Comprehensive Study of the Action of Corrosion Inhibitors Based on Quaternary Ammonium Compounds in Solutions of Hydrochloric and Sulfamic Acids

Abstract

Acid treatments are one of the methods for intensifying oil and gas production. Corrosion is a significant factor affecting the performance of oilfield equipment. There are many different methods of preventing corrosion, but corrosion inhibitors are most commonly used in industry. The protective effect of the inhibitor is directly determined by the effective adsorption of surfactants on the metal surface. For an indirect assessment of the adsorption of the inhibitor, a comprehensive research method is proposed based on the determination of the interfacial tension of acid compositions and steel plates’ contact angle of wetting after corrosion tests. It was found that in hydrochloric acid the adsorption of the inhibitor IC-1 reaches a maximum after 6 h, an increase in the acid concentration in the range of 5–15% wt. has a negative effect on the activity of the inhibitor. For sulfamic acid, the maximum adsorption of the IC-2 inhibitor is observed after 24 h, an increase in acid concentration has a positive effect on surfactants.

1. Introduction

One of the methods for intensifying oil and gas production is acid treatment [1,2,3]. For that purpose, a wide range of acids is used: hydrochloric, sulfamic, acetic, citric, mud, and others, which are highly corrosive. It is known that hydrochloric acid is most often used in acid treatments. It can act as the base of the acid composition and can also be used in small quantities together with other acids to enhance their dissolving ability during treatments of both carbonate and terrigenous collectors. The advantages of hydrochloric acid are its availability and low cost. However, it has a number of disadvantages, such as high reactivity [4], which does not allow acid to penetrate deep into the formation, high corrosion of steel [5,6], sediment formation, and secondary sedimentation. Sulfamic acid is currently a promising option for acidizing low-temperature carbonate reservoirs. Its application makes it possible to increase the efficiency of treatment of the bottomhole formation zone due to its lower dissolving capacity. Moreover, the corrosion rate of steel, using acid compositions based on sulfamic acid, is several times lower than that of hydrochloric acid [7].

Corrosion is a significant factor affecting the performance of oilfield equipment. Corrosion damage is often the main reason for the failure of oil and gas equipment at objects of transportation and storage of compositions used for acid treatments. During the formation of corrosion products and their further dissociation, iron ions appear in the system, which can lead to the formation of stable emulsions, sedimentation, and salt deposits [8].

There are a large number of different methods to prevent the occurrence of corrosion and protect against its effects, in particular, the use of protective coatings [9], the manufacture of special alloys, as well as electrochemical methods of protection. However, corrosion inhibitors are most widely used in industry [10]. The widespread use of corrosion inhibitors in the oil and gas industry is explained by the fact that at a low cost, it is possible to slow down corrosion destruction, even if constructions or equipment are in operation [11,12].

It is known that organic compounds with heteroatoms, such as N, S, and O, and heterocyclic compounds with a polar functional group and a conjugated double bond with a high electron density have high anticorrosive properties. Corrosion inhibition is directly determined by the effective adsorption of surfactants on the metal surface due to the physical and chemical blocking of surface-active sites exposed to corrosive environments. Physical adsorption is usually carried out due to Van der Waals forces and electrostatic interactions between polar or charged functional groups and the polar surface of steel [13]. When the acid dissociates, an excess of ions is formed. In acidic compositions, that excess forms a double electric layer with negative charges on the surface [14,15,16]. As a result, protonated inhibitor molecules, with their positive charge, are adsorbed on the steel surface due to electrostatic interaction [17,18] with the orientation of non-polar groups towards an aggressive media.

The second type of adsorption is chemisorption, which is the result of the distribution or transfer of a charge from inhibitor molecules to the metal surface to form a chemical bond. Nitrogen, oxygen, or sulfur atoms included in the active base of inhibitors have free lone pairs of electrons that can interact with the d-orbitals of iron, forming a coordination-type bond [19]. They are active centers for adsorption on the metal surface, thus forming a tightly packed hydrophobic barrier with outwardly directed hydrocarbon radicals, which prevents the diffusion of water, sulfamate ions, halide ions, hydrogen ions, oxygen to the metal surface [20] and, due to this, retains metal surface from the destructive effects of the environment [21].

Corrosion inhibition is usually evaluated based on the adsorption of surfactant inhibitor molecules on the surface of metal objects. However, it is known [22] that corrosion products film starts to form on the metal immediately at the moment of its contact with an aggressive medium. This film can have protective qualities and lead to a decrease in the corrosion rate. Thus, the main goal of this work was to study the effect of acid concentration on the activity of corrosion inhibitors. An integrated approach to assessing the effectiveness of inhibitors made it possible to establish the maximum protective effect of inhibitors in time, which is the scientific novelty of this work. The approach was based on the determination of adsorption from the values of interfacial tension, contact angle of wetting, and calculation of the Gibbs adsorption energy.

