Studies on the inhibitory effect of different surfactants on ammonium chloride corrosion

Deposition of ammonium salts and associated corrosion occur widely in refining equipment. To prevent clogging by these salts, water is often injected to dissolve them. However, if an insufficient amount of water is injected, a strong corrosive solution can form and cause perforation and leakage of the equipment.¹ Corrosion caused by ammonium chloride (NH₄Cl) solution is related to many factors. For example, hydrogen sulfide (H₂S) in the system can affect the reaction kinetics of the corrosion of carbon steel in NH₄Cl solution,² and the H₂S concentration can affect the anode reaction and accelerate the corrosion rate. At 10 ppm H₂S, a dense passivation film can form on carbon steel, which will prevent its corrosion and dissolution in NH₄Cl solution. With increasing H₂S concentration, the passivation film gradually loses its integrity, which leads to intensification of the electrochemical reaction.³ The pH of the NH₄Cl solution affects the corrosion rate by changing the ability of Cl⁻ to penetrate the passivation film.⁴ In addition, the flow rate also has a significant effect on corrosion. At higher flow rates, the material is subjected not only to electrochemical corrosion, but also to fluid erosion.⁵ The concentration of NH₄Cl solution has different effects on the corrosion of different materials, and the degree of corrosion of different materials at the same concentration varies.^6,7 It has been found that ultraviolet irradiation can affect the deposition of NH₄Cl, accelerating the anodic reaction of corrosion products with semiconductor properties, giving the material surface a larger carrier density and higher conductivity, improving the transmission ability of electrons in the rust layer, and thus accelerating the corrosion rate.⁸ The corrosivity of NH₄Cl solution is related to the acidity of the solution after hydrolysis. To reduce the rate and degree of corrosion by acid solutions, corrosion inhibitors are often injected.^9–12 Generally, a given corrosion inhibitor has an optimum concentration and temperature that maximizes its effectiveness.¹³ The corrosion inhibitor cetylpyridinium chloride has a good inhibitory effect against the corrosion of carbon steel in chlorine-containing solutions. Adding the synergist Zn²⁺ to the corrosion inhibitor can effectively suppress the anode reaction and slow the corrosion of carbon steel.¹⁴ Corrosion inhibitor solutions to which nonionic surfactants have been added have a good inhibitory effect on the corrosion of carbon steel in HCl solution.¹⁵ The inhibitory effect of amphoteric inhibitors on the corrosion of carbon steel in an HCl medium is related to their adsorption capacity on the liquid–solid surface, and the immersion time of the carbon steel in the solution and the temperature of the solution affect the inhibitory effect.¹⁶ Quaternary ammonium cationic surfactants have a good inhibitory effect on the corrosion of materials in alkaline media, and this effect is related to the length of the alkyl chain. As the alkyl chain becomes longer, the number of active molecules adsorbed on the solid surface decreases, which reduces the effectiveness of inhibition.^17,18 The addition of positively charged nanoparticles to a medium containing an anionic surfactant not only increases the critical micelle concentration of the surfactant, but also reduces the interactions among the molecules of the active agent. This enables the formation of a denser adsorption film on the metal surface and thus improves corrosion inhibition.¹⁹ 

A number of studies of the effects of concentration, pH, flow rate, and dissolved substances on corrosion by NH₄Cl solutions have been carried out. However, there have been few studies focusing on the inhibitory effects of different types of surfactants on NH₄Cl solution corrosion. Therefore, in this paper, wetting experiments with NH₄Cl solutions and electrochemical measurements are conducted to investigate the effects of different surfactants at different temperature and concentrations on NH₄Cl corrosion.

