Microscopy and Microspectroscopy of Aluminum and Ferrous Alloys and Their Surface Treatments

Electrochemical and surface analysis are commonly combined in order to understand the role of alloying elements on corrosion behavior such as the active-passive transition, passivity, and pitting resistance. Additionally, these techniques may be combined to study the effectiveness of surface treatments for corrosion protection, such as chromate conversion coating (CCC) of aluminum alloys. Electrochemical polarization diagrams provide a general guide to the influence of alloying additions on corrosion behavior, but surface compositional analysis by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), or Auger electron spectroscopy (AES) can provide information to interpret those diagrams from an atomic scale mechanistic perspective. The unique speciation data obtainable by XPS can often elucidate multiple roles played by a given element, while depth scale information can indicate the time line associated with the development of surface films. The following surface layers have been attributed to the passive process: barrier oxide layer, salt deposit layer, and alloy surface layer. Depth scale information is commonly developed through ion beam sputter etching, usually by argon ion beam etching in combination with SIMS or AES and, to a lesser extent, by XPS.