The structure and barrier properties of the oxyhydroxide layers on a carbon steel surface covered with electroactive polyaniline were investigated. Two types of polymer structure differing in degree of macromolecular order were prepared by simultaneous (fast polymerization) or dropwise (slow polymerization) mixing of reagents. A larger amount of the most stable FeOOH modification was formed on steel covered with slowly polymerized sample during treatment in the corrosion-active medium. Amorphous rust products with weak barrier properties were observed in the sample prepared by fast polymerization. Additionally, barrier activity of dedoped polyaniline was studied with SEM, WAXD, and electrochemical methods.
Due to their unique physicochemical properties and easy synthesis, the electroconducting polymers attract considerable attention as potential components of electronic devices [
It needs to be pointed out that in some cases it is possible to use pure conducting polymer as a protective coating. It was proposed [
Although various conducting polymers such as polypyrrole [
The aim of this work is to investigate chemical composition, crystalline structure, and barrier properties of inorganic films on the carbon steel/polyaniline interface formed with PANI prepared by two different procedures: dropwise mixing of reagents (slow polymerization) and simultaneous addition of an oxidant to the monomer solution (fast polymerization). The effect of dedoping of PANI on the products of its interaction with the steel surface is also discussed.
Ammonium peroxydisulfate (APS) and aniline were purchased from Sigma-Aldrich and used as received. Sodium chloride and sulfuric acid were obtained from Vekton (Russia). Carbon steel (steel 08 GOST 1050-88) plates were polished with abrasive paper (P500 grit) and then washed with water and alcohol. All solutions were prepared in distilled water.
PANI dispersions were synthesized by chemical oxidative polymerization of aniline monomer (0.15 mol L−1) in the H2SO4 water solution (1 mol L−1) with APS as an oxidant. The mole ratio of monomer to oxidant was equal to 1 : 1.125. The monomer and oxidant solutions were thermostated separately at 35°C and then mixed together either simultaneously (sample P1) or dropwise (sample P2). After mixing, the solutions were left for 2 hours on the magnetic stirrer. Then the polymer was filtered and washed with the sulfuric acid solution in order to remove the residual ammonium sulfate. The polymer samples were dried in vacuum at room temperature during 48 hours. For dedoping of PANI, the powder of P2 was dispersed in NH4OH water solution with concentration of 1 mol L−1 and stirred for 1 hour. Then the solid polymer phase was separated by filtration, washed, and dried. The dedoped sample is denoted in text as P2d.
The polymer powders were dispersed in water under the ultrasonic treatment and casted onto metal surfaces (4-5 mg cm−2) with subsequent drying on air. Then the samples were placed in the corrosion-active 3.5 wt% NaCl solution in water. Those samples are denoted in text as “Fe-xxx,” where “xxx” is P1, P2, or P2d.
PANI powders were dispersed in water in the ultrasonic bath, casted on the metallic table in the same way as for the corrosion experiment, and studied with scanning electron microscope ZEISS MERLIN (Germany) at the voltage of 10 kV. Due to electroconductivity of PANI, the high quality images of the polymeric layers could be obtained without metal covering.
Initial PANI powders and oxide layers on the steel surface were investigated with wide-angle X-ray diffraction (WAXD) using a D2 PHASER diffractometer (Bruker, Germany) equipped with a CoK
The degree of crystallinity of the samples (
Barrier properties of the films were characterized by potentiodynamic experiment on the steel electrode with PANI coating using the P30-J potentiostat (“Elins,” Russia). The experiments were carried out in a three-electrode cell with silver-chloride reference and Pt counter electrodes at 1st, 4th, and 8th days after immersion of the samples into the corrosion-active medium. The values of polarization resistance (
Morphology of the polymer particles formed in the samples P1 and P2 (simultaneous and dropwise addition of the oxidant to the monomer solution, resp.) studied with scanning electron microscopy (SEM) is shown in Figure
SEM images of PANI layers on metal surface. PANI prepared with simultaneous P1 (a, b) and dropwise P2 (c, d) addition of oxidant. Images (e) and (f) correspond to the dedoped polymer P2d.
