Research on non-toxic inhibitors is of considerable interest in investigations into the replacement of hazardous classical molecules. This paper reports the action of four amino acids containing sulfur on the corrosion of mild steel in phosphoric acid solution with and without Cl À , F À and Fe 3 ions near and at the corrosion potential (E corr ) using both the polarization resistance method and electrochemical impedance spectroscopy (EIS). Both cysteine and N-acetylcysteine (ACC) showed higher inhibition ef®ciency than methionine and cystine. Adsorption of methionine onto a mild steel surface obeys the Frumkin adsorption isotherm and has a free energy of adsorption value (DG°a ds ) lower than those obtained in the presence of cystine, cysteine and ACC whose adsorption isotherms follow that of Langmuir. Both F À and Fe 3 ions stimulate mild steel corrosion while Cl À ions inhibit it. The binary mixtures of methionine, cysteine or ACC with Cl À or F À ions are effective inhibitors (synergism) while the combinations of the amino acid with Fe 3 or the ternary Cl À /F À /Fe 3 mixture have low inhibitive action (antagonism). EIS measurements revealed that the charge transfer process mainly controls the mechanism of mild steel corrosion in phosphoric acid solution in the absence and presence of the investigated additives. The mechanism of corrosion inhibition or acceleration is discussed.

The effect of benzenethiol and its methyl, amino, and carboxylic ring substituted derivatives, benzylthiol and thioglycollic acid on the electrochemical and corrosion behavior of zinc in CH3000H, H2SO4, and HCI solutions was studied using the galvanostatic technique. In CH3000H, compounds which function by an adsorption mechanism were found to have inhibitive effects on the corrosion of zinc, while those functioning by surface chelation were ineffective. In H 2SO4 and HCI, with the exception of o-methylbenzenethiol (H 2SO4 and HCI) and benzylthiol (H2SO4 ), all investigated compounds were found to accelerate zinc dissolution. Adsorption of the inhibitors followed the Langmuir isotherm, and the mechanism of both the hydrogen evolution reaction and zinc dissolution were found to be the same in the uninhibited and inhibited states. Experimental Corrosion InhibitorsReagent grade (Fluka AG, Buchs SG Switzerland) benzenethiol ortho, meta, and para methylbenzenethiols, ortho and para aminobenzenethiols, thiosalicylic acid and its methyl ester, benzylthiol and thioglycollic acid were used without further purification. The inhibitor solution was prepared by dissolving the appropriate amount in 10 ml of redistilled methanol. The desired volume was added to the electrolyte (200 ml).Electrolytes 0.5N CH3000H (pH 3.0), 0.5N H 2SO4 (pH 0.8), and 0.1N HCI (pH 1.1) were prepared from the Analar reagent and bidistilled water and were deaerated by 02 free nitrogen for at least 6 hours. The pH of these solutions was adjusted by adding NaOH to the desired value. Zinc SpecimensSpecimens were cut from extra pure zinc rods 0.5 cm in diameter. These were cast from extra pure granular zinc (Merck) by vacuum melting) and were inserted into Teflon mounts so that the flat surface was in contact with solution.

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