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White spots on anodizing parts

We have an issue of white spots (undyed) on anodized aluminum parts. Both Secondary Electron Imagining (SEI) and RobinsonBackScattered Electron Imaging (RBSEI) did not detect any difference in the composition and the texture of the white spot and non white spot. In addition, parts did not corrode in a salt spray test. These tests show that the white area is actually anodized but does not have the dye. We use 9020, which is same as 3005 alloy, but the ratio of Iron to Silicon is controlled much tighter. The shells are drawn. Our vendor uses polypro tanks for dyeing and titanium racks. We believe that the drawn shells have crack lines and these lines trap sulfuric acid during anodizing, which prevent dye to enter the pores. Our vendor has introduced Nitric rinse after anodizing to drain the trapped sulfuric acid. The Nitric rinse improved the yield slightly but not to an acceptable level. Any ideas what is causing these white spots? Greatly appreciate any help!

Mehmet Akbas
- Lincoln, Rhode Island, USA
2000

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You seem to be on the right track. As far as using nitric as a"neutralizing" agent to sulfuric, it works, but you still have a case of a potential acidic layer hindering the dye from entering the pores of the anodized material. This is a more cost effective solution and it works by neutralizing the sulfuric... baking soda [in bulk on eBay or Amazon [affil links] . Yes, common cooking baking soda will nullify the acid on the parts and will make difficult to dye parts capable of being dyed. As far as the spotting goes, besides your assumption, which I believe to be the real reason there could also be a buildup of chlorides in the dye tank(s). Have the anodizer try the parts in a fresh dye tank, if this problem Gus for a period of time and then seems to show up again, I'd say that's exactly the problem. Good Luck to you sir

Matthew Stiltner
- Toledo, Ohio
2000

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Interesting enough, The Canning Handbook [on eBay, Amazon, AbeBooks affil links] says that a white powdery"bloom" on surface of articles anodized in sulfuric acid is caused by too high a temperature or current density, or local overheating due to lack of agitation.

Prevent by cooling electrolyte, cut down voltage, or correct agitation.

  
Tom Pullizzi
Falls Township, Pennsylvania
2000