The sample P1 contains a large amount of smooth rod-like particles with the length in the range of 150–250 nm and diameter of about 50–100 nm (Figure
Dropwise addition of the oxidant to the monomer solution leads to the considerable changes in the polymer morphology. The SEM images of the sample P2 taken with a different magnification are shown in Figures
Dedoping of PANI leads to the appearance of the particles with a less pronounced rod-like morphology as seen in Figure
The crystalline structure of the PANI samples was studied by WAXD. The results are presented in Figure
X-ray diffraction patterns of PANI powders prepared by simultaneous mixing of reagents, P1 (a), or dropwise addition of the oxidant, P2 (b), and by dedoping of P2 sample, P2d (с).
When the method of mixing of reagents is changed to the dropwise addition of the oxidant (P2), the pattern of the polymer demonstrates an increased intensity of the peaks at
The WAXD pattern of the dedoped sample P2d is shown in Figure
Figure
Composition (in volume%) and structural parameters ofinorganic layers at the PANI-Fe interface before and after corrosion experiment.
Sample | Phase | Volume, % | Crystallite size, nm |
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Fe-P1/before | Melanterite (FeSO4 × 7H2O) | 85 | 34 |
Rozenite (FeSO4 × 4H2O) | 15 | 75 | |
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Fe-P1/after | Goethite ( |
— | — |
Rozenite (FeSO4 × 4H2O) | — | — | |
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Fe-P2/before | Rozenite (FeSO4 × 4H2O) | 81 | 61 |
Melanterite (FeSO4 × 7H2O) | 9 | 54 | |
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Fe-P2/after | Goethite ( |
81 | 13 |
Akaganeite ( |
14 | 12 | |
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Fe-P2-b/before | No inorganic phases detected | ||
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Fe-P2-b/after | Lepidocrocite ( |
73 | 19 |
Goethite ( |
14 | 11 | |
Magnetite (Fe3O4) | 9 | 25 |
X-ray diffraction patterns of the samples Fe-P1 (1), Fe-P2 (2), and Fe-P2d (3) taken prior to (a) and after (b) corrosion experiments.
It was found that both of the doped PANI samples P1 and P2 (Figure
The phase composition of the inorganic layer after corrosion experiment appears to be different for all samples (Table
In the samples Fe-P2 and Fe-P2d, the sharp diffraction peaks are observed (Figure
Polarization curves of the steel electrodes with the PANI coatings were measured at the 1st, 4th, and 8th day of their keeping in the corrosion-active medium. The values of
Polarization resistance
Sample |
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Fe | 37.1 | 24.4 | 22.5 |
Fe-P1 | 7.9 | 18.1 | 29.9 |
Fe-P2 | 12.4 | 16.4 | 83.3 |
Fe-P2d | 1.4 | 41.4 | 67.8 |
The
For the steel samples covered with PANI, the value of
The polarization curves measured after 8 days of exposure in the corrosion-active medium are shown in Figure
Polarization curves for pure Fe covered with PANI dispersions at 8th day of keeping in 3.5% NaCl.
Polyaniline dispersions were synthesized by two procedures: simultaneous (fast polymerization) and dropwise (slow polymerization) mixing of reagents. Lower polymerization rate permits to achieve a higher ordering in the arrangement of polymer chains, which leads to the increase in aspect ratio of the polymer particles. The coatings prepared from a polyaniline doped with sulfuric acid promote the formation of ferrous sulfate on the carbon steel surface. Different crystalline modifications of iron oxyhydroxides are formed during treatment of the samples in the corrosion-active solution (3.5% NaCl in water). Deposition of a more ordered polymer onto the steel surface leads to the formation of a large amount (81%) of the crystalline goethite phase which is the most thermodynamically stable rust product. Covering the steel with PANI prepared by common procedure (fast polymerization) leads to the formation of amorphous rust layer. For the inorganic layer formed under the polymer prepared by dropwise oxidant addition (slow polymerization), the electrochemical measurements show highest barrier properties which is confirmed by the data of structural investigations.
In a highly ordered dedoped PANI, the degree of the polymer crystallinity decreases from 31 to 9%, and the rod-like structures disappear. Compared with the doped polyaniline, the dedoped sample displayed lower protective properties, which were still considerably higher than those of the doped PANI prepared by simultaneous mixing of reagents.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by Russian Foundation for Basic Research (Grant 14-03-31411 mol_a). M. Sokolova acknowledges Saint Petersburg State University for the Research Grant (no. 12.50.1195.2014). The experimental work was facilitated by the equipment of the Center for X-Ray Diffraction Studies of Nanotechnology Interdisciplinary Center and Centre for Innovative Technologies of Composite Nanomaterials of Saint Petersburg State University.