Anodizing defects: stripes from polishing, brush, chemical segregation

 

For successful application anodized aluminum, especially for decorative purposes, its surface should have a perfect appearance. However, the quality of anodized profiles is often overshadowed by defects in the form of alternating dark and light stripes..

Defects in the anodizing of aluminum profiles in the form of strips may have as a result preliminary machining of the surface of the profiles. This treatment is done to improve the appearance of the surface of the profiles or to give it any texture.

These defects usually occur due to errors at this stage of production., although as in the case of segregation bands, the cause of their occurrence at this stage may be previous stages of production.

Defects from mechanical polishing

English term

Mechanical polishing defects

defect Source

Surface preparation

The manifestation of the defect

Anodizing

Details of the defect

Different zones of polished and anodized extrusions show different gloss levels or irregular longitudinal defects are visible on the anodized surface (figure 2.1).

defect-anodirovaniya-polosy-polirovaniyaFigure 2.1

Preventing defect

  • These defects can be removed by re-polishing and anodizing..
  • The cause of defects in mechanical polishing is a violation of the polishing technology or the presence of polishing residues on the surface of the profile before its anodizing.

Strips from brushing

English term

Linishing operations streaks

defect Source

Surface preparation

The manifestation of the defect

Mechanical Preparation - Etching – Anodizing

Details of the defect

Strips after satin operation – brushing – may occur as dark stripes, usually in the direction of pressing as shown in the figure 2.2. They are associated with the microscopic roughness of the pressed surface..

defect-anodirovaniya-polosy-satinirovaniyaFigure 2.2

Preventing defect

  • These bands result from local surface damage due to wear or damage to grinding wheels or improper satin technology., for example, excessive pressure, resulting in local overheating.
  • This defect can be avoided by using the right surface treatment technology..

Segregation Stripes

English term

Segregational streaking.

defect Source

Ingot (billet) - Pressing.

The manifestation of the defect

Travlenye - Anodyrovanye.

Details of the defect

Segregation strips of this type are not visible in the pressed state of the material.. They look like segregation bands of type defects 1, but unlike them appear only after mechanical polishing and subsequent anodizing. Sometimes they can be removed by significant machining.. However, this is not always possible., for example, when trying to remove matrix traces results in a "woodgrain". On pic. 2.3 “wood texture” shown on etched and anodized profile, arising after selective polishing. Segregation bands result from different etching sensitivity of different sections of the profile, having a different metallurgical structure due to ingot segregation or alloy contamination from metal residues in the container or matrix ports.

defect-anodirovaniya-polosy-segregaciya-2Figure 2.3

Preventing defect

  • Profiles with such a defect cannot be “saved” by re-polishing or grinding.
  • This defect can be avoided by improving the ingot casting system to minimize reverse segregation, as well as pressing technology so, to prevent the flow of contaminants into the extruded profile.

Source: Barry R. Ellard, Aluminum Extrusion Technology, 2000