Assessment of the corrosion protection of aluminium substrates by a Mg-rich primer: EIS, SVET and SECM study

Magnesium-rich coatings have the capability of providing sacrificial corrosion protection to aluminium substrates and therefore present a new and challenging field of development. In this work the mechanism of protection of an aluminium substrate by a Mg-rich coating was investigated using the scanning vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM). The SVET has shown the evolution of the pit activity with time under sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. The study was complemented by classical electrochemical techniques, namely electrochemical impedance spectroscopy and open circuit potential measurement.The results show that magnesium acts in a first stage by both preventing nucleation of pits and inhibiting the growth of the already existing ones, whereas at a later stage the precipitation of a porous layer of magnesium oxide at defective areas leads to some degree of barrier protection.