Corrosion Behavior of Polypyrrole Films with Insulated Substrate in Alkaline Aqueous Solutions
Authors: Jin Zhen Zhen , Qiu Yu Bin , Guo Xing Peng 
Published: February 2020

The corrosion behavior of polypyrrole (PPy) films on Pt substrate in various media, the galvanic interactions between PPy and different metal substrates and the involved mechanisms have been investigated in our previous works. These works suggest that the metal substrates, even like inert Pt, may have influence on the corrosion of PPy, because they provide an electronic pathway with lower resistance so as to possibly change the electrochemical processes on PPy films. At presents, there are few reports on the corrosion behavior of PPy films without metal substrates under open circuit states and the effect of metal substrates on the corrosion of PPy is still not well understood.

In this paper, PPy/pTS films were prepared on a 13Cr stainless steel (SS) plate and peeled to make PPy electrodes on an insulated glass substrate (glass//PPy/pTS electrodes). The glass//PPy/pTS electrodes with different thickness (2-6 μm) were prepared and corroded in aerated and de-aerated 0.1 M NaOH solutions for different time (t). Then the corrosion extent of PPy (r
corr) was evaluated by the percentage loss of its cyclic voltammetry (CV) current capacity after corrosion (measured in de-aerated 0.1 M KNO3), i.e. (Q
0-Q
R)/Q
0, where Q
0 is the CV current capacity of the freshly prepared PPy film and Q
R is the CV current capacity of the corroded PPy film, and the corrosion rate of PPy (v
corr) defined as the change rate of r
corrwith corrosion time was also calculated. Furthermore, the changes in the electrochemical impedance, morphology, chemical composition and structure of the corroded glass//PPy/pTS electrodes were investigated using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. In order to verify the effect of metal substrates, the corrosion of PPy/pTS films with Pt and 13Cr SS substrates under the same conditions were also investigated.

The results indicate that the corrosion of the PPy/pTS films depends greatly on the film thickness, dissolved oxygen (DO) and the property of substrates. In aerated 0.1 M NaOH, the v
corr of PPy is much larger than that in the de-aerated solution. The corrosion process on the glass//PPy/pTS electrodes includes electrochemical processes, resulting from various galvanic cells formed on PPy (especially oxygen concentration difference cells), and chemical process owing to the nucleophilic attack of OH-. Furthermore, the corrosion process can be separated into two corrosion stages. The v
corr in the first stage is comparatively slow, because OH- is difficult to enter the inner layer of the compact PPy/pTS film and also difficult to exchange with doped pTS- in the PPy film. So the electrochemical processes should be dominant in this corrosion stage. However, as the extension of corrosion time, the PPy/pTS film becomes so loose and porous that the corrosion media penetrates the whole film. Therefore, the v
corr in the second corrosion stage is very fast. In de-aerated 0.1 M NaOH, the main corrosion process is the chemical process from the nucleophilic attack of OH-, so the v
corr of PPy is very slow. With the increase in the film thickness, the PPy/pTS film becomes more compact and the corrosion media will take much longer time to enter the inner layer of the PPy film, so the v
corr of PPy decreases obviously. It should be noted that the v
corr of the glass//PPy/pTS electrode is one order of magnitude larger than that of the Pt//PPy/pTS electrode under the same condition, which should be attributed to the stronger adhesive force between PPy film and Pt substrate as well as the more compact structure of the PPy/pTS film formed on Pt substrate. When the SS//PPy/pTS electrodes corroded in the same solution, there are strong galvanic interactions between PPy/pTS film and the 13Cr SS substrate, in which the PPy/pTS film suffers cathodic protection while the 13Cr substrate is corroded. Apparently, the metal substrate can influence the structure of the formed PPy film and the electrochemical processes in the corrosion of PPy film.