Inhibition Localized Corrosion of N80 Petroleum Pipeline Steel in NaCl-Na2S Solution Using an Imidazoline Quaternary Ammonium Salt

Abstract

In this paper, the local corrosion inhibition effect of imidazoline on N80 oil pipeline steel in a NaCl-Na₂S solution was studied by the simulated blocking tank cell method, and the corrosion processes of the cathode and anode in the blocking zone were simulated. The blocking corrosion behavior of the pipeline tubing steel N80 in simulated corrosion solutions without and with different concentrations of an imidazoline corrosion inhibitor was studied by chemical analysis and electrochemical analysis. The results show that in the three solution systems, after the anode polarization of the occluded cell, the solution in the occluded region is acidified, the pH value decreases sharply, the migration of Cl⁻ and S²⁻ increases, and the concentration is increased in the blocked area. After adding the imidazoline corrosion inhibitor, the imidazoline inhibitor can reduce the migration of small-radius anions (Cl⁻ and S²⁻) to the occluded area, inhibit the acidification of the solution in the occluded area, and prevent the dissolution of metals in the occluded area. As a result, the corrosion of the occluded area is slowed down due to the change in the chemical and electrochemical state of the occluded area. In the three corrosion solution systems of 2% Na₂S + 5% NaCl, 2% Na₂S + 8% NaCl, and 2% Na₂S + 10% NaCl, the imidazoline corrosion inhibitor can form an adsorption film on the metal surface, thereby increasing the polarization resistance and decreasing the corrosion rate. The addition of an imidazoline corrosion inhibitor can significantly increase the kinetic constant of anode polarization, which can effectively inhibit the local corrosion of N80 steel in these corrosion systems.

1. Introduction

With the continuous development of oil and gas fields, their reserves are decreasing sharply year by year, so there is an urgent need to adopt production stimulation measures to improve oil and gas field recovery [1]. Subsequently, various chemical agents and CO₂ flooding technologies have been applied to oil and gas fields, and with the increase in injection volume and type, the washing efficiency and sweep coefficient have increased, there has been a volume expansion of crude oil, a decrease in viscosity, and an enhancement of fluidity, all of which have contributed to achieving the purpose of increasing production [2,3]. However, in the process of oil and gas exploitation and transportation, some chemical agents and CO₂ gases are dissolved in the formation water, forming certain corrosive media that will cause serious corrosion once in contact with pipelines and equipment for a long time [4,5]. In the process of oil production, metal pipes are susceptible to corrosion due to environmental factors and high concentrations of ions in the surrounding solution. The extent and rate of corrosion are influenced by different environmental conditions. Elevated temperatures enhance the reactivity of the metal surface, facilitating its reaction with corrosive substances and exacerbating corrosion. In high-pressure environments, electrochemical corrosion and flow corrosion rates increase as electrolytes and solid particles have a higher likelihood of reacting with metals, leading to increased corrosion levels. The water content in oil well production fluid significantly impacts corrosion, as it contains electrolytes that accelerate electrochemical corrosion occurrence and intensify flow corrosion. Oxygen and carbon dioxide act as common electrolytes that expedite electrochemical corrosion, particularly when combined with high water content which further aggravates the corrosive effects. The primary mechanism behind this type of corrosion is chemical in nature, primarily involving active hydrogen sulfide gas volatilized from crude oil and water, along with oxygen, carbon dioxide, and other gases entering the tank through breathing valves on the top of tanks, or through openings such as measuring holes or light transmission holes. Once inside the tank, they condense into acidic solutions that directly react with metals resulting in the formation of ferrous sulfide, ferrous oxide, and ferrous carbonate iron trioxide compounds. In recent years, the content of S²⁻, Cl⁻, and CO₂ in China’s oil and gas has become higher and higher, which has greatly increased the sensitivity of oil pipelines and equipment to local corrosion, especially small-hole corrosion. Once pitting corrosion occurs in oil pipelines, the corrosion system of small anodes and large cathodes is formed, which will accelerate the dissolution of metals in small holes. In this case, the service life of oil pipelines and equipment will be greatly shortened, and the cost of oil extraction will be increased. According to authoritative research estimates, if anti-corrosion technology is used properly, it can reduce these corrosion losses by 30~40% [6,7].

