The Influence of the Alkyd Composite Coatings with Polyaniline Doped by Different Organic Acids on the Corrosion of Mild Steel

1. Introduction

Mild steels due to their good mechanical characteristics, low price, etc. are common construction materials in many industries and different applications. [1,2]. Unfortunately, it has low corrosion stability and is easily corroded in many different environments. Consequently, mild steel must be protected against corrosion, and one of the most common methods is the application of different organic coatings [3]. But over time, coatings protection characteristics decay, so many researchers investigated different additives to prolong protection, and among them conducting polymers with a main focus on polyaniline [4]

Polyaniline (PANI) due to its easy chemical or electrochemical synthesis, inexpensive monomer, distinct electrical, redox, and acid-base characteristics in addition to its various potential uses in many sectors, PANI was the subject of considerable research in the past [5,6]. Among the other applications, a lot of effort was in the field of corrosion protection of metals and alloys. The main focus was on steel [7,8,9], and some other metals, like aluminum [10], magnesium alloys [11,12] copper [13], etc.

Two basic polyaniline forms, the polyaniline emeraldine salt form (PANI-ES) and the polyaniline emeraldine base form (PANI-EB), are being investigated for their use as anti-corrosion coatings. Some studies have confirmed the non-conducting nature of PANI-EB and its barrier effect. On the other hand, some researchers found that PANI-EB had an anodic protective effect, while others found inconclusive results, so it is crucial to keep researching the different forms of composite PANI-based coatings on steel [7,14,15,16,17]. The composite organic coatings containing different forms of PANI are widely investigated in the corrosion protection of metals [7,18], but the mechanism of the corrosion protection of mild steel is still elusive [7,19].

In our previous paper [20], the PANI composite with alkyd-based paint by different oxidation states doped with organic acids is qualitatively investigated using the linear polarization methods. It is observed that determined polarization resistance decreases with a decrease in the initial oxidation state of the polyaniline. Also, the paint blistering and delamination are suppressed to a large extent in comparison with the base coating. So, to extend that work, in this paper, we present results of the quantitatively determined corrosion rate using the ASTM International 1,10-phenanthroline standard test method and determined rust appearance on the base and composite coatings surfaces. Also, the plausible mechanism of corrosion protection is discussed in detail.

2. Materials and Methods

Polyaniline in the powdered form is produced using the chemical synthesis route suggested by the International Union of Pure and Applied Chemistry (IUPAC) guidelines [21]. In the procedure, the 0.22 mol of aniline monomer (20.5 g or 20.8 cm³, p.a. Sigma Aldrich, previously distilled under a low-pressure condition) and 0.22 mol HCl (8 g or 26 cm³ of 37 wt.% HCl, p.a. Merck) are well mixed at ambient temperature in 500 ml of distilled water to produce anilinium hydrochloride. Following vigorous stirring, 500 ml of 0.22 mol HCl containing 0.275 mol (62.81 g) of ammonium persulfate, (NH₄)₂S₂O₇, is slowly added. After 24 hours of mixing with a magnetic stirrer, the obtained green powder is filtered, repeatedly washed with 0.1 M HCl, distilled water, and acetone, and then finally dried for 24 hours in an oven at 60^oC. To form the polyaniline emeraldine base (PANI-EB), a portion of the as-synthesized polyaniline emeraldine salt powder (PANI-ES) is immersed in 1 M NH₄OH during 24 hours under stirring conditions. The reported electrical conductivity of PANI-ES prepared by using recommended IUPAC procedure is 4.4 ± 1.7 S cm^–1 (average of 59 samples) and for the PANI-EB electrical conductivity is 3×10^-9 S cm^-1 [22]. The resulting polyaniline emeraldine base powder is separately exposed in the solutions containing 0.87 M sulfamic, 1 M succinic, 1 M citric, and 1 M of acetic acid in order to produce the reprotonated polyaniline, by the route described by Stejskal et al. [22]. Typically, 100 cm³ solutions of the appropriate acids are 24 hours continuously mixed with 2 g of the PANI-EB, and after that filtrated, rinsed with distilled water, acetone, and dried during 48 hours. Scheme 1 illustrates the reprotonotation of the polyaniline emeraldine base to emeraldine salt form with acids (HA), while Table 1 presents the characteristics of the physical-chemical data of used acids.

