Corrosion Protection of N80 Steel in Hydrochloric Acid Medium Using Mixed C₁₅H₁₅NO and Na₂WO₄ Inhibitors

Abstract

A novel inhibitor based on mixed Mannich base (C₁₅H₁₅NO) and Na₂WO₄ was developed for the corrosion prevention of N80 steel in hydrochloric acid solution. Infra-red spectrum, electrochemical measurements, X-ray Photoelectron Spectroscopy, and Scanning Electron Microscopy were used to understand the inhibition efficiency and mechanism. The results showed that the mixed inhibitors reduced the corrosion current density and increased the interface resistance. The inhibition efficiency is the highest when the ratio of C₁₅H₁₅NO to Na₂WO₄ is 1:1 in the mixture. Observed from the surfaces, the number of pits and small cracks was reduced on the surface in the presence of the optimized inhibitors. The inhibition film can successfully hinder the chloride ions from reaching the bulk steel.

1. Introduction

N80 steel is a commonly used material for pipelines for the petroleum and natural gas industry. This material suffers from serious corrosion problems when exposed to hydrochloric acid medium during industrial processes, such as acid cleaning, acid picking, acid descaling, and oil well acidizing [1,2,3,4].

In recent years, applying inhibitors has attracted a great deal of attention because of its advantageous properties, including its environmental friendliness, high efficiency, and convenience in application. Inhibitors can be divided into two types: organic and inorganic. Most inhibitors are organic compounds containing nitrogen, oxygen, or sulfur atoms [5,6,7,8,9,10,11]. It is generally believed that the organic molecules can produce a barrier layer through adsorption at the metal-solution interface [12,13], thus hindering the transfer of electrons between the metal substrate and the corrosion solution [14]. More and more researches have revealed that a single inhibitor has a limited effect, and mixed inhibitors are able to improve the protection effectiveness. For example, Meng et al. studied the inhibitor mechanism of Mannich base (C₁₅H₁₅NO) and Thiourea in a gas-field wastewater. The results showed that a bi-layer inhibitor film with an inner layer of Thiourea molecules and an outer layer of C₁₅H₁₅NO displayed a greatly improved inhibition efficiency [15]. Shibli et al. discussed the co-inhibition characteristics of sodium tungstate (Na₂WO₄) with potassium iodate. It was concluded that the presence of an oxidizing agent like KIO₃ can enhance the inhibition of tungstate [16]. Huang et al. [17] analyzed the inhibition performance of Na₂WO₄ and sodium lauroyl sarcosine for carbon steels in seawater. They revealed that the adsorption of sodium tungstate on the surface of carbon steel followed the Langmuir adsorption mechanism, and mixed inhibitors had a better corrosion performance than Na₂WO₄ alone [17]. Although both C₁₅H₁₅NO and Na₂WO₄ have been used as corrosion inhibitors in hydrochloric acid, most of them were used alone or in combination with other substances. Until now, little attention has been devoted to the investigation of the inhibition effect of C₁₅H₁₅NO with Na₂WO₄ for N80 steel in hydrochloric acid medium.

The objective of this paper is to discuss the inhibition mechanism of mixed C₁₅H₁₅NO and Na₂WO₄ for N80 steel in an acidic medium. Optimization of the C₁₅H₁₅NO:Na₂WO₄ ratio was explored. Infra-red (IR) spectrum, electrochemical tests, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to assist in the understanding of the inhibition effectiveness and corrosion prevention mechanism.

2. Experimental Procedures

The chemical composition of N80 steel is shown in Table S1. The concentration of Na₂WO₄ (AR, ≥99.5%, Shanghai Sinopharm Group, Shanghai, China) is 0.0002 M. C₁₅H₁₅NO was synthesized based on the conventional method [18]. The ratio of the C₁₅H₁₅NO to Na₂WO₄ in the mixture was setting as 20:1, to 10:1, 2:1, 1.25:1, 1:1, and 0.67:1, respectively. Two types of specimens were used: one is the circular columns with dimensions of Φ14.5 × 5 mm² for electrochemical measurements; the other is a cuboid block with dimensions of 5 × 5 × 3 mm³ for micrographic analysis. All specimens were manually polished by metallographic emery papers with a grit size from 400 to 800, 1000, and 1200. Afterward, they were immersed in petroleum ether for 10 min, washed thoroughly with distilled water, degreased with acetone, washed again with bidistilled water, and ultimately dried at room temperature [19].

