Spangle and shiny metal - Architectural anodising

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Can anyone tell me possible sources of "Spangle" (and shiny metal)?

These defects have occurred in ANOTHER anodising plant that is used by one of our extrusion customers.
We do have an anodising plant ourselves and I have reworked metal in OUR plant.
I therefore cannot comment on critical issues in the other plant as regards, etch times, etch temperature, etch chemistry or impurity levels in the baths. All of which, can contribute to shiny metal.

This defect (or I now believe something similar) has occurred on a derivative of architectural grade alloy 6063.

Background
The anodising process in BOTH plants is Type II sulphuric acid for architectural products.
The Zn content of OUR alloy is <0.01% (Spangle reported to occur above 0.03%).
The Zn content of OUR etch is 0.3ppm (Spangle reported to occur above 50ppm, or according to another report, above 6ppm).
So, both levels of Zn are well within specification.
Also, the additive in the long life etch (which ties up excess Zn, is well above suppliers minimum specification).

It is worth noting that reworking through OUR anodising plant did decrease the shininess of the metal to an acceptable level of matt, and to a lesser extent the Spangle.
Also, by linishing the metal back to the base metal before rework, I could re-anodise in OUR plant to produce matt metal, without Spangle.

Note
I have seen commentaries regarding grain effect on marine grade aluminium and "high" Aluminium in the etch (the figure quoted of 50g/l max is well above the tried and tested figure of 125+ g/l in the current formulations of long-life etch.
So, these causes of Spangle/preferential grain boundary attack do not seem applicable to this situation.

At present, I do not think this is the typical "Spangle" defect, but may be related to Mg2Si precipitate at the grain boundaries????

I would appreciate any comments about this little conundrum.

Best regards,

Martin Webb
- Hamilton, New Zealand

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Spangle or Galvanizing or fish scaling can come from an alkaline etch when the dissolved aluminum is too high.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note:
   Mr. Probert is the
   author of Aluminum How-To / Aluminio El Como

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Robert, Thanks for your input to my question.
What level of Al do you consider to be too high?
Thanks,

Martin Webb
- Hamilton, New Zealand

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The limiting level is a function of several parameters including: alloy, tome, temperature, free alkali,etc

Start geetting concerned at 20 gm/L, at 50 gm/L I am sure you have a problem. Go ahead and decant 30% right now and rebuoild to normal free alkaline livel, then keep a log of surface area versus dissolve.

Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina