A Proposal for AFOSR
Computational Design of Coatings to Optimize Under Paint
Corrosion Resistance Using Commonly Applied Corrosion Control
Strategies
John R. Scully1 and Robert G. Kelly2
1-Principle Investigator
2 - Co-Principle Investigator
Center for Electrochemical Science and Engineering
Department of Materials Science and Engineering
University of Virginia,
Charlottesville, VA 22911 USA
Submitted to:
Major Jennifer Gresham, Ph.D.
AFOSR Program Manager
Chemistry & Life Science
875 North Randolph St.
Suite 325, Room 3112
Arlington, VA 22203
(703) 696-7787 office
(703) 696-8449 fax
jennifer.gresham@afosr.af.mil
1
Table of Contents
1.0 Objective
p. 3
2.0 The Need and Opportunity
p. 3
3.0 Past Work on Organic Coating Degradation by Under-paint Corrosion
p. 5
3.1 Initiation of Under paint Corrosion in the Case of Intact Coatings
p. 5
3.2 Mechanisms of Paint Failure by Corrosion in the Presence of Defects
p. 6
3.3 Delamination, Filiform, and Scribe Scribe on Age Hardened Al Alloys used in Aerospace
Applications
p. 7
3.4 Strategies for Abatement of under paint Corrosion of Al-based Alloys
p. 8
3.5 Modelling of under paint Corrosion and Scribe Creep
p. 9
4.0 Proposed Approach:
p. 12
4.1 Overall
p. 12
4.2 Galvanic Corrosion Model for Paint Delamination
p. 14
4.3 Damage Model for Paint Delamination
p. 16
4.4 Experimental Approach for Paint Delamination
p. 16
5. Response to AFOSR and DOD needs
p. 20
6.0 Program Leveraging:
p. 20
7.0 References Cited
p. 20
2
1.0 Objective
The objective of this research is to develop a galvanic corrosion model of under paint
corrosion. Subsequently, this model will be used to elucidate effective combinations of
corrosion protection strategies and optimal coating and metal surface properties that minimize
the galvanic cells responsible for under paint corrosion at a model coating defect. The system
modelled computationally and experimentally will be a scratch through an organic coating to a
metal surface resulting in its exposure to electrolyte. This defect helps to establish a galvanic
cell between an anode (head) and cathode (tail) whose location depends on the exact
coating/metal system. The four common strategies of corrosion protection by coatings
(sacrificial cathodic protection, active corrosion inhibition, coating barrier, and coating
adhesion functions) will be considered in this model defect. Optimal properties of aircraft-type
epoxy primer coatings and metal surfaces for corrosion abatement will be ascertained by
exercising the model over a large number of relevant conditions. This objective will be
accomplished by computationally varying in a systematic fashion various traditional generic
physical and intrinsic attributes that limit the galvanic current between anode (head) and
cathode (tail), given that this corrosive process governs corrosion damage by under paint
corrosion and scribe creep. Coordinated experimental work involving model scratches will
provide the computational model with accurate boundary conditions, focus the model on issues
of importance, and provide the means to validate model predictions. In this way, a robust
elucidation and quantification of requirements for optimization of resistance to coating
delamination by corrosion will be gained. The model should be flexible and generic enough to
allow adaptation to a variety of coatings and substrates. Particular emphasis will be placed on
the protection of legacy-type precipitation hardened aluminium alloys at generic coating
defects such as scratches.
2.0 The Need and Opportunity
The DOD faces several severe challenges related to corrosion in the 21st century, as
detailed in a recent Defense Science Board Report on Corrosion Control1. These include
continued use of aging assets beyond original lifetimes which introduces advanced stages of
corrosion-related damage, the reliance on older legacy alloys with less than optimal intrinsic or
extrinsic corrosion protection strategies, and the Federally mandated replacement of traditional
(but very effective) inhibitor systems such as chromate with environmentally friendly
alternatives.2 This situation will impact chromate pigmented paints and chromate conversion
coatings which are the primary means of corrosion protection of aerospace structural Al alloys.
The design of new organic primer coatings for corrosion protection to aluminium alloys
can be accomplished via conventional and/or exotic strategies. Conventional approaches
include improved combinations of barrier, adhesion, sacrificial anodic, and active corrosion
inhibition strategies, whereas more exotic strategies, not yet typically implemented, include
corrodant sequestering, buffering of the corrosive solution formed at the coating-metal interface
and changes in the semi-conductor nature of the surface oxide and its ability to support the
electron transfer reactions that allow a galvanic corrosion cell to operate.3 Another exotic area
3
is electroactive coatings that can galvanically couple with the metal and can release a corrosion
inhibitor via an electrochemically triggered process.
The fact that these coating strategies can work is not the area of scientific debate. In
fact, we do not propose to duplicate these efforts nor conduct proof-of-concept type
experiments with novel strategies. Instead, one relevant debate centers upon what can be done
to change the empirical design process in a way that provides meaningful generic guidance on
useful coating attributes of broad applicability to the DOD coating community.
To date, a plethora of new technologies have emerged in the area of environmentally
friendly primer coatings, each with its own merits and disadvantages. Some new inhibitor
“cocktails” have been incorporated into coatings and, in some cases, are capable of
performance almost equal to that of traditional chromate bearing systems4. Impressive advances
in coating adhesion have been made also.
Unfortunately, much of this coating corrosion work is empirical in nature as measured
by scribe creep rate and visual appearance after standardized accelerated testing. The exact
reasons for success or failure of a given pigmented primer (where the soluble pigment provides
an inhibitor) on a given metal are uncertain. This uncertainty arises from the fact that either the
scientific foundations for and/or quantifiable measures of needed attributes for success in each
of the necessary stages such as storage, triggered release, transport and mechanisms of
inhibition have not been elucidated and established that lower the galvanic corrosion rate
between the metal under the paint and the scratch. In other words, success or failure is judged
by overall performance (such as corrosion-induced scribe creep at a scratch after an ASTM B117 salt spray test) without a clear understanding of why a given strategy was inadequate. A
good example of the shortcomings of the Edisonian approach to testing is a recent study of a
primer organic coating system containing a chromate replacement by a major aerospace
company. The primer with an active corrosion inhibitor performed well in the case of a 2024T3 precipitation age hardened alloy, but poorly in the case of 7075-T6, another alloy commonly
used in aerospace structures. The reasons for this discrepancy are not known at this time, but
clearly the beneficial inhibitor concentration for the 2024-T3 is not achieved for the other alloy.