The electrochemical experiments were conducted in a nitrogen environment, using the CS350 electrochemical workstation (Wuhan CorrTest Instruments Corp., Ltd., Wuhan, China) with a three-electrode system. The sweeping rate during polarization measurements was 0.5 mV·s⁻¹, and the potential was changed from –500 to +500 mV vs. E_oc. Electrochemical Impedance Spectroscopy (EIS) measurements were carried out at the open-circuit potential for alternating voltage amplitudes of 10 mV over a frequency range of 100 kHz to 10 mHz. The IR spectrum was recorded on a Perkin Elmer FTIR (PerkinElmer, Waltham, MA, USA) pls check instrument with a resolution of 4 cm⁻¹, and the aperture was set as 2.5 mm. XPS was taken by a Shimadzu-Kratos AXIS Ultra ^DLD (Kratos Analytical Ltd., Kyoto, Japan) which uses Al Kα as the excitation source, and operated in the constant 1486.7 eV analyzer energy mode with a pass energy of 50 eV. The sputtering speed was 2.4 nm s⁻¹ and the angle between the sample and the ion gun was 30°. The accuracy of the reported binding energy is ±0.1 eV. The C 1s peak at 284.6 eV, from adventitious carbon, was used as the reference for all spectra. Quantification of the atomic layer composition and the spectral simulation of the experimentally observed peaks were performed using Thermo Avantange software (V4.88) [20]. SEM images of N80 steel samples were taken by ZEISS SIGMA (ZEISS, Oberkochen, Germany). More details about the experimental procedures are available in section S1 of the Supporting Information.

The inhibition efficiency, η is calculated by:

$$\mathsf{\eta }=\frac{I_{\mathrm{corr}\left(0\right)}-I_{\mathrm{corr}(i)}}{I_{\mathrm{corr}(0)}}$$

(1)

where I_corr(0) is the value of the corrosion current density before adding inhibitor molecules and I_corr(i) is the value of the corrosion current density after adding the inhibitor. The surface coverage (θ) is an important parameter for the evaluation of the quality of the corrosion inhibitor film, and it can be calculated by:

$$\mathsf{\theta }=1-\frac{R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}$$

(2)

where $R_{\mathrm{ct}}^0$ is the charge transfer resistance of the blank electrode and R_ct is the charge transfer resistance after adding the inhibitor to the solution.

3. Results and Discussion

3.1. IR Spectrum Analysis of C₁₅H₁₅NO

The IR spectrum of the prepared red-brown C₁₅H₁₅NO liquid is shown in Figure 1.

The characteristic absorption peak of the synthesized product at 1402.21 cm⁻¹ indicates the existence of the C–N bond. The strong characteristic absorption peak at 1688.66 cm⁻¹ convincingly demonstrates the existence of carbonyl. The N–H stretching vibration absorption peak of secondary amine emerges at 3438.98 cm⁻¹, which infers the existence of the C₁₅H₁₅NO structure in the synthesized product. The normal skeleton stretching vibration of the aromatic ring has four bands. They are respectively located at 1450, 1500, 1585, and 1600 cm⁻¹, which is one of the important signs to determine the presence of the benzene ring. Since there are peaks at 1449.83, 1508.36, 1582.49, and 1611.72 cm⁻¹ corresponding to the above four bands from 1425 to 1650 cm⁻¹, the existence of the benzene ring in the synthetic product can be confirmed. Simultaneously, the C–H stretching vibration absorption peak of the benzene ring arises at 3095.66 cm⁻¹. Therefore, the synthesized red-brown liquid is C₁₅H₁₅NO solution.

3.2. Electrochemical Analysis

3.2.1. Potentiodynamic Polarization Studies

The potentiodynamic polarization curves for the N80 steel with different inhibitors are shown in Figure 2.

The corrosion current density is one of the significant indexes used to evaluate the corrosion resistance of the material. The greater the corrosion current density is, the more serious the corrosion of the material is [21]. From the polarization curve, it can be seen that the current density decreased after the inhibitors were added. The slope change of the anodic and cathodic poles indicates that the mixed inhibitors can hinder the anodic and cathodic reactions simultaneously, i.e., the compound inhibitor is a mixed-type inhibitor.

3.2.2. EIS Studies

Nyquist and Bode plots acquired for the N80 steel electrode are shown in Figure 3.

