Fig a presents the typical structure

Both the Ecorr for Cr6+-treated ENP coating and MCC thin film shift toward negative values, this negative shift phenomena are far different from the conversion coating on magnesium alloy [30]. The Ecorr of Cr6+-treated ENP coating shifts from –0.401 to –0.562 V, suggesting that the obtained Cr6+ conversion coating disennobles the corrosion potential of the surface. This negative shifting affects mostly the anodic portion of the Tafel plots by reducing the anodic current, because the oxidation reactions on the conversion coating are effectively limited due to staying at lower potential than before.