Empirical testing cannot illuminate this point. Thus,
the optimal combination of attributes for corrosion
resistance is often unclear, and primers as well as
surface treatments are often designed by trial-and-error
using qualitative guidelines such as evidence of
minimal corrosion after a salt spray test after a
prescribed exposure time (i.e., 1000-3000 hrs, etc.).
Figure 1. Non-chromate primer paint on
sulfuric acid anodized 7075-T6 (left) after
salt spray test compared to anodized 2024T3 (right).1 The reason for the improved
corrosion resistance on the 2024-T3 is
unknown but not 7075 is unknown.
Progress in effective coating design is often
frustrated by this “lessons learned” approaches as the
lesson “learned” is rarely clear. Not surprisingly, there
is difficulty in predicting how coating formulation
changes or changes in the alloy substrate will affect
primer performance towards corrosion without longterm empirical testing.
4
Notable exceptions to the completely Edisonian testing of the past are the recent work
of Wang for inhibitor release,5 Presuel-Moreno for metallic coatings6 and Sinko7 as well as
Kendig for organic coatings in which inhibitor release from organic coatings was confirmed
along with inhibition of the oxygen reduction reaction.8 Sinko made an early attempt to define
desired organic coatings inhibitor attributes, but no scratch modelling was attempted where the
inhibitor functions he specified were put to the test. Thus, failure to protect a defect under a
given set of circumstances cannot be attributed to inadequate release and transport or the need
to achieve a high inhibitor concentration, or reduced inhibitor effectiveness, etc. The result of
such uncertainty is that broad applicability of an inhibitor strategy to situations outside the
tested conditions cannot be assumed. For instance, the impact of different environment severity
or a change in an alloy cannot be forecasted.
This study seeks to both explore the necessary attributes of traditional corrosion
protection strategies in the case of a model coating defect as well as ultimately provide
knowledge and a coating evaluation toolset that are portable and capable of contributing to the
evaluation of other systems. A generic coating defect such as a scratch in a coating exposing
the metal substrate at such a defect will be considered. Two stationary configurations will be
considered with the aim of modelling the effect of coating and surface attributes on the
galvanic corrosion cell created by the defect electrically connected to the buried interface under
the coating and the associated chemistry and electrochemistry differences:
(1) A defect in a coating exposing the metallic surface to corrosive solution with an
adjacent intact coating with high degree of adhesion over the remaining coated
“buried” metal in the adjacent painted region.
(2) A defect in a coating exposing the metallic surface, with a partially delaminated
coating forming a creviced zone with a crevice gap between the metal and the adjacent
detached painted region.
Before describing the research to be undertaken in these model configurations, it is
necessary to describe the past work in the area of coating delamination by under paint corrosion
leading up to the assertion that the proposed work can enhance the understanding of the under
paint corrosion process and elucidate the factors that control the delamination of organic
coatings from defects.
3.0 Past Work on Organic Coating Degradation by Under-paint Corrosion
3.1 Initiation of under paint Corrosion in the Case of Intact Coatings
It is now well known that under paint attacks starts with an intact paint system in the
presence of microscopic defects and involves three stages: (i) formation of galvanic cells at a
local scale between microstructural features in the Al-based alloy under the coating where
adhesion has been compromised and bulk electrolyte has entered9,10 (ii) separation of anode and
cathode on the metallic surface into distinct spatial zones and a subsequent galvanic current
5
between the two such that local coating delamination is driven by either the accumulation of
reaction products at the delamination front (i.e., OH- in the case of steel delamination ahead of a
scratch through a coating) or is a direct consequence of the corrosion, itself (i.e., anodic
undercutting at the head of the corrosion filament in the case of Al filiform corrosion).11 The
third stage involves a more complex chemo-mechanical disbonding process where the adhesive
forces between the metal and organic coating are degraded both by chemical attack as well as
by products of the galvanic corrosion reaction and mechanically by the wedging forces
introduced by voluminous corrosion products.12 There are variations on this basic mechanism,
but this description is accurate and enables the process to be captured in a galvanic couple
model.
3.2 Mechanisms of Paint Failure by Under-paint Corrosion in the Presence of Defects
Numerous papers indicate that the coating delamination process of coated metal
surfaces can be described by the formation of a galvanic cell between the active defect and the
delamination front as described above.13,14,15 The driving force for such a galvanic cell is the
potential difference established between the anode and cathode zones formed because of the
distinct chemical differences in solutions formed at the defect and under the polymer film and
thus the differences in electrochemistry at the buried interface compared to the exposed metal
(Figure 1).16 This electrochemical cell leads to electrons and cations being transported to the
local cathode (orange zone) and Cl- ions being transported to the local anode (blue zone) as
shown in Figure 1. Metal cation hydrolysis may also occur at the anode. At the cathode site,
electron transfer reactions (ETR) such as O2 reduction (ORR) occur which raise the pH.
In the case of paints on steel, oxygen transport through permeable coatings and its
reduction at the metal/polymer interface is coupled and balanced by oxidation of iron at the
scratch.3 O2 migrates through the coating along
Defect
Delaminated zone
with water at either microscopic coating
O2
O2
O2
defects or through the intact polymer (Figure
1). Depassivated iron corrodes at exposed
Fe
Polymer
NaCl
Na
defects such as the physical scratch shown in
OH
Steel
e
Figure 1, where it is exposed to a high Clconcentration. Cathodic delamination of the
Delaminated zone length, l
organic coating on the steel ahead of the
Figure 2. Coating delamination in the case of an
scratch occurs due to alkaline attack of the
organic coating on steel by a mechanism of cathodic
polymer/metal bonds by OH- produced from
disbondment. A galvanic corrosion cell is established
ORR or by its reaction intermediates (e.g.,
between the prevailing anodic site at the scratch
H2O2).17 The adhesive bond between the
(blue) and the cathodic site (orange) under the O2
+
permeable coating. Cations such as Na must
polymer and metal is disrupted near the head of
migrate/diffuse along the interface to maintain charge the delaminated zone (orange). This type of
neutrality. The anodic reaction at the physical defect attack is called disbondment, delamination or
(blue) is coupled to the cathodic reaction at the
scribe creep. The delaminated zone of length, l,
delamination site.
moves from left to right with exposure time.
