Combined in situ spectroscopic ellipsometry and electrochemistry have been employed to monitor, in real-time, the formation of trivalent Cr conversion coatings on polished Al substrates at applied sample potentials. It is found that the formation kinetics and chemical composition of the film can be controlled by adjusting the anodic and cathodic reactions. The growth kinetics are accelerated at more positive anodic potentials or more negative cathodic potentials. At more negative potentials, the percentage of chromium in the coating is found to increase, while the zirconium percentage decreases.

Scanning Kelvin probe force microscopy (SKPFM) was used to quantitatively measure the surface potentials (also referred to as Volta potential) on different intermetallic particles relative to the matrix in AA7xxx aluminum alloys, in combination with scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). Both Cu and Fe-rich intermetallics showed rather high, positive Volta potential differences (~ +400 mV), while Mg/Si-rich one exhibited a negative potential (~ -250 mV) relative to the matrix for the Al alloys investigated. Large variations in the measured Volta potential, which appeared to be independent of particle size, were observed for these Cu and Fe-rich (Al7Cu2Fe) and Mg/Zn rich (Mg2Zn) particles in the custom Al-Zn-Mg-Cu alloy. The effects of different surface preparation on the Volta potentials of Cu/Fe-rich and Mg/Si-rich intermetallic particles were investigated. The results indicate that the influence of surface preparation is complex, but largely depends on the type of intermetallic particles, and whether there is any particle dissolution during the surface preparation.