2. Materials and Methods

2.1. Objects of This Study

Plates made of St3 steel were used as metal samples for the corrosion experiments. The dimensions of the steel used in the gravimetric experiments were 5 cm × 2 cm × 0.2 cm. The main elements that the steel consisted of are shown in

Table 1. Chemical composition of steel.

Steel Grade	Content of Chemical Elements, % wt.
	Carbon	Manganese	Silicon
St3	0.14–0.22	0.30–0.60	<0.05

.

Aggressive media were aqueous solutions of hydrochloric and sulfamic acids at a concentration rate of 5%, 10%, and 15% wt. Hydrochloric acid solutions were prepared by diluting chemically pure 37% wt. HC1 with bidistilled water. Sulfamic acid solutions were prepared by dissolving chemically pure NH₂SO₃H in bidistilled water.

Earlier, at the Scientific and Educational Center “Promyslovaya khimiya”, a large number of acid corrosion inhibitors of various brands was studied, both Russian and foreign. Inhibitors were tested in the concentration range from 0.005 to 1.00 wt%, taking into account the concentrations recommended by the manufacturers and technical documentation. In the work, inhibitors were identified that reduce the corrosion rate of steel to the maximum permissible value (0.2 g/m²∙h) during the 24 h for hydrochloric and sulfamic acids. Numerous experiments at various concentrations of acids made it possible to establish the inhibitor, designated IC-1, as the most effective in hydrochloric acid solutions. The active basis of IC-1 is a mixture of urotropine and quaternary alkylarylammonium compounds, and it demonstrates the greatest protective effect in solutions based on hydrochloric acid at a concentration of 0.3–1.0% wt. According to its technical characteristics, inhibitor IC-2 is designed to slow down corrosion in sulfuric acid, which makes it possible to use in sulfur-containing acids. Studies have shown the expected high protective effect of IC-2 in solutions based on sulfamic acid. The IC-2 consisted of alkylaryl-substituted quaternary ammonium compounds. It reduced the corrosiveness of sulfamic acid at a concentration of 0.4–0.5% wt.

2.2. Experimental Details

2.2.1. Determination of Interfacial Tension

The interfacial tension of acid solutions with the addition of the inhibitors under study was determined at a temperature of 20 °C on a Dataphysics OCA 15 Plus device for drop shape analysis. As a hydrocarbon medium, at the border with which the interfacial tension was studied, n-octane was used. Acidic compositions were placed in a cuvette; the syringe was filled with n-octane, controlling the absence of air bubbles inside. The syringe and cuvette were installed in the required areas of the device. The equilibrium state of adsorption/desorption of surfactants on the droplet surface was expected when the values of interfacial tension reached a plateau. The calculations were carried out using special software according to the Laplace–Young method. Reliability of results was confirmed by several parallel tests. The permissible measurement discrepancy did not exceed 0.05 mN/m or 0.5%.

2.2.2. Determination of Corrosion Rate by Gravimetric Method

The dissolution rate of steel was determined at a temperature of 20 °C by the gravimetric method, which is based on measuring the weight loss of plate samples made of St3 steel during their placement in acid compositions, followed by an assessment of the inhibitor protective effect. To confirm the reliability of the results in each acid composition, two parallel tests were carried out. The exposure time was 3, 6, 18, and 24 h. To prepare for the experiment, steel plates were polished with silicon carbide abrasive paper, degreased with acetone, and dried.

The massometric indicator of the corrosion rate was calculated using Formula (1):

$${\mathcal{V}}_{c inh }=\frac{m_1-m_2}{S·t},$$

(1)

where ${\mathcal{V}}_{c inh}$ is the corrosion rate in the presence of an inhibitor, g/(m²·h); $m_1$ is the mass of the plate before exposure, g; $m_2$ is the mass of the plate after exposure, g; S is the surface area of the plate, m²; t is the test time, hours.

The protective effect of the inhibitor was calculated by Formula (2):

$$Z=\frac{{\mathcal{V}}_c-{\mathcal{V}}_{c inh }}{{\mathcal{V}}_c}·100\%$$

(2)

where Z is the protective effect of the inhibitor, %; ${\mathcal{V}}_{\mathrm{c}}$, ${\mathcal{V}}_{c inh }$ are the corrosion rate of steel without the addition of an inhibitor and in its presence, respectively, g/(m²·h).

2.2.3. Determination of Corrosion Rate by Electrochemical Method

The electrochemical method includes the measurement of the polarization resistance of steel electrodes with a help of portable device “Monicor-2M” at a temperature of 20 °C. Based on the data obtained, the values of the corrosion current, the corrosion current density, and the protective effect of the inhibitors were calculated. The experiment time was 24 h, the readings of the device were recorded starting from the moment of dipping the used electrodes in an aggressive environment. The area of electrodes made of St3 steel was 4.7 cm². In preparation for the experiment, the electrodes were polished with silicon carbide abrasive paper, degreased with acetone, and dried. In each acid composition, two parallel tests were carried out to confirm the reliability of the results obtained.

