Characteristics of corrosion inhibition admixtures in OPC paste with chloride additions: Part I: Chemistry and electrochemistry

The chemical modfications to OPC paste and pore fluid compositions produced by 1–5% nitrite, chromate and phosphate–ased admixes have been studied. Admixture partitioning and modes of corrosion inhibition have been determined. Constitutional differences between the solid paste with and without chloride additions were analysed, as well as pH, admixture anion and chloride ion concentrations of the extracted aqueous phase. Electrochemical rest potential measurements and linear polarization methods were employed to assess inhibitor efficiency, using mild steel electrodes embedded in cement paste cylinders in accelerated experiments at 50°C and 100% humidity. Admixture anion partitioning between the solid paste and pore fluid phase is variable. The CrO₄ ion concentration in the pore fluids remains low; under 10% of the total is soluble, as compared to about 30% for nitrite and phosphate ions. Free CI ion levels in the pore fluid with and without admixture are found to range between 55 and 75% of the total after 75 days cure but the [Cl]/[OH] and [Cl]/[admix] ratios tend to be relatively constant after 28 days. Corrosion rate determinations indicate that phosphate and nitrite–based admixtures become effective as inhibitors during setting, but chromate begins to be effective only after about 28 days. This delayed action of the chromate ions may be linked to the initially high [Cl]/CrO₄] ratio, which was about seven. he apparent differences in modes of inhibition result from the extent to which admixture partitioning modifies the chemical and electrochemical environment surrounding the steel reinforcement.