Humic acid: A new corrosion inhibitor of zinc in chlorides

Aiming to simulate the corrosion of Zn-coated steels in soils containing high quantities of organic matter, we studied the corrosion of Zn in 0.1 M NaCl containing 2 g•L−1 humic acid (HA). Up to around 4 h exposure, HA slightly promotes corrosion, while after 20 h, it acts as an inhibitor reaching efficiencies around 90%, as inferred from EIS and mass loss data. Voltammetry and EIS evidenced the slow formation of a secondary passive layer that explains the HA inhibition. FTIR and XPS spectroscopy showed that the additional layer contains Zn humate, which blocks the lateral growth of pits, as observed by SVET (scanning vibrating electrode technique).