rust in test-tubes of HCl FeO or Fe3O4?? I am doing experiments on steel nails in HCl solutions with a pH of about 1.5 - 2. The rust is black in colour, and is not attached to the surface of the nails. It stays at the bottom of the test-tube. Is this FeO or Fe3O4. Why is it that the rust I am getting in solutions of just salt is not the same colour?. And why is it red and attaching to the nail?