On a recent Friday afternoon I was called away from my office on an
emergency, leaving behind my thermos mostly full of coffee. When I
arrived back at my office on Monday and emptied out the thermos
bottle, I discovered that the bottom was covered in little black
squiggly marks.

I tried washing it out using a narrow brush, but the marks remain. Is
my thermos permanently stained? Or worse, was the steel pitted by the
acidic coffee? Is that possible?

Thanks.