*Article* **Corrosion Inhibition Mechanism of Ultra-High-Temperature Acidizing Corrosion Inhibitor for 2205 Duplex Stainless Steel**

**Danping Li 1,2,\*, Wenwen Song <sup>3</sup> , Junping Zhang <sup>4</sup> , Chengxian Yin <sup>1</sup> , Mifeng Zhao <sup>3</sup> , Hongzhou Chao <sup>3</sup> , Juantao Zhang <sup>1</sup> , Zigang Lei 1,2, Lei Fan <sup>1</sup> , Wan Liu 1,2 and Xiaolong Li <sup>1</sup>**


**Abstract:** The acidizing corrosion inhibitors reported so far have a poor effect on duplex stainless steel in high-temperature and high-concentration acid systems and cannot effectively inhibit the occurrence of selective corrosion. In this paper, a new acidizing corrosion inhibitor was designed, which was mainly composed of Mannich base and antimony salt. The inorganic substance in the corrosion inhibitor had good stability at high temperatures and could quickly form a complex with the metal matrix to enhance the binding ability. The organic substance can make up for the non-dense part of the inorganic film. The properties of developed corrosion inhibitors were analyzed by quantum chemical calculation, molecular dynamics simulation, and scanning electron microscopy. The results showed that a double-layer membrane structure could be constructed after adding the corrosion inhibitor, which could play a good role in blocking the diffusion of acid solution at high-temperature. The uniform corrosion rate of 2205 duplex stainless steel after adding acidizing corrosion inhibitor immersion in a simulated service condition (9 wt.% HCl + 1.5 wt.% HF + 3 wt.% CH3COOH + 4~6 wt.%) at 140 ◦C, 160 ◦C and 180 ◦C for a 4 h test is 6.9350 g·m−<sup>2</sup> ·h −1 , 6.3899 g·m−<sup>2</sup> ·h <sup>−</sup><sup>1</sup> and 12.1881 g·m−<sup>2</sup> ·h −1 , respectively, which shows excellent corrosion inhibition effect and is far lower than that of the commonly accepted 81 g·m−<sup>2</sup> ·h <sup>−</sup><sup>1</sup> and no selective corrosion could be detected.

**Keywords:** duplex stainless steel; selective corrosion; high-temperature acidification; corrosion inhibitor; molecular dynamics simulation

#### **1. Introduction**

With the increasing demand for energy, the exploration and development of oil and gas fields have gradually developed from conventional working conditions to the harsh working conditions of high-temperature and high-pressure or even ultra-high-temperature and high-pressure. The high-temperature and high-pressure well is defined with wellhead pressure and bottom hole pressure greater than 70 and 105 MPa, respectively, and the bottom hole temperature greater than 150 ◦C by the International high-temperature and high-pressure Well Association, the wells with wellhead pressure greater than 105 MPa, bottom-hole pressure greater than 140 MPa and bottom-hole temperature greater than 175 ◦C is defined as ultra-high-temperature and pressure wells [1]. At present, hightemperature and high-pressure oil and gas wells in the world are mainly distributed in the Gulf of Mexico in the United States, the North Sea in the United Kingdom, Southeast Asia, Africa, and the Tarim Basin, the South China Sea and Sichuan in China [2–5]. The acidizing operation temperature of oil wells in the North Sea has exceeded 160 ◦C [6]. In China, representative ultra-high-temperature and high-pressure wells are mainly distributed in the Tarim Basin of Xinjiang, with deep burial depths and extremely harsh service conditions. The bottom hole temperature usually exceeds 180 ◦C [7,8]. In addition, the acid with

**Citation:** Li, D.; Song, W.; Zhang, J.; Yin, C.; Zhao, M.; Chao, H.; Zhang, J.; Lei, Z.; Fan, L.; Liu, W.; et al. Corrosion Inhibition Mechanism of Ultra-High-Temperature Acidizing Corrosion Inhibitor for 2205 Duplex Stainless Steel. *Materials* **2023**, *16*, 2358. https://doi.org/10.3390/ ma16062358

Academic Editor: Amir Mostafaei

Received: 20 February 2023 Revised: 7 March 2023 Accepted: 10 March 2023 Published: 15 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

extremely low pH value in the process of reservoir reconstruction and the CO<sup>2</sup> and Cl− in the fluid produced in the later production process all put forward high requirements for the corrosion resistance of oil pipes. According to the SUMITOMO oil well pipe material selection spectrum [9], duplex stainless steel should be used for H2S partial pressure ≤0.1 bar when the temperature is over 180 ◦C, while nickel base alloy with higher corrosion resistance should be selected for H2S partial pressure > 0.1 bar, and the traditional super 13Cr stainless steel could no longer meet the requirement of the working conditions.

Generally, a passive oxide (mainly Cr2O3) layer could be formed on the stainless steel to resist various corrosion, and the absence or breakdown of this layer could result in accelerated corrosion rates [10]. Stainless steel is mainly divided into the following categories: ferritic stainless steels, austenitic stainless steels, martensitic stainless steels, duplex stainless steels, precipitation-hardening stainless steels, and Mn-N substituted austenitic stainless steels [11]. Due to the particularity of its composition, duplex stainless steel has the characteristics of ferrite and austenite stainless steel. Duplex stainless steel has higher plastic toughness, and its weldability and intergranular corrosion resistance are significantly stronger than that of ferritic stainless steel. Compared with austenitic stainless steel, the yield strength, intergranular corrosion, and Cl− corrosion resistance of duplex steel is much better. In addition, duplex stainless steel also has the advantage of better resistance to stress corrosion, pitting, and crevice corrosion [12].

