Natural organic antioxidants from green tea inhibit corrosion of steel reinforcing bars embedded in mortar

Abstract

Natural antioxidants with multiple polar atoms and electron-rich bonds are potential mixed-type corrosion inhibitors and ‘green’ alternatives to commercial inorganic corrosion inhibitors. Green tea is one of the richest sources of natural antioxidants. Therefore, this paper compares the corrosion inhibition efficiency (IE) of green tea extract (GT) against commercial calcium nitrite corrosion inhibitor (CI), on steel reinforcing bars (rebars) embedded in mortar. Rebar corrosion was accelerated with impressed current and cyclic wetting-drying, and corrosion development was measured with linear polarization resistance. Selected CI dosages encompassed four chloride-to-nitrite ratios, and the IE of CI and GT were compared at: 1) similar concentration and 2) equal volume. At similar concentration, CI and GT demonstrated a similar IE (51–70%). However, at equal volume, GT exhibited a higher IE than CI (75–80% vs. 14–24%), due to a significant increase in rebar polarization resistance and reduction in iron oxidation rate (anodic slope). Meanwhile, oxygen reduction rate (cathodic slope) and open circuit potential did not change significantly. The changes in anodic and cathodic slope without a significant open circuit potential change suggested that GT was a mixed-type corrosion inhibitor as hypothesized, which formed a protective layer on rebar surface and increased the polarization resistance of the rebars. Based on liquid chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry analyses, it is hypothesized that (-)-epigallocatechin gallate, (-)-epicatechin gallate, and catechin or (-)-epicatechin are responsible for the anti-corrosion activity of GT as a mixed-type corrosion inhibitor.