Spangle, the “galvanised” defect of anodized aluminum
See also:. also Classification of defects in anodizing aluminum
Spangle
Source
Etching:
- composition of alkaline solution
- increased zinc content in aluminum alloy
Appearance
Overlapping of Different Corrosion Cells
Nonuniformity of the surface of an aluminum profile after etching, ranging:
- from a “grainy” or “galvanized” surface (Figures 1-2)
- to a surface with pronounced preferential grain etching (Figures 3-4).
Figure 1 – The typical visual spangle defect [1]
Figure 2 – Normal surface versus spangle surface [1]
Figure 3 – Varying level attack after caustic etching.
Preferential grain attack [1]
Figure 4 – Surface of extruded 6063 aluminum alloy profile
with a “spangle” defect.
Steps between grains 5 microns high are visible
(x 2000) [2]
The cause of this defect is:
- the presence of zinc as an impurity in the aluminum alloy or alkaline etching solution.
- etching aluminum profiles in an alkaline solution of increased concentration or at elevated temperatures.
Hollow aluminum profiles are more susceptible to this defect than solid profiles because they tend to have a larger grain size, and also because 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 [3].
This aluminium anodizing defect, “Spangle”, most often occurs:
- When using fresh etching solutions, for example after regeneration of the solution
- If the aluminum alloy has a high zinc content
- With an increased content of free zinc in an alkaline solution.
Prevention
- The limit of zinc content in aluminum alloys, for example, in alloys 6060 and 6063, is below 0,03% [1-3] (Figure 5)
- Maintaining the content of free zinc in the alkaline etching solution does not exceed 50-60 ppm (0.0050%) [1-3].
Figure 5 – The minor “Spangle” anodizing defect
on extruded profiles of aluminum alloy 6063
with zinc content 0,04% [2]
This defect does not occur where “long-life” alkaline solutions (“E6 solutions”) are used, since they contain sulfides and nitrates that bind free zinc. For this reason, many “long-life” alkaline pickling solutions turn very black.
Sources:
1. Extrusion Defects Fundamentals & Solutions for Optimum Finish – Columbus, Ohio – May 16-17, 2017 / Jerome Fourmann, Rio Tinto
2. The Identification and Prevention of Defects on Anodizing Aluminium Parts /T. Short, 2003
3. Defect Analysis in Anodizing / Bаrry R. Ellard, Aluminum Extrusion Technology Seminar, 2000