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Large Plate anodizing problems

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Q. Hi we have inconsistency of color throughout the plate:
1. Different color side to side
2. Horizontal lines
3. Vertical "flame-like" discoloration

61610-1a 61610-1d 61610-1b 61610-1c

for Shops, Specifiers & Engineers

"Aluminum How-To" -- Chromating, Anodizing, Hardcoating by Robert Probert (2005)
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"The Metallurgy of Anodizing Aluminum" by Jude Mary Runge (2018)
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"The Surface Treatment & Finishing of Aluminium and Its Alloys" by Wernick, Pinner & Sheasby (1959 - 2001)
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"Surface Treatment and Finishing of Aluminum" by Robert King (1988)
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"Anodic Oxidation of Aluminum and Its Alloys" by V. F. Henley (1982)
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"The Technology of Anodizing Aluminum" by Brace (1979)
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"Aluminum Finishes Process Manual" by Reynolds Aluminum (1973)
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"The Anodic Oxidation of Aluminum" by A. Jenny (1940)
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"Anodizing and Coloring of Aluminum Alloys" by S. Kawai (2002, VERY RARELY avail)
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Alloy is 5052, and has a brushed finish

Kai mizrahi
- Mexico city
November 18, 2023

A. Hello Kai,
The problem could primarily just be poor rinsing, but I'm guessing you've already considered that.

Is the coloration an organic dye or two-step electrolytic dye?
The plates appear to be somewhere around 3 foot high x 9 foot wide? How thick are they? -- which can become important because current distribution can be an issue on large thin plates. Are you sure the racking arrangement has sufficient current-carrying capacity? The cathodes look about 50X as robustly sized as those hangers :-)

Those vertical flames are certainly not random, but are a reflection of some physical situation which should be identifiable from an inspection of the site.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Q. Hi Ted

Thanks for your insight.
1. We are using cobalt for the color.
2. The vertical "flames" we think are caused by bubbles and they appear during the anodizing process.
3. The plate is 18 gauge so it's thin
4. We think we have good contact through the plate
5. Size of sheet is 4x10'

A. Using profiles on top and bottom to minimize bending,
B. Checking thee pvc tube for same bubble force along the tub,
C. Checking the pH of the cobalt tub.

Kai Mizrahi [returning]
- Mexico City
November 20, 2023

A. Hi again.
If you're going for 12 ASF on an area of 4' x 10' x 2 sides, that would be 960 amps, whereas the current-carrying cross sectional area of an 18 ga x 4' sheet is 0.048" x 48" = 2.304 sq. from each end.

Although that sounds to me like it shouldn't be a problem, people with substantially more experience than me (like Robert Probert) frequently warn about the difficulties of anodizing thin sheets due to current distribution problems, so I would suggest that those profiles you are planning to add along the top and bottom be used to carry current as well, not just support the sheet against bending, at least on a trial basis. Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

thumbs up sign Thanks Ted, we will try larger contact and let you know the results.

Kai Mizrahi [returning]
- Mexico
November 21, 2023

A. Hi Kai

In case the alloy can be changed, try with 1200 series, and you will be amazed with the even and uniform finish. Sheets with other alloys usually tend to react differently.


Sanjay Goel
- india
December 27, 2023

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