Sophisticated variations of this mechanism exits in the case of zinc-coated steel, which depend
upon whether the scratch through the organic coating penetrates to the zinc or to the steel
surface.18,19 The rate of the OH- or intermediate production is directly related to the galvanic
2+
+
-
-
6
current between the distinct anode and cathode formed in this cell and the position of the
anodes and cathodes may shift to sites under the mature delamination zone.
In the case of delamination of organic coatings on Al alloys in the presence of a
macroscopic defect such as a scratch (Figure 2), the mechanism of delamination is associated
with anodic undercutting of the delamination front via the formation of an acidified, rapidly
dissolving anode at this site. Local anode
Defect
Delaminated zone
and cathodes formed between the alloy
O2
O2
O2
matrix and chemical non-uniformities
such as intermetallic compound-based
Al
Polymer
NaCl
Cl
constituent particles20 transition into an
OH
Aluminum
e
anodic head (orange zone) and a cathodic
tail (blue) on an mm-length scale. Some
argue that the cathode is distributed
Figure 3. Coating delamination in the case on an organic
coating on Al by a mechanism of anodic undercutting
symmetrically about the anode head as
similar to crevice corrosion except that reactants can be
well as in the tail, but sealing of the defect
transported through the coating perpendicular to the
tends to arrest under paint corrosion while
coating interface. The defect is shown in blue.
impermeable coatings have little effect
with an open defect suggesting a dominance of the tail and physical defect as a cathodic site.21
The anode becomes acidified and Cl- rich due to metal cation hydrolysis and migration of Cl- to
maintain charge neutrality, respectively. The wake of the delaminated site is the dominant
cathode site because of the high oxygen permeability through the thin electrolyte film that
exists at this site, and the absence of poisoning of ETR by adhesives or blockage of the metal
ETR sites by a polymer. As mentioned above, symmetrically distributed cathodes about the
head can also be considered, but have been ruled out in some cases.22 The intact paint decreases
this ORR due to blockage of copper sites via the polymer and low oxygen permeability through
the polymer layer. The associated attack is called scribe creep, coating delamination by under
paint corrosion, and filiform corrosion.
+3
-
-
-
The latter stages of organic coating delamination are complicated by the action of
voluminous corrosion product formation created under the coating. These corrosion products
exert mechanical stresses capable of exceeding the bond strength of the paint. This advanced
stage, thus, involves electrochemical and chemical reactions as well as mechanical forces.
3.3 Delamination, Filiform, and Scribe Creep on Precipitation Age Hardened Al Alloys used
in Aerospace Applications
As discussed above, under paint corrosion in Al alloys most likely occurs by anodic
wedging23 and anodic undercutting.24,25,26,27 According to this mechanism, the head and tail
form the anode and cathode, respectively. These anode/cathode positions have been confirmed
by Scanning Kelvin Probe measurements at the sites where scribe-creep was observed under
polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate coatings on steel28,29 and commercial
polymer coatings on AA2024-T3.30 Model Al-Cu alloys were created by depositing a regular
array of Cu islands on high purity Al.31 When coated with an organic coating, micron scale
pitting at Cu dots transitioned to the formation of mm-scale low pH anodes at the filiform tip
and high pH cathodes at filiform tails. In fact, filiform tracts were arrested at the ends of the
7
arrays of Cu dots suggesting the importance of cathode sites that support fast electron transfer
reaction (ETR) rates in close proximity to anode sites.
Alloy composition in Al-based aerospace alloys, particularly Cu and Fe content, have a
dominant effect on filiform growth rates when grown from physical scratches.32,33 Moreover,
the same surface preparation and organic coating type and thickness are known to result in
drastically different undercoating corrosion rates in Cu-containing 2000 series Al alloys
compared to 5000 series Al alloys.34,35 Indeed, under paint corrosion susceptibility of high-Cucontaining 2xxx and 7xxx alloys is greater than that of 6xxx with its lower Cu content. 5xxx
alloys with nil-copper perform even better, indicating that the overall performance of coated
metals is linked to the corrosion and electrochemical ETR properties of the substrate alloy.36,37
Therefore, a key aspect of a galvanic couple model to address coating delamination is
to incorporate the electron transfer reactions that occur both at the defect and under the
polymer-coated heterogeneous alloy as a function of technological variations.
3.4 Mechanisms of Abatement of under paint Corrosion of Al-based Alloys
Anodizing and conversion coatings are known to minimize, but not eliminate, filiform
corrosion on AA2024-T342. Chromate pigmented paints act to protect IMC and poison
cathodic ETR as well as anodic ETR. Chromate conversion coatings perform the same function
to a lesser extent, but improve adhesion. Again the galvanic current between distinct anode and
cathode zones drives the delamination process via the anodic undercutting mechanism and the
beneficial action of chromate or surface treatment can be interpreted within such a framework
as limiting the cathodic reaction rate in this cell. Papers by Afseth, et.al.36 and Zhou, et.al.37
focus on the role of alloyed manganese and the effects of surface conditioning or treatment on
FFC resistance. It has been shown that alkaline cleaning of the surface leads to increased FFC
growth rates. Decreased severity of attack occurs in the filament tail when deoxidizing is
conducted after alkaline cleaning.38 Chromate conversion coatings also lead to a decrease in
the FFC growth rates.39,40,41 The decreased rate has been related to the adhesion of the coating
as a result of these surface pretreatments. Filiform corrosion is known to proceed at enhanced
rates on Cu-bearing Al alloys even when chromate conversion coated44. Organic and anion
exchange pigments were also found to inhibit filiform corrosion on AA 2024-T3. These effects
were interpreted through a decrease in the Volta potential or depression in free corrosion
potentials.42
Therefore, one conclusion is that many surface engineering, cladding and alloy
modifications may be interpreted largely (although not necessarily entirely) based on primarily
their effect of ETR rates that enable the galvanic cell formed. Second, surface engineering
measures can affect coating adhesion and third, they can sequester or otherwise limit ion
migration needed to complete the electrochemical galvanic cell. Regarding coatings, it is clear
that coating barriers that are highly resistive can limit the ionic current between the anode and
cathode whether local or in spatially distinct zones. Permeation barriers limit transport of
cathode reactants such that cathodic sites are limited exclusively to the defect. Clearly,
chromate pigment suppresses cathodic reaction rates in the tail and defect but also may be of
benefit towards anodic sites in the head. Chromate conversion coatings may limit filiform
8
corrosion by improvement in adhesion. Adhesion promoters create greater adhesive forces and
also can be interpreted as limiting ion transport parallel to interfaces, imposing increased
resistance between the anode and the cathode, thus reducing the galvanic current.
