Brightdip anodize 2011

[QualitymanPMF]

I need some advice. We are trying to Bright Dip and Clear Anodize 2011 aluminum and get consistent brightness and color. Our Bright Dip solution is at optimum pH, specific gravity, and temperature but it is still hit and miss. Some time it is bright and some time it is very frosted. I think it is an alloy problem. The question is can 2011 be Bright Dipped consistently or is the alloy not suitable for this process.

 

[unclesyd]

Looking at my little book it has a “D” by 2011 AL on A through D scale. It is the high copper that's giving the problem. Also the lead doesn't help.

Here is a company that knows about every trick in the trade on anodizing and brightening AL.

http://www.metalast.com/index2.html

 

[TVP]

unclesyd is correct, alloy 2011 is a poor choice for obtaining a cosmetically appealing anodized layer. Metlast is definitely a good resource to contact regarding this problem.

 

[QualitymanPMF]

Thank you for your postings. This confirms the research that I have done and the results that we have found. It is that 2011 will not Bright Dip, 2024 will once but can’t be reworked, and 6061 is the best bet for Bright Dip Anodize.

 

[kenvlach]

Hi,
Recommended alloys for bright anodizing are 5357, 5457, 5557, 6463, 7016 and 7029. This is based on primarily on experience with automotive trim using phosphoric-nitric acid bright dips.
From The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn., p. 139 (2001).

Your 2011 results, some good and others frosted, indicate that the bright dip solution parameters are out of control and/or there are microstructural variations in the alloy, e.g., any kind of sizeable segregation. The mention of pH suggests you're using a brightening solution other than the phosphoric-nitric acid type. I suggest getting some technical support from your supplier. They will all run free samples, so submit several candidate alloys.

The microstructures of 2011 and 2024 are described and shown on pages 70 and 74-79 in Aluminum: Properties and Physical Metallurgy (1984). Both alloys have primary solidification intermetallics which cannot be fully solutionized afterwards. Those of 2024 are more complex due to greater alloying, but 2011 also contains lead and bismuth. These free-machining additives solidify last within the dendrite interstices, but can form spheroidal globules if the initial solidification is slow and/or during solutionizing. I haven't bright dipped any Al, but have lots of anodizing experience. Most intermetallics and all foreign inclusions can form anodize defects when their size approaches the anodize thickness, so generally a problem above 1 micron and definitely a problem at 1 mil (0.001”). Good results require 1) clean alloy, 2) rapid initial solidification, 3) lots of deformation (to break up segregation) and 4) tight limits on the solutionizing treatment (especially, not overheating).

I presume that you are only doing a flash anodize, so the coloration due to the Cu isn't noticeable. I will remind you that the anodize on 2xxx alloys is softer and less corrosion resistant than on other alloys.

Hope this helps,
Ken

 

[AlanD]

At last something on my thesis topic.

The quick reason why aluminum copper alloys are unsuitable for anodizing is that the copper is preferentially etched from the alloy during the anodizing process. If copper is in the grain boundaries, this etching will leave the structure weak and susceptible to crevice corrosion and alsoleave the anodized layer porous. A black copper oxide (CuO) also forms on the anodized layer and can also disrupt the continuity of the layer as well as the visual quality of the layer.

 

[kenvlach]

Hey AlanD,
Still work hardening your anodized Al 6061-T6 sailboat mast?
Should be ready for typhoon winds by now!

We agree that Al-Cu alloys are inferior for anodizing, but yet QualitymanPMF sometimes obtains good results with Al 2011. The 'frosting' problem is with the bright dip stage. So could be either the bright dip chemistry or metallurgical segregation (or both). Comments?

Ken V.

 

[AlanD]

The 2024 has a lower copper concentration than 2011 which may be impacting on the ability to bright dip. My investigation didn't look at which which process (bright dip or anodizing) the copper was etched from alloy being anodized. But from memory, the black precipitate did initially form during the etching process and was progressively worse as the percentage of copper was increased. The quality did vary, even from samples sliced off the same ingot and anodized at the same time.

My personal feeling is the cosmetic quality is dependant upon many factors, and will involve both metallurgical and chemistry factors. However, there are probably far more experienced people here who can comment with more authority than I can.