In this work, a cuboid of No. 20 carbon steel with dimension 15 × 15 × 2 mm³ was selected for the contact angle measurements and a cylinder with diameter φ = 4 mm for the electrochemical measurements. The carbon steel had a chemical composition of 0.21% C, 0.58% Mn, 0.012% P, 0.008% S, 0.22% Si, 0.01% Ni, 0.01% Cr, 0.01% Cu, with the balance being Fe. Before the experiment, the surface of the carbon steel was polished with silicon carbide abrasive paper and then cleaned with alcohol and deionized water. To ensure the accuracy of the measurements, strict cleanliness and smoothness of the material surface were maintained. The five solutions tested all contained 5 wt. % NH₄Cl, either without surfactant (solution a) or with 2 wt. % surfactant: dodecyl dimethyl benzyl ammonium chloride (1227, solution b), alkyl polyglycoside (APG, solution c), dodecyl dimethyl betaine (BS12, solution d), and secondary alkyl sodium sulfonate (SAS, solution e), All materials used in the experiments were obtained from the Changqing Petrochemical Branch Company. A droplet of volume 2 µl was dripped from the top of the material surface, and the contact angle (as shown in Fig. 1) was measured using a video contact angle tensiometer (Kruss DSA100, with an accuracy of 0.01 mN/m). The surface tension was measured by the hanging drop method, and the video contact angle tensiometer was calibrated with deionized water. The surface tension of the water at room temperature was 72.8 mN/m, and the measurements commenced after the instrument had been calibrated.

The electrochemical measurements were conducted with a three-electrode system using an electrochemical workstation (Princeton VersaSTAT MC, with an accuracy 0.2% of reading). The working electrode was a No. 20 carbon steel rod with a diameter of 4 mm. The outside of the rod was sealed with polytetrafluoroethylene, and only the cross section was in direct contact with the solution. The platinum sheet with an area of 1 cm² was used as the counter-electrode, and Ag, AgCl/(3.5M KCl) was used as the reference electrode. The potential scan range measured by the polarization curve was −250 to 250 mV, and the scan rate was 0.17 mV/s. Electrochemical impedance measurements were carried out by the potentiostat method, with a frequency setting range of 10⁻² to 10⁵ Hz and an amplitude of 5 mV.

To study the wettability of the test material by the five test solutions, the contact angles on the No. 20 carbon steel were measured using the video contact angle tensiometer as shown in Fig. 2, and the values of the contact angle and surface tension are listed in Table I. From Fig. 2, it can be seen that the NH₄Cl solution without surfactant has the largest contact angle of α = 98.3° which indicates that it does not wet the surface of the test material, and the material surface is hydrophobic. After the addition of surfactant, the contact angle between the solution and the surface of the test material is significantly reduced, and the droplets spread more easily on the surface of the test material. The smallest contact angle of the five solutions on the surface of the test material is that of solution e (containing SAS), α = 12.7°.

The contact angles of the NH₄Cl solutions containing an ionic or amphoteric surfactant are significantly different from those of the other solutions. The reason is that the molecular distances in the NH₄Cl solution without surfactants covering the material surface are relatively short, and the liquid exerts a stronger attractive force on the molecules over the material surface. Thus, both the contact angle and surface tension between the solution and the test material are larger. When an ionic or amphoteric surfactant is added to the solution, it will ionize and form charged surfactant ionic groups. The surfactant ionic groups are tightly combined with the ions in the solution and reduce the electrostatic repulsion between adsorbed molecules. Surfactants are closely packed and arranged at the interface, and thus the contact angle between the solution and the test material is smaller.²⁰ Moreover, the movement of molecules from the material surface into the liquid is weakened, and the number of molecules on the material surface is increased. The effective distance between molecules is decreases, the attraction between molecules is weakened, and the surface tension is reduced.²¹ 

