The Inhibitive action of the extracts of Adathoda vasica, Eclipta alba, and Centella asiatica on the corrosion of mild steel in 1N HCl has been studied using weight loss method, electrochemical methods, and hydrogen permeation method. Polarization method indicates that the plant extracts are under mixed control, that is, promoting retardation of both anodic and cathodic reactions. The impedance method reveals that charge-transfer process controls the corrosion of mild steel. The plant extracts obey Langmuir adsorption isotherm. Theoretical fitting of the corrosion data to the kinetic-thermodynamic model was tested to show the nature of adsorption. Physisorption mechanism has been proposed for the inhibition action of these plant extracts. The protective film formed on the surface was confirmed by SEM. From hydrogen permeation method, all the plant extracts were able to reduce the permeation current. Results obtained in all three methods were very much in good agreement in the order Eclipta alba > Adathoda vasica > Centella asiatica, and, among the three plant extracts studied, the maximum inhibition efficiency was found in Eclipta alba which showed 99.6% inhibition efficiency at 8.0% v/v concentration of the extract. 1. Introduction Mild steel was the material of choice due to its characteristics of wide application in motor car bodies, machines, gears, pipes, tanks, and so forth and in most of the chemical industries. Hydrochloric acid and sulphuric acids are the medium generally being used for pickling mild steel. About 90% of pickling problems can be solved by introducing appropriate pickling inhibitor to the medium. The recent and growing trend is using plant extracts as corrosion inhibitor. Owing to strict environmental legislation, emphasis is being focused on development of naturally occurring substances as corrosion inhibitors . Recently, many plant extracts have been reported to be very effective corrosion inhibitors for the protection of mild steel in acidic media [2–19]. In this study, the inhibition effect of the leaf extracts of Adathoda vasica (Adathodai), Eclipta alba (Karisalankanni), and Centella asiatica (Vallarai) on the corrosion of mild steel in 1N hydrochloric acid was investigated using weight loss method, electrochemical methods, and hydrogen permeation method. There was no literature report on the studies of corrosion inhibition effect of the above plant extracts on mild steel in hydrochloric acid medium previously. From literature survey, it were found that the six plants selected for investigation was found to contain some
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