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"Cause of white spots on dyed 2024 aluminum"

Current question :

April 23, 2021

Q. We have recently encountered a problem with anodizing a 2024 part and dyed it in blue. The part has white spots which are powdery and can be wiped off. Wiping off the white powdery spots will leave dark spots behind which cannot be wiped off. Here is some basic information:

Material: 2024
Type II sulfuric acid anodizing (18 V for 60 mins)

Other parts (6061) that were anodized and blue dyed on the same load turned out to be fine.

I am new to the industry and wonder what might have caused the problem and what things I should investigate or try. thanks

Jason Chow
- Calgary Alberta

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April 24, 2021

A. You need a 5 to 10%/vol nitric acid post dip to get the white aluminum sulfate out of the pores. Try it in a bucket. The 2000 alloys require higher voltage than the 6000s. At 180 g/L free acid, 2024 needs 21 volts for a reasonable pore, at your 18 volts the pore is large enough for the aluminum sulfate to pop out.

robert probert

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

April 26, 2021

Q. Thank you Robert and Ted for your responses. What baffles me is that we ran the "same" part before and did not have this white powder and dark spots issue. I am attaching the pictures here "before wiping off the white spots" and "after wiping off". The "after wiping off" picture was taken of a part different than the "before wiping off" part due to the symptom being more pronounced.

Jason Chow
- Calgary AB

Ed. note: No pics rec'd, Jason. Please email them to mooney@finishing.com.

Previous closely related Q&A's:


Q. I'm having trouble with white spots when anodizing and dying 2024 T3 aluminum. Usually the parts are machined all over and seem to have little pin holes where the spots are. What am I doing wrong? Am I zapping the parts with too much electricity? My current solution is at 19% and I am anodizing at 16 volts. I have a 1000 amp rectifier.

Please forward any solutions.

Bill Smith
anodizing shop - Trinity, North Carolina


? How many minutes do you anodize and at what temperature?

Dado Macapagal
anodizing shop - Toronto, Ontario


A. Hi, Bill. You might want to consult Robert Probert's "Where Do White Spots Come From" in our on-line library. Good luck


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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