In situ observations of pitting corrosion as it spreads across the surface of stainless steel were carried out. We apply two different imaging methods simultaneously, viz., ellipsomicroscopy for visualizing changes of surface film properties and contrast-enhanced microscopy for monitoring nucleation and reactivation of metastable corrosion pits. A correlation between oxide film weakening caused by individual pits and the nucleation of subsequent pits was found. The existence of front propagation as a component of the transition to pitting corrosion shows that characteristics of this process are consistent with the behavior of stochastic reaction-diffusion systems and that the onset of corrosion is a cooperative critical effect. (c) 2006 The Electrochemical Society.