Greetings,
Mr. dj. Yup, HCl will attack the Al as evidenced by the vibrant reaction on a bare spot BUT it did not appear to react with the "deposit" (no bubbling visible with eye magnification). I understand the Al oxide protective layer scenario but replacement of the widow is out of the question and as I mentioned, scraping is nigh on next to impossible.
Since the two acids I tried do not seem to work, I might try a strong base (Easy Off oven cleaner or a straight lye solution). I do have access to glacial acetic and percloric (conc) acids so that may be next before the base...Thanks. Hydrogen peroxide perhaps?
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RTF
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