Natural formation of protective film for mild steel corrosion in sodium chloride containing saturated carbon dioxide medium using lanthanum (III) chloride

Carbon dioxide (CO₂) corrosion on metallic materials, especially mild steel, is the major concern to various industrial sectors as continuously exposed to different environmental conditions, which causes damages related to loss of materials, environment and finance. Finding an effective method to improve corrosion resistance, enhance durability, and even reduce maintenance costs for mild steel-based components is always an urgent requirement. Beyond increased applicability and extended lifespan of steel-based infrastructures, maintaining sustainability and environmental friendliness of the methods used is relatively important. Motivated by this direction, this work uses lanthanum (III) chloride (LaCl₃) as a corrosion inhibitor on mild steel in the 0.01 M NaCl solution saturated with CO₂. By combining surface characterizations and electrochemical measurements, electrochemical behaviors of mild steel and the effects of the inhibitor on electrochemical activities of steel are clearly demonstrated. Surface characterizations including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) are applied to observe and analyze the steel surface after immersion in the prepared solution without and with varying concentrations of the inhibitor for 72 hours. The results show the film formation of lanthanum-based compounds on the steel surface probably reduces localized corrosion and minimizes porous flakes on the steel surface. Additionally, the electrochemical measurements, including potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS), indicate that LaCl₃ acts a mixed-type inhibitor that prevents both anodic and cathodic reactions on the metal surface in the electrolyte, with the highest inhibition efficiency achieving 95.97 ± 0.41% at 2.4 mM LaCl₃. The film formation of the inhibitor based on the heterogenous monolayer adsorption is demonstrated by isotherm adsorption. Moreover, the wire beam electrode (WBE) mapping based on current density is used and the results obtained show the inhibitor can influence on random distribution of anodic and cathodic sites on the steel surface, suggesting localized corrosion inhibition of LaCl₃ in CO₂-saturated NaCl solution.