In order to slow down and prevent the corrosion of transmission pipelines and equipment, corrosion inhibitors are often added [8,9]. Corrosion inhibitor technology has the characteristics of strong corrosion inhibition ability and low economic cost, so it is widely used in various fields. Corrosion inhibitor molecules usually have both polar and non-polar groups, and there are elements such as N, O, P, and S in the polar groups, all of which contain lone pairs of electrons and are highly electronegative. The corrosion inhibitor is firmly adsorbed on the surface of the metal by polar groups, while the non-polar groups are arranged in the corrosive medium, which effectively isolates the contact between the metal and the corrosive medium on the one hand and hinders the diffusion of corrosion reaction products. On the other hand, the electrical double-layer structure of the metal/solution interface is also changed, and the activation energy of the corrosion reaction is increased, thereby reducing the corrosion rate of the metal, and finally inhibiting the corrosion reaction [10]. Imidazoline and its derivatives are considered to be one of the most effective corrosion inhibitors for carbon steel corrosion in highly corrosive environments and are widely used in the field of oil and gas pipeline corrosion and protection due to their green environmental protection and excellent biodegradability. Zhang et al. [11] studied the corrosion inhibition behavior of imidazoline thiourea and its derivatives on pipelines in a 5% NaCl corrosive medium and theoretically calculated that this corrosion inhibitor can effectively inhibit the reaction process of the anode and cathode on the metal surface, thereby improving the corrosion inhibition efficiency. Zhao et al. [12] made thioureylimidazoline and studied its corrosion inhibition performance against CO₂ at 358 and 298 K, and the results showed that the corrosion inhibitor was a mixed corrosion inhibitor that mainly controlled the anode process and had good corrosion inhibition behavior against Q235-A at both temperatures. Wang [13] studied the synergistic effect of imidazoline corrosion inhibitors and cationic twin surfactants in anti-corrosion behavior by using the hanging piece weight-loss method, the potentiodynamic polarization method, and the electrochemical impedance method. The results showed that the mixture of imidazoline corrosion inhibitor and cationic twin surfactant had a good corrosion inhibition effect on carbon steel in oilfield-produced water. Gharbi et al. [14] prepared an FYR-CO₂ corrosion and scale inhibitor (composition: 2-alkyl-1-hydroxyethyl-3-hydroxypropyl imidazole sodium phosphate, alkylphenol ethoxyethylene ether, formaldehyde, metropine, FC-N01 fluorocarbon surfactant, and sodium hexametaphosphate). The indoor evaluation found that the corrosion inhibition and scale inhibition rates were more than 90%. After field application, the iron ion reduction was up to 99.8%, and the corrosion rate was also significantly reduced, with an average corrosion inhibition rate of 87.0%. Chauhan et al. [15] developed a GTH solid corrosion inhibitor with alkynyl methylamine and its quaternary amine salt as the main corrosion inhibitor, amimidazoline as the compound corrosion inhibitor, and a variety of auxiliary components and processing aids. Based on the daily fluid production of the well, when the corrosion inhibitor concentration was maintained at 13 mg/L, the corrosion inhibition rate of the produced water was 83% and the corrosion rate was 0.023 mm/a. This anti-corrosion method was applied to 35 wells in the Gudao Oilfield, and the pump inspection interval of 32 wells was extended from four months to more than eight months.

