There is no simple rule of thumb for shielding. In some cases (e.g. Al) oxidation leads to passivation, in some cases (e.g. Fe) it leads to crumbling of the material. With respect to magnetism, you would have to apply crystal field or ligand field theory in order to check whether the electron configuration of the metal ion in a given charge state would raise the expectation of a diamagnetic, paramagnetic or ferrimagnetic material.