Growth Behavior of Initial Product Layer Formed on Mg Alloy Surface Induced by Polyaniline

The quality of the interfacial protective layer induced by polyaniline (PANI) has been claimed to play a crucial role for the enhanced corrosion protection of Mg alloy, but its growth behavior is not well understood. Here some composition, structure and growth kinetics of the protective layer formed at the PANI-emeraldine base (EB) coating/AZ91D Mg alloy interface were investigated to explore the growth mechanism. Upon immersion in 0.5 M NaCl solution, the growing interface layer under EB coating exhibited a fast passivation rate and an increased corrosion resistance, which was largely influenced by ion concentration. XPS depth profiles showed that the EB-induced layer was a mixture of MgO and Mg(OH) 2 , in which no significant bi-layer structure existed and MgO was dominant throughout the bulk film. These observations suggest that the interaction between EB and Mg can promote the faster growth of a stable MgO-rich layer mixed with Mg(OH) 2 probably by solid reaction. Meanwhile less hydration of MgO and dissolution-precipitation reaction occur, thus leading to less Mg(OH) 2 in the outer layer.