A biodegradable and green organic compound octadecyl dimethyl benzyl amm-onium chloride (ODBAC) was used as an efficient inhibitor for cold rolled steel (CRS) in phosphoric acid (H

3

PO

4

). The mechanism of adsorption and film formation of ODBAC on CRS was studied through experimental and theoretical calculations. The weight loss method shows that the inhibition efficiency of ODBAC can reach 92.01% at a concentration of 10 mg·L

-1

. The adsorption of ODBAC on the CRS surface conforms to the Langmuir isotherm model

which is a mixed adsorption mainly based on physical adsorption. The X-ray photoelectron spectroscopy (XPS) and contact angle results confirmed the existence of the ODBAC film and steel surface's hydrophobicity has been significantly enhanced. Electrochemical test results reveal that the film's formation mainly inhibits the cathodic corrosion reaction and effectively increases the charge transfer resistance. Quantum chemical calculations have found that N18 in ODBAC and C24 and C25 on the benzene ring are the key active adsorption sites. Molecular dynamics simulation results indicate that ODBAC can sharply reduce the free fraction volume to 8% and inhibit the diffusion of corrosion particles

meaning that the formed ODBAC film makes it difficult for corrosion particles to penetrate

thus improving the corrosion resistance of CRS in H

3

PO

4

.