The present investigation includes the study of corrosion inhibition effect of N-(1H-Indol-3-ylmethylene)-nicotinamide (IMN) and N-(3,4,5-trimethoxy-benzylidene)-nicotinamide (TMN) on mild steel in 0.5？M HCl by mass loss and electrochemical measurements. The obtained results showed that inhibition efficiency increased with the increasing concentration of inhibitors and decreased with increase in temperature and immersion time. Potentiodynamic polarization curves indicate that both IMN and TMN were acting as mixed type of inhibitors. Langmuir adsorption isotherm model was employed to determine the equilibrium of adsorption for inhibiting process in both inhibitors. Nyquist plots revealed that, as the concentration of the inhibitors increases, double layer capacitance ( ) and corrosion current ( ) decreases, while polarization resistance ( ) increases. Various thermodynamic parameters for the adsorption of inhibitors on mild steel were computed and discussed. The passive film formed on the metal surface was characterized by FTIR, EDX, and SEM. 1. Introduction Mild steel (MS) is cheap, strong, stiff, and widely used engineering material because of its good mechanical properties. It is the most important structural material exposed to outdoor conditions especially in acidic media [1] where corrosion is considerably more severe. Because of the general aggressiveness of acid solutions, inhibitors are commonly used to reduce the corrosive attack on metallic materials [2, 3]. Therefore, selection of an appropriate inhibitor ranging from rare earth elements [4] to organic compounds [5–8] for specific environment and metal is of great importance. In other words, the efficiency of these compounds is attributed to their steric factors, functional groups, lone pairs of electrons present on the hetero atoms, and pi-orbital character of donating electrons which determine the type of interaction between organic molecules and the metallic surfaces. The presence of corrosion inhibitors in a little amount blocks the corrosion sites and enhances the adsorption process, thereby increasing the life time of the metallic materials [9, 10]. A review of the literature reveals that the applicability of organic compounds as corrosion inhibitors for MS in acidic media has been recognized for a long time. A large number of organic compounds, particularly those containing nitrogen, oxygen, or sulphur in a conjugated system, are known to be applied as inhibitors to control acid corrosion of iron and steel. The inhibition process has been shown to occur via inhibitor adsorption, and
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