The polarization curves of No. 20 carbon steel in the NH₄Cl solutions with different surfactants are shown in Fig. 3, and the polarization parameters are listed in Table II. It can be seen from the figure that the corrosion potential in an NH₄Cl solution containing a cationic or amphoteric surfactant is shifted in the negative direction. The corrosion current density is larger than that in an NH₄Cl solution without surfactant, and the corrosion tendency becomes greater. By contrast, the corrosion potential in an NH₄Cl solution containing a nonionic or anionic surfactant is shifted in the positive direction, and the self-corrosion potential of the solution becomes larger. The corrosion current density is lower than that in an NH₄Cl solution without surfactant, and the corrosion tendency is slowed down. The reason is that the surface-active group of a cationic surfactant contains a positive charge. However, the anion in an amphoteric surfactant molecule is a carboxyl group, and the cation is a quaternary ammonium group. In an acidic medium, it appears as a cationic surfactant ([RN⁺(CH₃)₂CH₂COOH]Cl⁻), which is soluble in water. This acts in the same way as the electrical charge of the active component, H⁺, of the corrosive medium. The offsetting effect in the electric double layer around the anode is weak, and the adsorption of the surfactant on the anode surface is hindered. A nonionic surfactant like APG does not dissociate in solution and is not affected by the electrolytes in the solution. It adsorbs on the anode surface and forms a protective film to prevent oxidation of the anode. In addition, the pH of APG is between 11.5 and 12.5, which neutralizes the H⁺ in the solution and inhibits the electron-withdrawing reaction of the cathode. Therefore, it inhibits corrosion, and the corrosion current density is decreased. Anionic surfactants like SAS have surface-active groups with negative charges and attract positive charges in the electric double layer, as shown by the green polarization curve in Fig. 3. The mass transfer of cathode H⁺ is hindered, which affects the polarization of hydrogen removal and slows down the degree of hydrogen evolution corrosion. Thus, SAS has a certain inhibitory effect on corrosion of No. 20 carbon steel in NH₄Cl solution.

Figure 4 presents electrochemical impedance spectroscopy (EIS) results in the form of Nyquist plots of the electrochemical impedance of No. 20 carbon steel in NH₄Cl solutions containing different surfactants, and the corresponding equivalent circuit is shown in Fig. 5, where R_s is the internal resistance of the solution, R_ct is the charge transfer resistance, and C_d is the electric double-layer capacitance. It can be seen that the complex impedance plane for the different types of surfactants is composed of a single capacitive reactance arc with only one time constant, and the NH₄Cl solutions containing different types of surfactants have different capacitive reactance arc radii. Figure 4 shows that in a solution containing an ionic surfactant charged groups are produced by ionization in water, and the capacitive resistance arc radius is small. The electron transfer process on the surface of the electrode is subject to less impedance. The anode loses electrons and is prone to oxidation reactions, and the electrode is prone to corrosion. From the values of the EIS parameters in Table III, it can be seen that a solution containing a nonionic or amphoteric surfactant has a larger capacitive reactance arc radius, which indicates that the charge transfer resistance R_ct is larger, the electron transfer resistance is stronger, and the electric double-layer capacitance C_d is reduced. This shows that the adsorption of surface-active groups on the surface of the electrode will hinder charge transfer in the electric double layer, thereby increasing the polarization resistance and inhibiting the oxidation reaction at the anode and thus the corrosion of the electrode. These EIS results for different types of surfactants show that nonionic surfactants provide the best inhibition of corrosion.

On the basis of the above analysis of the effects of different types of surfactants on corrosion inhibition, different concentrations of the nonionic surfactant APG were selected for quantitative analysis: 5, 50, 250 ppm, and 0.1% (i.e., 1000 ppm). The polarization curves obtained by electrochemical measurements are shown in Fig. 6, and the related polarization parameters are listed in Table IV.

Four groups of measurements were carried out. It can be seen from Fig. 6 that the corrosion current density does not decrease with increasing concentration. The self-corrosion potential is largest and the corrosion current density smallest for an APG content of 5 ppm. This surfactant has an amphiphilic structure. When the concentration of the surfactant is too high, its physical and chemical properties change in such a way that the surface-active groups cannot effectively come into contact with water molecules, and so they no longer play an effective role. Moreover, when the concentration of surfactant is increased, the adsorption of surface-active groups on the electrode surface also tends to become saturated. With increasing concentration, the distances between the surface-active groups in the electric double layer are decreased. The adsorption efficiency on the electrode surface is reduced, and so the inhibition of the anode reaction is weakened.

As shown in Fig. 7, the corrosion resistances of NH₄Cl solutions containing different concentrations of APG are significantly different. When the APG content is 5 ppm, the charge transfer resistance R_ct is larger than for the other concentrations, and the electron transfer process on the electrode surface is subject to a greater resistance, and thus the corrosion inhibition is the most effective. When the concentration of APG is increased from 50 ppm to 0.1%, the charge transfer resistance gradually increases, and the corrosion inhibition is enhanced, as can be seen in Table V. At these three concentrations (50 ppm, 250 ppm, and 0.1%), the adsorption efficiency of the surfactant from the bulk solution to the electrode surface does not reach a maximum, and so the corrosion inhibition is slightly poorer than for the 5 ppm concentration (Table VI).

