*2.1. Solution*

The test solution was 3.5 wt% NaCl (analytical-grade reagent) solution; to avoid introducing extra chloride ions, deoxidized dilute sulfuric acid and sodium hydroxide were used to adjust the pH to 3.10, 5.15, and 7.18. The pH meter was PHS-25 (Shanghai INESA & Scientific Instrument Co., Ltd., Shanghai, China). The solution was deoxygenated with N2 for 4 h and then pumped in CO2 for 12 h before testing.

#### *2.2. Corrosion Scale Preparation*

An autoclave experimental setup (FCZ3-24/320, Dalian Science and Trade Experimental Equipment Co., Ltd., Dalian, China) was used to prepare the corrosion scale. The CO2 pressure in the autoclave was maintained at 5 MPa. The material was L360 pipeline steel, and the test sample was machined to the dimensions of 50 mm × 10 mm × 3 mm, with a chemical composition (wt%) of C 0.16%, Si 0.45%, Mn 1.60%, P 0.025%, S 0.015%, V 0.06%, Nb 0.05%, Ti 0.04%, and Fe the balance. The samples were sequentially ground with 400, 800, 1200, and 2000 grit SiC papers, rinsed with distilled water, dehydrated in alcohol, and dried in cool air. The fused sublimation sulfur was coated on the electrode surface, and its thickness was controlled within 1.0–1.5 mm by 2000# sandpaper grinding (coating thickness was determined using a vernier caliper) to ensure that there were no cracks in the sulfur layer and the working area of the electrode was covered by the sulfur layer. The average deposition mass of the sample was calculated to be 0.9312 g using the subtraction method.
