THE SYNERGISTIC EFFECT OF HALIDE-IONS AND SOME SELECTED THIOLS AS A COMBINED CORROSION-INHIBITOR FOR PICKLING OF MILD-STEEL IN SULFURIC-ACID-SOLUTION

The effect of halide ions on the electrochemical and corrosion behavio ur of mild steel in sulphuric acid solution with and without some sele cted thiols, viz. 2-mercaptobenzoxazole, 2-mercaptobenzothiazole and 2 -mercaptobenzimidazole, has been studied. Each of the halide ions and the investigated thiols were found to inhibit the corrosion process of mild steel when added separately to the corrosive medium, while the i nhibition efficiency of thiols is higher than that of halides. It was found that the presence of halide ions together with the different thi ols in the corrosive medium decreases the inhibition effect, specially at low concentrations. The surface coverage values which are obtained both from weight loss and polarization studies are approximately the same. The decrease in corrosion rate associated with an increase in th e cathodic overvoltage and a positive shift in the corrosion potential , in the presence of the investigated thiols, denotes inhibition of a mixed type (predominantly anodic). Synergism of the investigated thiol s and halides is discussed from the viewpoint of a model of co-adsorpt ion of halide ions and thiols. Adsorption of the inhibitor was found t o follow the Langmuir isotherm.