The UV-vis spectra of all PANI samples in a powdered form dispersed in distilled water (~15 mg in 20 cm³) are obtained using LLG uniSPEC 2 spectrophotometers (Germany). The water dispersions of the polyaniline are prepared by vigorous sonication during 30 minutes, and waiting for the precipitations of larger particles for one hour.

The composite coatings containing reprotonated PANI are prepared using the commercial finishing paint for steel, “Professional emajl lak”, Nevena Color Xemmax, Serbia, based on an alkyd binder and white pigments in organic thinners, containing 67% of solids. The mass of solids is determined by measuring the mass of wet and dry paint. The composite coating is prepared by mixing 10 g of the base paint with 5 wt.% of well grind reprotonated PANI powder, 0.33 g based on dry paint, with a particle size in the range of 0.5 to 1.0 μm. The base and composite coatings are applied using a doctor-blade method on the properly prepared samples of the low carbon, ≤ 0.13 C, mild steel (ANSI 1212) with the dimensions of 4.5 cm × 5 cm, with an exposed area of 22.5 cm². The backsides and the edges of the steel samples are isolated with ~200 µm thick epoxy coating. After proper drying at room temperature during 48 hours, the average coating thickness of ~25±5 μm is measured using Byko-test 4500 FE/NFe (Germany) thickness tester. The samples are separately immersed in a 200 cm³ laboratory glass beaker containing 3% NaCl.

The corrosion rate, expressed as the average corrosion current density, of the mild steel samples painted by the base and composite coatings, as a mass of the iron in a beaker with a volume of 200 cm³ of 3% NaCl solution, is determined after 96 hours of corrosion using the ASTM International 1,10-phenanthroline standard method [23]. The 1,10-phenanthroline reacts with ferrous ions generating an intensely colored red complex. In a typical route, after 96 hours of the corrosion, when the first corrosion product on the samples is observed, the 10 cm³ of stack solutions are added to 100 cm³ volumetric flasks containing 0.25 mL of concentrated sulfuric acid and ~20 cm³ of distilled water. Then 1 cm³ of the hydroxylamine solution, 100 g dm³, 10 cm³ of the 1,10-phenanthroline solution, 1 g dm³, and 8 cm³ of the sodium acetate solution, 1.2 M is added, respectively, and filed to 100 cm³ with distilled water. The mass of the iron in the corrosive solutions is determined 96 hours after the immersion of the samples, by recording the UV-visible spectra using a LLG uniSPEC 2 spectrophotometer (Germany), and determining the value of the absorbance at 508 nm of investigated solutions and using the calibration line of the standard solution. Samples are left to corrode for a total of 150 hours, and the corroded area is determined from the sample images using ImageJ software [24]. For the preparation of the iron standard test solutions and determining the calibration line from UV-visible spectra, the appropriate amounts of ferrous ammonium sulfate hexahydrate, Fe(NH₄)₂(SO₄)₂×6H₂O, (Aldrich, p.a., USA) that 1.000 g contains 0.1443 g of iron, is used. The iron standard solution is prepared by dissolving 0.7022 g of ferrous ammonium sulfate hexahydrate in 500 mL of distilled water containing 20 mL of concentrated sulfuric acid and diluting to 1 L with distilled water. 100 mL of this solution is then diluted to 1 L. For the determination of the calibration curve, an adequate volume of the standard solutions is used to prepare 100 cm³ of the Fe²⁺ in the range of 18.6 μg to 0.186 mg.

