THE COLLOIDAL-CHEMICAL BASIS OF THE EFFECT OF INHIBITORS ON THE CORROSION RATE OF VARIOUS STEEL SAMPLES

ABSTRACT

To study the colloidal-chemical basis of the inhibitors effect  on the corrosion rate of various steel samples, experiments were conducted at a temperature of 343 K using inhibitor solutions at different concentrations (100, 150, 200, 250, and 300 mg/l). Based on the experimental results, the activation energy (Ea), enthalpy (∆H), and entropy of corrosion were determined. These values were calculated using the Arrhenius equation, derived from the corrosion rates obtained gravimetrically in a 15% hydrochloric acid solution. The results showed that the activation energy (Ea) of the inhibitor-free solution was 29.52 kJ/mol, whereas it increased to 52.13 kJ/mol at an inhibitor concentration of 300 mg/l. It was observed that an increase in inhibitor concentration led to an increase in activation energy. This increase in activation energy indicates a slowdown in the corrosion process. Additionally, these findings suggest that the formation of a protective layer on the metal surface under the influence of the inhibitor significantly reduces the corrosion rate.

АННОТАЦИЯ

С целью изучения коллоидно-химических основ влияния ингибиторов на скорость коррозии различных образцов стали были проведены эксперименты при температуре 343 К с растворами ингибиторов различной концентрации (100, 150, 200, 250 и 300 мг/л). На основе результатов экспериментов были определены энергия активации (Ea), энтальпия (∆H) и энтропия коррозии. Эти показатели рассчитаны с использованием уравнения Аррениуса на основании скоростей коррозии, полученных гравиметрическим методом в 15%-ном растворе хлористоводородной кислоты. Результаты показали, что энергия активации (Ea) раствора без ингибитора составляет 29,52 кДж/моль, тогда как при концентрации ингибитора 300 мг/л она увеличивается до 52,13 кДж/моль. Было установлено, что увеличение концентрации ингибитора приводит к росту энергии активации. Увеличение энергии активации указывает на замедление процесса коррозии. Кроме того, эти результаты свидетельствуют о том, что под воздействием ингибитора на поверхности металла образуется защитный слой, что значительно снижает скорость коррозии.

Keywords: GXMA (guanidinochloride methylacrylate), corrosion rate, activation energy, chemical adsorption, gravimetric analysis, protection degree.

Ключевые слова: ГХМА (гуанидинохлорид метилакрилат), скорость коррозии, энергия активации, химическая адсорбция, гравиметрический анализ, степень защиты.

INTRODUCTION

The effectiveness of corrosion inhibitors largely depends on their ability to adsorb onto the metal surface. There are two main types of adsorption: 

1. Physical adsorption: This process occurs through weak Van der Waals forces between the molecules and the surface. 

2. Chemical adsorption: This process takes place due to the formation of chemical bonds between the molecules and the metal surface. 

Corrosion inhibitors adsorb onto the surface, protecting the metal from direct contact with the chemical environment. To understand adsorption and evaluate its effectiveness, the theory of isotherms is utilized[1-3]. 

Several adsorption isotherms are used to describe the adsorption process, including the Freundlich isotherm. The Freundlich adsorption isotherm is expressed by the following general equation: 

θ = K_ads. • Cⁿ                                                                                            (1)

Here: 

- θ — the fraction of the metal surface covered, 

- C — the concentration of the inhibitor in the solution, 

- Kₐ_ds — the adsorption constant (associated with the adsorption capacity), 

- n — an empirical parameter that defines the intensity of adsorption.

Although the Freundlich isotherm provides a simplified representation of the adsorption process, more complex isotherms, such as Langmuir and Temkin, can also be applied to gain a deeper understanding of the phenomenon. The Freundlich model is commonly used to describe adsorption occurring on heterogeneous surfaces, and its results are based on empirical data related to adsorption [4-6].

MATERIALS AND METHODS

To investigate the inhibitory properties of GXMA, steel of grade St-20 was used. The composition of this steel is as follows: Iron (Fe) – 98%; Carbon (C) – 0.20%; Manganese (Mn) – 0.50%; Silicon (Si) – 0.17%; Chromium (Cr) – 0.25%; Copper (Cu) – 0.2%.

Coupons (plates) with dimensions of 2x4x0.45 cm were prepared from the steel sample. The surface of the steel coupons was cleaned using SiC abrasive paper. The cleaned coupons were degreased in 96% ethanol and then dried with acetone. Each coupon was tied with a thread and placed into experimental solutions. A total of 6 steel coupons were used in the experiment:

• Coupon 1: Placed into a 15% HCl solution without an inhibitor (control group).
• The remaining 5 coupons: Placed into 15% HCl solutions containing varying concentrations of GXMA.
• The solution temperature was maintained within the range of 343–363 K (70–90 °C), and the experimental volume was 100 ml.

