The shape and ultimate stability of corrosion sites is highly sensitive to the competition between active dissolution and passivation. In situ methods, especially those involving synchrotron X-rays, are key to understanding both passivation and dissolution processes. The growth of passive oxide films can be a very rapid solid state process, often with good epitaxy between the metal and the oxide. The rate of dissolution is highly sensitive to the local solution chemistry as well as the interfacial potential, and may be affected by the formation of salt layers. X-ray imaging, scattering and spectroscopy can shed light on these processes, and their interdependence during the development of localised corrosion sites.