*3.3. Corrosion Inhibition Mechanism Analysis at High-Temperature and High-Pressure* 3.3.1. Effect of Temperature on Performance of Corrosion Inhibitor

Figures 5–7 show the uniform corrosion rate, inhibitor film thickness, and local corrosion depth of 2205 duplex stainless steel in acid solution at 140 ◦C, 160 ◦C, and 180 ◦C, respectively. It can be seen that the uniform corrosion rate shows no significant difference between 140 ◦C and 160 ◦C but rises sharply at 180 ◦C, which is twice that at 160 ◦C, which is far lower than the commonly accepted 81 g·m−<sup>2</sup> ·h −1 [41] and 26.8053 g·m−<sup>2</sup> ·h <sup>−</sup><sup>1</sup> of the 2205 duplex stainless steel in the reported 120 ◦C 15 wt.% HCl + 1.5% wt. HF + 3 wt.% HAC + 5.1 wt.% acid corrosion inhibitor [12]. The thickness of both layers showed a decreasing trend with the increase in temperature. When combined with the local corrosion and the morphology of the corrosion inhibitor film, it can be seen that the density of the film decreases with the increase in temperature.

**Figure 5.** Uniform corrosion rate of 2205 duplex stainless steel at different temperatures.

**Figure 6.** Thickness of corrosion inhibitor film on 2205 duplex stainless steel at different temperatures.

**Figure 7.** Localized corrosion depth of 2205 duplex stainless steel in acid solution system at different temperatures ((**a**) 160 ◦C; (**b**) 180 ◦C).

No obvious localized corrosion was observed on the specimen surface after the 140 ◦C test. But the sample surface appeared to have localized corrosion at 160 ◦C and 180 ◦C tests. The maximum localized corrosion depth of the 160 ◦C sample is 22 µm. The average localized corrosion depth is 10 µm. The depth of the corrosion pit is mainly concentrated in 5~10 µm. The maximum localized corrosion depth of 180 ◦C sample surface is 37 µm. The average localized corrosion depth is 16 µm. The depth of the corrosion pit is mainly concentrated at 11~15 µm. It can be seen that the localized corrosion depth increases with the increase in test temperature.

Figure 8 shows the SEM morphology of the corrosion inhibitor film at different temperatures. At 140 ◦C, the inner and outer films of the corrosion inhibitor on the surface of the sample were relatively intact without obvious damage. With the increase in temperature, the corrosion inhibitor film was damaged, and the inner and outer films on the surface of the 180 ◦C sample were damaged. Thus, localized corrosion occurred.

**Figure 8.** Micromorphology of corrosion inhibitor films on the surface of 2205 duplex stainless steel sample at 100× magnification ((**a**) 140 ◦C the outer film, (**a,**) 140 ◦C the inner film, (**b**) 160 ◦C outer film, (**b,**) 160 ◦C the inner film, (**c**) 180 ◦C outer film, (**c,**) 180 ◦C the inner film).

The EDS results of the main element contents of outer and inner films are displayed in Table 5. The main elements of the film are C, O, Sb, Cl, Fe, Sand Cr, and the content of C in the outer film is much higher than that of in the inner film, while the content of Sb in the inner film is much higher than that of in the outer film. The contents of C and Sb indicate that the outer layer is mainly organic film, and the inner layer is mainly inorganic salt. The content of Cl− in the outer film is higher than that of in the inner film, indicating that the corrosion inhibitor film has an excellent blocking effect in the acid solution, which is consistent with the simulation results (see Table 6). The content of Fe in the inner film increases with the increase in temperature, indicating that the corrosion resistance of the film is reduced.


**Table 6.** Main element contents EDS analysis result of outer and inner films at different test temperatures (wt.%).

## 3.3.2. Effect of Time on Performance of Corrosion Inhibitor

The uniform corrosion rate of 2205 duplex stainless steel after 4 h and 12 h tests in acid solution at 180 ◦C is shown in Figure 9. It can be seen with the extension of test cycles, and the corrosion rate generally shows a decreasing trend. Figure 10 shows the thickness of two corrosion inhibitor film layers at different test cycles. With the extension of the test cycles, the thickness of the two layers displays an increasing trend. According to the local corrosion situation and the micro-morphology of the inhibitor film, although the film thickness increases with the increase in temperature, the compactness decreases, and the localized corrosion density and depth increase.