In the context of a galvanic couple model for scribe creep, all of these abatement
strategies may be interpreted in either stage I, II or III as processes which interfere with the
formation, magnitude and sustainment of large galvanic couple currents between the head and
tail.
3.5 Modelling of Under paint Corrosion and Scribe Creep
Almost all studies of stage II coating delamination have resulted in phenomenological
descriptions of the mechanisms of coating delamination by under paint corrosion but have not
resulted in damage models that describe the time evolution of coating delamination as a
function of material, chemical, electrochemical and physical parameters. The Scanning Kelvin
probe method has brought about significant advances in understanding of under paint corrosion
mechanisms as well as the capability to map the movement in the delamination front in-situ in
real time. Scribe creep rates have often been described as following a t-1/2 rate where t is time.38
In one case, it has been proposed that the rate of scribe creep of galvanized steel with an
organic coating is governed by the transport of Na+ to the cathodic site at the delamination
front.43 This transport is rationalized to occur by both diffusion and migration and accounts for
the t-1/2 dependency. Replacement of Na+ with other cations has changed the transport mobility
and substantiates this claim. Therefore, coating and surface preparation methods that limit
anion or cation transport as shown in Figures 2 and 3 can limit the current density within such a
galvanic cell.
Alahar, Ogle, and Orazem provide one of the only cases where implementation of a
mass transport model, such as used in crevice corrosion, to describe the local current and
potential at the model organic coating delamination site on zinc electrogalvanized steel.44 Like
many steady state crevice corrosion models transport equations are solved, mass and charge
balance are maintained. Key features of the model for describing coating delamination include
a pH-dependent diffusivity associated with O2 permeability through an organic film whose
porosity increased with pH, pH dependent electrochemical reaction rates, and a electrochemical
reaction rate that depended on the degree of coverage of the metallic surface by the either
partially detached or detached polymer. The model was capable of describing the potential
distribution and pH in the delaminated zone and claimed to detect the position of the
delamination front. Successive runs of the model accounting for the pH distribution up to and
ahead of the delaminated coating created a moving delamination front. However, the model did
not include any sort of damage function that related the local electrochemical current or
chemistry to the loss of adhesion or propagation at the delamination front, and there was no
explicit criterion for failure. These are important limitations in this advanced model.
In previous AFSOR-funded work at the Univ. of Virginia (GRANT #F49620-02-10301), a phenomenological scribe creep model was developed that could account for a
number of surface treatment and metallurgical variables on the rate of scribe creep and explain
the time dependency of the process described in Figure 3. Scribe-creep experiments were
45
9
conducted on epoxy polyamide-coated (average coating thickness ~10 µm) AA2024-T3 in 80%
relative humidity at 25°C, 40°C, and 50°C with an intentional scratch through the coating into
the substrate. The effects of surface pretreatment and alloy aging that control the amount of
surface copper and alter intermetallic compound distributions on the rate of scribe-creep caused
by under-paint corrosion on coated AA2024-T3 were investigated. The effects of alloy aging
on the rate of scribe-creep caused by under paint corrosion on coated AA2024-T3 were also
investigated.
A galvanic couple exists between the anodic head and the cathodic tail of the scribecreep filament formed during under-paint corrosion. The galvanic current results in anodic
undercutting which is responsible for scribe-creep in Al alloys. Consider a cathodically
controlled galvanic corrosion process with a fast anodic reaction. The galvanic couple
relationship is affected by the cathode area, cathodic kinetics per unit area, anodic kinetics,
anode area, and the distance between the head (anode) and tail (cathode) as the filament grows.
At the galvanic couple potential, the sum of the anodic currents is equal to the sum of the
cathodic currents over all areas (Equation [1]). This global conservation of charge is always
true, however, the local current densities at the anode and cathode are not always equal.
Theoretically this means that if the area of the cathode (AC) increases at constant cathodic
kinetics per unit area, such as by Cu-replating, then the corrosion rate of the anode (ia) or the
anode area (Aa) or both must increase to maintain the balance of Equation [1]. Therefore, the
growth rate of scribe-creep when driven by such a galvanic current should decrease as the area
of the cathode decreases or as the cathodic kinetics per unit area (ic) are inhibited. Equation [2]
states the basic fundamental equation for a galvanic couple which states that the difference in
the mixed potentials of the anode and cathode (∆E) is equal to the sum of the anodic and
cathodic overpotentials plus the IR drop (IgalvanicRΩ) for the system.
∑ i a A a = ∑ i c A c = I galvanic
[1]
∆E galvanic = η a + η c + I galvanic R Ω
[2]
When the distance between the anodic head and cathodic tail increases, the ohmic resistance
(RΩ) between the anode and cathode of the galvanic couple also increases. The galvanic current
must then decrease (Igalvanic ↓ as RΩ ↑) because ∆E is fixed and is given as the difference in open
circuit potential between the anode and cathode (i.e., ∆E = Etail – Ehead). Assuming that the rate
of scribe-creep is proportional to the rate of galvanic corrosion (Igalvanic), many of the
pretreatment and metallurgical factors describing scribe-creep may be described within this
framework.
