Scanning Probe Studies of Water at the Aluminum Surface

Water adsorption on the passive aluminum surface was investigated as a function of relative humidity (RH) using atomic force microscopy (AFM). Aluminum surfaces exposed to the ambient environment for times longer than one month showed that water adsorbed by the formation of droplets ranging in lateral size from 20 nm - 1 µm at RH above 70%. A surface residue at the droplet sites was observed by scanning electron microsocopy and AFM following evaporation of these droplets as the RH was reduced. Aluminum surfaces with minimal exposure to the ambient environment did not show droplet formation at high RH, but rather a more uniform water layer as inferred from pull-off force measurements. Macroscopic contact angle measurements showed that hydrocarbon adsorption on the passive aluminum surface from ambient environment changes the surface from hydrophilic to hydrophobic in less than 24 hours.