Contrasting Behaviour of Mild Steel and Pure Iron towards Pickling Inhibitors

Long-chain n-alkyl-trimethyl- and -triethyl-ammonium sulphates give good inhibition of the corrosion of mild steel in 0·5 M H₂S0₄, but little or no inhibition with pure iron. This is probably because mild steel is negatively charged in O·5 M H₂SO₄ whereas pure iron ispositive.

Although the sulphates of n-alkyl-trimethy- and -triethyl-ammonium compounds give little inhibition of the corrosion of pure iron in 0·5 M H₂SO₄, the corresponding bromides give good inhibition. The experimental results suggest that there is a mutual stimulation of adsorption between the bromide and the quaternary ammonium ions.

Polarisation measurements show that the quaternary ammonium bromides give a higher proportion of cathodic inhibition with iron than with steel, presumably because of the more restricted cathodic reaction of iron.There are considerable differences between mild steel and pure iron in the effects of the quaternary ammonium bromides on the anodic and cathodic Tafel slopes. These results are discussed in terms of the different mechanisms of adsorption of the inhibitors on steel and iron.The positive charge on pure iron enhances the specific adsorption of Br⁻ and Cl⁻ ions, and they both give appreciable corrosion inhibition in 0·5 M H₂SO₄. The effect on mild steel may be complicated by competition with HS⁻ ions, since it is found that, whereas Br⁻ ions give some inhibition, the weakly adsorbed Cl⁻ ions actually cause acceleration of corrosion.Thiourea derivatives are effective pickiing inhibitors for steel, but at low concentrations they greatly stimulate the acid attack on pure iron. At higher concentrations, above about 10⁻⁵−10⁻⁴ M, the thiourea derivatives give good inhibition of the acid corrosion of iron.