Volume 2 Paper 30
Characterization of the Electrochemical Events at Intrinsic Breakdown Sites on Organically Coated AA2024-T3
A.M. Mierisch and S.R. Taylor
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JCSE Volume 2 Paper 30
Submitted 13th September 1999
Characterization of the Electrochemical Events at Intrinsic Breakdown
Sites on Organically Coated AA2024-T3
A.M. Mierisch and S.R. Taylor
Center for Electrochemical Science and Engineering,
University of Virginia, Charlottesville, VA 22903
An understanding of the events leading to the local breakdown of
organically coated alloys requires the use of local electrochemical and
chemical methods. Intrinsic (i.e. naturally occurring) breakdown
events on AA2024-T3 coated with a series of neat resins (vinyl,
polyurethane, and epoxy) were examined using Local Electrochemical Impedance
Mapping/Spectroscopy (LEIM/S) along with supportive techniques.
Several revealing events and processes were identified. Several types of
breakdown sites were identified that differed in both electrochemical and
underfilm chemistry characteristics. The time evolution of these defects was
observed as a rise in the local impedance in the first stages of
observation. Metastable behavior defects have also been observed.
Present effort is focussed on theoretical corroboration of the
interpretation of these events.
§2 The initial sites of corrosion on organically coated alloy substrates are
likely governed by either chemical heterogeneities within the organic
coating , heterogeneities within the alloy (e.g. intermetallics) [2-4],
or a juxtaposition of the two, assuming a physically uniform
film. An understanding of the relative contributions from these
processes to coating breakdown relies on the ability to obtain information
on the local chemical and electrochemical processes.
§3 Traditionally, the electrochemical investigation of organic coating
behavior has used global techniques to provide insight into the general
performance of a coating [5-10]. However, these measurements are
averaged over the entire surface and do not provide information about
individual sites of corrosion initiation. It is believed that
information about these sites will help govern the development of better
surface pre-treatment and coating chemistries. Thus, recent efforts
have turned towards developing methods that can measure the important
electrochemical events at these breakdown sites[11-16].
§4 This paper presents several observations of local electrochemical events
associated with the breakdown of intrinsic defects (i.e. naturally
occurring) on organically coated AA2024-T3 using Local Electrochemical
Impedance Mapping (LEIM) and Spectroscopy (LEIM) in conjunction with other
supportive methods. The literal translation of these data provides
insight into the initiation and evolution processes of localized underfilm
§5 The LEIM/S system used in this investigation employed the
five-electrode (split micro-reference electrode) configuration originally
designed by Lillard, Isaacs, and Moran , and further developed for
specific use on local defects in coated alloys  as described in detail
by Mierisch . The probe uses a 5-electrode configuration with two
teflon coated Ag/AgCl micro-reference electrodes, each 125mm in diameter, in
addition to the standard working, counter and reference electrodes. In
mapping, a single, small amplitude (ca. 15 mV) sine wave perturbation
of optimal frequency (generally 700 Hz) is applied to a DC potential set at
the global open circuit potential of the substrate. The local current
density is measured by measuring the potential gradient in solution above
the substrate between the two micro-reference electrodes and then converting
this potential difference through Ohm�s law to a local current
density. The local impedance (or admittance) is determined by
comparing the global voltage to the local current density, and an LEI map is
generated by moving the probe to discrete sites across the sample and
plotting the local admittance as a function of x,y position. LEI
spectroscopy operates by holding the probe over a particular x,y position
and scanning the sine wave excitation frequency, generally between 10KHz and
1Hz. In both cases the voltage signal from the split micro-reference
electrode is amplified to minimize the required excitation signal and
maintain a nondestructive character.
§6 All electrochemical measurements were performed on 1 mm thick AA2024-T3
cut into 6.5x6.5 cm panels coated with a neat organic resin. One of
several coating chemistries were applied depending on the particular
variable to be studied. These coatings were (a) vinyl VYHH copolymer
of polyvinyl chloride and polyvinyl acetate, (b) polyurethane coatings made
with either a 100% polyether polyurethane or a polyurethane composed of 50%
polyether and 50% polyester (both mixed with isocyanate, a dibutyl tin
dilaurate catalyst and methyl ethyl ketone solvent), (c) a two-part
polyester polyurethane comprised of Desmophen polyester polyol and desmodur
aliphatic polyisocyanate, and (d) a two-part epoxy comprised of bisphenol A-epichlorohydrin-based
epoxy resin solution with an Epi-cure fatty acid-polyethylenepolyamine based
polyamide mixture curing agent. Butyl cellosolve, in varying amounts,
was used as a solvent to adjust viscosity in coatings (c) and (d).
