The influence of some trivalent metal cations on the structure and corrosion properties of phosphates

Abstract

This study presents an investigation into the inhibitory properties of phosphate-based systems used for the protection of metals against corrosion. Various types of phosphates are examined, including ortho-, di-, and cyclophosphates, as well as their interactions with metals in aggressive environments. Special attention is given to the influence of iron modifier ions on the efficiency of corrosion inhibition. The potential of modifying phosphate systems to enhance protective properties is analyzed.

The aim of this research is to study and apply complex inhibitors that provide a high level of protection for steel structures against corrosion. The material used for the investigation is carbon structural steel of ordinary quality, compliant with the European standard S235JR (EN 10025-2). Various inorganic phosphate formulations modified with trivalent iron, aluminum, and lanthanum ions were selected as inhibitors.

Shifts in spectral bands were observed, corresponding to the deformation vibrations of hydroxyl groups and Al-O bonds. These shifts suggest interactions between the ionic dopants and the phosphate matrix. Specifically, bands shifted from 735 to 671 cm⁻¹. Similarly, the band associated with Al-O interactions changed from 530 to 555 cm⁻¹, while the HPO₄⁻ band shifted from 940 to 979 cm⁻¹ in the presence of Al³⁺ ions. Deviations in the vibrational frequencies of Al-O and HPO₄⁻ indicate changes in chemical bonding and increased reactivity of the inhibitors.

Scanning electron microscopy (SEM) provided high-resolution images and elemental composition of corrosion deposits on the steel surface. The method revealed the distribution of elements: oxygen (36.17%), iron (26.79%), phosphorus (4.72%), as well as minor amounts of sodium (0.78%) and aluminum (1.39%) in corrosion products, and allowed localization of protective layers.

The results of this study demonstrate that modifying phosphates with trivalent iron, aluminum, and lanthanum ions leads to significant changes in their inhibitory properties.

Keywords: corrosion, inhibitor, orthophosphates, degree of protection, depth index.