After putting HCl on a tap part that was really attacked by limescale, lots of gas began to form and limescale progressively disappeared, as expected.
However, the reaction left behind black soot-like deposits, on the cup holding the reaction but also on the sink (when washing away), that don't go away easily.
So, what could be this black deposit made of?
Also, the chromium-plated parts of tap began turning a bit red after exposure to HCl and the non chromium-plated parts turned black and slowly began to be covered with some kind of white powder (see the picture below). What is the material of the tap that would explain this behavior?