[en] Predictions for the corrosion behaviour of Zircaloy cladding are based on empirical models. This results in significant uncertainties for forecasts beyond existing data e.g. for high burn-up, or when there has been a change in operating conditions. To allow for a more accurate prediction of corrosion behaviour a better understanding of the mechanisms involved is required. A program has been initiated with the aim of developing a detailed mechanistic understanding of out-of-pile Zircaloy corrosion behaviour. This paper reports the results of isothermal exposures of Zircaloy-4 to PWR water at 350 Celsius degrees. A variety of analytical techniques have been employed to analyse the corroded specimens, including scanning and transmission electron microscopy, atom probe tomography, micro-beam synchrotron X-ray diffraction and electron energy loss spectroscopy. When the results of these techniques are compared, it becomes evident that the periodic transition from slow to fast oxidation rates results from the accumulation of stress relief processes in the metal (particularly plastic deformation, but also oxygen and hydrogen dissolution); at the metal-oxide interface (decomposition of the flat interface into undulations); and in the oxide (cracking). These reduce the in-plane compressive stresses near the metal-oxide interface but ultimately balance them with in-plane tensile stresses which encourage through-thickness cracking, and the percolation of the environment to the metal-oxide interface. This is noticed only if allowance is made for out-of-plane stresses in the thin oxide. (authors)