The operation of the corrosion rate indicator “Monicor-2M” is based on the Stern-Geary principle, obtained by differentiating the equation of the polarization curve near the stationary corrosion potential. It is based on the assumption that the relationship between the potential and the external current at a potential, close to stationary, is Linear (3).

$$I_{corr}=\frac{b_a·b_k}{2.303}\left(b_a+b_k\right)\left(\frac{dI}{dE}\right)=\frac{B}{R_p},$$

(3)

where I_corr is the corrosion current, mA; b_a and b_k are the coefficients of the Tafel equation and correspond to the slopes of the anode and cathode sections of the straight line in semilogarithmic coordinates, mV; B is the constant depending on the Tafel equation coefficients; R_p is the polarization resistance, mV.

The polarization of the working electrode in the cathodic and anodic regions from the corrosion potential was ±10 mV; the value of the coefficients b_a and b_k 120 mV; the duration of polarization was 30 s in the cathodic and anodic regions. The current density was calculated by Formula (4):

$$j=\frac{I_{corr}}{S},$$

(4)

where I_corr is the corrosion current, mA; S is the electrode area, cm².

Based on the obtained values of the corrosion current (Formula (3)), the depth index of the steel corrosion rate was calculated according to Formula (5):

$$P=11.7\ast \frac{I_{corr}}{S},$$

(5)

where P is the corrosion rate, mm/year; I_corr is the corrosion current, mA; S is the electrode area, cm²; 11.7 is the coefficient that takes into account the conversion of measurement units.

The conversion of the depth index of the corrosion rate into the massometric indicator was carried out according to Formula (6):

$${\mathcal{V}}_{c inh}=P·\frac{p}{8.76},$$

(6)

where ${\mathcal{V}}_{cinh}$ is the corrosion rate in the presence of an inhibitor, g/(m²·h), P is the corrosion rate, mm/year; p is the density of the corrosive metal, g/cm³; 8.76 is the coefficient that takes into account the conversion of measurement units.

Corrosion rate values, registered with the device for determining electrochemical corrosion “Monicor-2M”, allow determining the protective effect of the surface film of corrosion products, the protective effect of the inhibitor, and their general protective effect [22]:

$$Z_f=\frac{v_{c in}-v_{c t}}{v_{c in}}·100\%,$$

(7)

$$Z_{gen}=\frac{v_{c in}-v_{c 24 inh}}{v_{c in}}·100\%,$$

(8)

$$Z_{inh}=Z_{gen}-Z_f,$$

(9)

where $Z_f$ is the protective effect of the film of corrosion products, %; $Z_{gen}$ is the general protective effect, %;$Z_{inh}$ is the protective effect of the inhibitor, %; $v_{c in}$, $v_{c t}$, $v_{c 24 inh}$ are the corrosion rate at the initial moment of time, at a certain point in time, after a day in inhibited solution, respectively.

2.2.4. Determination of the Contact Angle

The angle of wetting steel plates St3 with water before and after corrosion tests in 3, 6, 18, 24 h was determined at a temperature of 20 °C using a Dataphysics OCA 15 Plus device for drop shape analysis. To confirm the reliability of the results, two parallel tests were carried out in each case. Using the obtained values, it is possible to calculate the difference in the wetting angles after the steel samples were placed in an aggressive environment with an inhibitor and without (Formula (10)):

$$∆\theta ={\theta }_{inh}-\theta ,$$

(10)

where ∆θ is the difference in contact angles of steel plates, °; θ_inh is the angle of wetting of steel plates after corrosion in an inhibited acidic environment, °; θ is the angle of the steel plates after the uninhibited corrosion in an acidic environment, °.

2.2.5. Determination of Gibbs Adsorption Energy

In the case of studying corrosion inhibitors from the values of the Gibbs adsorption energy, the possibility of their spontaneous adsorption on a metal surface, and the type of this adsorption are identified [23]. The Gibbs adsorption energy was calculated by Formula (11):

$$∆G_{ads}^0=-RTln\left(55.5K_{ads}\right),$$

(11)

where $∆G_{ads}^0$ is the Gibbs adsorption energy, kJ/mol; R is the universal gas constant, J/mol∙K; T is the temperature, K; 55.5 is the molar concentration of water in solution; mol/L; $K_{ads}$ is the adsorption equilibrium constant.

The equilibrium constant of adsorption was determined based on Formula (12):

$$\frac{{\mathrm{C}}_{inh}}{\phi }=\frac{1}{K_{ads} }+{\mathrm{C}}_{inh},$$

(12)

where $K_{ads}$ is the adsorption equilibrium constant; C_inh is the inhibitor concentration, mol/L; $\phi$ is the degree of surface coverage with inhibitor molecules, %.