§7 All panels were cleaned prior to coating by the following procedure and
then left to air dry: scrubbed with acetone, scrubbed with Alconox, cleaned
ultrasonically in hexanes for at least 5 minutes, then rinsed with
isopropanol. Ultrapure water was used for rinsing between steps.
The appropriate coating was then spin cast onto the substrate producing a
uniform dried film thickness of 5-20 microns depending on the
experiment. Freshly coated samples were dried in a dessicator for at
least 48 hours prior to use. A glass reservoir was affixed at its base
perimeter to the coating surface with RTV silicone. Samples were exposed
primarily to 0.6 M NaCl of ambient aeration, but other solutions including
0.1 M NaCl and 0.1 M KCl were used as needed.
Results and Discussion
Types of Defects
§8 An important observation made in early investigations  was that
several different types of defects occurred within a single panel. These
defects types were differentiated initially by the color that they attained
upon long-term exposure. Two such defects were classified as �red� and
�black� blisters. LEIM was used on examples of such blisters in
order to evaluate the electrochemical differences between them. Maps
of a red and black blister are shown in Figure 1.
§9 Figure 1: LEIM plots showing the differences in
electrochemical activity between black (left) and red (right)
blisters. Note the higher electrochemical activity of the red
blister. These AA2024-T3 panels were coated with vinyl VYHH to 10mm
thickness and exposed to 0.6 NaCl. (larger image)
§10 It should be noted that these types of blister (i.e. red and black) were
observed on all samples when exposed to chloride solutions regardless of the
coating chemistry, however differences in rate of formation, and extent may
vary. It is clear from the figure that the red blister is
electrochemically more active, while the black blister is relatively
passive. After several weeks, the coating was removed from the
substrate and the alloy surface at the blister sites examined by an optical
microscope. As suspected from the LEIM results, the red blister had
indeed incurred significant metal loss, whereas the black blister had caused
little damage. An example of the pitting caused by a red blister is
shown in an electron micrograph in Figure 2.
§11 Figure 2: Electron micrograph of a pitted region of the
substrate beneath the initiation site of a red blister. This substrate
had previously been coated with a 10mm thick polyester polyurethane
coating. The coating was removed for viewing in the electron
microscope. (larger image)
§12 It was hypothesized that LEIM/S could predict which initial defects might
become the severe red blisters or the more innocuous black blisters by
comparison of initial electrochemical activity to the eventual blister
formed. LEIS was performed on defects which were identified through
mapping in early, still microscopic stages. The results are shown in
Figure 3. The defect which would later become a red blister was even then
significantly more electrochemically active than the eventual black blister.
§13 Figure 3: LEIS shows that even in early stages, the red
blister is more electrochemically active than the black blister. The
smaller circle diameter on this Nyquist plot represents a lower pore
resistance. (larger image)
§14 In addition to differences in electrochemical activity, the blisters were
also examined for chemical differences. In one experiment, small
quantities of solution were drawn from several examples of each type of
blister and the ionic content of the underfilm solution was analyzed using
capillary electrophoresis (CE). Cu2+, Al3+, and Mg2+
ions were found in the solution of the red blisters whereas only small
amounts of Mg2+ and Zn2+ were found in the black
blister solution. The cations present in the red blister solution
suggest that active dealloying of the substrate or a particular phase within
the substrate has taken place [2-4,17]. These results demonstrate that the
electrochemical and chemical nature within very small (<1 mm)
blisters can be quantified and characterized.
§15 Differences in underfilm pH and potential also differentiate these
blister types and substantiate the LEIM/S findings. In a similar
experiment, local pH values of the underfilm solutions were obtained for
several examples of each type of blister. This was done by inserting
an Ir/IrO microelectrode into the blister. Prior to these tests, the
electrochemical activity of each blister was mapped by LEIM. Some of
the red blisters were found to be low in electrochemical activity, while
most were found to be high in activity as expected. In fact, the
blisters which were high in electrochemical activity were found to contain
solution at pH values of 3-5, whereas the blisters with low electrochemical
activity, presumably temporarily repassivated, contained solution of pH
8-9. Local pH measurement provides valuable information about the
underfilm solution, but it is also a destructive technique. Once the
coating has been breached, the defect will no longer develop as it would
have. However, this problem can be avoided through use of the
nondestructive LEIM technique.
