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In-situ scanning force microscopy was used to examine how corrosion at the nanometre scale depends on material microstructure. The results reveal an unusual porous corrosion morphology in partially nanocrystalline Al 87 Ni 8.7 Y 4.3 alloys exposed to dilute Cl - solutions at various pH values. This nanometre-scale porosity is absent in the fully amorphous state of the same alloy. This difference is surprising because the macroscopic corrosion behaviours of the two materials are similar. This work shows that the partially nanocrystalline material sustains nanometre-scale corrosion damage in addition to localized corrosion pits. We propose a mechanism for porosity development based on acid-induced depassivation of protective oxides and two key microstructural differences between the fully amorphous and the partially nanocrystalline alloys.