The adsorption and inhibitive action of hexadecylpyridinium bromide (HDPB) on the corrosion of cold rolled steel (CRS) in phosphoric acid produced by dihydrate wet method process (7.0 M H(3)PO(4)) was studied by weight loss, open circuit potential (OCP), potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and atomic force microscopy (AFM) methods. The results show that HDPB is a good inhibitor in 7.0 M H(3)PO(4), and its maximum inhibition efficiency (IE) is higher than 90% at little concentrations. The adsorption of HDPB obeys Langmuir adsorption isotherm equation. Thermodynamic parameters (adsorption enthalpy a dagger H (0), adsorption free energy a dagger G (0), and adsorption entropy a dagger S (0)) were calculated and discussed. Polarization curves show that HDPB acts as a mixed-type inhibitor in phosphoric acid. EIS shows that charge-transfer resistance increases with the inhibitor concentration, confirming adsorption process mechanism. The inhibition action of HDPB could also be evidenced by SEM and AFM images. A probable inhibitive mechanism is proposed from the viewpoint of adsorption theory.