In the case of scribe-creep by an anodic undercutting mechanism, the growth rate of the
scribe, dl/dt, is proportional to the galvanic current, Igalvanic, for the system as stated by Equation
[3].
 dl 
  ∝ (ν )I couple
 dt 
[3]
10
Here the factor υ numerically converts galvanic current (A/cm2) to scribe creep growth or
delamination rate.(cm/s). Assuming that the galvanic couple current is controlled by total
cathodic current, in one possible scenario for galvanic corrosion, then the growth rate of scribecreep area (l is the scribe-creep damage length and w is the width of the scribe area) is
proportional to the total cathodic reaction rate (Equation [4]). The scribe-creep area growth
rate is then a function of the product icathodeAcathode as shown in Eq. [5] and is inversely
proportional to the distance, l, from the anode at the head and the cathode at the tail raised to
some power n accounting for non-linearity (ln) in the RΩ vs. l relationship as shown by
Equation [5]. The scribe-creep length is seen to be a function of the cathodic reaction rate (
Icathode) and θCu, or the surface coverage of copper. Rearranging (Equations [6]-[7]), it is
observed that the length is directly proportional to t1/n+1 where n is a factor that describes how
much the galvanic current decreases with l (Equation [8]).
 i ×i
icathode =  L act
 i L + iact


+ i HER
 ORR
[4]
i
A
dA
dl
= wscribe
α I couple α cathode cathode
n
dt
dt
1scribe
n
lscribe
dl α
i cathode A cathode
dt
w scribe
[6]
n +1
i
A
t
lscribe
α cathode cathode
n +1
w scribe
 (n + 1) icathode Acathode 
l scribe α [ν ] 

wscribe


[5]
[7]
( 1n+1)
t
( 1n+1)
[8]
It can seen how either a t1/2 or slight deviation from t1/2 growth law can occur, depending upon
the value of n =1. This type of growth law in seen in a number of cases.38,45,46 From these
equations it should be possible to describe and then predict the growth rate for scribe creep or
delamination front that occurs by under-paint corrosion.
The derivation above could be used to explain the observation that the length, l, of the
scribe-creep at any given time was thermally activated and can be described by an Arrhenius
type relationship. The time dependency for scribe creep was expressed as tx as shown by
Equation [1]. Scribe-creep rates decreased with time as “x” was typically less than one (where
x=1/n+1).
l = kt x = k o t x exp
(− E RT )
[9]
11
In this expression, t is time, T is temperature, E is the activation energy for the scribe creep
process and R is the universal gas constant. Experiments show that the Cu content of the alloy
and pre-treatment strongly influence scribe creep. The pre-exponential term, k, was greatest for
the NaOH treatment followed by the as-received condition and the NaOH + HNO3 pretreatment
had the lowest k value. The effect of each surface pretreatment in enhancing or retarding
scribe-creep can be directly traced either to the initial level of Cu-replating that was introduced
by such treatment, or to its ability to allow the supply Cu for replating in the scribe-creep
filament wake. Cu replating increased Acath in equation [8]. When Cu was eliminated as an
alloying element, such as in the case of 99.99% Al, or when surface Cu was minimized at the
coating-metal interface, such as by HNO3 deoxidation pre-treatment of AA 2024-T3, scribecreep corrosion rates were lowered as Acath was lowered. It should be noted that the effect of
pretreatment was not traceable to coating adhesion, but was a result of a decrease in the
cathodic ORR rate in the scribe creep tail and at the defect, which supports anodic undercutting
at the head of the delamination front by supporting the galvanic corrosion cell discussed above.
Scribe-creep was, also, observed to be enhanced by exposure test temperature regardless of
surface pretreatment with activation energy, E, of 30-40 kJ/mol, as well as by artificial aging
and surface pretreatments. This activation energy was interpreted to relate to the effect of
temperature on cathodic ETR such as ORR under charge transfer control. For instance, icath in
equation 8 would likely be thermally activated with this type of activation energy.
This is one of the only scribe creep or delamination growth laws that can describe the
rate of coating delamination in term of simple material, surface and coating attributes.
4.0 Proposed Approach:
4.1 Overall
The overall approach in this proposal involves recognition that all three stages of
corrosion attack under paint discussed above involve formation of an electrochemical cell
between distinct anodes and cathodes. Either this galvanic cell reaction, or the reactions
themselves, or their by-products, drive corrosion-induced organic coating delamination. In
stage II, this electrochemical cell is formed with spatially distinct anode and cathode zones at
the head and tail of a filiform-type corrosion site. There are variations on this basic mechanism
depending upon the details, but this generic framework is applicable to all of those situations
discussed above.
The broad applicability of this mechanism enables the stage II process to be captured
in a galvanic model based on mixed potential theory, mass and charge conservation laws
yielding the current-potential and pH, and ion distributions associated with the electrochemical
cell formed at a coating defect. Such a mass transport model must be coupled with a damage
model which connects delamination or scribe creep to the galvanic current density at the
delamination front.
Therefore, stage II will be the focus the initial three year plan of the proposal. It is
likely the most important technological stage for engineered coatings on aerospace alloys since
all coatings contain defects which may shorten stage I and repair is often implemented in find-
12
it/fix it repair strategies before stage III becomes too advanced. Moreover, the mechanism of
attack is well-enough understood to be described in the form of a model.
Hence, this study will focus on:
(1) Establishing a galvanic corrosion model to describe galvanic interactions at a coating
delamination coupled with a scribe creep model that expresses the coating delamination
rate as a direct function of the galvanic current between the head and the tail/wake/defect
region.
(2) Exercising the model over a broad range on conditions to ascertain the optimal
combination of metal and coating properties needed to mitigate, hinder or inhibit the
formation and rate of galvanic corrosion between the head and tail.
(3) Performing experiments which (a) provide the fundamental data and (b) construct a
model delamination site to probe validate key aspects of commonly applied abatement
strategies as well as to validate modelling.
The output of this model (inhibitor concentration, local pH and potential and galvanic
current as a function of position under coating defects) will serve as inputs to recently
established coating delamination propagation rate law for Al alloys.38 This law describes the
rate of the scribe creep by a process of anodic undercutting at the coating metal interface driven
by galvanic corrosion between the filiform head and tail and accept various electrochemical
and physical inputs. This damage law accepts these various inputs and yields scribe creep
propagation rate as a function of them.
This damage output (i.e., L vs. t) would be exercised focusing on different coating
system attributes to illuminate what set of coating, alloy, surface engineering conditions
provide the best way to lower the rate of creep. In this way the impact of various coating
attributes (intrinsic and physical) on the damage kinetics will be assessed over a broad range of
model conditions.
The key question to be addressed through experiments and modelling will be how
can (a) coating barrier, (b) coating or conversion coating properties that provide an active
corrosion inhibitor, (b) adhesion by bond promoters, or (c) sacrificial cathodic protection, or (d)
surface engineering of precipitation age-hardened alloys be manipulated to minimize the
galvanic current between the head and the tail/defect of the corrosion damage site associated
with stage I and stage II of the under paint corrosion process.
