The dual character of an aluminium—thiourea mixture in an NaCl aqueous solution—an inhibitor and an accelerator of corrosion for low carbon steel

Abstract

The effect of temperature on the corrosion inhibition of low carbon steel in 31.3 g l −1 NaCl solution has been investigated. A mixture of aluminium sulfate and thiourea (A + T) was found to be an effective corrosion inhibitor at temperatures <50°C, but both A + T and A alone accelerate corrosion at temperatures >60°C. Pits formed in the presence of A alone at both high and low temperatures. The film formed on unhibited steel, identified by infrared spectroscopy, was lepidocrocite (γ-FeOOH). The film formed in the A and A + T systems was a form of hydrous aluminium hydroxide containing weakly bound molecular water with the structure similar to pseudo-boehmite. X-ray diffraction showed that at low temperatures the surface layer produced in A was in amorphous form and that in A + T possesses the crystalline characteristics of υ-Al 2O 3. From the results of pH monitoring on the steel surface and in the bulk solution, the possible mechanisms for the dual character of A + T have been proposed. The inhibition of A + T at low temperatures is due to the formation of crystalline υ-Al 2O 3 which provides a corrosion barrier; the acceleration of A + T at high temperatures is not only due to the non-protective film which has been peptized in a charged amorphous gel, but is also due to the extra attack of the acid, presumably induced by the dissociation of water and the hydrolysis of polynuclear aluminium hydroxide complexes.