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Acid vs. Alkaline Etch before Anodizing or Chem-Filming Aluminum?

(-----) June 16, 2021

Q. I need some opinions on etching before anodizing/chem film. What are the advantages of an acid etch compared to a caustic etch? What are the drawbacks of each type?? And your choice?

Pretreatment for anodizing for MIL-A-8625 Type I II or 3 generally is mild alkaline clean-rinse-caustic etch-rinse- deoxidize-rinse-anodize. I know several of the major companies in aerospace don't give much leeway on this sequence or products used, but if a company had design authorization to specify the sequence and materials what would you use for chromic anodizing? What are your thoughts on using an acid cleaner/etch and eliminate the caustic etch and deoxidizer? That would mean acid etch-rinse -anodize?

drew nosti
Drew Nosti, CEF
Anodize USA
supporting advertiser
Ladson, South Carolina

July 19, 2021

A. As I know, caustic etch has two major drawbacks:
1. Caustic etches aluminum, but leaves other metal inclusions, like copper and zinc. The following deoxidation step should dissolve them, but this dissolution could be not completed and this will cause incomplete anodizing and failure by corrosion/pitting in salt spray and in service.
2. Also caustic etch causes intergranular corrosion/etching much more than acid etch (deoxidation). IGC will reduce the mechanical strength of the part and promote cracks formation and progress.
Possibly, these are the reasons for aerospace companies not to allow caustic etch on their parts. It is not safe enough. Their process is mild alkaline clean (non-etching)-rinse-deox-rinse-anodize. It is proven for decades as good and safe process.

Alex Sirota
Plating Process Engineer (retired) - Jerusalem

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