**Figure 9.** Uniform corrosion rate at different test cycle.

**Figure 10.** Thickness measurement results of corrosion inhibitor film at different test cycles.

The cross-section and surface micromorphology of the inhibitor film at different test cycles shows in Figure 11. The two film layers are relatively complete in the fresh acid medium at 180 ◦C, which has a good protective effect on the substrate, but the local film layer is damaged, which leads to pitting corrosion on the surface of the sample. Although the thickness of the film in the 12 h test was significantly larger than that in the 4 h test, the compactness of the film was not significantly improved, and the degree of damage was more serious than that in the 4 h test.

**Figure 11.** Micromorphology of the inner film of the corrosion inhibitor on the surface of the samples at different test cycles magnified by 100× ((**a**) test cycle 4 h; (**b**) test cycle 12 h).

Figures 12 and 13 display the statistical results of pitting depth and the surface microscopic corrosion morphology of samples after different test cycles. It can be seen from Figure 12a that the maximum pitting depth is 37 µm, the average pitting depth is 16 µm, and the depth of the corrosion pit is mainly concentrated at 11~15 µm at 180 ◦C for 4 h. The maximum pitting depth is 64 µm, the average pitting depth is 23 µm, and the depth of the corrosion pit is mainly concentrated in 21~25 µm at 180 ◦C for 12 h (see Figure 12b).

**Figure 12.** Normal distribution of pitting depth on the surface of samples at different test cycles (**a**) 4 h; (**b**) 12 h.

**Figure 13.** Micromorphology of sample surface magnified by 100× under optical microscope at different test cycles: (**a**) 4 h, (**b**) 12 h.

#### 3.3.3. Corrosion Inhibitor Layer Analysis

In the acid solution system with high temperature and high concentration, the common organic corrosion inhibitor is seriously desorbed at high temperature, and its adsorption capacity differs greatly from that of austenite and ferrite, which is prone to selective corrosion and difficult to play a good protective role. The corrosion inhibitor in this study constructs a double-layer membrane adsorption structure, which uses inorganic substances with good stability at high temperature to form a complex with the metal matrix to enhance the binding ability with the metal matrix and maintain the high-temperature stability of the film. However, the inorganic film is not dense enough; organic film is used to supplement, as shown in Figure 14a. The microscopic morphology of the film cross-section after adding corrosion inhibitor in the 180 ◦C acid solution is displayed in Figure 14b. It can be seen the inhibitor has a double-layer membrane structure, which is consistent with the expected results of the inhibitor mechanism. Inorganic salt has a good binding ability with metal matrix at high-temperature. However, the film is easy to crack at high-temperature and forms a channel for acid diffusion. The crack defect could be repaired by the organic film, making the corrosion inhibitor film more dense and better protective for the matrix.

The cross-section of the line scan results of the corrosion inhibitor film is shown in Figure 15. The outer film and the inner film is mainly composed of element C and Sb, respectively, which is consistent with Table 5. From the results of line scanning, it can be seen that the organic film and the inorganic salt film penetrate into each other and combine closely.

**Figure 14.** Structure of high-temperature acidizing corrosion inhibitor double-layer film ((**a**) schematic diagram, (**b**) section SEM).

**Figure 15.** Line Scanning Results of Element Distribution of Corrosion Inhibitor Film.

Figure 16 shows the cross-section morphology of 2205 duplex stainless steel after testing in 180 ◦C acid solution with a corrosion inhibitor. No selective corrosion was observed. Figure 17 shows the XRD analysis results of the sample surface after the 180 ◦C test without corrosion inhibitor and with corrosion inhibitor. The peaks of the austenitic phase are much stronger than that of the ferritic phase in tests without the corrosion inhibitors. However, the peaks of the ferritic phase are as strong as those of the austenitic phase when the corrosion inhibitor is added. The results show that without adding a corrosion inhibitor, selective ferrite corrosion could be observed in high-temperature and high-concentration acid solutions, and selective corrosion is effectively inhibited after adding a corrosion inhibitor.

**Figure 16.** Cross-sectional morphology of 2205 duplex stainless steel sample without corrosion product film after testing in 180 ◦C acid solution with corrosion inhibitor.

**Figure 17.** XRD analysis results of samples without and with corrosion inhibitor ((**a**) 180 ◦C without inhibitor, (**b**) 180 ◦C with inhibitor).

