In contrast to iron scales from cast iron water mains, scales developed on household galvanized iron carry a substantial load of heavy metals, especially lead (Pb). The distribution of this Pb can be imaged by using back-scatter electrons (BSE) in an electron-beam instrument such as a scanning electron microscope (SEM). Larger atoms scatter more incident electrons, therefore the intensity of the backscattering is proportional to the mass of the atom. Lead, at an atomic weight of 207.2 is so much greater than Fe at 55.9, that any Pb particles in an Fe matrix will show as bright spots against a dark background in BSE images. See example below.



Looking Deeper - Energy-Dispersive Analysis (EDAX)