Tannins as interphase corrosion inhibitors for aluminum in near‐neutral chloride solutions

The addition of tara and quebracho colorado tannins remarkably reduces the corrosion rate of aluminum in near‐neutral and naturally aerated NaCl solutions. The experimental results (potentiodynamic polarization curves, corrosion potential measurements, scanning electron microscopy–energy‐dispersive X‐ray spectroscopy [SEM‐EDS] analysis on exposed surfaces) show that both tannins behave as a mixed‐type corrosion inhibitor of aluminum. In the absence of tannins, the SEM‐EDS analysis of the exposed samples demonstrates the accumulation of corrosion products and the existence of chlorinated compounds adsorbed on the metal surface. In the samples immersed in both tannins solutions, a more homogenous and compact layer is formed over the aluminum surface. Furthermore, in these layers chlorine was not found, revealing a high blocking capacity of the chloride adsorption when the tannins are present. Taking into account these findings, an inhibitory mechanism of aluminum corrosion is proposed. This mechanism is interphase inhibition, which involves the adsorption and incorporation of tannin molecules during the corrosion products passive layer formation, resulting in a protective three‐dimensional structure that retards both anodic and cathodic reactions.