Atomic force microscopy, a powerful, high-resolution imaging technique for determining the structure of surfaces in gaseous and liquid environments, was used to examine the reactivity of an electropolished surface of a naturally aged aluminum-copper-magnesium alloy (2024-T3) in aqueous hydrochloric acid (0.01, 0.1, and 1 ill). When first exposed to acid, the matrix dissolved uniformly. Dissolution then accelerated and pits formed predominately in the vicinity of the second-phase precipitates. The pits developed into characteristic intergranular damage : i.e., elongated pits (incipient corrosion cracks) along grain boundaries. Postexperimental ex situ energy dispersive x-ray analysis and Auger electron spectroscopy were employed to characterize the composition of the various surface features of corroded samples.