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Rem: Revista Escola de Minas

Print version ISSN 0370-4467

Rem: Rev. Esc. Minas vol.66 no.2 Ouro Preto Apr./June 2013 



Electrochemical behavior of indole for AISI 430 stainless steel in changing the media from 1 mol L-1 H2SO4 to 1 mol L-1 HCl


Comportamento eletroquímico do indol para o aço inoxidável AISI 430 com a mudança do meio de H2SO4 1 mol L-1 para HCl 1 mol L-1



Martha TussoliniI; Aline ViomarII; André Lazarin GallinaIII; Everson do Prado BanczekIV; Maico Taras da CunhaV; Paulo Rogério Pinto RodriguesVI

IMaster in Applied Chemistry UNICENTRO - State University in the Midwest, Guarapuava - Paraná, Brazil.
IIMaster in Bioenergy UNICENTRO - State University in the Midwest, Guarapuava - Paraná, Brazil.
IIIMaster in Bioenergy UNICENTRO - State University in the Midwest, Guarapuava - Paraná, Brazil.
IVDoctor in Science UNICENTRO - State University in the Midwest, Guarapuava - Paraná, Brazil.
VMaster UNICENTRO - State University in the Midwest, Guarapuava - Paraná, Brazil.
VIDoctor in Science. UNICENTRO - State University in the Midwest, Guarapuava - Paraná, Brazil.




The adsorption of organic substances in metals and in different media has been attributed to different effects, such as the carbon chain size, type or number of heteroatom in the molecule structure, reactivity, the steric effect, among others. The objective of this work is to study the electrochemical indole behavior for AISI 430 stainless steel (SS), when changing the study media from H2SO4 1 mol L-1 to HCl 1 mol L-1. The experimental techniques used are as follows: the open-circuit potential, anodic and cathodic potentiodinamic polarization, electrochemical impedance spectroscopy, optical microscopy and scanning electron microscopy. These results suggest that the adsorption of indole in the presence of SO4-2 is lower than in the medium containing Cl-, which is justified by the indole action as a weak acid, because it has only a single pair of electrons, which is displaced and takes part in the π-electron aromatic system; thus the first step of the indole reaction in acid media is the protonation and generation of an indole cation.

Keywords: Corrosion inhibitor, adsorption, protonation.


A adsorção das substâncias orgânicas nos metais, em diferentes meios, tem sido atribuída a diferentes efeitos, tais como: tamanho da cadeia carbônica, tipo ou número de heteroátomo presente na estrutura da molécula, reatividade, efeito estérico, entre outros. O objetivo desse trabalho é estudar o comportamento eletroquímico do Indol (C8H7N), para o aço inoxidável AISI 430, quando se muda o meio de estudo de H2SO4 1 mol L-1 para HCl 1 mol L-1. Experimentalmente foram empregadas as técnicas seguintes: medida de potencial de circuito aberto, curvas de polarização potenciodinâmica anódica e catódica, espectroscopia de impedância eletroquímica, microscopia óptica e eletrônica de varredura. Esses resultados sugerem que a adsorção do indol, em presença de SO4-2, é menor em relação ao meio que contem íons Cl-, fato justificado pela ação do indol como ácido fraco, pois o mesmo possui apenas um único par de elétrons, o qual é deslocado e participa do sistema π-elétron aromático. Dessa forma, a primeira etapa da reação do indol, em meio ácido, é a protonação e a geração de um indol cátion.

Palavras-chave: Inibidor de corrosão, adsorção, protonação.



1. Introduction

The metallic corrosion is a technological subject well studied since the economic loss of companies and governments in the replacement of corroded metal parts are quite significant (Anunziato, 2010). To minimize the problem of corrosion, corrosion inhibitors are used; these are substances that when added to aggressive media minimize the oxidation reactions (Rodrigues, 1998). There are several studies about inhibitors, mainly on the adsorption effect of these compounds considering the influence of the heteroatom present in the carbon chain, the polar group effect, the electron density in aromatic compost, the position of the heteroatom in the aromatic ring, the nature and charge of the metal surface, the type of electrolyte, among others (Rodrigues, 1997). It was observed that inhibition depends on the chemical and electronic structures of the compounds, the latter being more important when different azoles were studied, but with similar surface areas, differing only in the number and type of heteroatom, the oxidation of carbon steel in media 1 mol L-1 HCl (Popova, 2006). The study of the indole effect on the corrosion behavior of 316 L solution of sodium chloride 0.3 mol L-1 at different pH values (4, 8 and 10) showed that change occurs in inhibitory efficiency (Düdükcü, 2004). For these changes, the objective is to study the electrochemical behavior of Indole (C8H7N) for AISI 430 SS, when changing the aggressive medium from 1 mol L-1 H2SO4 to 1 mol L-1 HCl (Tussolini, 2010).


2. Materials and methods

The electrochemical behavior was evaluated using the following techniques: open circuit potential (Eca) tests, anodic and cathodic potentiodynamic polarization measurements electrochemical impedance spectroscopy (EIS); the surface of the sample was analyzed by scanning electron microscopy (SEM). All tests were performed at 22 ± 2ºC. In electrochemical tests, a three-electrode cell was employed using: platinum as the counter-electrode with a 35 cm2 average area; saturated mercury sulfate (Hg/Hg2SO4) and silver-silver chloride (Ag/AgCl) as reference electrodes; and a working electrode of (square plate 430 steel Stainless (SS) with a 2 cm2 average area).

Before tests were carried out, the working electrode was polished with SiC sandpaper with grit sizes 320, 400, 600 and 1200 mesh successively; after that it was rinsed with ultrapure water and dried in a cold air jet. A scanning electron microscope Philips brand, model XL 30 was used for SEM, and the analysis was followed by polishing with diamond paste 1µ. All solutions used were prepared with analytical reagents and ultrapure water. The concentrations of sulfuric acid and hydrochloric acid were 1 mol L-1 in the presence and absence of indole at 1x10-3 5x10-4 and 1x10-4 mol L-1 concentrations.

Tests of the open circuit potential (Eca) were obtained performing a stable potential of ± 5 mV in the interval time of 30 minutes before each test electrochemical impedance spectroscopy was carried out. The curves of anodic and cathodic potentiodynamic polarization were obtained in the two media studied with 1 mV /s scan rate. The electrochemical impedance spectroscopy curves were obtained from the kinetic equilibrium with ± 10 mV potential perturbation.


3. Results

The open circuit potential curves of 430 SS in different media are shown in Figure 1.

The reason why two different reference electrodes were used is different media - so in the H2SO4 media, the MSE employed contains sulfate ions, and in the HCl media, SCE was employed that contains chloride ions; the common ion present in the reference electrode and in the media allows to minimize de junction potential. The Ecorr values for the system in the absence and presence of indole in 1 mol L-1 H2SO4 and 1 mol L-1HCl media are presented in the Table 1.

In Figures 2 and 3 the anodic potentiodynamic polarization curves for 430 SS are presented in the absence and presence of indole in 1 mol L-1 H2SO4 and 1 mol L-1 HCl media.

In Figure 4 the potentiodynamic cathodic polarization curves for 430 SS are shown in the absence and presence of Indole at different concentrations in 1 mol L-1 H2SO4 and 1 mol L-1HCl media.

Tables 2 and 3 show the coverage degrees (θ) in the media of 1 mol L-1 HCl and 1 mol L-1 H2SO4 in the presence of Indole. These values of θ were calculated at different voltages from the Equation 1 (Tussolini, 2007):


jc/i= current density in the presence of indole.

js/i= current density in the absence of indole.

The EIS measurements were carried out in the corrosion potential in 1 mol L-1 H2SO4 and 1 mol.L-1 HCl media in the presence and absence of Indole 1x10-3 mol L-1, the curves are presented in Figure 5.


4. Discussion

Figures 1.A and 1.B show that in the presence of Indole in the first few minutes, the potential decreases, after that time, more positive values are reached where it stabilizes. This behavior is probably due to rupture of the pre-existing oxide layer before immersion in the metal, which accelerates the process of hydrogen evolution and after these initial minutes the potential tends to rise due to the generation of new oxide layer in the presence of the corresponding indole.