The method of adding corrosion inhibitors to protect oil pipelines and equipment from corrosion is widely used due to its low cost, simple and convenient operation, etc. [16]. As an important oil well casing material, N80 steel is widely used in oilfield development. However, due to the complexity of the oil well environment and the presence of high temperature and high-pressure conditions, N80 steel is susceptible to corrosion damage in the simulated solution occlusion zone, resulting in a shortened life of the steel [17]. Therefore, it is of great theoretical and practical significance to study the corrosion behavior of the imidazoline quaternary ammonium salt corrosion inhibitor on N80 steel in the simulated solution occlusion zone, so as to improve the corrosion resistance of N80 steel and prolong its service life [18]. As a high-efficiency adsorption corrosion inhibitor, imidazoline derivatives contain imidazole rings and quaternary ammonium salt groups in their molecular structure, which can form a dense protective film on the metal surface and block the further erosion of the metal by the corrosive medium. They have the advantages of a high corrosion inhibition rate, a good corrosion inhibition effect, and a low effective dosage, and are often used as acidizing corrosion inhibitors in petroleum exploitation [19,20]. It has been reported that imidazoline corrosion inhibitors can effectively inhibit the uniform corrosion of metals, but the corrosion inhibition effect of such corrosion inhibitors on local metal corrosion has been rarely reported. Therefore, in view of this situation, imidazoline was used as a corrosion inhibitor to study the corrosion inhibition of the local corrosion of N80 in a Na₂S + NaCl solution system by simulating the blocked cell formed by small holes. Combined with chemical analysis methods and electrochemical analysis techniques, the corrosion behavior of tubing steel N80 in the corrosion solution of different concentrations of imidazoline corrosion inhibitor was studied, which provided a reference for the small-hole corrosion protection of pipelines in oil exploitation.

2. Materials and Methods

2.1. Specimen and Corrosion Medium Preparation

N80 steel (mass fraction, %, C 0.42, Si 0.24, Mn 1.55, P 0.012, S 0.004, Cr 0.051, Mo 0.18, V 0.005, Ni 0.034) was applied as the experimental material. A mixture of 2% Na₂S solution with a 5%, 8%, and 10% NaCl solution, respectively, was used as the corrosion medium in this investigation. The corrosion media used in this experiment were divided into two groups: one group was the above-mixed solution, and the other group was the mixed solution with the imidazoline corrosion inhibitor containing 0.2%, 0.5%, and 0.8% in mass fraction.

2.2. Simulated Occluded Cell for Corrosion Experiment

The test work is divided into two parts. The first part is the simulation experiment toward pitting corrosion using the self-designed occluded cell in Figure 1. Figure 1 shows the cell used in this simulation experiment, and the details of the cell are referred to in our previous work [21]. The processing size of the anode sample used for the corrosion test in the simulated block area is 10 mm × 50 mm × 5 mm, and the surface is polished with 1000# abrasive paper step by step until it is smooth. The polished sample was washed with distilled water and absolute ethanol and then dried at room temperature. One end of the sample is connected to an electrical wire, and the rest of the sample is sealed with epoxy resin which is immersed into the test solution with an exposed working surface of 1 cm². The outside cathode sample is prepared in the same way. Note that the exposed area ratio of the specimens inside and outside the occluded cell is about 1:70. In the evaluation experiment when the current density is 1 mA/cm² and the duration is 8 h, the corrosion inhibition performance of the imidazoline-type quaternary ammonium salt corrosion inhibitor is better, and the reaction degree of the corrosion inhibitor in solution is more adequate. After setting up the cell device, the anode polarization current with a DC current of 1 mA/cm² for simulating a small pit was applied and the simulated polarization reaction was continued for 8 h at 50 °C. The second part is the electrochemical tests. The polarization curve and AC impedance were conducted on the solution inside/outside the occlusion system using a CS350 electrochemical workstation (Wuhan Koster Instrument Co., Ltd., Wuhan, China). At the same time, the Cl⁻ concentration was calibrated using continuous potentiometry with a standard AgNO₃ solution according to the standard GB/T15453-2018 [22]. According to GB/T223.68-1997 [23], the S²⁻ concentration was obtained by the titration of a standard Na₂S₂O₃ solution.

The corrosion inhibition rate was calculated using the polarization curve test by the following formula:

$$\eta =\frac{(I_{corr}-{I^{\prime }}_{corr})}{I_{corr}}\times 100\%$$

(1)

• η—Corrosion inhibition efficiency, %;
• I_corr—Corrosion current density in blank solution, μA·cm⁻²;
• I′_corr—Corrosion current density after addition of corrosion inhibitor, μA·cm⁻².