As shown in Figure 3a, the small radius of the capacitive arc for the blank sample indicates that the resistance of the corrosion reaction is small. This is mainly due to the corrosive Cl⁻ in the corrosion solution, which can easily diffuse through the metal-solution interface and cause damage to the surface of N80 steel [22,23]. Furthermore, each impedance spectrum is not a complete semicircle, which may be owing to the dispersion effect [24]. After adding mixed inhibitors with different ratios, the capacitance arc radius becomes larger. The large capacitive arc radius indicates that the corrosion inhibitor forms a protective film on the surface of N80 steel to increase the charge transfer resistance. The capacitive arc in the high-frequency region represents the characteristics of a metal surface film [25]. What is more, the maximum radius of the capacitive arc appears when the inhibitor ratio is 1:1, which shows that the corrosion process on the surface of N80 steel has been strongly restrained in this case. As shown in Figure 3b, the |Z| versus logf plot in the medium frequency range is an oblique line with a 45° slope, and the relative maximum phase angle in the Bode plots is smaller than 90°, which indicates that the system has good capacitance characteristics [26]. The R(Q(R(QR))) equivalent circuit diagram (with minimum error) is used in this paper, and the fitting data is shown in

Table 1. Electrochemical parameter of N80 steel obtained from the EIS Equivalent electrical circuit.

Sample/mol·L–1			Rs/Ω·cm2	Cf/S·sn·cm−2/×10−5	n1/0 < n < 1	Rf /Ω·cm2	Cdl/S·sn·cm−2/*10−2	n2 /0 < n < 1	Rct/Ω·cm2	θ
HCl	C15H15NO	Na2WO4
4.865	0	0	0.6821	2.375	0.9656	4.102	0.057	0.6694	39.34	–
	0.004	0.0002	0.8403	1.833	0.8728	3.789	0.089	0.5647	102.1	0.615
	0.002	0.0002	0.7169	2.184	0.8899	3.064	0.046	0.6856	83.99	0.532
	0.0004	0.0002	0.7487	2.177	0.9937	2.901	0.011	0.5495	114.8	0.657
	0.00025	0.0002	0.8093	2.169	0.8972	3.918	0.035	0.5889	92.24	0.574
	0.0002	0.0002	0.7571	2.484	0.9844	5.217	0.034	0.6518	260.9	0.849
	0.00013	0.0002	0.8079	1.89	0.9423	5.589	0.034	0.634	139.2	0.717

.

From Table 1, the charge transfer resistance, R_ct, changes after adding different concentrations of C₁₅H₁₅NO and Na₂WO₄. The value of R_ct reflects the speed of the electrochemical reaction at the electrode solution interface [27,28]. When the metal surface is covered with the corrosion inhibitor film, the barrier effect of the film on the charge transfer changes the impedance response of the electrode-solution interface. This increase of R_ct after adding mixed inhibitors is due to the enhanced capacitive arc radius in the high-frequency area and the decrease of the double-layer capacitance. Meanwhile, since the dielectric constant of organic molecules is smaller than that of water [29], the double-layer capacitance is smaller than that of the control electrode. Therefore, the corrosion inhibitor film acts as a barrier to hinder the diffusion of Cl⁻ from the solution to the metal surface, and slows down the rate of the corrosion reaction. The data in Table 1 reflect that the charge transfer resistance is the largest when the concentration ratio of C₁₅H₁₅NO to Na₂WO₄ is 1:1 (the concentrations of C₁₅H₁₅NO and Na₂WO₄ were both 0.0002 mol·L⁻¹). The result proves that the effect of mixed inhibitors is the best at the 1:1 concentration ratio. A comparison with other kinds of inhibitors is shown in Table S3. The cyclic voltammetry of N80 steel in mixed inhibitors proved that the corrosion inhibitor film structure is quite stable at different applied potentials (Figure S1). Figures S2 and S3 show that the mixed-inhibitor can greatly improve the performance and obeys the Langmuir adsorption isotherm on the N80 steel surface in the HCl solution. Based on the calculations, the highest value of θ is 0.849. Furthermore, θ has a good consistency with η, which was obtained from the polarization curve in Table S2.

3.3. Analysis of Scanning Electron Microscopy

SEM images for N80 steel are shown in Figure 4. As indicated in Figure 4a, the morphology of the N80 steel sample without the inhibitors was damaged by corrosion with pits, cracks, and corrosion products observed on the surface of N80 steel. When separately applied, the inhibitor film of C₁₅H₁₅NO was flaky without full coverage (Figure 4b), and the inhibitor film of Na₂WO₄ was layered and unevenly distributed (Figure 4c). However, the morphology of the N80 steel specimen in the mixed inhibitors was greatly improved. Figure 4d demonstrates a much improved corrosion resistance when the mixed inhibitors were used. Many particles appeared on the much smaller flaky structure, and the small cracks disappeared although there were still few pits on the surface. In summary, the combination of C₁₅H₁₅NO and Na₂WO₄ can more effectively prevent the corrosion of N80 steel in 15% hydrochloric acid medium.

3.4. X-ray Photoelectron Spectroscopy

3.4.1. XPS Survey Spectra Studies

The XPS survey spectra of the N80 steel are shown in Figure 5.

