Atomic force microscopy is used to characterize the nucleation and beginning growth of chromate conversion coating on polished Al 2024-T3 alloy surfaces. Rapid nucleation occurs at different rates on the Al matrix and on two distinct intermetallic phases. One phase, assigned as a Cu-Fe-Mn aluminide, reaches full conversion coating coverage faster than the matrix while the other, assigned as a Cu-Mg aluminide, is covered much slower than the matrix.