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(Top Left) Illustration of a generic oxidized metallic surface containing second phase particles (blue). (Bottom Left) Cross section of surface at row indicated by arrows (top) showing a fictional variation in the thickness of the oxide covering metal particles. (Right) Cartoon and mathematical relation demonstrating basis of Fowler-Nordheim theory.

INL is studying the effects of passive oxide films to determine whether such structures contribute to localized corrosion.

We are using Current-Sensing Atomic Force Microscopy (CSAFM) and Conductivity AFM (C-AFM) on passive oxide films to determine their electrical characteristics and possible links to localized corrosion breakdown.

Ex situ AFM topography (left) and conductivity (right) images of AA6061-T6 after 1 hour exposure in 100 mH HCl. Electron conductivity is highest at the edges of an Al-Fe-Si inclusion. Specimen was imaged at a bias of -100mV.

In our experiments, a sample is biased to some potential while the conductive AFM tip is held near ground. The AFM is operated in contact mode where the probe tip is in constant contact with the specimen. Local electric fields at the point of contact depend upon the potential difference between the sample and AFM tip as well as the medium (thickness and permeability) through which current (electrons) may travel (see Figure 1). Of course the sample bias, whether positive or negative, with respect to ground (AFM tip potential) determines the direction of the field and the flow of electrons from either sample to tip, or tip to sample.

From the Fowler-Nordheim relationship, it is possible to use the tunneling current (IT) for calculating a value for the film thickness. Additionally, we use our SECM to examine preferential sites of electrochemical activity in both corrosive and non-corrosive solutions. At present, our work is been focussed on ex situ measurements with commercial aluminum (AA2024-T3, AA6061-T6, and AA7075) alloys, 300 series stainless steels, and Ni-Gd based systems. We also have recently begun acquiring in situ measurements of these systems to help us develop a more realistic dynamic picture of the connection between electron tunneling and localized corrosion events. Much of the basis for this study emanates from work performed in Henry White's group at the University of Utah. Further work in their group has shown a correlation between the electrochemical activity and breakdown sites on the surface under corroding conditions.

Apparati: For more information about these instruments, see the capabilities page.

• Molecular Imaging PicoSPM
• Digital Instruments Multimode Nanoscope IIIa
• Cypress Systems EI-400 Bipotentiostat
• Homebuilt SECM
• FEI Environmental Scanning Electron Microscope (ESEM)
• Perkin-Elmer Phi 4300 Scanning Auger Microprobe (SAM)
• Perkin-Elmer Phi 5300 X-ray Photoelectron Spectrometer (XPS)
• Nikon LS-400 Light Optical Microscope

Peer-Reviewed Publications:

P.J. Pinhero, T.E. Lister, T.L. Trowbridge, R.E. Mizia, “Analysis of Local Defects in Surface Films on Commercial Alloys Using Conductive Atomic Force Microscopy (C-AFM),” NACE Corroson/2003, Paper 03380, San Diego, CA.