The degree of surface coverage was determined by Formula (13):

$$\phi =\frac{∆m-∆m_{inh}}{∆m}·100\%,$$

(13)

where $\phi$ is the degree of surface coverage with inhibitor molecules, %; $∆m, ∆m_{inh}$ are the weight loss of steel plates during corrosion without the addition of an inhibitor and in its presence, respectively, g.

3. Results and Discussion

3.1. Determination of the Critical Concentration of Micelle Formation of Inhibitors in the Studied Acids

It is believed [24] that the critical micelle concentration (CMC) is an important factor in determining the effect of surfactant inhibitor concentration on adsorption and corrosion retardation. The authors of scientific articles [25,26] determined that there is a maximum decrease in the corrosion rate with an increase in the inhibitor concentration to CMC. A further increase in concentration does not make a direct contribution to the additional protection of the metal against corrosion [13] and may have a negative effect on the values of the corrosion rate.

Based on the information provided, it was decided to calculate the CMC by the interfacial tension values for IC-1 and IC-2 in aqueous solutions of hydrochloric and sulfamic acids. The obtained CMC values made it possible to determine the concentration of inhibitors required to obtain high protective properties.

Measurement of the interfacial tension index of the analyzed inhibitors was researched at concentrations of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 1.0% wt. in hydrochloric and sulfamic acids at the border with n-octane according to the method in Section 2.2.1. From the graph of the interfacial tension values dependent on the logarithm of the inhibitor concentration, the CMC values were calculated. Figure 1 shows the obtained values of the interfacial tension index for the corrosion inhibitor IC-1 in hydrochloric acid (a–c) and IC-2 in sulfamic acid (d–f).

The obtained data on the determination of interfacial tension in acid solutions adding an inhibitor showed that in hydrochloric acid in the entire studied concentration range (Figure 1a–c), CMC of the IC-1 was about 0.5% wt. A change in the acid concentration has practically no effect on the analyzed indicator of the critical micelle concentration.

In 5% sulfamic acid (Figure 1d–f), the CMC for the IC-2 inhibitor was approximately 0.4% wt., and with an increase in the acid concentration to 10 and 15% wt. the CMC value was reduced to 0.2% wt. This corresponds to the data obtained in [27,28] since the ability to reduce the critical concentration of surfactant micelle formation with an increase in the acid concentration is considered to be a well-known feature of sulfamic acid [29]. This is explained by the tendency of the sulfamate anion to “salting out the surfactant” and thereby reduce the CMC and the interfacial tension of surfactant solutions [30].

Thus, studies to determine the critical concentration of micelle formation IC-1 in aqueous solutions of hydrochloric acid make it possible to establish the inhibitor concentration needed to achieve high protective properties. This concentration corresponded to CMC and was 0.5%. For the inhibitor IC-2, the CMC was in the range from 0.2 to 0.4% wt. Based on that, for the effective action of the inhibitor in all concentrations of sulfamic acid, 0.4% wt was chosen for further studies.

3.2. Corrosion Rate Study

3.2.1. Gravimetric Studies

Based on GOST R 9.905-2007 and technical regulations of companies operating in the oil and gas industry, it is believed that the highest acceptable level of corrosion of St3 steel in inhibited acid, tested for 24 h at 20 °C, is 0.2 g/(m²∙h). To establish the concentration of inhibitors at which a given value of the corrosion rate is achieved and to test the concentrations selected in Section 3.1, the corrosion rate of St3 steel was determined by the gravimetric method described in Section 2.2.2, within 24 h according to Formula (1). The tests were carried out in solutions of hydrochloric acid, adding the corrosion inhibitor IC-1 at a concentration of 0.3, 0.5, and 1.0% wt. and sulfamic acid adding the inhibitor IC-2 at concentrations of 0.4 and 0.5% wt.

Figure 2 shows the results of the corrosion rate when various concentrations of the inhibitor were added to aqueous solutions of hydrochloric acid 5%, 10%, and 15% wt.

Earlier [27], it was noted that an increase in hydrochloric acid concentration in solution reduces the activity of cationic surfactant molecules due to an increase in the concentration of hydronium ions formed during the dissociation of hydrochloric acid [31]. According to the data obtained (Figure 2), an increase in the concentration of hydrochloric acid negatively affected the activity of the inhibitor, and the corrosion rate increased in the range of 5 to 15% wt. HCl.

The increase in the concentration of the inhibitor IC-1 from 0.3 to 1.0% wt. led to a decrease in the corrosion rate at all concentrations of hydrochloric acid to the maximum permissible value of 0.2 g/(m² h). At the same time, an increase in the inhibitor concentration from 0.5 to 1.0 wt.% had an insignificant effect on a decrease in the corrosion rate, which is consistent with the data obtained in [13,32].

