The morphological changes of the liquid and solid phases occurring during the corrosion of aluminum by sulfuric acid were imaged in situ with nanometer resolution using scanning polarization force microscopy and conventional atomic force microscopy. The rate of reaction between the acid and the metal is highly dependent on the relative humidity. At high relative humidities, a dramatic spreading of the liquid droplets is observed that is indicative of sulfuric acid-induced aluminum corrosion. The liquid layer consists of a mixture of aluminum sulfate product and sulfuric acid. At low relative humidity, an aluminum sulfate precipitate is formed which segregates from the unreacted liquid acid. This phase separation inhibits further corrosion, in agreement with the results of previous experiments using infrared reflection absorption spectroscopy. The results of these experiments offer new insights into the mechanisms of chemical corrosion at gas-liquid-solid interfaces.