Reason analysis of black spots appearing on anodization film of pure aluminum clad 2A12 aluminum alloy sheet and countermeasures
[Introduction]Plenty of pinpoint-like black spots appeared on the surface of pure aluminum clad 2A12 aluminum alloy sheet after sulfuric acid anodization and sealing.It is very important to solve this problem for guiding the anodization production.[Method]The micromorphologies and elemental compositions of anodization film and substrate at black spot and normal area were compared by electron scanning microscopy(SEM)and energy-dispersive spectroscopy(EDS).The causes for the formation of black spot were analyzed according to the actual production,and corresponding verification tests were conducted.Some countermeasures were presented.[Result]There were some carbon-containing inclusions on the surface of pure aluminum clad 2A12 aluminum alloy sheet,which didn't participate in the reactions related to film formation during anodization.Therefore,it was proposed to treat the substrate with a mixture of nitric acid and hydrofluoric acid prior to anodization.The black spot defects were eliminated.[Conclusion]The state of substrate is one of the important factors affecting the sulfuric acid anodization of pure aluminum clad aluminum alloy sheet.Not only the process parameters of each procedure but also the quality of substrate should be strictly controlled.
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