Summary for alkaline etch of aluminum Discussion
Alkaline etch of aluminum is discussed as a possible cause of a marbled finish seen on some 6000-series alloys before anodizing. Andy Eben says he has not seen the issue in earlier work and asks whether others have, how to avoid it, and whether certain materials can be identified in advance. Willie Alexander replies that the problem could be either the material or the etch, and suggests ruling out the etch by preparing a fresh 5-gallon bucket of etch solution and testing a part in each bath; if the test bath works well, he recommends decanting 30-60% or all of the old etch and replenishing with fresh chemistry. He also asks for a picture to help diagnose the defect. Andy later says the issue has been discussed elsewhere, but the etch explanation is still disputed, and he is leaning toward a material-related cause that seems to be becoming more common.
Q. I have been involved with metal finishing for a good number of years and never see it all.
In recent years, I see a marbling in some aluminum alloys (which I think are 6000 series) when alkaline etched prior to anodizing. I don’t remember this from prior experience. Has anybody else seen this? How are you getting past this problem? Can you somehow know before processing which materials may do this?
AJE
Andy Eben
– Denver, Colorado, USA
A. Andy
It could be the material or your etch. To rule out the etch, make up a 5-gallon bucket of your etch solution, and etch a part in each. If the test solution produces good results, consider decanting 30-60% (or all)of your etch and adding fresh chemistry.
A picture would help.
Willie Alexander
– Colorado Springs, Colorado
November 9, 2012
Q. Thanks for your input, Willie. I have captured a pic of this condition.
This has been addressed in another thread here and the etch argument has surfaces but not agreed upon. I am leaning towards a material problem that is becoming more rampant but not sure.
Andy
Andy Eben
– Denver, Colorado, USA