Both of these aluminium anodising defects are associated with disturbances in electrical contact during the anodizing process.
- Anodizing burn
- Loss of electrical contact
This is frequent “electrical” anodising defect. A soft, powdery anodic coating on aluminum profiles or, in more serious cases, complete dissolution of the coating to produce an electropolishing effect on the aluminum surface at the defect site. Anodic burn occurs when the current density is excessive and the anodic solution is inadequately mixed (Figure 1).
This anodising defect does not occur with the correct anodizing technology:
- correct hanging of profiles to ensure uniform current distribution
- good movement of the electrolyte through the sample
- the correct ratio of current density, composition of the anode solution and temperature.
Loss of electrical contact
Racking – Anodizing
Light grey or dark gray surface of aluminum profiles, on which die lines or streaks are visible. They may have an iridescent appearance.
To demonstrate this difference in surface appearance, Figure 2 shows:
- an aluminum extrusion that has been partially etched and anodized to normal anodic film thickness (left)
- the other half of the extrusion has been left in the anodic solution without current (right).
A sample surface that has not been properly anodized shows the rainbow colors characteristic of a very thin anodic coating. This effect is less noticeable if electrical contact is lost midway through the anodizing process. In that case the defect will only be detected by measuring the actual thickness of the anodizing coating.
- The aluminum profile can be mechanically processed, if possible, and re-anodized.
- With well-developed technology for racking aluminum profiles, this electrical anodising defect will be reduced to a minimum.
1. Defects Analysis in Anodizing /Bаrry R. Ellard – Aluminum Extrusion Technology Seminar, 2000