During the whole production cycle of the well, applying a strong acid (such as 10– 28 wt.% HCl) to increase the output of the reservoir is inevitable. The passive film of duplex stainless steel could be destroyed under such harsh conditions. Duplex stainless steel has good corrosion resistance in general conditions, but it will exhibit selective corrosion of different phases in specific conditions [13–25]. Due to the different crystal structure and chemical composition of α and γ phases, which shows different electrochemical potential in a specific solution, a micro-electric couple will be formed and cause selective corrosion [26,27]. For example, the anodic polarization curve of 2205 duplex stainless steel sample in acid solution usually shows two anodic dissolution peaks, the lower and the higher peaks are the ferrite phase and austenite phase activation peaks, respectively. The ferrite phase is preferentially dissolved when the applied potential corresponds to the lower peak. Similarly, the austenite phase is preferentially dissolved when the applied potential is close to the higher peak [16]. Yau and Streicher [24] found that ferritic selective corrosion of FeCr–10%Ni duplex stainless steel occurred in reducing acid. Sridhar and Kolts [25] discovered that the austenitic phase selective corrosion of high nitrogen content duplex stainless steel occurred in sulfuric acid and phosphoric acid, while the ferrite phase selective corrosion occurred in hydrochloric acid. Tsai et al. [18] showed that the activity of ferrite was stronger than austenite in 2 mol/L H2SO<sup>4</sup> + 0.5 mol/L HCl solution; however, austenite was in a more active state and is preferentially corroded as the anode in 1.5 mol/L HNO<sup>3</sup> solution.

Acid corrosion inhibitors must be used to reduce the corrosion damage of steel in service [26]. At present, most of the research on acidizing corrosion inhibitors for duplex stainless steel was mainly focused on the acid solution system under 120 ◦C [12,27–30] or high-temperature, low-concentration acid [31]. Hirotaka et al. [28] adopted 7 wt.% HCl at 60 ◦C as the test medium to simulate the acidizing bottom-hole conditions of onshore oil fields in Japan. The corrosion inhibition mechanism of 25 wt.% cinnamaldehyde + 20 wt.% long-chain alkyl imidazoline + 55 wt.% methanol plus CuI/KI/Cu/CuCl2/CuSO<sup>4</sup> on 2205 duplex stainless steel in an acid system was studied, the results showed that the corrosion inhibition rate was only 34.9 wt.%, and the ferrite selective corrosion could be observed, and the corrosion inhibition rate reaches more than 98% after the addition of Cu compound, and no selective corrosion could be seen. Wang et al. [12] showed that the corrosion rate of 2205 duplex stainless steel was 26.8053 g·m−<sup>2</sup> ·h <sup>−</sup><sup>1</sup> after adding corrosion inhibitor in 10 wt.% HCl + 1.5% wt.HF + 3 wt.% HAC solution at 120 ◦C, but ferrite selective corrosion could be seen on the surface of the sample. Du et al. [31] showed that the corrosion rate of

2205 duplex stainless steel could be controlled at 40.3092 g·m−<sup>2</sup> ·h −1 in the 4 wt.% HCl + 3 wt.% CH3COOH + 5 wt.% QASE + 1 wt.% Sb2O<sup>3</sup> at 200 ◦C test environment.

Molecular simulation techniques play an important role in the modern characterization of materials [32], which are beneficial to the study of corrosion behavior by increasing the understanding of chemical and physical processes at the molecular and atomic levels. Cl− has a great influence on metal pitting corrosion, and the molecular dynamics (MD) simulations method is also used to study the pitting behavior of Cl− on metals [32–36]. By using molecular dynamics simulation, Sepehr Y et al. [33] studied the pitting corrosion behavior of nano-diamonds. Chen et al. [34], using MD simulation, found that the mixed corrosive anions (Cl<sup>−</sup> and HSO<sup>3</sup> −) adsorbed much more strongly to the passive film, and their diffusion coefficient was also significantly improved compared to the solution containing only one kind of anion.

The above-mentioned acidizing corrosion inhibitor had a certain corrosion inhibition effect on 2205 duplex stainless steel. However, there have been no reports on corrosion inhibitors suitable for ultra-high temperature and high-concentration acid solution systems. Herein, it is essential to develop a corrosion inhibitor suitable for the ultra-hightemperature and high-concentration acidizing environment and to clarify its corrosion inhibition mechanism, which is critical to prolonging the service life of duplex stainless steel. In this paper, quantum chemical calculation and molecular dynamics simulation were applied to design a new acid corrosion inhibitor, and the suitability was further verified by high-temperature and high-pressure corrosion simulation tests. This research has great engineering application value to broaden the selection of materials for ultra-high-temperature and high-pressure wells and reduce the cost of oil and gas field string.
