A major advantage of x-ray techniques is that they allow in situ studies of the processes taking place. A range of applications of synchrotron x-rays for the study corrosion will be briefly reviewed. These include novel atmospheric measurements using ionize air to determine Volta potentials usually made using Kelvin probes, and scanning x-ray fluorescence techniques for the analysis of corrosion products at the corroding interface and within solution. Emphasis will be given to the measurement of passive oxide films. These are a few nanometers thick and responsible for corrosion resistance. Examples using x-ray absorption will be given of the chemistry of passive oxide films on iron and stainless steels, their electrochemical formation, dissolution, and the behavior of chromium, the critical additive to stainless steels.