It is probably "aluminum oxide", which is a component of many sands and 70% of the moon. Mine looked similar (~1 mo ago), though slightly less corroded. I started with a flat file, then 60 grit sandpaper. You could also chuck it in a vise and use a wire brush in a drill. No leaks at my radiator hose connection.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans