On the corrosion inhibition of iron in hydrochloric acid solutions, Part I: Electrochemical DC and AC studies

3.1 Potentiodynamic polarization measurements

Some corrosion phenomena can be explained in terms of electrochemical reactions. It follows then, that electrochemical techniques can be used to study these phenomena. Measurements of current–potential values under carefully controlled conditions can yield information on corrosion rates, coatings and films, passivity, pitting tendencies and other important phenomena. Potentiodynamic anodic polarization is the characterization of a metal specimen by its current–potential relationship. The specimen potential is scanned in the positive – going direction. These measurements are used to determine corrosion characteristics of a metal specimen in aqueous environments. A complete current–potential plot of a specimen can be measured in a few hours or in some cases in a few minutes (Basics of corrosion measurements, 1982).

When an iron specimen is immersed in a corrosive medium (HCl in this case), both reduction and oxidation processes occur on its surface. Typically the iron specimen oxidizes (corrodes) and the medium (HCl in this case) is reduced. This means that the hydrogen ions are reduced. The iron specimen can function as both anode and cathode, and both anodic and cathodic currents occur on the specimen surface. Any corrosion processes that occur are usually a result of anodic currents. When an iron specimen is in contact with a corrosive liquid and the specimen is not connected to any instrumentation – as it would be “in service” – the specimen assumes a potential (relative to a reference electrode) termed as the corrosion potential, E_corr. A specimen at E_corr has both anodic and cathodic currents present on its surface. However, these currents are exactly equal in magnitude so there is no net current to be measured. The specimen is at equilibrium with the environment (even though it may be visibly corroding). E_corr can be defined as the potential at which the rate of oxidation is exactly equal to the rate of reduction. Experimentally one measures polarization characteristics by plotting the external current response as a function of the applied potential. Since the measured current can vary over several orders of magnitude, usually the log current function is plotted vs. potential on a semi-log chart. This plot is termed as a potentiodynamic polarization plot. In this study we will use Tafel plot to describe the polarization characteristics of the corrosion system. The corrosion current density, i_corr, is obtained from Tafel plot by extrapolating the linear portion of the curve to E_corr, as shown in Fig. 1. The corrosion rate can be calculated from the corrosion current by using Eq. (1), Basics of corrosion measurements, 1982.

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Figure 1

Potentiodynamic polarization curves of iron immersed in 1.0 M HCl and different concentrations of 4-methyl pyrazole (4MP): (a) Blank, (b) 1 × 10⁻³, (c) 3 × 10⁻³, (d) 5 × 10⁻³, (e) 7 × 10⁻³, (f) 9 × 10⁻³, (g) 10 × 10⁻³.

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Tafel plots can provide a direct measure of the corrosion current, which can be related to the corrosion rate. Tafel plots for the iron specimen immersed in 1.0 M HCl in the absence and in the presence of different concentrations of 4-methyl pyrazole is presented in Fig. 1.

Fig. 1 shows that the addition of (4MP) to the acid solution shifts the anodic polarization to more positive and the cathodic polarization to more negative values. The effect of (4MP) is more pronounced on anodic polarization than on cathodic polarization. Values of the electrochemical parameters and the percentage inhibition efficiency δ% are given in Table 1. The inhibition efficiency calculated from potentiodynamic polarization measurements, δ% is given from the Eq. (2) Abd El-Rehim et al., 1999:

By Analysis of Fig. 1, we can determine different corrosion kinetic parameters for iron in 1.0 M HCl in the absence and presence of different concentrations of (4MP). These kinetic parameters are reported in Table 1.

It is seen from Table 1 that the addition of (4MP) reduces the values of $i_{\text{corr}}$ to an extent, which increases with increasing concentration. This indicates that (4MP) inhibit the corrosion of iron in HCl solutions. Inspection of Table 1 shows that the maximum inhibition (lowest corrosion current density) is achieved at the highest concentration. It can also be seen that increasing concentrations of (4MP) shift the corrosion potential E_corr to less negative values, thereby showing that (4MP) is adsorbed on the iron surface and this kind of inhibitor behaves mainly as anodic-type inhibitors. The anodic and cathodic Tafel constants (b_a and b_c) do not change significantly with increasing concentration of the inhibitor, i.e. (4MP) affects both the anodic and cathodic overpotentials and shifts Tafel lines parallely in both directions. This indicates that this inhibitor at varying concentrations do not alter the reaction mechanism. This suggests that the inhibiting action of such compounds occurs by simple site blocking of the electrode, thus decreasing the surface area available for corrosion reactions (Khaled, 2003). The values of Tafel slops (b_a and b_c) are in good agreement with the values reported previously for iron in 1.0 M HCl (Grauer et al., 1982). Table 1 also shows that by increasing (4MP) concentration inhibition efficiency increases and corrosion rate decreases.

Fig. 2 shows the effect of (4MP) on the corrosion rate of iron in 1.0 M HCl at 25 °C. The corrosion rate decreases with an increase in the concentration of (4MP), presumably because of adsorption on the iron surface. Fig. 2 also shows that the inhibition efficiency δ% increases with increasing concentration of (4MP).

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Figure 2

The corrosion rate/mpy and the inhibition efficiency δ% for iron in 1.0 M HCl containing different concentrations of 4-methyl pyrazole (MPA) (from potentiodynamic data).

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Sieverts and Lueg firstly and systematically investigated the dependence of the corrosion rate of iron in acids upon the concentration of the organic inhibitors (Sieverts and Lueg, 1923). They established that at constant temperatures the curves representing this relationship (Fig. 2) have the form of adsorption isotherms if the concentration of the inhibitor is plotted on the horizontal axis and the values of the inhibition efficiency on the vertical axis. If, on the other hand, the rates of corrosion are plotted as ordinates, the curves assume the form of adsorptions turned through 180^o. This led Sieverts and Lueg to suggest an adsorption mechanism for the action of organic corrosion inhibitors in acids, and many latter works have supported this hypothesis (Putilova et al., 1960).

3.2 Electrochemical impedance spectroscopy

The popularity of the EIS technique for studying corroding systems is growing. EIS technique uses the rapid relaxation phenomena, which can be varied over a wide range of frequency. Impedance is determined by applying an ac voltage at a specific frequency and measuring the associated current. From the voltage and the current, impedance can be calculated for that frequency. By means of EIS technique a spectrum of impedance of the system under a series of frequencies can be observed.

Fig. 3 shows the Nyquist plots of iron in 1.0 M HCl without and with various concentrations of 4MP (10⁻³–10⁻² M) at 25 °C. The impedance diagrams obtained are not perfect semicircles and this difference has been attributed to frequency dispersion (Mansfeld et al., 1982). The charge transfer resistance R_ct is calculated from the difference in impedance at lower and higher frequencies, as suggested by Harnyama and Tsuru (Tsuru et al., 1978). To obtain the double layer capacitance C_dl, the frequency at which the imaginary component of the impedance is maximum (−Z”_img), is found and C_dl values are obtained from the Eq. (3) Ross Macdonald, 1987.

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Figure 3

Complex-plane impedance of iron in 1.0 M HCl in the presence of different concentrations of 4-methyl pyrazole (4MP) at 25 °C, (a) Blank, (b) 1 × 10⁻³ M, (c) 3 × 10⁻³ M, (d) 5 × 10⁻³ M, (e) 7 × 10⁻³ M, (f) 9 × 10⁻³ M, (g) 10 × 10⁻³ M.

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From the charge transfer resistance we can calculate the inhibition efficiency of the corrosion of iron, as in Eq. (4) Abd El-Rehim et al., 1999.

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Figure 4

The inverse charge transfer resistance 1/R_ct and the inhibition efficiency φ% for iron in 1.0 M HCl containing different concentrations of 4-methyl pyrazole (4MP) (from impedance data).

