Solder corrosive?

Is it true that if you heat solder (40 tin/60 lead/rosin core) to a
point where it has blackish/brownish droplets on it, those droplets
are corrosive to the solder/metal? The piece is made of base metal.
If so, is it possible to rub it with something which would clean off
the droplets enough to stop this reaction?

Thanks so much!