Extraction of Sophora japonica L. leaf extract and its application for corrosion inhibition of X52 steel in hydrochloric acid

X52 steel exhibits a high susceptibility to corrosion within hydrochloric acid environments. In order to formulate an effective and eco-friendly corrosion inhibition strategy, a phytochemical-rich Sophora japonica L. leaf extract (SJLE) was acquired by ultrasound-assisted extraction. Electrochemical tests, surface analysis, and theoretical calculations were employed to evaluate its performance. The results showed that 500 ppm SJLE alone achieved 72.83% corrosion inhibition efficiency, while a combination with 10 mM (1660 ppm) KI significantly enhanced this to 92.48%, with a synergism parameter confirming strong positive synergy. Surface characterizations confirmed the formation of a dense, uniform protective film, and quantum chemical calculations revealed that the main active components, quercetin and rutin, adsorbed onto the Fe(110) surface primarily through electron donation from O- and C-centered functional groups. This work demonstrates the promising potential of the SJLE-KI system as an eco-friendly and high-performance corrosion inhibitor for carbon steel in acidic media.