Origin of stains on anodized aluminium profile

The origin of stains on the surface of anodized aluminium profile was investigated. Stains were visible on the surface under the specific angle of observation. Anodized surface and layer were investigated by scanning electron microscopy, metallography of cross section of the layer was performed and EDS analyses were done. Presented are results of investigation. No difference was observed by scanning microscope between the surface with and without stain. The metallography of cross section of anodized layer revealed prevailing compact layer, only in the region with the stain a small pores were observed below the surface. The pores were not connected to the surface. The EDS analyses detected in the pores the presence of oxygen, sulphur, fluorine and nickel. Sulphur confirmed the sulphuric acid was used as electrolyte at anodizing process, but fluorine and nickel are evidence that pores were cold sealed in solution containing NiF2. Investigations confirmed that apparent stains were an optical effect, a consequence of different refraction of the light due pores in the layer. The main reason for the rests of pores in anodized layer was not optimised anodizing process.