SVET and SECM imaging of cathodic protection of aluminium by a Mg-rich coating

The mechanism of cathodic protection of an aluminium substrate by a Mg-rich coating was investigated using localized techniques. Both scanning vibrating electrode techniques (SVET) and scanning electrochemical microscopy (SECM) were used to investigate the processes occurring at the surface of exposed metal when electrically connected or disconnected in a galvanic couple with the Mg-rich coating. The SVET has shown the evolution of the pit activity with time under conditions of sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. It was shown that the cathodic protection provided by magnesium to aluminium substrates acts by both preventing pit nucleation and inhibiting the growth of the pre-existing pits.