Time Evolution of Defect Sites
The time evolution of electrochemical defect sites was studied in further
detail. Defects were monitored over time using LEIM to detect changes
in electrochemical activity. A representative time series of maps for
a single defect is shown in Figure 4.
§16 Figure 4: LEIM of the evolution of a red blister. Note
the fluctuation in electrochemical activity representative of metastability.
At 52 hours, the open circuit potential dropped dramatically, suggesting a
possible breach in the coating. (larger image)
§17 Red blisters generally show metastable behavior, with periods of
temporary repassivation or decreased activity occurring between periods of
increasing electrochemical activity. Such metastable behavior has been
suggested in the literature but has not been specifically
investigated. It has been suggested  that the biphasic nature
parallels capacitance changes in the coating. Metastability could also
be caused by accumulation of corrosion product periodically dispersed by the
convection of hydrogen evolution. The former possibility will be
investigated by comparing LEIM of a gold disk electrode with and without a
coating. Reference standard gold disk electrodes have been created
using Si/SiO wafers with embedded gold electrodes. These standards
will be discussed in a future paper.
§18 In later stages, metastability is visually observable as growth
spurts. The blisters experience rapid growth (visible growth within
1-2 hours) followed by lengthy periods of no outward growth (12-24
hours). In earlier stages, however, when these changes are not
visually apparent, such a localized event can only be measured by local
electrochemical methods. The observed changes in growth and in pH
value of the solution render the possibility unlikely that metastability of
the blisters might be due to changes in coating properties such as water
uptake. However, preliminary studies have shown that a low (3-5) pH
solution increases the rate of transfer of ions across the coating
interface. It is possible that variations in pH affect the activity of
the blisters and could contribute to metastability.
§19 Repassivation, or �healing� has been suggested to occur on chromate
conversion coated (CCC) aluminum alloys and cited as an asset of these
coatings . LEIM was used to study the effect of chromate added to
solution on the activity of red blisters. In Figure 5, LEIM shows the
change in electrochemical activity before and after chromate solution is
§20 Figure 5: LEIM of a red blister before and after a chromate
solution was added to the bulk electrolyte. 1.2 M Na2CrO4
was added to 0.6 M NaCl. After chromate is added, the admittance
decreases, in agreement with a rise in open circuit potential. After
24 hours, the admittance begins to rise, although open circuit potential
alone does not indicate this change. (larger
§21 Once the chromate was added, the open circuit potential rose to a more
noble value as expected, and LEIM showed a decreased electrochemical
activity in the blister. LEIS also showed a decrease in
electrochemical activity. However, while the open circuit potential
remained relatively constant suggesting a continuing repassivation of the
blister, LEIM and LEIS showed that, in fact, electrochemical activity began
again to increase within the blister. Future studies will focus on
metastability of defects on substrates coated first with CCC�s and then
with organic coating. Although several investigations have studied CCC
coated substrates [20-22], the effect of an organic coating on CCC coated
substrates has not been investigated from a local electrochemistry
Delineation of Anodic and Cathodic Regions
§22 In addition to characterizing overall properties of individual
blisters, LEIM was also used to distinguish between anodic and cathodic
regions of red blisters. It was observed that blisters initiate at a
given site, and then branch into one or more secondary lobes that grow away
from the initiation site. LEIM showed that the initiation site is
electrochemically active, while the secondary lobes are relatively
inactive. An example of this is shown in Figure 6.
§23 Figure 6: An optical micrograph of a red blister with
initiation site (lighter region to right) and secondary lobe (darker region
to left). LEIM shows the higher activity in the initiation
region. Local underfilm open circuit potentials, also shown, confirm
the higher activity in the initiation region. (larger
§24 Energy dispersive spectroscopy (EDS) also revealed a higher amount of Cu
and lower amount of Al and Mg in the general area of initiation (with Cu
deposited just outside the initiation pit region). Local open circuit
measurements, made with a Ag/AgCl microelectrode similar to the one used in
pH measurements, showed a more anodic potential within the initiation region
and a more noble potential in the secondary lobe. Although further
studies are required to draw an absolute conclusion, it can be presumed that
the initiation region is the anode and the secondary lobe/lobes the
cathode. The contributing factors to the blister front advancement are
presently under investigation.