The optical micrographs are acquired using an optical microscope Olympus CX41 equipped with a digital camera and connected to the PC. The image of the whole samples is obtained by PANASONIC DC-FZ82 digital camera

3. Results

3.1. Characterization of the Samples

The UV-visible spectra of the PANI samples, sonicated in distilled water, in the range of the wavelength from ~200 to 1100 nm are presented in Figure 1. The as-synthesized PANI in the emeraldine salt form (PANI-ES), Figure 1a, has an absorption peak at 365 nm that is connected to the π–π* transition inside the benzenoid ring (B). The peak at 440 nm can be connected to the polaron–π* transition inside the quinoid structure (Q), followed by a broad tail associated with polaronic structures [25,26]. The spectra of the PANI in the emeraldine base form (PANI-EB) among peak at ~365 nm, have a strong broad peak with maximum absorbance at ~700 nm that is ascribed to the molecular excitation connected with the quinoid-imine structure [26], Figure 1a. The ratio of absorbance for PANI-ES of quinoid, polaron–π* transition at 440 nm to benzenoid, π–π* transition at 365 nm bands can be taken as a measure of the oxidation state in the PANI conducting polymer [27,28,29,30,31]. All four reprotonated samples have a well-defined absorption maximum at 365 nm from π–π* transition inside the benzenoid ring. Polyaniline doped with sulfamic acid has a shoulder at 440 nm from the polaron - π* transition, while the other three samples have an increased absorbance, Figure 1b.

Figure 2 shows the polyaniline doped with sulfamic acid, particle size distribution in the composite coating, shown in the inset of Figure 2, determined using ImageJ software. It can be seen that main particle sizes are ~13±8 μm, while some agglomerates have much higher dimensions, up to 110 μm. The high agglomeration tendency of the PANI is affected by the inflexibility in the conjugated double-bond backbone and is caused by the high density of electrical charges inside the conducting polymer matrix [32]. Consequently, it is difficult to make a proper dispersion of PANI in the classical organic polymer paint which is one of the main issues in the application.

3.2. Corrosion Behavior

The UV-visible spectra of the standard solution are shown in Figure 3a. The linear dependence of the absorption of standard solutions at 508 nm, shown in the inset of Figure 3a, is in agreement with the Lambert–Beer law. The dependence of the absorption at 508 nm over iron (iron ions) mass in grams can be given as A = 1680 m(Fe²⁺). From the Figure 3b and the inset, it can be seen that maximum absorption that corresponds to the highest iron mass in the solution, possesses base coating, then composite coatings with acetic acid; citric acid; succinic acid, and finally sulfamic acid doped polyaniline.

Figure 4 shows the images of the samples before, and after immersion in corrosive media, 3% NaCl, during 150 hours. The bottom images show the graphical presentation of the corroded area of the samples determined by ImageJ software. By the visual inspection, it is obvious that the steel sample with the base coating is most corroded than composite coatings with acetic acid; citric acid; succinic acid, and at last with sulfamic acid doped polyaniline, which is in agreement with UV-vis measurements.

After exposure to corrosion media for 150 hours, the optical images of the samples with base coating and composite coating with polyaniline doped with sulfamic acid are taken and shown in Figure 5. As can be seen, the base coating is covered with huge blisters, while the composite coating shows only traces of corrosion products. Other composite coatings show the appearance of the corrosion product, but with microscope examinations, no blisters formation or delamination is observed.

4. Discussion

Protonated polyaniline theoretically can exist in three fundamental forms; fully reduced leucoemeraldine, half-oxidized emeraldine, and fully oxidized pernigraniline. From that three forms only emeraldine salt is conducive [33]. The doping degree, y, represents the average number of doped anions per polymer unit, four monomers, in the polymer chain. For example, emeraldine salt has consisted of four monomers in the polymer units and two anions, Scheme 1, so the doping degree is 0.5. Taking into account that PANI prepared following the IUPAC procedure has a doping degree of 0.5, the ratio of absorbance at 440 nm and 365 nm can be used to estimate the doping degrees of the reprotonated samples using the following equation:

$$y=\frac{(A_{440/365})\times 0.5}{1.02}$$

(1)

The estimated average doping degrees of the reprotonated samples are shown in Figure 6. It can be seen that sulfamic acid doped PANI has an average doping degree of 0.27; PANI-succinic acid 0.18, PANI-citric acid 0.15, and PANI-acetic acid 0.13.