For instance, the combination of attributes that optimize a coating–inhibitor system
will be critically assessed. In the case of active inhibitor release, a computational study of
storage, triggered release, delivery and assessments of the fate of the inhibitors that ultimately
control the electrochemical and chemical mechanism(s) associated with active corrosion
inhibition would be undertaken to illuminate the inhibitor storage, release, and transport
properties needed.
13
The desired attributes of sacrificial coatings (such as Mg pigmented organic coatings
under development) will include the ability to polarize a large scratch relative to critical
potentials (e.g., high throwing power), a low sacrificial species self-corrosion rate, and high
sacrificial species capacity. The physical attributes to be considered include barrier and
adhesive properties of the primer, and the electrochemical properties of the magnesium
pigmented paint.
Barrier properties can be investigated by considering intrinsic barrier properties and
physical properties such as coating thickness. These will be studied by exercising the
continuum scale galvanic corrosion model to be developed at UVa over a broad range of primer
conditions.
4.2 Galvanic Corrosion Model for Paint Delamination
Equation 8 provides a conceptual framework for the construction of a delamination law.
The framework needs to have its parameters (e.g., icathode, Acathode, and υ) quantified based upon
a coordinated modelling and experimental program in order to separate the variables which can
affect the galvanic current, and thus scribe creep. The modelling work will allow the relative
contributions of the different attributes to the delamination rate to be isolated in ways that are
not possible experimentally. In this way, optimization of these parameters can be achieved and
trade-offs made apparent and quantifiable. The experimental work will not only provide
reality-based boundary conditions for the modelling, but also will allow validation of
predictions.
The delamination system illustrated in Figure 3 represents a distributed galvanic couple
operating under occluded conditions. The galvanic interaction between the anodic head and the
cathodic tail/open defect is similar to that of a crevice exposed under atmospheric conditions
wherein the size of the external cathode is limited. The material within the delaminated region
will support electrochemical reactions that are affected by the potential distribution as well as
the chemistry distribution, as is the case for crevice corrosion. At UVa, we have used analytic
and numerical solutions to these types of problems for several years, including studies of the
release of inhibitors from organic and metallic coatings and their transport to coating defects.5,6
The key distinguishing characteristics of our modelling approach are (a) the use of
experimentally-derived electrochemical kinetics rather than theoretical Tafel behavior, (b) the
imposition of the conservation of charge for the complete anode/cathode system as opposed to
assumed potentiostatic conditions at the mouth of a defect, and (c) the use of state-of-the-art
physical chemistry software1 to calculate the chemical distributions and transport properties of
the concentrated electrolytes present within the occluded region. The outputs of our models are
the current, potential, and chemistry distributions within the occluded region as a function of
time, or at steady state. The galvanic current which drives scribe creep is simply the spatial
integration of the current density distribution over the region between the anode and the
cathode.
For the proposed work, we would extend our modelling to the case of a crevice with
limited, but pH- and time-dependent permeability to water and oxygen, as would be the case
1
OLI Systems, Morris Plains, NJ
14
for an organic coating undergoing under paint corrosion. For example, the effective diffusivity
of oxygen through a section of coating could be described by:
Deff = Dpolymer[1– a(pH-12)*(t-to)]
[10]
where to is the time at which that portion of the coating experiences a pH equal to 12 and a is a
damage potency factor. Thus, at shorter times, that section of polymer would have a state of nil
polymer degradation with associated low diffusivity and ionic mobility.
Two cases would be studied in the first three years of this program in which static
physical arrangements corresponding to the initiation and early propagation of delamination
would be characterized. These would form the basis for a fully integrated model of filiform
corrosion damage evolution when linked with data modelling studies of filiform corrosion
being performed at UVa under ONR sponsorship.
Case 1: In this case no fully delaminated region exists. Instead, the defect alone is in
contact with an intact polymer covered by a thin electrolyte (not shown). Thus, this case
probes the factors controlling the initiation of under paint corrosion. The galvanic interaction of
interest is between the defect, exposed to a solution with a high concentration of oxygen, and
the region of the coated metal adjacent to it. The coated region has lower oxygen content due
to the restricted diffusion, and thus would become the anode. The galvanic current can be
calculated given electrochemical kinetics appropriate for the two regions, their dependence on
solution composition, knowledge of the ionic conductivity and oxygen diffusivity of the
polymer coating, and the release and transport properties of any inhibitors in the coating. All of
these values are available through the literature or standard experimentation. The magnitude of
the galvanic current is the output of the calculation. Thus, its dependence on material
properties (i.e., pH-dependent electrochemical kinetics, polymer diffusivity and ionic
conductivity, inhibitor release and transport) can be quantitatively evaluated. An understanding
with respect to the relative contributions of these variables to scribe creep initiation resistance
will be gained. This model will be very similar to recent implementations to study inhibitor
storage and release from organic5 and metallic Al-Co-Ce coatings.5,6
Case 2: Once under paint corrosion has stabilized past the conditions described in Case
2, a new set of conditions must be addressed. Figure 4 shows geometry of the so-called stage II
delamination front based on the work of
Defect
Delaminated zone
Alahar, Ogle, and Orazem44. In this
Polymer
case, there is a distribution of damaged
NaCl
Metal
polymer due to the distribution of
exposure times to the solution within the
Delaminated
Front
Semi-intact Fully-intact
delaminated zone. The coating near the
defect has been substantially degraded
Figure 4. One-dimensional model of delaminated organic
whereas that in the semi-intact zone is
coating on a metal with a defect at the far right.
only beginning to suffer from exposure
to chemical attack.
The galvanic
current between the defect and the anode at the tip of the under paint corrosion will be
influenced by the length of the delamination zone, the conductivity of the solution under the
15
coating, the coating and the electrolyte layer on top of the coating. In addition, the differences
in solution composition between the defect and the delamination tip will be substantial, leading
to potential gradients that depend on the electrochemical kinetics of the substrate in those two
solutions.
Mitigation of propagating under paint corrosion will be investigated by
computationally varying the controlling parameters. The base case for each will be derived
from experimental measurements of polarization behavior, coating diffusivity and conductivity,
and using standard methods for dealing with the effect of porous corrosion products on
transport parameters.
