[alexxx]

Hello. I would like to know which reaction is the one Iwill describe, which elements are present,  and, very important,  if it is toxic, harmful.

When mixing bleach and  common sewing pins(I think, nickel ) into a bottle, after some months there is a black dense, liquid. Pins are near destroyed.

Thank you

[Enthalpy]

Sewing pins are made of steel. Usually high-carbon, unalloyed steel, tempered and cold-drawn for extreme hardness.

In water, it rusts. Bleach makes it worse and can add some new compounds.

In reasonable amounts, I wouldn't worry too much about toxicity. Don't swallow it, avoid eye contact, wash you hands.

But more generally, don't experiment with bleach. It's the one household chemical that kills most people. Never mix it with anything else.