The corrosion inhibition properties of 2-hydroxy-1-(2-hydroxy-4-sulfo-1-naphthylazo)-3-naphthoic acid (HHSNNA) for C38 steel in HCl 1 M solution were analyzed by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and gravimetric methods. It was found that the presence of HHSNNA hugely decreases the corrosion rate, while its inhibition efficiency (E%) increases with concentration to attain 93.8 % at 10−3 M. The high inhibitory efficiency of HHSNNA was related with the adsorption of HHSNNA molecules at the metal/solution interface and a protective film formation. Adsorption behavior of the inhibitor on the surface followed the Langmuir adsorption isotherm. This compound is classified as the mixed type corrosion inhibitor with predominant control of the cathodic reaction. Data, obtained from EIS measurements, were analyzed to model the corrosion inhibition process through an appropriate equivalent circuit model. © 2014, The Indian Institute of Metals - IIM.