- alkaline solution composition
- high zinc content in aluminum alloy
Surface heterogeneity aluminum profile after etching, from a "grainy" or "galvanized" surface to a surface with selective grain etching (Figure 1).
The cause of this defect is the presence of zinc as an impurity in the aluminum alloy or alkaline pickling solution.. This defect can also occur during etching of aluminum profiles in an alkaline solution of high concentration or at elevated temperature..
Hollow aluminum profiles are more prone to this defect., than solid profiles, because, what do they have, usually, larger grain size, and also because, that when etching the inner surface of the profile, an increased temperature of the solution occurs due to the limited circulation of the etching solution in its cavity.
Anodizing defect “galvanization”(“tinsel”) most often occurs:
- When using fresh pickling solutions, for example, after solution regeneration
- If aluminum alloy has a high zinc content
- With an increased content of free zinc in an alkaline solution.
- Limitation of zinc in aluminum alloys, for example, alloys AD31, 6060 and 6063, below 0,03 % (drawing 2) 
- Maintenance of the content of free zinc in alkaline etching solution is not higher 50 ppm (0,0050 %) .
This defect does not occur there., where "long-playing" alkaline solutions are used (“E6 solutions”), since they contain sulfides and nitrates, that bind free zinc. For this reason, many “long-playing” alkaline pickling solutions turn very black.
Soft, powdered anode coating on aluminum profiles or, in more serious cases, complete dissolution of the coating with the effect of electro-polishing of the aluminum surface at the defect site. Anodic burn occurs with excessive current density and inadequate mixing of the anode solution (Figure 2).
Figure 3 
This defect does not occur with the correct anodizing technology:
- correct hitching of profiles to ensure uniform distribution of current,
- good movement of the electrolyte through the hitch, and
- correct current density ratio, composition of the anode solution and temperature.
Loss of electrical contact
Loss of electrical contact
Hanging - Anodizing
Light- or dark gray surface of aluminum profiles, on which traces of a matrix or strip are visible and which may have a rainbow tint.
To demonstrate this difference in surface appearance, on the image 4.8 aluminum profile shown, half of which is etched and anodized to normal anode thickness (left), and the other half is left in the anode solution without connecting the current (right).
Figure 4 
Sample surface, which was not properly anodized, shows rainbow colors, characteristic of a very thin anode coating. This effect is less noticeable., if the electrical contact was lost in the middle of the anodizing process - in this case, the defect is detected only by measuring the actual thickness of the anode coating.
Aluminum profile can be machined, if possible, and re-anode. With well-developed technology for hanging aluminum profiles, this problem is minimized.
1. Bаrry R. Ellard, Aluminum Extrusion Technology, 2000
2. T. Short, The Identification and Prevention of Defects on Anodizing Aluminium Parts, 2003