Synchrotron X-ray microtomography has been used to study the distribution of Y in the aerospace magnesium alloy WE43 and its role in the morphology of corrosion attack. A three-dimensional Y map was obtained by acquiring tomographic images above and below the Y K-edge at 17 keV. Tomographic images from a heavily corroded as-cast WE43 sample showed that Y-rich regions of the matrix slow the propagation of corrosion. In situ time-dependent tomographic images of corrosion of heat-treated WE43 show that homogenization of the distribution of Y decreases the corrosion rate, but both general attack and pitting is still observed.