Natural organic antioxidants from green tea form a protective layer to inhibit corrosion of steel reinforcing bars embedded in mortar

In our previous work, we have demonstrated that at equal volume, green tea extract (GT) exhibited a higher inhibition efficiency (IE) than commercial calcium nitrite corrosion inhibitor, on corrosion of steel reinforcing bar (rebar) embedded in mortar (75–80 vs. 14–24%). GT behaved as a mixed-type corrosion inhibitor which increased rebar polarization resistance (Rp); indicating that it formed a protective layer on rebar surface. In this paper, the formation of a protective layer was confirmed with visual inspection and microscopic examinations. In presence of this layer, rebar corrosion was reduced. This is evidenced by a reduced rebar weight loss and further indicated by a similar chloride permeability between control and GT concrete, and a similar corrosion rate (CR) between control reinforced mortar and reinforced mortar incorporating solely the residual solid of GT. The similarities ruled out the plausible higher IE of GT due to an improved physical barrier of mortar/concrete against corrosion. Analyses with energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD), and Fourier-transform infrared spectroscopy (FTIR) suggested that the layer was enriched with calcium, specifically the calcium carbonate polymorphs. Although the formation of protective layer and increase in Rp were influenced by the magnitude of antioxidant activity, in this study GT has shown a better IE than CI. This should encourage more studies on the IE of sustainable ‘green’ corrosion inhibitors in concrete.