Those black spots on the metal. What is that?

A disease.? Granted I beat up on that old Stanley chisel building fences, a
deck and a shed and there was a neglected rust spot on it for awhile. But I
cleaned it up along time ago. Now the metal at that spot has sort of eroded
away a little and there's a black spot there. Doesn't look like rust.. I
sanded on it today with some wet and dry sand paper some came off. What's
with the black. Is this black rust?


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