37x64R "Federal Laboratories,Inc.Pittsburgh,PA.U.S.A."

This cartridge spent some time in dirt/ground, I cleaned it up a bit. The deep gashes in its body show some white compound laying below the surface. What lies below the outer silvery layer?

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My guess, it looks like aluminum corrosion…?
I have seen badly corroded aluminum handgun frames that looked like that beneath the finish.
Either that or the contents are leaking through?