The Role of Humic Acid in the Corrosion of AZ31 Magnesium alloy

Magnesium alloys used as sacrificial anodes can interact with humic acids (HAs) in soil and water environments. Therefore, these natural organic molecules can have an impact on the corrosion behaviour of magnesium materials. In this work, the aim was to investigate the effect of lignitic HA on the corrosion of AZ31 magnesium alloy in 3.5 % NaCl at different pH (5, 7 and 9). The results showed that HA increased the corrosion rate about four times at pH 7, while it weakly decreased the corrosion rate at pH 5 and 9. The acceleration of corrosion is probably related to the depletion of Mg ions by forming complexes with HA instead of producing Mg(OH)2. The increase in corrosion resistance may be related to the reduced stability of the complexes and adsorption on the AZ31 alloy surface at pH 5 and 9. Surface analysis suggested that the presence of HA promotes the formation of Mg(OH)2 nanoparticles.