Effect of oxide film on Volta potential of aluminium

摘要

Atomic Force Microscopy (AFM) coupled with Scanning Kelvin Probe ForcernMicroscopy (SKPFM) has been used for years to characterize the surface topographyrnchanges and Volta potential difference (VPD) distribution for different metals andrnalloys before and after corrosion exposure. Aluminum alloys have been paid anrnattention because of their heterogeneous nature. The presence of differentrnintermetallic inclusions in aluminum matrix shows a good contrast of VPD comparingrnwith the surrounding aluminum matrix. However, the surface of alloys was found tornhave different properties after corrosion exposure. In particular, it is resulted in thernsignificant change of VPD over the intermetallics and surrounding aluminum matrix.rnDecrease of VPD (I.e. increase of the nobility) over the intermetallics could bernexplained by the enrichment of the nobler elements in it. However, the reason of thernVPD increase on the adjacent aluminum matrix is still unclear.rnThis work brings new results which can contribute to understanding of such arnphenomenon. The suggestion was that in places around cathodic intermetallics oxidernfilm thickness increases because of the localized anodic polarization of surroundingrnaluminum matrix. In the present paper, AFM coupled with SKPFM technique wasrnused to estimate how the formed oxide film on the metallic aluminium can influencernits Volta potential.rnThe oxide films were prepared in different electrolytes with Ph close to 7. Therndifferent terminal voltage was used to produce oxide films with different thickness.rnImpedance spectra were taken on prepared samples aluminum samples and fittedrnusing appropriate equivalent circuit. To estimate oxide film thickness, values of therncapacitance of oxide films calculated from the Impedance measurements were usedrnin a model of flat condenser. The results have shown decrease of the VPD (increasernof the nobility comparing to pure aluminum substrate) with the increase of aluminumrnoxide film thickness confirming our suggestion that Volta potential increase aroundrnintermetallics can be related to the changes in the surrounding oxide film.