The nature of «black spots» formation on the surface of large parts made of aluminum alloys in the process of anodic oxidising

The causes of black spot defects detected on the surface of large parts made of aluminum alloys (D16chT, 1973T2, V95pchT2, V95pchT3) following anodic oxidation   are investigated in the paper. A hypothesis of such defects appearing during the dissolution of intermetallics on the surface of a part in the process of etching during anodizing was put forward and tested. A technique was developed for this purpose the essence of which was that an intermetallic was detected on a polished sample and using a microhardness tester the area including the intermetallic was marked to facilitate its subsequent detection at the end of the specimen etching. After the etching of the specimen during periods of different duration in an alkaline bath of the anodizing line the area containing an intermetallic previously labeled using the microhardness tester was detected by microscopic observations and photographed. The duration of the etching process was 120 seconds and consisted of 8 cycles. Metallographic tests of the specimens. revealed the fact that in the process of etching of an aluminum alloy dissolution (etching) of intermetallic particles opening out on the surface of the part takes place. Subsequent anodic oxidation leads to the visualization of defects which have the form of black spots.