4.3 Damage Model for Paint Delamination
The determination of galvanic current based on the finite element model may be
coupled with the damage process controlled by under paint galvanic corrosion rate. For
instance, if this galvanic corrosion process is controlled by the cathodic reaction rate and ohmic
voltage, scribe length L can be described by equation [10] or similar
 (n + 1) icathode Acathode 
l scribe α [ν ] 

wscribe


( 1n+1)
t
( 1n+1)
[10]
From these equations it should be possible to describe and then predict the growth rate
for paint filaments or delamination based on under-paint corrosion. It should also be possible
to anticipate what metallurgical, pretreatment or inhibitor factors could impact scribe-creep. It
can also be seen how improved adhesion might help because in the case of better adhesion a
given increase in the right hand side of Eq. 10 would produce less increase in lscribe at the same
reaction rates as represented in this model by a decrease in the value assigned to υ.
To exercise the coupled model, the open circuit potentials and E-I kinetics for
unpretreated AA2024-T3, NaOH pretreated AA2024-T3, and pure Cu can be measured and
used to obtain Icouple via the modelling described above. The drop in Igalvanic and icathode can be
computed by recomputing Icouple as l increases. υ can be determined via experimental scribe
creep data and simultaneous measurement of Icouple from instrumented scribe creep experiments
by comparison of dl/dt to Icouple. Scribe creep rates versus time can then be estimated for a
variety of material and coating variables. By taking new and existing data from an artificial
delamination site, it should be possible to determine approximate values of υ that equate the
measured galvanic corrosion rate to the measured scribe creep rate.
4.4 Experimental Approach for Paint Delamination
16
Experimental data of relevant electrochemical behaviour and coating properties are
needed. Two types of data are needed. These include:
1) Data that can serve as input to the model to provide boundary conditions and
unknown parameters that are needed to complete the computations for case (1) and
(2)..
2) Experimental scribe creep data under model experimental conditions for
computational model verification and fine tuning.
4.4.1 Experimental data for model input
Some of this information can be obtained from mining the literature from previous
studies at UVA and elsewhere.47 Storage capacity, leach rates and triggering of released
inhibitors will be
2.5
Aliquot
measured
100 ppmCe standard
2.0
experimentally using
1.000
4.0x10
0.227
0.120
Ce
UV-vis spectroscopy
1.5
0.092
and
capillary
3.0x10
0.067
electrophoresis
to
1.0
investigate inhibitor
2.0x10
0.5
0.030
release
from
pigmented paints in
1.0x10
0.0
both
acidic
and
0.017
200
250
300
350
400
alkaline
solutions
0.0
0.0
-0.5
-1.0
-1.5
-2.0
λ (nm)
representative
of
η(V)
various
corrosion
(a)
(b)
sites
on
Al
Figure 5. Example of UV-vis based detection of Ce+3 released (aliquot) from an
precipitation
age
Al-Co-Ce metallic coating in contact with AA 2024-T3. (b) Predicted E-I
hardened alloys. An
relationships for AA 2024-T3 modelled as a heterogeneous electrode containing
example of such
non-reactive Al oxide and Cu-rich islands simulating S-Al2CuMg particles in the
release detected from
case where the size, spacing of Cu-rich islands are varied and the electrolyte
boundary layer thickness was fixed.47
an Al-Co-Ce coating
is shown in Figure 5a. Figure 5b shows an example of the type of cathodic kinetics that can be
implemented to describe the rate of ETR reaction associated with Al-Cu-Mg alloys as a
function of water layer thickness and copper coverage. This can be repeated at relevant pH
values. Critical inhibitor concentrations will be determined independently for model input
based on levels required to inhibit corrosive processes on Al alloys.48,49 Mechanistic
understanding of ETR on Al alloys under organic coatings will be obtained from E-I curves
established in-situ under polymer coatings with various copper coverages and various degrees
of coating damage. Both the effect of polymer coverage and the copper coverage on ETR will
be considered. Some of the data needed regarding the effect of copper coverage on ETR by
ORR on 2024-T3 have been previously developed.47 In addition, separate studies will be
conducted to describe E-I behaviour with inhibitors of various concentrations in simulated
3+
-4
"ideal", Levich
d=11µm; h=9µm
-4
d=7µm; h=13µm
d=6µm; h=14µm
3+
d=10µm; h=10µm
d=4µm; h=16µm
d=3µm; h=17µm
-4
ΘCu
i (A/cm2)
Absorbance
d=8µm; h=12µm
-4
17
filiform head (acidic, high Cl- and tail solutions (alkaline). The latter will achieve
characterization of the inhibitor behaviour over a range of pH.
4.4.2 Experimental study of scribe creep
Coating delamination in the presence of a galvanic cell between a defect and coated
2024-T3 will be evaluated using scribe creep testing. Delamination will be measured by
scanning Kelvin Probe (SKP) analysis of potential distributions2 perpendicular to the scribe
creep artificial scratch or by image analysis of scribe creep on planar 2024-T3. An alternative
configuration will use model Al-Cu substrates (such as Al with copper replated via CuSO4
solution) and/or AA 7075 panels (or model Al-Zn-Mg alloys) with and without roll-bond
applied cladding. Exposure studies will then be performed on artificially scratched, and coated
planar electrodes with artificial defects produced following the method shown in Figure 6a. The
method of Ogle may be used to facilitate easy separation of anodes and cathode for inventory
of Igalvanic.15 In this situation, a model configuration may be attempted where the artificial
scratch contains significant replated copper (or pure copper) in contrast with a starting
conditions where a matching alloy is present at the scratch and cathode sites may be distributed.
AA2024-T3 panels (or model versions) will be coated with a translucent epoxy polyamide
coating similar to aircraft coating similar to aircraft primers such as equal weights of Epon
resin 1001-CX-75 (Shell) with
Epi-Cure 3115 X73 curing
agent (fatty acid-polyethylene
polyamine based polyamide
mixture, Shell) and 5 wt%
Butylcellosolve. The coating
will be applied using a spin
coater and will be placed in a
dessicator out of the light for
at least one week for curing.
The average coating thickness
will be approximately 10-100
µm and controlled within ±10
(b)
(a)
18
µm.
Full or alternate
Figure 6. (a) Method of Strattmann and co-workers used to create an
artificial scratch by inserting tape along the right side of the panel prior to immersion in various HCl or
coating. (b) Method of Ogle, et al.,15 where the galvanic current between
NaCl solutions will create a
the head and tail can be inventoried by electrical separation of the corrosive environment at the
artificial scratch from the delaminated region. In the UVA proposal, the
artificial defect as shown in
delaminated region would be instrumented with separately addressable
electrodes buried under the coating and accessible for AC and DC Figure 6b. The rate of scribecreep will be determined and
electrochemistry.
compared with the galvanic
current. For post-test analysis of these panels, the coating will be removed using a tape pull
method while the coating was still moist from exposure.