The corrosion inhibition rate was calculated using the AC impedance method by the following formula to calculate:

$$\eta =\frac{(R_p-R_p^0)}{R_p}\times 100\%$$

(2)

• η—Corrosion inhibition efficiency, %;
• R_p—Polarization resistance after addition of corrosion inhibitor, Ω·cm⁻²;
• R_p⁰—Polarization resistance in blank solution, Ω·cm⁻².

3. Results and Discussion

3.1. The Solution State Inside and Outside the Occluded Area without Adding Corrosion Inhibitor

Table 1. Solution composition inside and outside the occluded zone after simulating occluded cell corrosion of N80 steel in Na₂S + NaCl solution systems.

Anodic Current Density (mA·cm−2)	Solution System	Solution Position	Cl− Concentration (mol·L−1)	S2− Concentration (mol·L−1)	pH
I = 1	2% Na2S + 5% NaCl	inside	1.2411	1.1440	4.66
		outside	1.1081	0.9948	13.05
	2% Na2S + 8% NaCl	inside	1.3981	1.3206	4.82
		outside	1.2388	1.1005	12.28
	2% Na2S + 10% NaCl	inside	1.5708	1.3448	5.95
		outside	1.1501	1.1023	12.92

shows the pH value and the Cl⁻ and S²⁻ concentration changes of the solutions inside and outside the block area after 8 h of anodic polarization with N80 steel at 50 °C in the 2% Na₂S + 5% NaCl, 2% Na₂S + 8% NaCl and 2% Na₂S + 10% NaCl corrosion solutions.

As can be seen from Table 1, in the solution systems of 2% Na₂S + 5% NaCl, 2% Na₂S + 8% NaCl, and 2% Na₂S + 10% NaCl, the concentration of Cl⁻ and S²⁻ in the occluded area increased due to the ion enrichment of the occluded solutions, and concentration phenomena occurred. The concentration enrichment times of Cl⁻ concentration in the occluded area were 1.12, 1.13, and 1.37, respectively. The enrichment concentration times of S²⁻ concentration in the occluded area were 1.15, 1.20, and 1.22 times, respectively. Moreover, the pH values of the solutions in the occluded area decreased sharply, from 13.05 to 4.66, 12.28 to 4.82, and 12.92 to 5.95, respectively, indicating the acidification of the solutions. These phenomena occurred for the following reasons: When access is gained to a simulated occluded cell specimen, the anode current is imposed on the simulated occluded cell specimen under the rigorous exposed area ratio (1:70) and increases due to the internal and external specimen exposed area ratio of 1:70, thus, small anode corrosion in a high concentration cell occurs. With the increase in the dissolution of the metal in the occlusion area, the concentration of the metal cation concentration increases [24]. The following hydrolysis, and high concentrations of the metal cation hydrolysis in the block area, result in a high concentration of H⁺, resulting in a sharp decrease in the pH of the solution. Due to the increase in positive charge in the occlusion zone, the electropositivity of the solution in the occlusion area increases. In order to maintain the electroneutrality of the solution, Cl⁻ and S²⁻ in the external solution migrate to the occluded area, and concentration anion enrichment occurs [25] Generally speaking, chloride ions increase the corrosion rate of metals because chloride ions increase the conductivity of the solution, increase the activity of H+ in the solution, enhance the conductivity, prevent the formation of dense FeS₂, and accelerate corrosion. However, when chloride concentrations are high, metal corrosion slows down. The reason is that chloride ions have a strong adsorption capacity, which adsorbs onto a large amount of the metal surface, completely replacing the H₂S and HS⁻ adsorbed on the metal surface, so the corrosion is slowed down.

3.2. Changes of Cl⁻, S²⁻ Concentration and pH Value in the Occluded Area with Different Amounts of Corrosion Inhibitors

Table 2. Solution composition of the occluded area in Na₂S+NaCl solution systems with different concentrations of corrosion inhibitors.