The coupons remained in the solutions for 300 hours, during which the corrosion process and the efficiency of the inhibitor were monitored. At the end of the experiment, the condition and mass of the coupons were analyzed. The procedure was carried out in the following sequence:

1. Cleaning: The coupons were cleaned with a special brush to remove corrosion residues from the surface.

2. Washing: The coupons were first washed with distilled water and then with acetone.

3. Drying: The coupons were dried until their mass remained constant. This process was performed under controlled drying conditions.

The corrosion rate and protection degree of the steel samples were determined using the gravimetric method. The obtained results are presented in Table 1 below.

Table 1. 

The relationship between the concentration of GXMA corrosion inhibitor and its anti-corrosion characteristics

To study the adsorption process of corrosion inhibitors in detail, several isotherms have been applied, as each isotherm allows for the characterization of different aspects of adsorption. Below are the isotherms and their key features:

1. Langmuir Adsorption Isotherm:

                                                                              (2)

His isotherm assumes that adsorption occurs as a single-layer, ideal process at uniform adsorption sites. It considers that all adsorption sites possess identical energetic characteristics. Additionally, it is applied to describe homogeneous adsorption on metal surfaces.

2. Temkin Adsorption Isotherm:

                                                                          (3)

This isotherm accounts for changes in surface activity during the adsorption process. It provides information about the interactions between adsorbing molecules and the energetic variations of adsorption sites on the metal surface. It is used in cases where there are interactions between the adsorbing species and the metal surface.

3. Dubinin–Radushkevich (D–R) Isotherm:

                                                   (4)

This isotherm is applied to distinguish between physical and chemical adsorption. It helps identify the type of adsorption process by analyzing the average value of adsorption energy. It is commonly used to study the combination of physical and chemical adsorption on heterogeneous surfaces.

4. Elovich Adsorption Isotherm:

                                                                   (5)

This model describes systems where the activation energy of adsorption changes over time. It is used to study complex and kinetically controlled adsorption processes.

5. Flory-Huggins Adsorption Isotherm:

                                                         (6)

This isotherm  considers the interactions between molecules on the surface and the probability of surface coverage. It is applied in cases where the adsorbing molecules interact with each other [7-9]. 

One of the key conclusions of the Langmuir model is that each adsorption site can bind to only one molecule, meaning that adsorbate molecules do not interact with each other and are evenly distributed on the adsorbent surface. 

RESULTS AND DISCUSSION

The results obtained using the gravimetric method are presented in Table 2. The GXMA inhibitor was tested at concentrations ranging from 100 mg/l to 300 mg/l at temperatures of 70 ºC (343 K), 80 ºC (353 K), and 90 ºC (363 K). The obtained results were then verified through several adsorption isotherms.

Table 2.

Values of inhibition coefficient (γ), full surface coverage (θ), and protection degree (Z) determined gravimetrically for GXMA corrosion inhibitor in 15% HCl medium at various temperatures over 300 hours

To determine the conformity of the experimental results with theoretical studies, the data were analyzed using various adsorption isotherm equations. At temperatures of 343, 353, and 363 K, plots of log(C/θ) versus log(C) were generated, resulting in linear relationships, as shown in Figures 1.a, 1.b, and 1.c.

These linear correlations indicate that the adsorption of the GXMA inhibitor onto the metal surface follows a specific adsorption isotherm model, confirming the theoretical validity of the experimental data.

Figure 1.a. The Langmuir adsorption isotherm plot for the GXMA inhibitor at 343 K on the steel surface

Figure 1.b. The Langmuir adsorption isotherm plot for the GXMA inhibitor at 353 K on the steel surface

Figure 1.c. The Langmuir adsorption isotherm plot for the GXMA inhibitor at 363 K on the steel surface

The R² values obtained at these temperatures were 0.99875, 0.9833, and 0.99744, indicating a strong correlation with the Langmuir adsorption isotherm.

The effect of various concentrations of the corrosion inhibitor in a 15% HCl solution on steel samples at 343 K was studied, and the thermodynamic parameters were calculated. The results obtained for different concentrations are presented in table 3.

Table 3.

Below are the values of activation energy () for the corrosion process of steel samples in the presence of various concentrations of GXMA at 343 K:

The results confirm that the anticorrosive efficiency of guanidine hydrochloride methyl acrylate (GXMA) inhibitor in an acidic medium is directly dependent on its concentration.

CONCLUSION

In the study, GXMA demonstrated the highest efficiency at a concentration of 300 mg/l, where the activation energy of the inhibition process reached 52.13 kJ/mol. Based on the obtained results, the activation energy (𝐸𝑎) without the inhibitor was recorded at 29.52 kJ/mol, whereas it increased to 52.13 kJ/mol at a GXMA concentration of 300 mg/l. This increase in activation energy with higher inhibitor concentrations indicates a significant reduction in the rate of the corrosion process.

Moreover, these findings highlight that the increase in activation energy corresponds to the formation of a protective layer on the metal surface due to the inhibitor's action, which significantly slows down the corrosion process.

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