Volume 6 Preprint 58
The Visual Determination of Exfoliation Rate of Al Alloy Slices in Humidity
X. Zhao and G. S. Frankel
Keywords: Al alloy, exfoliation corrosion
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Volume 6 Paper C134
The Visual Determination of Exfoliation Rate of Al Alloy
Slices in Humidity
X. Zhao and G. S. Frankel
Fontana Corrosion Center, The Ohio State University, 477 Watts Hall, 2041
College Rd., Columbus, OH 43210, USA, firstname.lastname@example.org
A new technique, Exfoliation of Slices in Humidity (ESH), was developed for the
determination of exfoliation corrosion (EFC) susceptibility and quantification of
EFC kinetics. Two AA7178 plates taken from the wingskin of a retired KC135
airplane were used as test samples. Samples were pretreated by potentiostatic
polarization in chloride solution, during which they developed a corrosion
morphology of combined sharp IGC and selective grain attack.
exposure to high humidity after pretreatment of properly oriented and
unconstrained samples resulted in the development of EFC. The EFC kinetics
were determined by measuring the width of the unattacked region of the
sample. The different behavior of the two plates during outdoor exposure at
Daytona Beach was reflected by the ESH results.
These results show the
capability of the ESH test to discriminate between plates of varying
susceptibility and to quantitatively determine EFC rates.
Keywords: Al alloy, exfoliation corrosion
Exfoliation corrosion (EFC) is a form of intergranular corrosion that occurs on
the surface of wrought aluminum alloys with elongated grain structures. The
susceptibility of Al alloys to EFC can be assessed by exposure testing. ASTM G34, known as the EXCO test, involves exposure to an oxidizing acidic chloride
solution and comparison of the resulting surface to standard photographs [1,2].
Other tests involving exposure to aggressive environments have been devised,
including the ASSET and MASTMAASIS tests [3,4]. The behavior of Al alloys in
these accelerated environments has been correlated to long term exposure in
less-aggressive natural environments [5-8]. Determination of EFC kinetics is
useful for the development of predictive failure models, but no standardized
tests exist for determination of EFC rates. Robinson and coworkers have used a
deflection technique to quantify exfoliation extent and determine EFC kinetics
In this technique, the effective remaining load-bearing section of
specimens having undergone EFC is determined from their compliance under
The rate of EFC can be assessed from periodic
However, the deflection technique is valid only when the
thinning of the specimen during corrosion is uniform.
A technique that is
effective and also simple is required in order to assess EFC kinetics and further
improve the understanding of EFC.
In this work, a new test for the
determination of EFC susceptibility and quantification of EFC kinetics is being
The Exfoliation of Slices in Humidity (ESH) technique involves
exposure of properly-oriented free standing alloy pillars to high humidity
following an initial electrochemical pretreatment.
Test samples were machined from a piece of
a wingskin of a retired KC135 airplane. The
attached to an underlying support beam by
The piece of wingskin was
exposed in the uncoated condition at an
Daytona Beach, FL. As shown in figure 1, the
two plates in the test piece exhibited vastly
Figure1.Daytona Beach Exposure
Sample. 9 Month Exposure Of Bare
Material (Al and Steel Rivets).
cosmetic surface attack.
different exfoliation behavior during the
exposure near the seacoast: one of the
plates exfoliated badly next to the steel
rivets and the other plate only developed
The two plates are referred to as “good” and “bad”
plates in reference to their EFC susceptibility.
Samples were machined from the “good” and “bad” plates in the shape of
rectangular pillars. The pillars were 3-4 cm long, oriented such that the long
axis of each pillar was in the longitudinal orientation of the microstructure
(along the rolling direction). The pillar thickness was around 1 mm, and the
pillar thickness was oriented in the plate transverse direction.
The width of
each pillar was the full plate through-thickness in the short transverse
direction: 4.1 and 4.6 mm for the “good” and “bad” plates respectively. The
pillar edges, which were the original outer surfaces of the plate, were lightly
All other faces were ground to 800 grit.
pretreatment was 7 h in 1 M NaCl at a potential of –710 mV SCE. Following the
pretreatment, the sample was rinsed with DI water and placed in a humidity
chamber, consisting of a sealed beaker containing a saturated salt solution at
room temperature (22-25C).
Several salts were used to create a range of
constant humidity: sodium sulfate (Na2SO4), ammonium chloride (NH4Cl),
potassium iodide (KI), potassium carbonate (K2CO3), and calcium chloride
The humidity expected above saturated solutions of these salts as
reported in the CRC Handbook  and the values measured by an RH meter
were close, as shown in table1.
Table.1 Humidity associated with saturated salt solutions.
%RH (from CRC )
%RH (measured) at RT
93 at 20oC
79.3 at 20oC
56.2 at 100oC
32.3 at 20oC
Following the electrochemical pretreatment, the samples exfoliated when
exposed to a high humidity. The EFC started at the outer edges and moved
inward. The contrast between at the boundary of the outer exfoliated and inner
unattacked regions was sufficient to allow tracking of the EFC kinetics by
analysis of digital photographs of the sample taken through the glass walls of
the humidity chamber. The photographs recorded the pillar face that showed
the transverse section of the microstructure.