Current (mA·cm−2)	Solution System	Inhibitor Concentration (wt%)	Cl− Concentration (mol·L−1)	S2− Concentration (mol·L−1)	pH
I = 1	2% Na2S + 5% NaCl	0	1.2411	1.1440	4.66
		0.2	0.9967	0.7768	5.09
		0.5	0.9546	0.7533	5.38
		0.8	0.9022	0.7380	5.83
	2% Na2S + 8% NaCl	0	1.3981	1.3206	4.82
		0.2	1.2420	0.8806	5.23
		0.5	1.2100	0.8442	5.46
		0.8	1.1500	0.8311	6.08
	2% Na2S + 10% NaCl	0	1.5708	1.3448	5.95
		0.2	1.4057	0.9067	6.21
		0.5	1.3655	0.8956	6.42
		0.8	1.3430	0.8812	7.05

shows the changes in Cl⁻, S²⁻, and pH values in the block area under the conditions of an anode polarization current of 1 mA·cm⁻² and a polarization temperature of 50 °C after adding imidazoline corrosion inhibitors of different concentrations to the solution systems of 2% Na₂S + 5% NaCl, 2% Na₂S + 8% NaCl, and 2% Na₂S + 10% NaCl, respectively. As can be seen from Table 2, the addition of the imidazoline corrosion inhibitor changed the Cl⁻ concentration, S²⁻ concentration, and pH value in the occlusion area when compared with the solution without a corrosion inhibitor.

Figure 2 and Figure 3 show the concentration change trend of Cl⁻ and S²⁻ in the occluded area after polarization with the addition of different concentrations of corrosion inhibitors, respectively. The 5%, 8%, and 10% in the graphs represent 2% Na₂S + 5% NaCl, 2% Na₂S + 8% NaCl, and 2% Na₂S + 10% NaCl, respectively. In the three solution systems, it can be seen that the Cl⁻ and S²⁻ concentrations in the occlusion zone gradually decrease with the increase of the concentration of corrosion inhibitor, indicating that the driving force of the anion migration from the solution outside the occluded area decreases. This means that the electropositivity of the solution decreases. The electropositivity of the solution is derived from the hydrated cations produced by the dissolution of metals, indicating that the corrosion dissolution of the inner specimen metal is reduced, and thus the electromigration of Cl⁻ and S²⁻ is reduced. It can also be seen from Figure 2 and Figure 3 that, in the three solution systems, the concentrations of Cl⁻ and S²⁻ in the occluded area decrease rapidly after the addition of 0.2 wt% imidazoline, and then the trend of the concentration reduction of Cl⁻ and S²⁻ slows down as the concentration of the corrosion inhibitor increases.

The special chemical conditions in the occlusion area make the pH of the solution significantly lower than that of the external solution (Table 1). In the study of the corrosion inhibition performance of the imidazoline corrosion inhibitor on the solution, the metal corrosion reaction process of the steel sheet as an anode is usually as follows: Fe → Fe²⁺ + 2e⁻. The H⁺ produced by the hydrolysis of Fe²⁺ will enhance the acidity of the solution. Thus, the acidity of the blocked area will increase. This will further accelerate the metal dissolution and decrease the pH value [26].

As shown in Figure 4, after the addition of a certain concentration of imidazoline to the bulk solution, the pH values of the solution in the occluded area increase in comparison to that of the solution without the inhibitor. Additionally, the pH values gradually increase with the increase in the inhibitor concentration, indicating that the addition of a corrosion inhibitor to bulk solutions can prevent the acidification of the solution in the occluded area. Thus, the corrosion of the metal in the occluded area is slowed down and the metal in the occluded zone is protected.

3.3. Polarization Curve Analysis

Using the simulated occluded cell device, the simulated occlusion solution obtained after 8 h of occlusion cell corrosion in three solution systems with the addition of different concentrations of corrosion inhibitors is taken out. The polarization curve is then obtained for N80 petroleum-specific steel at 50 °C in the as-prepared occluded solution, as shown in Figure 5. The kinetic parameters, such as the anodic polarization dynamics constant βa, the cathodic polarization kinetic constant βc, the corrosion current I0, the self-corrosion potential E0, and the corrosion rate, are fitted on these polarization curves. The corresponding corrosion inhibition rates are calculated and shown in

Table 3. Electrochemical parameters fitted from the polarization curves.

