Caustic etch is a precursor for NDI - particularly Dye-penetrant inspections. The electrochemical difference between the second phases and dispersoids in the grain boundaries of Al-Zn-Mg and Al-Cu-Mg very high strength aluminum alloys is known to present a risk during caustic etching. This has long been known to be an issue, and it has been such a legacy issue that it is difficult to find any literature with details beyond "Do not etch more than 2 or 3 times."
Does anybody know of or possess literature detailing testing that quantitatively delineates microstructural deterioration from caustic etching? Any information in this regard will be helpful as Coast Guard aviation looks to refine our NDI specifications.
Thanks!
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William Weimer
Senior Metallurgist and Materials Engineer
United States Coast Guard
Virginia Beach VA
6147475647
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