DEFECTS

Defects

Surface Bollosity

A typical defect of 7000 series alloys (rarely observed in other alloys), consisting of swelling of the superficial oxide layer.

This phenomenon is caused by the presence of oxides or other heterogeneous particles trapped in the surface (cortical) areas of the extruded material, often associated with gas inclusions. During the thermal cycles involved in oxide formation and sealing, these inclusions tend to migrate toward the surface, generating localized swelling of the anodized layer.

These “bubbles” create weak points in the oxide layer. When subjected to abrasion (e.g. with abrasive paper), they may rupture, exposing the underlying base metal.

Various corrective actions can be evaluated on a case-by-case basis to minimize or mitigate the issue. However, since these defects often result in actual micro-craters in the base material, simply removing the oxide layer is not sufficient. Proper mechanical reworking is required before retreatment.

Flames, Streaks or Bands

This defect is often not visible before anodizing and may only become evident after surface preparation (etching or pickling). After anodic oxidation, it appears as areas with different shades (lighter or darker) compared to the surrounding surface.

The root cause is typically linked to an uneven distribution of intermetallic phases or heterogeneous particles within the base material.

Components affected by this defect are difficult to recover through anodizing alone. Certain finishing techniques may reduce its visual impact, but complete elimination is rarely achievable.

Welding Effects

Welding on aluminum components intended for anodizing can lead to aesthetic inconsistencies, even when performed correctly.

It is essential to use a filler material compatible with the base alloy. However, due to the thermal stresses introduced during welding, the affected area may exhibit differences in microstructure, resulting in visible color variations after anodizing.

Pre-Treatment Corrosion Dots

Aluminum, particularly 7000 series alloys, is prone to spontaneous oxidation. If components are left wet after machining (with water or machining oils), corrosion may occur.

In some cases, corrosion is visible and can be removed through appropriate pre-treatment finishing. In other cases, corrosion defects may be subsurface and only become evident after anodizing, leading to an irregular or defective oxide layer.

The only effective solution is:

• Deoxidizing the component

• Performing mechanical reworking to restore surface uniformity (if tolerances allow)

• Reapplying the anodizing treatment