Solution System	Temperature (°C)	Inhibitor Concentration (wt%)	βa (mV)	βc (mV)	E0 (mV)	I0 (A/cm2)	Inhibition Efficiency η
2% Na2S + 5% NaCl	50	0	191.56	32.73	−7.885 × 102	2.56 × 10−5	---
		0.2	322.43	35.88	−7.819 × 102	8.85 × 10−6	76%
		0.5	119.69	126.94	−6.223 × 102	5.78 × 10−6	88%
		0.8	326.02	232.31	−6.133 × 102	5.33 × 10−6	93%
2% Na2S + 8% NaCl	50	0	196.65	36.531	−8.875 × 102	1.24 × 10−5	---
		0.2	490.59	28.46	−8.845 × 102	4.92 × 10−6	60%
		0.5	255.08	31.38	−8.277 × 102	4.62 × 10−6	63%
		0.8	160.34	71.63	−7.736 × 102	1.94 × 10−6	84%
2% Na2S + 10% NaCl	50	0	213.04	65.87	−8.656 × 102	8.13 × 10−5	---
		0.2	112.07	67.40	−7.783 × 102	1.94 × 10−5	65%
		0.5	175.60	118.81	−7.276 × 102	9.80 × 10−6	77%
		0.8	181.48	111.04	−6.829 × 102	5.54 × 10−6	79%

.

It can be seen from Figure 5 that, in the three solution systems with the addition of a corrosion inhibitor, the corrosion rate of N80 steel in the simulated occlusion area decreased significantly. Among them, the highest corrosion inhibition efficiency was achieved when adding 0.8 wt% imidazoline corrosion inhibitor to the 2% Na₂S + 5% NaCl solution system. The corrosion inhibition efficiency of the inhibitor to the occlusion area reached 93%. In the 2% Na₂S + 8% NaCl and 2% Na₂S + 10% NaCl solutions, the corrosion inhibition efficiencies of the corrosion inhibitors to the occluded area were calculated to be 84% and 79%, respectively.

With the addition of the corrosion inhibitor, the self-corrosion potential was positively shifted. More importantly, the cathodic polarization kinetic constant βc was smaller than the anodic polarization kinetic constant βa, indicating that the anodic reaction was the control step in this electrochemical reaction [27]. Both βc and βa showed an increasing trend with the increase in the amount of inhibitor, suggesting that the addition of the inhibitor had a certain inhibitory effect on both the anode and cathode reaction processes, while the inhibitory effect on the anodic reaction was relatively greater. This is also the reason for the positive shift in self-corrosion with the increase in the amount of the inhibitor [28].

3.4. Electrochemical Impedance Spectroscopy Analysis

Figure 6 shows the electrochemical impedance spectra (EIS) obtained in the simulated block solution after the simulated blocked cell corrosion at 50 °C in the three solution systems. As can be seen from Figure 6, the Nyquist plots mainly present arcs on the real axis in all cases, which deviate from the semicircles. The deformed semicircles are called “dispersion effects” [29] and are related to the inhomogeneity of the measured electrode surface, the adsorption layer on the electrode surface, and the difference in the conductivity of the solution.

The EIS with dispersion effect can be equated to a circuit model with the interface capacitor, in which the C_dl is connected in parallel with the polarization resistor Rp and then connected in series with the solution resistor Rs, as shown in Figure 7. The EIS parameters fitted by the equivalent circuit are shown in

Table 4. Electrochemical parameters fitted from EIS by an equivalent circuit.

