Abstract
This investigation was made of the dark spotted area on the integrally colored anodic oxidation coatings of an extruded 6063 alloy deposited from organic acid baths.
This spotted area was likely to appear on a large sized extrusion. The hardness of that area was little changed by ageing and black smut was formed by etching it with sodium hydroxide solution.The same phenomenon also appeared in the laboratory on the alloy, which had been heat-treated to precipitate β'-phase. It was considered that the spotted area was attributed to an abnormal precipitate by local slow cooling of the extrusion. It was also considered that the area was derived from the co-existence of different stages of ageing in the precipitate. It was confirmed in the laboratory that the current density was considerably varied in different stage of ageing in integrally colored anodic oxidation coatings.