Figure 3 shows the results of the corrosion rate when various concentrations of IC-2 inhibitor were added to aqueous solutions of sulfamic acid 5%, 10%, and 15% wt.

The research results presented in Figure 3 show that an increase in the concentration of sulfamic acid led to a decrease in the corrosion rate, which is also consistent with the data obtained in [27]. This can be explained by the activity of the acid itself due to the possibility of its existence in two forms [33]: neutral and zwitterionic, with surface activity, as well as the positive effect of sulfamic acid on the effectiveness of surfactants [28].

An increase in the concentration of the inhibitor IC-2 led to an increase in the corrosion rate at all concentrations of sulfamic acid. In some studies [14,16,19], this was explained by the formation of bilayers of inhibitor molecules at a concentration higher than the CMC, since in a saturated solution, non-adsorbed and adsorbed molecules interact with each other, which leads to desorption of particles from the metal surface.

Based on gravimetric studies of the corrosion rate, it can be concluded that selected concentrations of inhibitors (Section 3.1) IC-1—0.5% wt. and IC-2—0.4% wt. showed the best results.

3.2.2. Electrochemical Research

The idea behind electrochemical corrosion rate testing is in establishing the values of the corrosion current flowing between two electrodes in an acidic environment. Using the technique, a low voltage is applied to the electrodes and measures the resulting current which emerges from the corrosion processes described above. At a high intensity of corrosion of electrodes, metal ions Fe²⁺ easily enter the solution, and a low voltage on the electrodes causes a high current proportional to I_corr. If the corrosion rate is not high, ions enter the solution slowly, and the same voltage causes a low current. Based on this marker, the current density is calculated, that is, the charge passing through the cross-sectional area unit of the electrode. It is known [34,35] that the lower the corrosion current density, the more effective the inhibitor action due to a decrease in the active electrode surface during adsorption of inhibitor molecules on it [36]. To confirm the inhibitor concentrations specified in Section 3.2.1, the values of the corrosion current density of steel electrodes were calculated applying the method in Section 2.2.3 for 24 h according to Formula (4). The tests were carried out in solutions of hydrochloric acid with the addition of the corrosion inhibitor IC-1 at a concentration of 0.3, 0.5, and 1.0% wt. and sulfamic acid adding the inhibitor IC-2 at concentrations of 0.4 and 0.5% wt.

Figure 4 shows the dependence of the corrosion current density in the presence of various concentrations of the inhibitor IC-1 on the concentration of hydrochloric acid (a) and IC-2 on the concentration of sulfamic acid (b).

From the data obtained in Figure 4, it can be seen that the corrosion current density grew with an increase in the acid concentration, which can be explained by the increasing number of hydrogen ions that were restored at the cathode [37]. Adding inhibitors, a significant decrease in the current density was noticed due to the adsorption of inhibitor molecules on the surface of steel electrodes [36]. In the case of hydrochloric acid (Figure 4a), as in gravimetric tests, the most effective concentration of IC-1 was 0.5% wt., since an increase in the inhibitor concentration to 1.0% wt. did not lead to a significant decrease in the current density. A study of the effect of different concentrations of IC-2 in sulfamic acid (Figure 4b) showed that at an inhibitor concentration of 0.4% wt., the greatest decrease in the corrosion current density was observed, compared with a concentration of 0.5% wt.

Thus, the study of the corrosion rate by the electrochemical method is consistent with the data obtained during the gravimetric test (Section 3.2.1). The highest efficiency of surface-active corrosion inhibitors was confirmed at CMC concentration.

3.3. Determination of the Protective Effect

3.3.1. Gravimetric Studies

In practice, an indicator of the protective effect is also used in addition to evaluating the effectiveness of inhibitors by the values of the corrosion rate. The dynamics of the corrosion process in time is also an interest to study. That is how the protective effect indicators of corrosion inhibitors were calculated on the basis of the method in Section 2.2.2 using a Formula (2) at 3, 6, 18, and 24 h after impacting with acid compositions. The studies were carried out for acid compositions based on hydrochloric and sulfamic acids 5, 10, 15% wt. with the concentrations of inhibitors selected in Section 3.2.1 of 0.5% wt. for IC-1 in hydrochloric acid and 0.4% wt. for IC-2 in sulfamic acid.

Figure 5 shows the data about the protective effect of the IC-1 inhibitor in hydrochloric acid solutions over time.

From the data obtained, it can be seen that the protective effect of inhibitors varied depending on the time of the study. The maximum protective effect was achieved in all hydrochloric acid solutions after 6 h. Further, at 18 and 24 h, the values of the protective effect slightly decreased, which can be explained by various factors and competing processes taking place in time.

Figure 6 presents data about the protective effect of the IC-2 inhibitor in sulfamic acid in time.

From the data in Figure 6, the dependence on time was revealed to increase the protective effect of the inhibitor in sulfamic acid. The least protective effect in the entire range of sulfamic acid concentrations was observed after 3 h. Further, over time, the effectiveness of the inhibitor increased and reached a maximum after 24 h.

