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Defect in shiny anodized product

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"The Surface Treatment & Finishing of Aluminium and Its Alloys" by Wernick, Pinner & Sheasby (1959 - 2001)
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"Anodic Oxidation of Aluminum and Its Alloys" by V. F. Henley (1982)
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"Aluminum Finishes Process Manual" by Reynolds Aluminum (1973)
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Q. I have a problem in shiny anodized product -- appears on surface after cleaning and polishing.

61433-1a 61433-1b

Cold stamped aluminum, then mechanically polished, and then transferred to the anodizing line
degreasing
HNO3 (50-60 °C)
polishing (85% H3PO4, 15% HNO3, 110°C)
desmut
anodizing (15 min, 20 °C, 190 g/l H2SO4)

Mai Thi Ngoc ÁNh
staff - nam dinh, Vietnam
May 4, 2022

A. What is the alloy? Heat treated? Suspect Chloride contamination from the water, or iron drag into the anodizing from the deox.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Q. How is the iron concentration in the deoxidation tank and in the anodizing tank analyzed?

S Llinares
- Spain
November 27, 2023

Q. I have a similar problem on the bright anodized product. The top of the parts has a defect that does not go away when rubbing. The defect appears more in the upper rows of the rack and always on the top of the pieces. Sometimes it also appears on the sides of the rack. It appears before anodizing.

bright dip anodizing problem 2c bright dip anodizing problem b

Our process is:
We use alloy 5657
bright dip (H3PO4, HNO3 and H2SO4)
deox (diiron trisulphate, sulfuric acid, nitric acid)
anodizing (195 g/L, 20 °C, 17 min)

S Llinares
- Spain
November 16, 2023

A. S Llinares

It looks like the Brite Dip solution is drying on the part(s) prior to rinsing. Consider a faster transfer to the rinse, or add a spray rinse over the Brite Dip tank. If you are operating the BD at >100 °C, water added via rinsing should boil off in a timely manner.

Willie Alexander
- Green Mountain Falls, Colorado
November 19, 2023

Insightful answer, Willie. I bet you're right.

Luck & Regards,

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

Q. Thank you. I will try adding a spray rinse over the Bright Dip tank.
I have read that you can increase the wetting additive in the Bright Dip, do you know what wetting additive to use?

S Llinares [returning]
- Spain
November 20, 2023

A. S Llinares:
We used an R5 Bright Dip many years ago. No additives required other than seeding with a little copper. I am unaware of any wetter you could use. Sorry.

Willie Alexander
- Green Mountain Falls, Colorado
November 21, 2023

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