corrosion protection performance. However, after one-month immersion in HCl solution, the copolymer coating was affected and some fine cracks are visible (Fig. 13b). The one-month immersion in HCl solution did not cause any significant damage to the nanocomposite coating and more or less a defect free surface was obtained (Fig. 13d).

Fig. 13
figure 13

SEM images of (a) and (b) Poly(AN-co-OA) copolymer-coated steel before and after 30-day immersion; (c) and (d) Poly(AN-co-OA)/ZnO nanocomposite-coated steel before and after 30-day immersion in 0.1 M HCl

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Conclusions

  1. 1.

    Copolymer Poly(aniline-co-o-anisidine) and nanocomposite Poly(aniline-co-o-anisidine)/ZnO were synthesized by chemical oxidative copolymerization.

  2. 2.

    A strongly adherent dark blue or dark black colored coating of copolymer and nanocomposite coating, respectively, was successfully obtained by solution evaporation. The nanocomposite coating was observed to be more dense and uniform than copolymer coating.

  3. 3.

    The results of immersion tests indicate that the corrosion rates for nanocomposite-coated steel are significantly lower than copolymer-coated steel in all the corrosive media under investigation.

  4. 4.

    The results of OCP measurements show noble potentials for copolymer and nanocomposite coatings compared to uncoated steel. However, the shift in noble potential is more pronounced for nanocomposite coating than copolymer coating in all the corrosive media subjected to investigation.

  5. 5.

    The electrochemical parameters as obtained from potentiodynamic polarization measurements indicate substantial reduction in I corr and corrosion rates for both copolymer and nanocomposite coatings.

  6. 6.

    The presence of scribed mark on the coating does not significantly affect the integrity of either copolymer or nanocomposite coating.

  7. 7.

    Owing to the good performance of the Poly(aniline-co-o-anisidine)/ZnO nanocomposite coating in different corrosive environments, the same may be considered for future industrial assessment.