Characteristics of corrosion inhibition admixtures in OPC paste with chloride additions Part II: Microstructures and mechanisms

The changes in ordinary Portland cement paste micro-structure resulting from admixed nitrite, chromate and phosphate-based corrosion inhibitors have been investigated by analytical electron microscopy techniques. The mechanisms by which these admixtures inhibit corrosion are also examined in accelerated experiments, made at 50°C, using pure iron powder embedded in the mix and from the chemical and electrochemical characteristics of the pastes reported in Part 1 of this Paper. Both the morphology and distribution of the principal products of cement hydration, notably calcium hydroxide [Ca(OH)₂] and calcium-silicate-hydrate (C-S-H), are found to be dependent on the admix anion type. Thus the admixes condition characteristic differences in paste microstructures. Energy-dispersive elemental X-ray mapping indicates the precipitation of large quantities of chromium salt in the cement matrix and adjacent to the embedded iron particles, while the phosphate admix appears to enhance the formation of lath-shaped crystals of Friedels salt. The effect of microstructural changes on the processes of corrosion inhibition are discussed.