The properties of surfactants are affected by temperature, and therefore, to determine the influence of temperature on the corrosion inhibition performance of NH₄Cl solutions containing different concentrations of APG, electrochemical measurements were carried out at 40 and 90 °C. The polarization curves are shown in Fig. 8, and the corresponding polarization parameters are listed in Table IV. At both 40 and 90 °C, the self-corrosion potential of the NH₄Cl solution containing 0.1% APG is the highest, indicating that solutions containing a high concentration of surfactants have a lower corrosion tendency. At a given concentration of APG, the corrosion current density increases as the temperature rises. The reason is that with increasing temperature, the solubility of the surfactant becomes poorer and its adsorption capacity is decreased. At both temperatures, the corrosion current density of NH₄Cl solution containing 5 ppm APG is the smallest, indicating that this concentration provides the maximum adsorption efficiency on the anode surface and the strongest film formation effect, thereby effectively inhibiting the anode reaction and thus corrosion of the electrode. At the higher temperature, the mass transfer rate of the surfactant between the bulk solution and the electric double layer is increased, and adsorption and desorption occur frequently, causing fluctuations in the inhibition of the redox reaction. Therefore, at 90 °C, the polarization curve appears jagged, and the corrosion current density increases rapidly, and therefore there is a sharp reduction in corrosion inhibition.

EIS results for NH₄Cl solutions containing different concentrations of APG at different temperatures are shown in Fig. 9. It can be seen that charge control occurs mainly in the high-frequency region, which affects the oxidation reaction at the anode and the polarization of the cathode by blocking electron transfer. In the low-frequency region, temperature changes affect the mass transfer of surface-active groups in the solution. Therefore, control is mainly through diffusion. At different temperatures, as shown in Table VII, the NH₄Cl solution with 5 ppm APG has the largest charge transfer resistance and the best corrosion inhibition effect. As the temperature increases, the charge transfer resistance of a given solution decreases, and the corrosion tendency of the solution is increased. The solubility of APG is affected by temperature. When the temperature increases, the solubility becomes poorer. APG is dissolved as a result of hydrogen bonding, and hydrogen bonds are broken in a higher-temperature solution. As the solubility of APG decreases, its adsorption on the anode surface is weakened. Besides this, when the temperature is higher, hydrolysis in the NH₄Cl solution progresses, and the H⁺ content of the solution is increased, resulting in rapid changes in mass transfer in the electric double layer, and the corrosiveness of the solution is increased. As the solubility of APG decreases, its degree of binding with H⁺ is reduced and its adsorption efficiency is decreased. Thus, the degree of inhibition of the oxidation reaction at the anode is weakened, and the possibility of corrosion of the electrode is increased.

A series of experiments have been performed to analyze the wettability and inhibitory effects of different types of surfactants on corrosion by ammonium chloride solutions. The following conclusions can be drawn:

1. Nonionic surfactants can achieve corrosion inhibition by reducing the contact angle between the solution and the material and increasing the wettability to form a film on the surface of the material and thereby inhibit anodic reaction.

2. The smallest corrosion current density and most effective inhibition of corrosion are achieved with a 5 ppm content of the nonionic surfactant APG in an NH₄Cl solution.

3. As the temperature increases, the corrosion inhibition by APG becomes poorer. APG at low concentrations has a better corrosion inhibition performance in different temperature ranges.

We gratefully acknowledge the support of the National Natural Science Foundation of China (Grant Nos. 52176048, 51806198, and U1909216).

The authors have no conflicts to disclose.

Dexiao Fu: Data curation (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Xishui Yu: Data curation (equal); Resources (equal). Xin Huang: Data curation (equal); Writing – review & editing (equal). Guofu Ou: Conceptualization (equal); Funding acquisition (equal). Tongzao Zhou: Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Zhenqian He: Conceptualization (equal); Data curation (equal); Software (equal).

The authors confirm that the data supporting the findings of this study are available within the article.