Inhibition Effect of <i>N</i>-Substituted Anilines on Iron Corrosion in Hydrochloric Acid

Inhibition effects of N-alkyl anilines C₆H₅NHC_nH_2n+1 and N-dialkyl anilines C₆H₅N(C_nH_2n+1)₂, where n=0-4, on the corrosion of 99.89% Fe were studied in 6.1M HCl by polarization measurement. The inhibition efficiency of these amines was discussed on the basis of general adsorption behavior by using some physical parameters. The alkyl anilines inhibited both the cathodic and anodic processes of the corrosion reaction. The anodic inhibition efficiency I_a is approximately expressed by the Hammett-like equation, <br>log(I_aA_o/I_aoA)=ρ*Σσ*+δE_s+Ψ_r+α<br>where zero subscripts refer to a reference inhibitor, A is the molecular coverage area of free amine, Σσ* and E_s denote the polar and steric substituent constants, and ρ*, δ, and α are constants, respectively. The value of Ψ_r, which is related to the delocalized electronic structures in the inhibitor molecule, was determined for the alkyl anilines. It was found that there is a minimum value of Ψ_r at 1-2 of n for each series of the alkyl anilines. This result was explained by steric hindrances of the alkyl branch to the formation of the resonance structures and to the adsorption on the metal surface. The cathodic inhibition efficiency was also found to be closely related to the Ψ_r value. The result suggested that the aniline inhibitors are adsorbed even on the cathode of the iron surface by the formation of a coordinate bond between the metal and the unprotonated molecules rather than by the coulombic attraction of the ammonium cation.