High-speed camera image of an anodised aluminium substrate (250 V, 0.125 A/cm−2, 3 min) after a short PEO process treatment (750 V, 0.125 A/cm^2, 40 µs) (on the top). Before the treatment, a part of the substrate (dark grey square) was covered with a gold layer of 50 nm thickness. The exposure time of the high-speed camera amounts to 0.1 ms. The zoomed image shows a part of the gold-covered surface and the corresponding SEM image is presented on the bottom. The dashed ellipse and rectangle in the SEM image show the areas of the microdischarges (areas of intensive emission) observed in the camera image on the top. The zoomed image (dashed rectangle) on the bottom presents the surface structure of erosion traces.