The corrosion-inhibiting effect of polypyrrole films doped with p-toluene-sulfonate, benzene-sulfonate or dodecyl-sulfate anions, as coating on stainless steel in NaCl aqueous solutions
Graphical abstract
Introduction
In recent years, there has been increased interest in the use of conducting polymers as protective coatings against the corrosion of metals and alloys in different electrolytic media. The conducting polymers are considered as an environmentally promising alternative against corrosion of metallic materials [1], [2], [3], [4], [5]. Polypyrrole (PPy) is currently considered one of the best organic materials with the ability to significantly reduce the corrosion of different metals and alloys such as aluminum [6], [7], copper [8], [9], mild steel [1], [10] and stainless steel [2], [11].
PPy is commonly synthesized by electrochemical [12], [13], [14] and chemical [15], [16] polymerization. However, it is known that the morphological and conducting properties of PPy films depend strongly on the synthesis conditions of the material. A characteristic fact is that the anion present in the electrolytic solution where the electropolymerization is carried out plays an important role in the doping–dedoping process presented by the PPy [17], [18], [19]. It has been proposed that when PPy is synthesized in an electrolyte media containing relatively small anions, such as chlorides, sulfates, nitrates, perchlorates, etc. the film presents a doping–dedoping process in which anions (X−) are exchanged between the polymer and the electrolytic solution. This process has been described as follows:
When the polymer is synthesized in a medium where there are voluminous monovalent anions such as pTS−, BS− or DS−, these anions are trapped within the polymer film. The anions are not interchangeable and as a result a conducting polymer grown in the presence of these anions presents a doping–dedoping process in which the ions that are exchanged correspond to the cations (Cm+) present in the electrolyte solution. This process has been described as follows:
The latter fact is important to take into account when aiming to establish a polymer film that can protect against corrosion of a metal structure that is subjected to an electrolytic medium containing aggressive anions such as chlorides. This type of polymer film has therefore been tested with a number of metals and alloys [11], [20], [21], [22], [23], [24].
The protective nature of these films has been measured using different techniques. The most commonly used are polarization curves and the evolution over time of open circuit potential. More recently, measurements of electrochemical impedance have also been used. With this technique it has been possible to obtain quantitative parameters for these processes for different metals coated with the polymer films. For example, analysis has been conducted on metals such as Al, Cu, SS, etc. coated with these polymers [8], [9], [25], [26], [27].
The contribution of this paper is to extend the study of the use of polypyrrole doped with bulky anions (p-toluene sulfonate, benzene sulfonate and dodecyl sulfate, Fig. 1) electrochemically deposited on stainless steel in order to evaluate its protective properties against corrosion in 0.1 M NaCl electrolytic solutions. The structural properties of the polymer films were analyzed by IR and UV–vis spectroscopy and the morphological characteristics by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical study of the corrosion-inhibiting effect of different polypyrrole films doped with anions on stainless steel was carried out in 0.1 M NaCl aqueous solutions. The techniques used in this study were the measurement of open circuit potential (EOCP), polarization curves (PC), cycle voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The latter technique was also used to compare the response of the SS/PPy (anion) systems with those obtained with the Pt/PPy (anion) systems.
Section snippets
Materials and methods
The composition of the stainless steel was C 0.03%, Si 1.0%, Mn 2.0%, P 0.045%, S 0.03%, Mo 2.2%, Ni 13.7%, Cr 18.0% and Fe 62.05%.
A conventional three-electrode electrochemical cell was used for the experiments. Before each measurement, the electrolyte solution was deaerated with argon for 45 min and then an inert atmosphere was maintained above the solution. The method of CV was used for polypyrrole electro-synthesis. The electropolymerization was performed at room temperature from aqueous
Morphological, compositional and optical characterization
Table 1 presents a summary of the positions of the signals observed in the IR spectra of the three polymers in the study. In all cases, the most important vibrational bands that identify the links between the structure of the polymers are the following: 935 cm−1 CH outside the plane [28], 1055 cm−1 CH within the plane [29], 1244 cm−1 CN aromatic [30], 1316 cm−1 stretch vibration of the monomer [31], 1446 cm−1 CC aromatic [32], 1560 cm−1 CC aromatic [33], 1600 cm−1 CH aromatic [29], and 1710 cm−1 CC
Conclusions
The data obtained under the study conditions used in this research confirm the protective nature of the polymer films deposited electrochemically on the stainless steel surface in the presence of the anions pTS, BS and DS. This protective characteristic was demonstrated by testing the systems in solutions of 0.1 M sodium chloride. However, due to the smaller molecular size of the dopant, the PPy-BS and PPy-pTS films presented the best protective behavior against the corrosion of the stainless
Acknowledgement
The authors acknowledge financial support from Dirección de Investigación of the Pontificia Universidad Católica de Valparaíso, Chile.
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