To determine the corrosion rate of the base and composite coatings, from the absorbance determined after 96 hours of exposition to the corrosive solution, and taking into account calibration curve A = 1680 m(Fe²⁺), and knowing that for the analysis is from 200 cm³ analyzed 10 cm³, the average corrosion current density is calculated using the modified equation of the Faraday law:

$$j_{\mathrm{corr}}=\frac{A_{508}\times 20\times n\times F}{1680\times S\times t\times M(\mathrm{Fe})}$$

(2)

where A₅₀₈ is absorbance after the time of immersion t = 96 h, inset in Figure 3b, S, cm², is the surface area of the samples, F = 26.8 Ah mol^-1 Faraday constant, and 20 is the conversion factor from 10 cm³ to 200 cm³. The calculated corrosion current density is shown in Figure 7a. It is obvious that corrosion current density closely follows the initial doping degree, and decreases with an increase of the initial doping degree. In order to compare the value of the polarization resistance with the determined j_corr, the same Figure 7a shows the data of the reciprocal values of the polarization resistance after 96 hours, extracted from Figure 4a from [20]. Because j_corr is proportional to R_p^-1 an excellent agreement of trend is obtained using those two methods. To evaluate the formation of rust onto the surface of the sample, Figure 7b shows the comparisons of corrosion current density with the corroded surface after prolonged immersion in 3% NaCl for 150 hours, obtained using ImageJ software, Figure 4. The close connections of the initial doping degree, j_corr, and appearance of the rust on the samples surfaces are obvious. For example, the base coating surface is covered with 20% of rust, while composite coatings with PANI-sulfamic acid only ~1%. Therefore, all of the investigations suggest that the initial PANI doping degree has a high influence on the corrosion protection of mild steel

The main degradation of mechanisms of the base organic coatings are delamination and the formation of blisters, as schematically shown in Figure 8 [3,20]. During the immersion in corrosive media, the development of the pores in the coatings occurs. In the case of delamination, in the two neighboring pores, iron is dissolved, and released electrons are transferred through metal to another pore where an oxygen reduction reaction occurs producing OH^– anions, and very fast alkalization of the pore occurs, reaching pH of ~14, that provoke loss of the coating adhesion. During blister formation, oxygen, and water, penetrate through the coating, oxygen is reduced to OH^–, and with dissolved iron form rust that lifts the coatings from the steel surface.

For polyaniline and polypyrrole is shown that could reduce oxygen to hydrogen peroxide and/or hydroxyl anions via two or four-electron path [34,35]. Hence, it can be suggested that polyaniline with a higher doping degree and higher conductivity could reduce the oxygen to mainly hydrogen peroxide anions, HO₂^–, lowering the possibility of pH increase and formation of blisters or delamination. Decreasing the doping degree conductivity of PANI also decreases, and parallel two and four oxygen reduction paths could occur, but with a much lower concentration of OH^– and a smaller tendency of coating degradation, as schematically presented in Figure 9. It should be also mentioned that formed hydrogen peroxide anions could react with Fe²⁺ to form a passive film of Fe₂O₃ via reactions:

Fe²⁺ + HO₂^– + H₂O → Fe₂O₃ + 3H⁺

(3)

and immediate reaction:

H⁺ + OH^– → H₂O

(4)

that again lowers the concentrations of OH^– ions in the coating pores. Also, the barrier effect on oxygen penetration due to the presence of the polyaniline particles in the composite coating should be taken into account.

5. Conclusions

In this paper, we investigated the synthesis of the polyaniline in the emeraldine salt form by the procedure suggested by IUPAC. Emeraldine salt form is deprotonated with ammonium hydroxide and reprotonated with four different organic acids. Using the UV-visible spectroscopy the doping degree is estimated. The corrosion performances of the base coating and composite coating with 5 wt.% of polyaniline doped with organic acids on mild steel are investigated by the means of in site determined iron concentrations in the corrosive media using ASTM 1,10-phenanthroline method and the corrosion current density is recalculated. It is also shown that the protection ability of composite coatings closely follows the initial oxidation state of polyaniline, and that polyaniline doped with sulfamic acid with doping degree of 0.27 have the best corrosion protection role. Blistering formations are suppressed in the composite coatings due to the changes in the oxygen reduction path from four to mainly two electron path. The mechanism of corrosion behavior is also discussed in detail.