3.3.2. Electrochemical Research

A distinctive feature of electrochemical studies is the ability to obtain results immediately after immersion of the electrodes in an aggressive medium solution, which makes it possible to evaluate the effect of the inhibitor and the formation of corrosion products at any time, as noted in the method in Section 2.2.3. As a result, using the obtained data, it was possible to calculate the corrosion rate indicators according to Formulas (5) and (6). Then using the method in Section 2.2.3 and those indicators, one can determine the values of the general protective effect (Formula (7)), the protective effect from the film of corrosion products (Formula (8)), and the protective effect directly of inhibitor (Formula (9)).

Figure 7 and Figure 8 show the IC-1 and IC-2 general protective effect (Z_gen), the protective effect of the film of corrosion products (Z_f), and the protective effect of the corrosion inhibitor (Z_inh) in hydrochloric and sulfamic acids, respectively, obtained by Formulas (7)–(9).

Based on the data obtained, it can be seen that a significant contribution to the suppression of corrosion processes was made not only by the corrosion inhibitor but also by the film of the formed corrosion products. In hydrochloric acid (Figure 7), with an increase in concentration, a decrease in the effect of the inhibitor on the corrosion process was observed. In the study where duration was the key, the greatest protective effect from the inhibitor was achieved after 6 h, which corresponds to the data of gravimetric tests in Section 3.3.1. The contribution of corrosion products film rose with an increase in the concentration of hydrochloric acid and the time of the experiment, which can be explained by its high corrosive activity, leading to the formation of a significant amount of corrosion products.

When studying the corrosion process in sulfamic acid using Figure 8, it can be seen that with an increase in the acid concentration, the activity of the inhibitor increased and, accordingly, its contribution to the protective effect. The protective effect of the inhibitor itself increased over time and reached its maximum value after 24 h. The contribution of the reaction products formed during corrosion decreased with a growth in the concentration of sulfamic acid and the duration of the experiment. Thus, taking into consideration all results obtained, differences were shown between the protective effect of the inhibitor and the corrosion products film. The protective effect from the inhibitor is understood by the adsorption of molecules on the surface of steel samples [25]. Further studies were focused on studying the features of the adsorption process of the inhibitor and the possible effect of the reaction products on it during acid corrosion.

3.4. Determination of Adsorption

3.4.1. Study of Interfacial Tension

One of the most important properties of surfactants is their ability to concentrate at the interface with a decrease in excess of free surface energy. During the interaction of the acid composition with the metal surface, the molecules of the surface-active inhibitor will be adsorbed on it [25]; thus, their concentration will decrease in the solution volume.

This decrease can be determined by the change in the value of the interfacial tension of the acid compositions at the interface with the hydrocarbon (n-octane) (method in Section 2.2.1) at the initial moment—before the interaction of the acid with the metal and after 3, 6, 18, 24 h, as described in the method in Section 2.2.2. However, it must be mentioned that there is a possibility of a decrease in the surface activity of substances due to the aging process over time.

Table 2. Interfacial tension of acid solutions without the addition of inhibitors at the interface with n-octane.

Interfacial Tension, mN/m
5% HCl	10% HCl	15% HCl	5% NH2SO3H	10% NH2SO3H	15% NH2SO3H
43.97	43.97	43.97	43.97	43.97	43.97

shows the interfacial tension indicators of aqueous solutions of hydrochloric and sulfamic acids at a concentration of 5, 10, and 15% wt. without adding an inhibitor.

Table 3. The values of the interfacial tension index of solutions of hydrochloric and sulfamic acids with the addition of corrosion inhibitors at the border with n-octane.

Time, h	Interfacial Tension, mN/m
	5% HCl	10% HCl	15% HCl	5% NH2SO3H	10% NH2SO3H	15% NH2SO3H
	IC-1			IC-2
0	2.37	3.91	6.38	2.82	2.47	2.06
3	2.38	3.90	6.40	2.80	2.48	2.05
6	2.35	3.91	6.39	2.81	2.46	2.05
18	2.31	3.95	6.40	2.79	2.47	2.06
24	2.40	3.92	6.38	2.83	2.47	2.05

shows the interfacial tension indicators of inhibited acid solutions at the interface with n-octane over time without interaction with the metal. The inhibitor IC-1 at a concentration of 0.5% wt was added to hydrochloric acid at all concentrations, and the inhibitor IC-2 at a concentration of 0.4% wt was added to solutions of sulfamic acid.

The values presented in Table 2 and Table 3 indicated that the inhibitors IC-1 in hydrochloric acid and IC-2 in sulfamic acid demonstrated high activity, which is shown by the decrease in the interfacial tension of acids at the interface with the hydrocarbon after adding inhibitors to them.

