SEM image of pretreated aluminium substrate (PEO, 750 V, 0.125 A/cm^2, 30 min) in the area of the additional square gold layer (5 mm × 5 mm × 50 nm) after an additional short PEO treatment (750 V, 0.125 A/cm^2, 40 µs). Shallow erosion traces can be observed near cracks in the previous PEO oxide coating.