#### **4. Discussion**

Figure 18 shows the corrosion mechanism of duplex steel in a high-temperature and high-concentration acid solution system. Minor differences could be seen in the element composition of 2205 duplex stainless steel. Cr, Mo, Ni, and N tend to be concentrated in the ferrite and austenite phases, respectively [26,42,43]. The distribution of elements in the phase is a key factor affecting selective corrosion. The 2205 duplex stainless steel is in an active state when working with high-temperature and high-concentration hydrochloric acid and hydrochloric acid plus hydrofluoric acid [12,26]. Because the potential of Cr is more negative than that of Fe, Cr is more likely to lose electrons and form ions than Fe, and the corrosion of ferrite is more serious than that of austenite. The possible chemical reactions are:

anodic [44]:

Cr <sup>→</sup> Cr3<sup>+</sup> <sup>+</sup> <sup>3</sup>*<sup>e</sup>* (7)

Fe <sup>→</sup> Fe2<sup>+</sup> <sup>+</sup> <sup>2</sup>*<sup>e</sup>* (8)

cathodic [12]:

$$\text{2H}^+ + e \rightarrow \text{H}\_2\tag{9}$$

**Figure 18.** Corrosion mechanism of duplex stainless steel in high-temperature and high-concentration acid solution system.

Figure 19 shows the corrosion inhibition mechanism of acid corrosion inhibitors in high-temperature and high-concentration acid solution systems. The passive film of 2205 duplex stainless steel was dissolved in those harsh environments. Using high-temperature acid corrosion inhibitors can promote the steel to form a protective film on its surface, thus isolating the metal from the acid solution system and reducing the corrosion process. The SEM (see Figure 14) and EDS (see Figure 15) results all confirmed that the inhibitor film shows a double layer, and the inorganic film combined with the substrate forms a coordination bond, which enhances the stability of the inhibitor film at high-temperature. The outer layer is an organic layer. The quantum chemical calculation results show that the organic corrosion inhibitor molecule mainly takes the benzene ring and the O and N atoms in the molecule as the main adsorption sites. The O and N atoms have solitary electron pairs, which will coordinate with the empty orbit on the metal surface in the anode region to form an adsorption film [45] or adsorb at the defects of the inorganic film layer so as to improve the overall compactness of the film layer. The results of molecular dynamics simulation of the diffusion behavior of corrosion medium particles in the organic corrosion inhibitor film also show a good blocking effect on the corrosion media and improves the corrosion resistance of the double-layer film. The simulation result is confirmed by the EDS test results in Table 5, the content of Cl− in the inner membrane is far lower than that of in the outer membrane, and the outer film provides a good barrier and supplement for the inner membrane. When the temperature rises from 140 ◦C~180 ◦C, the binding energy of the organic corrosion inhibitor decreases from 188.39 kcal·mol−<sup>1</sup> to 180.75 kcal·mol−<sup>1</sup> . The high-temperature corrosion simulation test results demonstrate that there is no obvious local corrosion on the surface of the sample at 140 ◦C, and there is obvious local corrosion on the surface of the test sample at 160–180 ◦C, and the pitting corrosion is deeper at higher temperatures.

**Figure 19.** Schematic diagram of corrosion inhibition mechanism of acidizing corrosion inhibitor for duplex stainless steel in high-temperature and high-concentration acid solution system ((**a**) 140 ◦C, 4h (**b**) 180 ◦C, 4 h, (**c**) 180 ◦C, 12 h).

The corrosion inhibitor bilayer film is relatively dense when the temperature is low (see Figure 19a). The inorganic film is cracked or damaged with the increase in the experimental temperature, and the organic matter is adsorbed, which plays a supplementary role in the film layer (see Figure 19b). However, the binding energy of the organic matter and the adsorption property is reduced with the increase in temperature. Parts of the damaged film could not be repaired, leading to the base metal contact with the acid solution, then localized corrosion occurred (see Figure 19c). Although a thicker film could be formed with the further extension of test cycles, a loose inorganic film would be formed in the inner layer, and the organic film on the outer layer was also not intact. Hence, the local corrosion depth and scope were greatly increased (see Figure 19c). To sum up, the compactness of corrosion inhibitor film will be affected by the higher testing temperature and longer cycles.

## **5. Conclusions**

In this paper, the corrosion inhibition performance of the self-designed ultra-high temperature acidizing corrosion inhibitor was studied by using quantum chemical calculation, MD simulation, SEM, EDS, and XRD. The main conclusions are summarized as follows:


**Author Contributions:** Investigation, D.L., W.S., M.Z., H.C. and Z.L.; Writing—original draft, D.L.; Methodology, J.Z. (Junping Zhang); Writing—review & editing, J.Z. (Junping Zhang), C.Y. and J.Z. (Juantao Zhang); Formal analysis, L.F., W.L. and X.L.; Project administration, L.F.; Data curation, W.L. and X.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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