Figure 5 shows the presence of the O element from the external environment and Cl element from the corrosion solution, together with elements from the steel, in the absence of inhibitors. The elements on the surface of metal were mainly C, N, O, Fe, and Cl after adding the C₁₅H₁₅NO [30]. The presence of N elements proves that C₁₅H₁₅NO was adsorbed on the surface of N80 steel, while the peak of Cl elements indicates that the film of C₁₅H₁₅NO only provides sufficient hindrance to the diffusion of Cl⁻. Therefore, the N80 steel surface was still subject to serious corrosion with this single inhibitor. The precipitation film contained C, Fe, Se, Cl, O, N, and Na after adding the Na₂WO₄ inhibitors. The W, Na, and Fe element peaks reveal the fact that FeWO₄ has been deposited on the metal surface of N80 steel. Similarly, owing to defects of the FeWO₄ precipitation film on the metal surface, there was a significant Cl peak. The XPS survey spectra in mixed inhibitors show that the major elements of the surface of the membrane formed by the complex inhibitor were C, N, O, Fe, Se, W, and Na. In this case, no obvious Cl peak was detected, indicating that the corrosion product contains no or a very small amount of chloride. This indicates that the film formed can successfully prevent the chloride ions from entering and effectively improve the corrosion efficiency.

In comparison with the content of Cl at different ratios of C₁₅H₁₅NO to Na₂WO₄ (Figure 6a), we find that the content of Cl will greatly change when the mixed ratio of the corrosion inhibitor changes. Furthermore, Cl was not detected when the ratio was 1:1, which indicates that this ratio is the most effective in corrosion prevention. The binding energy between Fe 2p and inhibitors, obtained by adsorption analysis, also verifies this conclusion (Table S4). Furthermore, the data with different plasma etch times (corresponding to different depths into the film) also indicate that mixed inhibitors can successfully hinder the Cl penetration, as shown in Figure 6b.

3.4.2. Analysis of Binding Energy of Film

The XPS survey spectrum and the spectrum of some elements on N80 steel in the mixed inhibitors with the ratio of 1:1 after a 250 s etching time are shown in Figure 7. In addition to the photoelectron spectra, the spectral lines of XPS also have satellite peaks, auger electron lines, spin orbit splitting (SOS), and ghost peaks [31].

As shown in Figure 7a, the C1s peak with a binding energy of 284.60 eV represents the C–C or C–H bond in the inhibitor molecule [32]. The peak from the N–H bond in the inhibitor molecule is located at 399.12 eV (Figure 7b), which is a strong evidence that the corrosion inhibitor molecules were adsorbed on the N80 steel surface [33]. In Figure 7c, binding energy at 528.98 eV is related to the dissolved oxygen in the etching solution, and the peak at 530.48 eV is ascribed to the C–O bond in the C₁₅H₁₅NO. According to the literature on XPS spectra of compounds containing different Fe valence values and the X-ray photoelectron spectroscopy manual, the Fe 2p₃ peak of element Fe is at 706.75 eV, and the Fe2p3 peak of Fe₂O₃ is at 710.70 eV. Their corresponding Fe 2p₁ peak spacing values are 13.2 and 13.6, which are at 719.45 eV and at 724.30 eV, respectively [34]. It can be inferred that in Figure 7d, the peaks between 706.52 eV and 719.52 eV come from elemental Fe, and the peaks between 709.228 eV and 722.62 eV belong to Fe₂O₃ formed by the reaction of elemental iron with oxygen on the surface of N80 steel [35,36,37]. The Na1s peak with a binding energy of 1072.94 eV in Figure 7e corresponds to Na⁺ in the inhibitor molecule. Figure 7f is the Auger electron energy spectrum of Fe, whose kinetic energy has nothing to do with the incident light hv. The characteristic peaks located at 784 eV and 846.0 eV indicate the simultaneous existence of Fe(II) and Fe(III) on the surface of N80 steel [38,39]. Therefore, the protection film mainly consists of the inhibitor molecules and iron oxide or ferrous hydroxide.

4. Conclusions

In this paper, the combined use of C₁₅H₁₅NO and Na₂WO₄ as a corrosion inhibitor for N80 steel in 15% HCl solution was investigated by electrochemical experiments and microscopic analysis. The mixed inhibitors of Mannich base and sodium tungstate have demonstrated excellent corrosion prevention, and the best corrosion inhibition efficiency is 96.19% when the mixing ratio of the two components is 1:1. The SEM results indicate a fine and dense surface structure when the optimal mixed inhibitor was applied, and the degree of corrosion was greatly reduced compared with an un-protected surface or when a single inhibitor was used. The XPS analyses show that the content of Cl on the film was much decreased in the mixed inhibitor solution. The mixed inhibitors are able to form a stable membrane structure, which can effectively protect the metal from severe corrosion.