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Metallic Contaminants in Sulfuric Acid Anodizing Bath

⇦ (tip: readers rarely show interest in abstract questions, but people's actual situations usually prompt responses)

Q. Does anyone know about the effects of metallic contamination in a type III sulfuric acid anodizing bath? We have been having a lot of trouble with the anodizing process recently. Analysis of the solution showed it contained copper, iron, zinc, and chromium. Does anyone know the acceptable limits for these in a sulfuric acid anodize bath? Does anyone know how this contamination affects the anodizing process? What type of artifacts will be visible in the anodized coating with respect to each of these elements? Any help or advice with this would be greatly appreciated.

Jessica Boike
- Madison Alabama
July 19, 2021

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby
pinner
on AbeBooks

or eBay or

Amazon

(affil links)

A. Hi Jessica. Pinner says iron at above 50 mg/L and copper at above 125 mg/L are troublesome ... that's all I could quickly find.

This has the makings of a really interesting discussion but I don't think it's quite there yet :-)
What kind of "troubles" have you been having recently? What are the levels of "copper, iron, zinc, and chromium" that you are seeing? What "artifacts" are visible? Thanks.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2021

A. Be sure you take the titration sample during electrolysis. The copper plates out just as soon as the current is turned on, unless you have one of these new pulse or super imposed AC power supplies. In the case of a Boeing spec, the limits are set so low that it is impossible to anodize the first 777 engine cowling (2024) without exceeding the limit on copper, unless you take the analysis sample under electrolysis.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

July 21, 2021

Hi,

Analysis showed less than 1 mg/L of copper, 77 mg/L of iron, 3.2 mg/L of zinc, and 8.1 mg/L of chromium. The cathodes are completely covered in what appears to be a copper immersion deposit. If we remove them, most of it rinses off and the rest can be removed by an etch and deox sequence. The copper deposit usually returns within one day or so. The anodized parts are grey instead of black after dye and seal, even though the thickness is acceptable. Coverage is poor in the low current density areas and the parts tend to burn in the high current density areas. We are also seeing some type of brown colored uneven streaking in the anodized surface.

Thank you,

Jessica Boike [returning]
- Madison Alabama
July 21, 2021

"The Technology of anodizing Aluminum"
by Arthur W. Brace

on AbeBooks

or Amazon

(affil links)

A. To add to Ted's response, Brace published more restrictive limits than Pinner has. Brace says the following will lower brightness and corrosion resistance ...
Fe above 50 ppm.
Cu+Ni+Mn total combined above 50 ppm

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

July 26, 2021

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