Abstract: This study investigates the adsorption mechanism, the film formation process, and the inhibition performance of benzotriazole (BTAH) on carbon steels with different grain sizes (i.e., 24.5, 4.3, and 0.6 µm) in 3.5 wt. % NaCl solution. The results demonstrate that grain refinement significantly impacts the adsorption and inhibition performance of BTAH on carbon steels. Ultra-refinement of steel grains to 0.6 µm improves the maximum inhibition efficiency of BTAH to 90.0 % within 168 h of immersion, which was much higher than that of the steels with 24.5 µm (73.6 %) and 4.3 µm grain sizes (81.7 %). Notably, grain sizes of 4.3 and 0.6 µm facilitate a combination of physisorption and chemisorption of BTAH after 120 h of immersion, as evidenced by the X-ray photoelectron spectroscopy (XPS) results and Langmuir adsorption isotherms, while BTAH adsorbed on carbon steels with a grain size of 24.5 µm through physisorption during the 168 h of immersion. Ultra-refinement of grains has beneficial impacts on promoting the formation of a stable and dense corrosion inhibitor film, leading to improved corrosion resistance and the mitigation of non-uniform corrosion. These advantageous effects can be attributed to the higher adsorption energy at grain boundaries (approximately -3.12 eV) compared to grain interiors (ranging from -0.79 to 2.47 eV), promoting both the physisorption and chemisorption of organic corrosion inhibitors. The investigation comprehensively illustrates, for the first time, the effects of grain size on the adsorption mechanism, film formation process, and inhibition performance of organic corrosion inhibitors on carbon steels. This study demonstrates a promising approach to enhancing corrosion inhibition performance through microstructural design.

Key words: Carbon steel, Corrosion inhibitor, Grain refinement, Microstructure, Adsorption mechanism