Inhibition of Low-Carbon Steel Corrosion in a Solutions of Mixture of Hydrochloric and Phosphoric Acids Containing Iron(III) Salts

The corrosion of low-carbon steel in 1 M HCl + 1 M H3PO4 solution containing Fe(III) salts is studied under conditions of natural and forced convection. In this environments, partial reactions of iron anodic ionization, H+ and Fe(III)-cation cathodic reduction on steel occur. The first two reactions pass under kinetic control; the last one is diffusion-controlled. A composition of 4.5 mM IFKhAN-92 + 0.5 mM KNCS + 200 mM urotropine is studied as a steel corrosion inhibitor in this environments. In the solutions under consideration, the three-component inhibitor slows down all partial reactions involving steel and, as a consequence, its corrosion, which is a result of the polymolecular protective layer formation on the metal surface. The empirical dependence of the steel corrosion rate on the flow intensity of such media is described by the linear equation k = kst + λw1/2, where kst is the steel corrosion rate in a static medium, w is the rotation velocity of the propeller mixer creating the medium flow, λ is the empirical coefficient characterizing the rate of increase of steel corrosion increment. The presence of a three-component-inhibitor additive in acid solutions containing Fe(III) salts reduces the values of kst and λ parameters significantly, indicating that it slows down the metal corrosion in both static and dynamic environments. The 1 M HCl + 1 M H3PO4 solution added with the studied three-component inhibiting composition is shown to be able using as a medium for cleaning low-carbon steel surfaces from mineral deposits, resistant to the Fe(III) salts’ accumulation.