Table 1 shows that the addition of Indole to the two media studied did not promote significant change in the Ecorr values, suggesting that the inhibitory action of azoles in relation to the oxidation of 430 SS in both media studies is mixed.

Figure 2 reveals that the addition of indole to the concentrations studied increases the current density (j) in the region of the anodic curve studied, suggesting the indole action as a catalyst of corrosion in the active region and transpassive for 430 SS in 1 mol L-1H2SO4 media. Figure 3 shows that the Indole concentration is the highest when more negative potentials decreased (j). And for the lowest concentration at lower potentials there was no change, yet at potentials above -0.26 V increases occurred (j). There is a breakdown of passive film, probably by the presence of Cl-.

In Figure 4.A, it can be observed that indole has a double behavior at lower potentials. In potentials lower than -1V, an increase occurs in the current density and when potentials are increased the current density decreases, sometimes acting either as a catalyst or as an inhibitor.

It is observed in Figure 4.B that the indole in the highest concentration reduces the current density in the region studied. While in lower concentrations of greater E >-0.62 V/SME, there is some increase in (j), suggesting a dual effect for these concentrations, catalyst and inhibitor, probably because at low concentrations there are more exposed active sites catalyzing the reaction rate of hydrogen detachment.

The indole in sulfuric acid media had an inhibitory effect for the cathodic overvoltage, while the anodic region to the inhibitory effect is only relevant at high voltages, as shown in Table 2, where η > 200 mV. In Table 3 it is possible to see the indole in the cathodic and low anodic overvoltage, the highest concentration showed a highly efficient inhibitor.

In Figure 5.A it is possible to observe that the impedance in the presence of indole is slightly larger than for the system with only 1 molL-1 H2SO4. The presence of an arc can be noticed: capacitive in high frequency and inductive in low frequency. In the first arc, the capacitor can be attributed to faradaic processes and the second inductive due to the indole presence.

Figure 5.B reveals that in 1 mol L-1 HCl the impedance in the presence of indole is significantly greater than in its absence. Featuring two arcs and the inductive arc is due to the adsorption of indole.

"Holes" characteristic of generalized attacks can be seen in figure 6.A, probably due to dissolution around the inclusions, the remaining area suggested generalized corrosion of 430 stainless steel in 1 mol L-1 H2SO4 . In the presence of Indole in figure 6.B, corrosion is lower than that observed for the system in its absence.

Figure 6.C shows that the corrosion is localized for 430 stainless steel type in the 1 mol L-1 HCl media. In Figure 6.D a minimization of corrosion of 430 SS in the presence of indole in HCl media is observed when compared to figure 6.C.

Comparing Tables 2 and 3, the results in hydrochloric acid 1 mol.L-1 media, the inhibitory efficiency in the presence of indole in the 1x10-3 molL-1 concentration is considerably higher when compared with the same concentration for the sulfuric acid media. This increase can be attributed to the fact that the nitrogen atom provides electrons to the aromatic ring, thus catalyzing the protonation process in hydrochloric acid media, which follows the mechanism suggested in Figure 7. It is suggested that the indole in protonated form can adsorb on electrode surface by simple electrostatic attraction of positively charged molecule, due to the chloride ions adsorbed in that metal surface.


5. Conclusions

The corrosion of 430 SS in sulfuric acid media is typically generalized, while in the hydrochloric acid media the attack is very aggressive. A considerable reduction of the localized corrosion was observed in the presence of indole.

EIS diagrams showed that for the H2SO4 and 1 molL-1 HCl media, the highest impedances were found in the presence of indole, being higher for the hydrochloric acid medium.

The potentiodynamic polarization showed that in the two media studied the presence of indole had no significant inhibitory effect in the anodic region. However, in the cathodic region, the highest concentration studied for the 1 molL-1 HCl media presented high inhibitory effect. It can be suggested that this behavior is due to the fact that the indole in this medium is protonated, which would facilitate the adsorption on metallic substrate in the presence of Cl-.


6. Acknowledgments

CNPq, CAPES, FINEP and Araucaria Foundation.


7. References

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Paper submitted to INOX 2010- 10th Brazilian Stainless Steel Conference, September 20-22, Rio de Janeiro, Brazil
Revised accepted December, 04, 2012.

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