According to the results received (Table 3), it can be concluded that the surfactants IC-1 and IC-2 did not lose their surface activity within 24 h. Thus, a possible increase in the values of the interfacial tension of acid compositions after corrosion tests will not be connected with a decrease in their surface activity during the day.

The results identifying the interfacial tension of the compositions after corrosion tests are shown in Figure 9a,b.

The growth of interfacial tension with time presented in Figure 9 confirmed the assumption about a decrease in the concentration of inhibitor molecules in solution due to their adsorption on the metal surface [13,20,25,26].

However, when considering the obtained data (Figure 9a), it was revealed that the activity of inhibitors decreased with an increase in the concentration of hydrochloric acid. Over time, the interfacial tension of inhibited hydrochloric acid solutions after contact with a metal surface changes in different ways. On the one hand, there was an increase in interfacial tension in the period up to 6 h, which correlated with the highest index of the protective effect for the IC-1 inhibitor in hydrochloric acid at this time (Figure 5). Further, there was a slight decrease in the values of the interfacial tension of acid solutions and, at the same time, a slight increase, which can be explained by various processes, such as the appearance of corrosion products in the solution volume and on the metal surface. All that has a different effect on the possibility of inhibitors adsorption on the surface [18,38].

Based on the data in Figure 9b, it can be concluded that a decrease in the values of interfacial tension with an increase in the concentration of sulfamic acid from 5 to 15 wt%. can be explained not only by a decrease in adsorption of the inhibitor but also by the ability of the acid itself to show surface activity due to its structure [27]. For these solutions containing the IC-2 inhibitor, after corrosion tests, an increase in the interfacial tension index during corrosion could also be noted. However, unlike hydrochloric acid, an increase in interfacial tension within 24 h indicated a gradual adsorption of surfactant inhibitor molecules on the steel surface.

3.4.2. Study of the Contact Angle of Wetting

The diphilic nature of surfactants explains their ability to adsorb at the interface; in the case of corrosion tests, as has already been mentioned, surfactant molecules will be adsorbed on the steel surface. In the process of corrosion, a film of reaction products is also formed as a result of the interaction of acidic compounds with iron. As a result, the contact of the metal surface with the acid decreases as there is an increase in hydrophobicity, which will lead to an increase in the contact angle [39,40,41].

To study the hydrophobization of metal plates in duration, the values of the contact angles of steel samples kept in an inhibited and uninhibited environment for 3, 6, 18, 24 h were calculated by the method in Section 2.2.4. To assess the contribution of a corrosion inhibitor adsorption to an increase in the contact angle without taking into account the effect of the formed corrosion products, the difference in the contact angles after corrosion with and without the addition of an inhibitor was calculated using Formula (10). The results obtained in our studies are presented in Figure 10a–f.

Studies in inhibited hydrochloric acid (Figure 10a) suggested that the hydrophobicity of steel plates increased after their interaction with inhibited acidic solutions, which is explained by the adsorption of inhibitor molecules on the surface [42]. An increase in acid concentration from 5 to 15% wt. led to a decrease in the contact angle, which can be explained by the negative effect of hydrochloric acid on the surface activity of surfactant molecules, as shown earlier [27]. In Figure 10b, one can see a significant decrease in the contact angle in a 5% hydrochloric acid solution without the addition of an inhibitor compared to the data in Figure 10a. However, with an increase in the acid concentration to 10–15%, the decrease in the contact angle became less. This can be explained by the formation of corrosion products that are adsorbed on the metal surface.

It is clear from the study of the contact angle during corrosion that in the presence of an inhibitor, not only the molecules of the surface-active inhibitor but also the formed corrosion products contributed to the surface protection, which correlated with the data of electrochemical tests presented in Section 3.3.2.

Based on the results obtained by assessing the contribution of adsorption of the inhibitor (Figure 10c), it can be concluded that in hydrochloric acid, the greatest effect of the inhibitor on the increase in the contact angle was observed after 6 h. It is at this time that the adsorption of surfactant inhibitor molecules on the steel surface reached its maximum, which was obtained earlier in Figure 9a. After that, the desorption process will prevail, which will lead to a slight decrease in the hydrophobicity of the surface, according to [14,43,44,45].

Studying the contact angle in inhibited sulfamic acid (Figure 10d), an increase in the hydrophobicity of steel samples surface during corrosion with an increase in acid concentration could be noted. This is explained by its positive effect on inhibitor molecules with an increase in acid concentration [17,18], which leads to better adsorption of the surfactant on the metal. Figure 10e also allows us to conclude that the effect of corrosion products on the increase in the contact angle was not as significant as their combined effect with the inhibitor. The results obtained confirmed the data of electrochemical tests of Section 3.3.2.

Evaluating directly the effect of the inhibitor on the value of the contact angle according to Figure 10f, it can be noted that the maximum adsorption on the metal surface was achieved after 24 h. The gradual growth of the adsorbed polar parts of the inhibitor molecules [46] can be explained by the lower aggressiveness of the acid itself and a slower formation of corrosion products.

