**2. Research Methods**

#### *2.1. Test Materials*

The special ultra-high-temperature acidizing corrosion inhibitor for duplex stainless steel was developed by China Petroleum Engineering Materials Research Institute Co., Ltd., Xi'an, China. and it is mainly composed of 40 wt.% Mannich base + 6 wt.% antimony trioxide + 15 wt.% hydrochloric acid + 5 wt.% OP + 34 wt.% DMF.

The test 2205 duplex stainless steel was developed and produced by Zhejiang Jiuli Special Material Technology Co., Ltd., Huzhou China. and its chemical composition was 0.019 wt.% C, 0.41 wt.%Si, 0.63 wt.% Mn, 0.023 wt.% P, 0.005 wt.% S, 2.38 wt.% Cr, 52 wt.% Mo, 5.47 wt.% Ni, 0.181 wt.%N and residual Fe. The size of the high-temperature and high-pressure corrosion test sample was cut from a pipe material with the dimension of 50 mm × 10 mm × 3 mm.

Before testing, the samples were ground by using the abrasive paper from 400 # to 800 # step by step, then immersed in anhydrous ethanol for 5 min by ultrasonic cleaning, dried with cold air, placed in a dryer for at least 1 h, weighed (accurate to 0.0001 g), and measured the size (accurate to 0.01 mm). The samples should be mutually insulated and installed on the fixture during the test. After testing, the acid solution on the surface of the sample was removed by deionization, the corrosion inhibitor film on the surface of the sample was removed by N, N–dimethylformamide, and then the sample was ultrasonically cleaned with anhydrous ethanol for 5 min, dried by cold air, placed in a desiccator for at least 1 h and weighed (accurate to 0.0001 g).

#### *2.2. High-Temperature and High-Pressure Corrosion Test*
