Comparative study of three sodium phosphates as corrosion inhibitors for steel reinforcements

A comparative study was performed on the inhibition of steel reinforcement corrosion using three soluble phosphates: sodium monofluorophosphate (Na2PO3F), disodium hydrogen phosphate (Na2HPO4) and trisodium phosphate (Na3PO4). Tests were carried out using ordinary Portland cement (OPC) paste specimens and OPC mortar specimens with embedded steel reinforcements. The corrosion inhibitors were deployed in two different ways: by immersion of OPC specimens in aqueous solutions containing the soluble phosphates (migrating corrosion inhibitor), and by addition of the phosphate powders to a fresh OPC paste (admixture corrosion inhibitor). After curing, the tested specimens were studied using X-ray diffraction, wavelength-dispersive electron microprobe analysis, linear polarisation resistance and electrochemical corrosion potential. A correlation was found between the phosphate content (by migration or admixture) in the OPC matrices and the steel corrosion rate.