Figure 2. AFM Images of an AA2024-T3 alloy sample. Inter-metallic particles are visible asbrighter areas (higher potentials) in the surface potential image (right). Topography (left) does
not distinguish between the matrix and the inter-metallic particles. 60µm scans. Reproduced bypermission of The Electrochemical Society.4

circuit bulk electrolyte measurements for
DI-H2O, and for the NaCl solution. The
AFM measurements correlate linearlywith the open circuit potentials, and canbe associated with the Volta potential ofthe samples. The open circuit potentials
measured in 0.5M NaCl are shifted in
the active direction (i.e., they are lower)
by around 200mV compared withmeasurements made in DI-H2O.
These plots establish that surface
potential measurements made with theAFM are reliable for establishing therelative nobility or activity of species,and can be compared with open-circuit
potential measurements made of thesame species in bulk electrolytes. The
main difference is that AFM surface
potential measurements may have very
high in-plane (X,Y) resolution, allowing
for deep sub-micrometer mapping ofregions with different potentials, as we
describe next.

AFM provides high-resolution
surface potential measurements
A unique feature of AFM surface potential imaging is that it maps the potential
locally, with a resolution that can extend
down to the nanometer-scale in theplane of the sample surface. Figure 2

shows the surface of an aluminum-based
AA2024-T3 alloy, commonly used inairplanes and extremely susceptible tocorrosion. The left and right images areTappingMode AFM topography andsurface potential maps, respectively,of the same area, 60µm on a side.
Lighter shades of color in surface
potential images correspond to higher
potential values. On this sample,
inter-metallic particles that are notevident in the surface topography areseen in sharp contrast to the embedding
matrix in the surface potential image,which is captured simultaneously withthe topography map. The surface
potential image pinpoints the location
and boundaries of inter-metallic
particles. These particles measure fromsub-micrometer to as large as 20µm
across, and here they exhibit a higher
potential than the alloy matrix. These
potential differences are responsible
for enhanced corrosion due togalvanic coupling between the different
areas. The cathodic reaction rates areenhanced at these particles, while
dissolution is stimulated at lower
potential sites in the matrix or at
active particles.
The surface potential image can also
shed some light on the significance ofthe features in the TappingMode AFM

Figure 3. SEM image of approximately
the same area as in Figure 2. EDS analysis
indicated that particles 1-5 are Al-Cu-(Fe,Mn)inter-metallics, and that A, B are Al-Cu-Mg
inter-metallics. Reproduced by permission ofThe Electrochemical Society.4

topography image. In Figure 2, several
pits are visible in the topography
image (left): one associated with theinter-metallic particle 5 in the surface
potential image, and one with particle
A. This sample’s surface was preparedby non-aqueous polishing to minimize
corrosion during preparation, and wasexamined in the as-polished condition.
The pits evident in the topography
image formed during polishing (despite
the non-aqueous polishing medium),
because the sample is extremely
susceptiable to corrosion during thepolishing process. The measuredpotential distribution (right) provides
important information regarding themost likely locations for the anodic
and cathodic reactions on the surface
during subsequent exposure to a
corrosive environment.

Comparing AFM surface
potential imaging with electronbased-probe techniques
Figure 3 is a scanning electron
micrograph (SEM) of approximately thesame area shown in the AFM images
in Figure 2. The contrast in the SEM
results from the difference in theelectron-scattering properties between
the particles and the matrix. EDS

analysis performed on different particles
confirmed that the regions of higher
potential in Figure 2 are associated withdifferent types of inter-metallic particles:
particles 1-5 are Al-Cu-(Fe,Mn), andparticles A and B are Al-Cu-Mg. In thesurface potential image in Figure 2,particles 4 and 5 measure at lower
potentials than particles 1-3; so, there
are apparently different types of Al-Cu(Fe,Mn) particles present in the matrix.
These are further evidence that surface
potential images can complement
information obtained from establishedanalytical techniques, and vice-versa.

AFM surface potential imagingreveals effect of processing stepExperiments have shown that the Mgcontaining particles will dissolve upon
exposure of the polished surface to achloride-containing solution, but that theattack takes some time to initiate.3
During this initiation time, the potentialof these particles decreases to the valueof the potential of the matrix.
Surface Potential imaging is extremely
sensitive to surface charge. The
as-polished AA2024-T3 samples arecovered by a native oxide layer formed
during polishing and subsequent
exposure to air. We imaged an areaof a sample surface, and identified inthat area several round Mg-containing
particles before and after removingsome of the oxide with Argon ion
sputtering (Figure 4). Auger analysis
performed simultaneously with thesputtering showed that the sputteringremoved only part (1-2nm) of thesurface oxide thickness. The sample
was then exposed to air again andthe surface potential was remappedwith AFM. The surface potential images
in Figure 4 show that after removing a
few mono-layers of the oxide from thesurface, the measured potential of theMg-containing particles shifted fromFigure 4. Surface Potential images of inter-metallic particles in AA2024-T3. The arrows point to
the location of Mg-containing particles. (Left) before, and (right) after partially removing (about1-2nm) the native oxide film by argon ion sputtering. 30µm scans. Reproduced by permission ofThe Electrochemical Society.5

being more noble than the matrix(higher potential, thus brighter colors in
the image), to being more active (darker colors). The location and the boundaries of these particles are visible with
sub-micrometer resolution. Surface
potential measurements using data histograms (Figure 5) quantify the shift in
potential to be from about 60mV higher
than the matrix to about 60mV lower.

When this sample was exposed toa chloride solution, these particles
dissolved immediately with no initiation
time required. The redistribution ofcharge during the partial removal of
the surface oxide film and re-growthin air resulted in activation of theparticles, and this had an immediate
consequence for the corrosion behavior.

5a.

5b.

Figure 5. Histograms of surface potential images in Figure 4. The round Mg-containing particles(correlating approximately to the position of red cursor in histograms) shifted from being about
60mV higher in potential than the matrix (green cursor in histograms), to being about 60mV lower
after partial native-oxide removal (bottom histogram). (The peak on the right side in both histograms
corresponds to the large, irregularly shaped particle, which is the dominant feature at the center ofboth images.)