Looking at presented results, it can be concluded that the study of adsorption based on the values of the contact angle of wetting makes it possible to analyze in detail the processes of adsorption of the inhibitor and the formation of corrosion products, and also completes the results of the adsorption analysis by interfacial tension. Furthermore, the time was determined at which the highest adsorption values of the corrosion inhibitor from acid solutions on the metal surface are achieved. For hydrochloric acid, it was 6 h; for sulfamic acid—24 h. At the next stage, a study was carried out to determine the type of adsorption.

3.4.3. Gibbs Adsorption Energy

Gibbs energy is a quantity whose change in the course of a chemical reaction is equal to a change in the internal energy of the system. It is believed that high negative values of the Gibbs energy indicate strong adsorption of the inhibitor on the steel surface [47], while the values of ΔG up to −20 kJ/mol refer to the process of physical adsorption, values of about −40 kJ/mol or more—to chemisorption due to shared use or transfer of electrons from a molecule to a metal surface, and values in the range from −20 to −40 kJ/mol correspond to a joint mechanism consisted of physical and chemical adsorption [15].

In order to determine the nature of adsorption of the inhibitor on the steel surface by the method in Section 2.2.5, using Formula (13), the degree of surface coverage with molecules of the inhibitor IC-1 at a concentration of 0.5 wt% was calculated in hydrochloric acid after 6 h of interaction with an inhibited acid composition. The same was done with the inhibitor IC-2 at a concentration of 0.4% wt. in sulfamic acid after 24 h.

From the received data, the values of the adsorption equilibrium constant were calculated by Formula (12), and their complete correspondence to the Langmuir adsorption isotherm was confirmed (RMSD R2 = 0.9999). The values of the Gibbs adsorption energy were calculated using Formula (11).

Figure 11a,b shows the calculated values of Gibbs adsorption energies of inhibitors IC-1 and IC-2 in the studied acids.

The values of the Gibbs adsorption energy obtained in Figure 11a were negative, which indicated significant adsorption of inhibitor molecules on the surface of steel plates. The presented indicators varied from minus 29.41 to minus 25.50 kJ/mol in the range of hydrochloric acid concentrations of 5–15% wt. In sulfamic acid (Figure 11b), the Gibbs adsorption energy values changed from minus 27.82 to minus 28.75 kJ/mol with increasing acid concentration. It can be concluded that the mechanism of adsorption of inhibitors IC-1 and IC-2 in the studied acids was complex and consisted of physical adsorption based on electrostatic interactions between the charged metal surface and organic corrosion inhibitors and chemicals [46], including the transfer and redistribution of the charge.

4. Conclusions

In this work, various factors influencing the process of suppressing the corrosion rate of steel were considered. In hydrochloric acid compositions, acid anions play the role of corrosion activators. The effect of cationic inhibitors on the corrosion rate is explained by the fact that their adsorption on the metal hinders the penetration of hydronium ions to the metal surface. The adsorption of IC-1 on the steel surface reached its maximum after 6 h. A further change in the protective effect is explained by the formation of corrosion products, the presence of which reduced the possibility of chemical adsorption of the inhibitor.

The maximum adsorption of the IC-2 on the metal surface in sulfamic acid was achieved after 24 h. The gradual growth of the adsorbed inhibitor molecules can be explained by the lower aggressiveness of the acid itself and a slower formation of corrosion products. Sulfamate ions help to suppress corrosion damage. With an increase in the concentration of sulfamic acid, the number of sulfamate ions rose. The increase in the protective effect can be explained by the possibility of the formation of complex compounds with the inhibitor molecules, which will be more strongly adsorbed on the metal surface. After 24 h, the maximum coverage of the metal surface was achieved by the formed corrosion products, sulfamate ions, and inhibitor molecules.

In general, it should be noted that changes in the protective effects of IC-1 and IC-2 inhibitors were influenced by the constantly occurring processes of adsorption–desorption of inhibitor molecules and corrosion products over time.

Based on the calculated values of the Gibbs adsorption energy, a probably mixed mechanism was suggested, including the physical and chemical adsorption of the inhibitors under study.

In this work, the factors influencing the process of suppressing the corrosion rate of steel were considered. As a result, an integrated approach was used to study the inhibition of a metal surface, based on the study of the interfacial tension of acid solutions before and after corrosion tests, the wetting angle of steel plates, and the Gibbs adsorption energy. Carrying out a complex of studies allowed us to establish that inhibitor IC-1 reached a maximum after 6 h, an increase in the acid concentration in the range of 5–15% wt. had a negative effect on the activity of the inhibitor, while the IC-2 inhibitor showed maximum adsorption after 24 h, an increase in acid concentration had a positive effect on surfactants.