Solution System	Temperature (°C)	Corrosion Inhibitor Concentration (wt%)	Cdl	Rp	Inhibition Efficiency η
2% Na2S + 5% NaCl	50	0%	9.41 × 10−4	408	---
		0.2%	4.40 × 10−4	832	51%
		0.5%	3.44 × 10−4	1158	65%
		0.8%	1.57 × 10−4	1615	75%
2% Na2S + 8% NaCl	50	0%	2.81 × 10−4	483	---
		0.2%	3.31 × 10−4	943	49%
		0.5%	7.28 × 10−4	993	51%
		0.8%	8.02 × 10−3	2101	77%
2% Na2S + 10% NaCl	50	0%	1.96 × 10−3	140	---
		0.2%	3.04 × 10−4	262	47%
		0.5%	2.17 × 10−2	332	58%
		0.8%	1.82 × 10−3	594	76%

. The inhibition efficiency obtained by the EIS method is calculated in Formula (2).

It can be seen in Figure 6 that, in three kinds of solutions, the impedance shape of the blocked area is close to the spectra of that without the corrosion inhibitor. With the increase in the concentration of the corrosion inhibitor, the impedance spectrum shape does not change, but the diameter of the fitting semicircle corresponding to the different concentrations of corrosion inhibitor in the impedance map increases. At the same time, the double-layer capacitance C_dl decreases, indicating that, after the addition of the imidazoline corrosion inhibitor to the native solution, the imidazoline corrosion inhibitor diffuses from the main solution into the occluded area and is adsorbed on the surface of the steel. The inhibitor molecules gradually replace the water molecules and the corrosive anions Cl⁻ and S²⁻ are adsorbed on the surface of the carbon steel to form a protective film, thus increasing the polarization resistance Rp and decreasing the double-layer capacitance.

As can be seen from Table 4, in the three solution systems, with the increase in the concentration of the corrosion inhibitor in the bulk solution, the polarization resistance of the corrosion system increases, and the corresponding inhibition rate of the corrosion inhibitor to the occlusion area gradually increases. With the increase in the Cl⁻ concentration in the systems the corrosion inhibitor still maintains a good corrosion inhibition effect on the occlusion area. In the 2% Na₂S + 5% NaCl system, the inhibition efficiency of the occlusion area can reach 75% with the addition of 0.8 wt% imidazoline to the bulk solution [30,31]. The corrosion inhibition efficiency of the occluded area can reach 77% with the addition of 0.8 wt% imidazoline to the bulk solution in the 2% Na₂S + 8% NaCl system. Additionally, 76% of the corrosion inhibition efficiency could still be reached in the 2% Na₂S + 10% NaCl system.

4. Conclusions

In this paper, imidazoline was used as a corrosion inhibitor, and the corrosion inhibition of this inhibitor on the local corrosion of petroleum special pipeline steel N80 in a Na₂S + NaCl solution system was studied using simulated occluded cells. The corrosion behavior of the oil pipeline steel N80 in the occluded corrosion solution with different concentrations of the imidazoline corrosion inhibitor was analyzed by combining chemical analysis methods and electrochemical analysis techniques. The following conclusions were obtained:

(1)

In the three solution systems, after the anode polarization of the occluded cell, the solution in the occluded area is acidified, and the pH value decreases sharply; the migration of Cl⁻ and S²⁻ is enhanced and enriched in the occluded area.

(2)

An imidazoline quaternary ammonium salt corrosion inhibitor can reduce the migration of small-radius anions (Cl⁻ and S²⁻) to the occluded area and inhibit the acidification of the solution in the occluded area simultaneously. As a result, the dissolution of metals in the occluded area is prevented, owing to the decrease in the corrosivity of the solution in the occluded area as a result of the change in the chemical state of the occluded area.

(3)

In the three corrosion solution systems of 2% Na₂S + 5% NaCl, 2% Na₂S + 8% NaCl, and 2% Na₂S + 10% NaCl, the imidazoline quaternary ammonium salt corrosion inhibitor can form an adsorption film layer on the metal surface to increase the polarization resistance and reduce the corrosion rate. The addition of a corrosion inhibitor can significantly increase the anode polarization kinetic constant, which can effectively inhibit the local corrosion of N80 steel in these corrosion systems.