Metallic Contaminants in Sulfuric Acid Anodizing Bath

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,