2
UVA CESE does not currently own a SKP system. Cooperative experiments with Martin Stratmann are planned
and a DURIP proposal will be written to acquire this instrument at UVa.
18
Specially designed instrumented scribe creep panels (Figure 7a) will also be utilized to
determine in-situ scribe creep rates (Figure 7b) and galvanic corrosion rates using embedded
electrodes and multiple electrode arrays as implemented previously.45 Scribe-creep will be
monitored in-situ utilizing a digital camera or the scanning Kelvin Probe method, exploiting the
translucent coating (Optical) or potential shape (SKP) to detect the under paint corrosion front
and the increase in length with time (Figure 7b). Scribe creep can also be characterized using a
variety of other methods including EIS, optical examination, and adhesion testing. The
electrodes shown in Figure 7a can be interrogated as in 7c to obtain the cathodic reaction rate
and open
Average Scribe-Creep Length (mm)
5
Instrumented and Non-instrumented panels
Exposed in 80% RH at 40oC and 50oC
4
3
2
1
0.0
0
20
40
60
Exposure Time (days)
1.91 cm
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
2
i (A/cm )
Sc ribe
10 holes (~ 263 µm dia)
Pulse Potential
(b)
(c)
1.91 cm
5.08 mm
8 holes drilled ~ 762 µm
apart (c enter to c enter) and
then a 9th hole ~ 9.208 mm
from first hole and a 10th hole
~ 13.08 mm from first hole
(center to center)
2024-T3 unpretreated instrumented panel in 80% RH
at 40oC and retreated every 3 days with 16 wt% HCl
(near 115 µm dia wires)
2024-T3 unpretreated instrumented panel in 80% RH
at 40oC and retreated every 3 days with 16 wt% HCl
(near 254 µm dia wires)
2024-T3 unpretreated non-instrumented panel in
o
80%RH at 40 C
2024-T3 unpretreated non-instrumented panel in
80%RH at 50oC
Pure Al (99.99%) in 80% RH at 40oC
80
E (V vs Hg/Hg2SO4)
0
10 holes (~ 411 µm dia)
wire #1 scratc
wire #2
wire #3
wire #4
wire #5
wire #7
wire #8
wire #9
-0.2
2.54 cm
(a)
Figure 7. (a) Instrumented scribe creep panel showing scribe creep up to and beyond buried AA 2024-T3
electrodes on 2024-T3 sheet.45 (b) Change in scribe creep length l vs. exposure time for epoxy coated 2024-T3
compared to pure Al. (c) Cathodic polarization data for 2024-T3 buried electrodes at various positions relative to
the scribe creep front and the defect. Electrode (wire) one is located at the scratch while wire 9 is buried under
the intact coating.
circuit potential as a function of position under the delaminated coating or buried under the
intact coating relative to the position of the defect or scratch. A mechanistic understanding of
various factors discussed during under paint corrosion will be accomplished via this suite of
experimental techniques. Electrochemical information obtained in this manner can either be fed
into the galvanic couple model as inputs or in the case of dl/dt and Igalvanic, used to validate it.
The first variables to be explored are copper coverage on the alloy or defect, organic
coating thickness, surface treatment and either inhibitor addition at the scratch or coating
pigmentation with such as with SrCrO4. Additionally, the literature may be mined to obtain
other experimental scribe creep data under other conditions. The basic approach could be
applied to other alloy classes (Fe-based) assuming mechanisms of under paint corrosion is
19
known and fits the description shown in Figures 2 and 3 and that damage laws have been
developed or can be adapted from the above.
5. Response to AFOSR and DOD needs
The potential outputs include development of a systematic fundamental understanding
of the desired coating and metal attributes that suppress galvanic cell current between the
anodes and cathodes responsible for galvanically-driven scribe creep and coating delamination.
This could improve upon empirical understandings and lessons-learned approaches used in
corrosion protection by coatings. This improved insight will enable improved coating design
and also highlight areas of research that require greater specific fundamental knowledge in
under paint corrosion that contribute to the galvanic corrosion framework.50 The quantified
attributes could serve as target goals to be sought for any new, environmentally friendly primer
or point towards surface engineering strategies to control electron transfer reactions at coated
metal interfaces. The generic methodology developed should eventually provide a coating
design toolset that is extendable and capable of contributing to the development of many primer
coating/surface preparation schemes via elucidation of the relative merits of each strategy and
quantifiable measures that can be taken to optimize them. Using the appropriate under paint
corrosion mechanism and associated damage model, the approach will be readily adoptable to
other substrates such as steels and other organic coatings in future research. A current goal is
not to predict exact delaminated coating damaged areas as a function of time given a variety of
environmental stresses, service conditions, coatings and alloys. This goal is not impossible but
is not the current focus.
6.0 Program Leveraging:
The project will be leveraged by on-going work at UVa CESE, UVA collaborations via
STTR and SBIR and collaborations with other institutions. Recommended start date is January
2008.
7.0 References Cited
1
Defense Science Board Report on Corrosion Control, Report A767824, Office of the Undersecretary of Defense
for Acquisition, Technology and Logistics, Washington, DC, July (2004).
2
U.S. Department of Labor, Occupational Safety and Health Administration, “Occupational Exposure to
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3
M. Stratmann, “2005 W. R Whitney Award Lecture: “Corrosion Stability of Polymer-Coated Metals-New
Concepts Based on Fundamental Understanding”, Corrosion Science, Vol. 61, No. 12, pp. 1115-1126, (2005).
4
See for instance several papers reported in: Proceedings of the 2005 Tri-Service Corrosion Conference, Orlando
FL, (2005).
5
H. Wang, F. Presuel-Moreno, R.G. Kelly, Electrochim Acta, 49, 239, (2004).
6
F. Presuel-Moreno, H. Wang, M.A. Jakab, R.G. Kelly, J.R. Scully, J. Electrochem. Soc. 153(11), (2006).
7
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20
10
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11
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13
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20
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21
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32
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34
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37
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38
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39
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40
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41
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42
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