Entrapped molybdate in phytate film and the corresponding anodic corrosion inhibition on AA2024-T3

A strong synergistic corrosion inhibition effect was observed when phytate and molybdate were combined and applied onto a AA2024-T3 substrate. pH 5 was found to be the optimum condition as revealed by free corrosion tests in 3.5% NaCl solution with 1mM phytate and 10mM molybdate. The inhibition mechanism was explored by electrochemical, microscopic, and surface analytical techniques. XPS, SEM-EDS, and Raman results show that both phytate and molybdate exist on the metal surface treated at pH 5, wherein both pit initiation and pit growth were suppressed; while only phytate was detected under pH 8 conditions, wherein only pit initiation was reduced. It is therefore hypothesized that phytate suppresses pit initiation while molybdate-phytate complex plays a major role in reducing both pit initiation and pit growth. The complexation between phytate and molybdate at pH 5 was validated by UV-vis spectroscopy, but no interaction was detected between these two species at pH 8. Combined with surface analysis, this supports that molybdate is entrapped and incorporated into the phytate film on the substrate at pH 5. Titration study results suggest that the buffering effect offered by the combination of phytate and molybdate around pH 5 may prevent local acidification/alkalinization and therefore reduce localized corrosion.