Abstract
The corrosion behaviour of mild steel has been evaluated by accurate weight loss measurements in aerated phosphoric acid/organic solvent mixtures. The media studied were butanol, butane 1,4-diol, isopropyl alcohol, butane 1,3-diol and propylene glycol. The results show that the organic solvents improved the corrosion resistance of mild steel to an extent depending on the concentration of the acid and the solvent as well as on the different protic and stereochemistry properties of the alcohols considered. The inhibition efficiency increased in the order: butanol > butane 1,4-diol > isopropyl alcohol > butane 1,3-diol > propylene glycol. The effect of chloride ions (HCl and CaCl2) on the rate of dissolution of steel in the presence of mixtures of phosphoric acid and alcohol has been investigated. Aqueous solutions containing Cl– ions inhibit the dissolution of steel in phosphoric acid, such inhibition being enhanced in the presence of propylene glycol or butanol. The corrosion rate of steel in phosphoric acid was also measured as a function of the concentration of aniline derivatives. It was found that 3% vol/vol inhibitor was sufficient to level off the protection efficiency. The order of inhibition efficiency was n,n-dimethyl aniline > n-methyl aniline > aniline. Good agreement was obtained between the kinetic–thermodynamic and Flory–Huggins models. At 35% and 50% phosphoric acid concentration, each molecule of the inhibitor occupied more or less than one active site on the metal surface depending on the molecular structure of the inhibitor. The corresponding negative values of the free energy of adsorption varied between 21.5 and 23.3 kJ/mol and 17.0 and 21.1 kJ/mol respectively for inhibitors at low and high acid concentrations.
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