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Sulfuric acid Anodized Aluminum and Aluminum Alloy Black

After sulfuric acid anodization, you often see black spots and black bars on the surface of aluminum parts. Today we will talk about the causes of blackening aluminum. Please criticize and correct any deficiencies.

1. Before the part is anodized, if the surface condition is not good, the blackening phenomenon after anodization will appear, which is generally expressed as irregular black spots. One is that there are corrosion spots on the surface of the part. After anodization, the corrosion spots cannot be effectively removed after the process is completed, resulting in film coverage. Generally there will be a film there, but after filling, it will be visible due to visual reflection The location of the corrosion point presents a black spot. The second is because there are attachments on the surface of the parts, metal shavings, etc., this is very small, there may be no film layer after the anodization.

2. Forging defects of parts, this is generally a regular black bar that is consistent with the forging streamline direction. The causes of this phenomenon are mostly forging defects, uneven forging, inconsistent crystal gaps, and selective corrosion during anodization.

  • Segregation of raw material components or aggregation of compounds, which generally appear as black areas after magnification. Metallographic and compositional analysis of the black area will reveal that the two structures are inconsistent, and there are differences in composition at the same time, resulting in inconsistent corrosion resistance in the two places. During the anodization process, selective corrosion occurs, that is, the corrosion resistance of the segregated location will decrease, where the corrosion is faster, and the corrosion area is formed, and the black area appears after the anodization.
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