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Figure 6

Impedance data for corroding iron at corrosion potential in 1.0 M HCl at 25 °C. Plot (A, C) shows the complex plan representation each data point corresponds to a different frequency; Plot (B, D) shows the magnitude and phase angel of the impedance as a function of frequency. Plot (A, B) for blank, and (C, D) for 10⁻³ M 4-methyl pyrazole (4MP)/1.0 M HCl. (+) curves are the nonlinear regreation to the data, open symbols are experimental data.

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Figure 5

Equivalent circuit model for iron/1.0 M HCl interface.

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Fig. 5 shows the equivalent circuit used to fitting the experimental impedance data which is described in detail elsewhere (Abd El-Rehim et al., 1999). Often a CPE is used in a model in place of a capacitor C_dl (double layer capacity) to compensate for non-homogeneity in the system (Abd El-Rehim et al., 1999). For example a rough or porous surface can cause a double layer capacitance to appear as a CPE with an n value between 0.9 and 1.

Excellent fit with this model was obtained with our experimental data (Fig. 6). It is observed that the fitted data match the experimental data, with an acceptable error.

In order to understand the mechanism of corrosion inhibitor, the adsorption behavior of the organic adsorbate (4MP) on the iron surface must be known (Al-Andis et al., 1995). The degree of surface coverage (θ) for different concentrations of (4MP) in 1.0 M HCl has been evaluated from EIS measurements using Eq. (5) Abd El-Rehim et al., 1999.

The data were tested graphically by fitting to various isotherms. A straight line was obtained on plotting θ vs. log C (Fig. 7) suggesting that the adsorption of (4MP) from 1.0 M HCl on an iron surface follow Temkin’s adsorption isotherm.

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Figure 7

Temkin adsorption plots for iron in 1.0 M HCl containing different concentrations of 4-methyl pyrazole.

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The Temkin isotherm characterizes the adsorption of uncharged molecules on a heterogeneous surface, where θ is a linear function of lnC (Morad and Kamal El-Dean, 2006)

The linear plot of θ vs. lnC as shown in Fig. 7 is in agreement with the Temkin equation. The calculated values of the adsorption parameters $f$ , $\text{ln}{K}_{\text{ads}}$ , and $\mathrm{\Delta }{G}_{\text{ads}}^{\text{o}}$ are 0.5, and −11.2 kJmol⁻¹, respectively. The negative value of $\mathrm{\Delta }{G}_{\text{ads}}^{\text{o}}$ implies that the adsorption of 4MP on the iron surface is allowed from the thermodynamics point of view and indicates that the inhibitor is weakly adsorbed. Values of $K_{\text{ads}}$ and $\mathrm{\Delta }{G}_{\text{ads}}^{\text{o}}$ were found to be 1.665 M⁻¹ to −11.2 kJ mol⁻¹, respectively.

The adsorption of 4MP on the metal surface can occur either directly on the basis of donor-acceptor interactions between the π-electrons of the heterocyclic compound and the vacant d-orbitals of iron surface atoms. Or the interaction of MPA with already adsorbed chloride (Hackerman et al., 1966). The performance of 4-methylpyrazole as an inhibitor in 1.0 M HCl can be explained in the following way. In aqueous acidic solutions, the 4-methylpyrazole compound exists either as neutral molecules or in the form of cations. Amines may be adsorbed on the metal surface in the form of neutral molecules, involving the displacement of water molecules from the metal surface and sharing of electrons between the nitrogen atoms and the metal surface (Khaled et al., 2009). Amines and heterocyclic nitrogen compounds may also adsorb trough electrostatic interactions between the positively charged nitrogen atom and the negatively charged metal surface (Khaled et al., 2009). The presence of lone pairs and π-electrons of the 4-methypyrazole compound enhanced the inhibitory effect of this compound.