Early Stages of Defect Formation
§25 Whereas later stages of blister development can be characterized by
compiling information obtained both from local electrochemical and chemical
methods and from optical and global methods, the earliest stages of
development and initiation of a blister require the use of local
electrochemical methods. By stepping the LEI probe over the surface of
a coated alloy panel, LEIM was able to reveal microscopic regions of defect
initiation. During this early stage of development, it was found that
the impedance had actually increased relative to the surrounding
region. LEIM of an example of this is shown in Figure 7.
§26 Figure 7: An example of the initial decrease in admittance,
or increase in impedance, in earliest stages of defects. This
AA2024-T3 panel was coated with vinyl VYHH to 10 mm thickness and exposed to
0.6 M NaCl solution.
§27 Several explanations for this increase in impedance have been
proposed. Although it was suspected that nonuniform field
distributions emanating from the defect may have given rise to the increased
impedance, this explanation was found to be improbable. The expected
field from an equipotential disk was derived and calculated for dimensions
similar to those in the experiment. It was shown that the
micro-reference electrodes would have to be very close to the surface
compared to the radius of the disk (or size of the disk is large relative to
the height of the micro-reference electrodes from the surface -unlikely in
early stages). If the probe were indeed within this realm, a dip in
admittance would originate from an increased base admittance value and
substantial peaks from edge effects at the perimeter of the disk would be
apparent. A representative plot of theoretical results that
would result in an admittance dip is shown in Figure 8. The normal
field over an equipotential disk is calculated for different heights above
the surface relative to the radius of the disk. Actual admittance
plots of defects did not display any of these features. Two additional
explanations for the increased impedance are water nucleation under the
coating and build-up of corrosion product at the initiation site .
§28 Figure 8: The normal field is calculated theoretically for
an equipotential disk at various heights (z) from the surface relative to
the radius of the disk (a). Note that a dip does occur when the probe
is very close to the surface, but this dip is accompanied by significant
edge effects and the dip does not extend below the baseline of the surface.
§29 The local nature of corrosion of coated alloys necessitates the use of
local electrochemical and chemical techniques toward understanding
individual breakdown events. LEIM/S is capable of characterizing in
situ the initiation of and changes in electrochemical activity of a
single defect without disturbing blister development. LEIM/S of local
breakdown sites has also confirmed previous observations of metastable
§30 LEIM has identified several different types of defects on coated
AA2024-T3 with the two most prevalent examples being a red and a black
blister. Through LEIM and LEIS the red blister was found to be more
electrochemically active both in early and later stages of development than
the black blister. CE analysis revealed that the underfilm solution of
the red blisters contained Cu2+, Al3+, and Mg2+
ions whereas the black blister only contained small amounts of Mg2+
and Zn2+ ions. LEIM showed that red blisters often
repassivated during development and that this repassivation was found to
correspond to an increase in local pH from an aggressive environment of 3-5
to a more benign environment of 8-9. Red blisters caused a visibly
severe amount of damage to the substrate as compared to black blisters.
§31 The metastable nature of defects on the coated panels was monitored by
LEIM. Several possible explanations for the metastability, including
changes in coating properties, accumulation of corrosion product, and
variation in local pH were discussed. The addition of chromates to the
bulk electrolyte caused an initial repassivation of a red blister.
Whereas open circuit potential measurements remained steady after the
addition of chromates, LEIM showed that repassivation was only temporary.
§32 Red blisters were observed to initiate in a given region and branch into
secondary lobes. LEIM showed that the initiation sites were anodic to
the secondary lobes, suggesting a possible cathodic head in growth.
Local OCP measurements and EDS measurements of the composition of the
affected substrate confirmed the differentiation of the red blisters into an
active initiation region with corrosion taking place and relatively passive
§33 LEIM revealed an initial increase in impedance of blisters in the
earliest stages of development. Possible explanations, including
nonuniform field distribution, water nucleation, and corrosion product
accumulation were presented. Nonuniform field distributions were found
to be an unlikely cause. Defects in such early, microscopic stages
were located through LEIM of the substrate surface.
§34 The authors would like to thank the Air Force Office of Scientific
Research (AFOSR) for their sponsorship of this project as well as Jiangnan
Yuan and Robert Kelly for their contributions of CE analysis.
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