Typical defect of 7000 series alloys (rarely found in other alloys) which consists of a swelling of the superficial oxide layers.
This defect depends on the inclusion of oxides, or other heterogeneous particles, in the cortical areas of the extruded, associated with gas. When the detail undergoes thermal swings in the formation and fixing stages of the oxide, these inclusions tend to migrate to the exterior areas, swelling the anodized protective layer .
These bulbs turn into oxide weakening areas and if scraped with an abrasive paper break, leaving the metal underneath uncovered.
There are several possible solutions to be evaluated from time to time to minimize or avoid the problem but, of course, these “bubbles” create real variable craters in the base material and it is not enough to remove the oxide layer and retrieve the pieces , They must be adequately prepared.
FLAMES, STREAK OR BANDS:
Unrecognized defect before anodic oxidation, sometimes traceable after decapitation. After anodizing, the appearance is characterized by lighter or darker areas than normal.
The origin should be sought in the distribution of undetermined intermetallic phases or, at times, of heterogeneous particles in the base material.
Hardly stripped particulars are recoverable by anodic oxidation, some finishes may make it less obvious.
Welding on an aluminum detail to be treated with anodic oxidation is a problem, if performed with accuracy, mainly aesthetic. It is important that they are made with the same alloy as the base material; In any case, the thermal stress under the affected area is likely to cause the welding in question to be different from the base material (different colour tone).
PRE – TREATMENT CORROSION DOTS:
Aluminum (especially the 7000 series alloys) is a material subject to spontaneous oxidation. If after mechanical working is left wet with water or working oils, it tends to corrode.
Sometimes corrosion is evident and it can be removed with proper finishing techniques before the treatments, and other corrosion defects (which may also be profound) are highlighted by the oxidation itself, resulting in defective surface layer.
The only solution is to deoxidize the details, work them to create homogeneity in the material (if the tolerances allow it) and then retreat them.