Insight into the Anticorrosion Properties of <i>Stephania kwangsiensis</i> Root Extracts as High-Efficiency Eco-Friendly Inhibitors for Carbon Steel in HCl Medium

In the present study, <i>Stephania kwangsiensis</i> root extracts (SKRE) were first investigated as an innovative, environmentally friendly corrosion inhibitor for Q235 steel in 1 M HCl medium by combining experimental measurements and theoretical calculations. It is indicated that the SKRE inhibitor, which represses both cathodic and anodic reactions, serves as a mixed type inhibitor. The corrosion inhibition efficiency (IE) at 100 mg/L SKRE exceeds 99% when using electrochemical techniques, demonstrating a preeminent anticorrosion capability in acid environments. The IE could still be beyond 92% after 5 days of immersion, suggesting an excellent long-term stability of the SKRE inhibitor. A considerably high IE (more than 98%) could also be achieved at various temperatures. The main active component, cepharanthine (CEPT), adsorbs on the steel surface in a parallel configuration. The presence of isoquinoline rings, dioxole rings, benzene rings, and oxygen atoms could donate profuse π-electrons and lone pair electrons to metals, which facilitates the stable adsorption of SKRE onto the steel surface.