Exfoliation tests of “good” and “bad” 7178 samples were performed in different
humidities created by saturated aqueous solutions of the salts in the above
table. Digital pictures were taken through the beakers every day to monitor the
progress of exfoliation corrosion.
Detailed metallographic analysis was performed on T, S, and L sections of the
“good” and “bad” plates. Samples were polished down to 1 micron and etched
by Keller’s etchant.
Results and Discussion
Figure 2 shows images of the “good” and bad plates from the AA7178 wingskin
sample exposed to 96% RH for 0-52 days
following electrochemical pretreatment.
After several days of high humidity
exposure (96% RH), EFC was evident on
both edges of the slice from the “bad”
plate and it continued to progress over
the exposure period of almost 2 months,
The behavior of the slice
removed from the “good” plate behaved
quite differently. Severe EFC formed on
one side of the sample (left side of
images in Figure 2b), while the other side
of the sample was practically unattacked.
The side of the slice that was not
attacked in the ESH test was the side of
the plate that was exposed upwards to
Figure 2. Image of AA7178 wingskin
sample exposed to 96% humidity
following electrochemical pretreatment.
(a) “Bad” sample. (b) “Good” sample.
Overall, the “bad” sample was attacked
much faster than the “good” sample. As
seen in Figure 2, corrosion product
oozed out of the sides (corresponding to
transverse face of the microstructure) of both good and bad samples upon
exposure to 96% RH. This deliquescence is evidence of the chloride corrosion
The kinetics of EFC can be determined by measuring the change in width of the
central unexfoliated region. Regions with different rates of attack are evident
as shown in figure 3. The EFC started out slowly, about 0.01 mm/day. Then,
after some delay time on the order of 2
Width Change (mm)
weeks, the rate increased to a higher
Bad sample 1
Good sample 1
Bad sample 2
Good sample 2
value, about 0.03 mm/day.
then decreased again to a lower value
of about 0.005 mm/day. This indicates
that there is a susceptible layer inside
different behavior of the two plates
during outdoor exposure at Daytona
Beach is reflected by the ESH results.
Figure 3. Exfoliation corrosion kinetics of
ESH testing was performed on “good”
AA7178 wingskin samples exposed to 96%
and “bad” samples under a range of
The two plates exhibited
In the lowest
humidity (30%, CaCl2), the surfaces of
Width Change (mm)
both samples were still rather shiny
after 60 days exposure, and no EFC was
evident. Clearly this humidity is below
the critical humidity of the corrosion
In 49% RH, some product
oozed out of the surface and EFC was
seen on the down side of the "good"
Figure 4. Exfoliation Corrosion kinetics of
sample after 3 days. However, the EFC
ceased after that time, perhaps as a
result of a drying out of the corrosion
AA7178 wingskin sample exposed to
product formed during the pretreatment as a result of equilibration with the
humidity in the chamber. In 65.1% RH, both sides of the “bad” sample exfoliated
gradually after around 1 week exposure even though no change was observed
at the beginning.
Above 49% RH, samples exfoliated more severely with
increasing humidity. The critical humidity for EFC propagation seems to be just
above 50%. The width of the unattacked region was measured at ten different
positions along the axis of each sample for RH values from 65-96%.
these measurements, the average rate of EFC was determined. The average EFC
rate was 3.0, 4.2, and 14.2, µm/day for the “bad” samples in 66, 76, and 96%
For “good” samples, the values were 0.61, 0.76, and 5.0
µm/day, respectively, for the same humidities. The EFC kinetics of wingskin
samples are shown in figure 4.
The microstructure of the two plates exhibit differences that correlate with the
EFC behavior. As shown in figure 5, the "bad" plate has a microstructure that
has more elongated grains. In contrast, the near-surface regions of the "good"
plate exhibit smaller grains with a smaller aspect ratio.
equiaxed microstructure should be more resistant to exfoliation (but should
exhibit faster IGC in the S direction).
There is no clear difference in the
microstructure of the near-surface up and down sides of the “good” sample to
help explain the different EFC behavior.
The exfoliation corrosion behavior of two plates taken from a retired KC135
was studied using a new approach.
Slices of the plates with particular
orientations were electrochemically pretreated in chloride solution and then
exposed to constant humidity.
The rate of exfoliation corrosion was
determined quantitatively by analysis of digital images of the slices.
results correlated with the exfoliation behavior during exposure to a seacoast
Figure 5. Metallographic sections of AA7178 wingskin plate. (a) “Bad” plate. (b) “Good” plate.
The sections are through thickness montages, starting at the right of the top image in each pair
and then wrapping around to end at the left side of the bottom image in each pair.
The authors acknowledge the support of AFRL with a contract through S&K
Technologies and are grateful to Mr. Jim Suzel from S&K Technologies for
providing the samples. The exposure testing at Daytona Beach was performed
by Dr. William Abbott of Battelle.
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