Characterization of Potential Inhomogeneities on Passive Surfaces by Scanning Kelvin Probe Force Microscopy

Potential mapping measurements have been performed on passive surfaces in air using an Atomic Force Microscopy based technique, the Scanning Kelvin Probe Force Microscope (SKPFM). A linear relation was found between the potential measured this way and the corrosion potential in aqueous solution for a range of pure metals. The SKPFM can be considered to map the practical nobility of the surface. A large difference between the potential on intermetallic particles surfaces and the matrix allows clearly identification of inhomogeneities in Al alloys down to a sub-micron level. The importance of the first surface layers on the measured potential is clearly demonstrated by the modification of the potential distribution observed on aluminum 2024-T3 alloy upon exposure to solution, as well as scratching or ion sputtering of a depth of a few nm. Al-Cu-Mg intermetallic particles are initially noble, and then activated upon removal of surface oxide layers. Non-homogeneous potential distributions are observed on Al-Cu-(Fe,Mn) particles.