Surface examination and analysis of steel inhibited by 1-hydroxyethylidene 1,1-diphosphonic acid in presence of zinc ions

Abstract

Surface examination and analysis of steel inhibited by different zinc–HEDP mixtures were carried out using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results obtained from these surface techniques combined with those obtained from different electrochemical techniques present better understanding of the mechanism of inhibition of mild steel by zinc–HEDP mixtures. It is shown that the mixture could inhibit the corrosion of mild steel by various mechanisms, the relative importance of which depends to a great extent on the solution composition, particularly the zinc/HEDP molar ratio. Inhibition can be due to the reduction of the anodic metal dissolution rate (0·9<molar ratio<2·7), the cathodic oxygen reduction reaction (molar ratio>2.7), or both (molar ratio=2.7). XPS analysis revealed that the layer formed on the mild steel surface in the presence of zinc–HEDP mixtures is essentially composed of Zn, P, O, and C. Iron is also detected in the metallic state and in the form of different oxides/hydroxides. This is indicative of the incorporation of zinc and HEDP molecules into the layer containing mixed oxides/hydroxides that developed on the mild steel surface. The intensity of the metallic iron peak as well as the amount of Fe decrease progressively with the zinc/HEDP molar ratio, indicating the growth of a thick surface film, as revealed by SEM examination. At low Zn/HEDP molar ratio, the Zn:P ratio in the surface layer is 1 : 1 suggesting the adsorption of the 2 : 1 zinc–HEDP complex (Zn₂H₋₁L⁻) on the mild steel surface. The amount of zinc in the surface layer increases with Zn/HEDP molar ratio, which suggests the formation of either the 2 : 1 complex as well as Zn(OH)₂ or the zinc salt of the complex.