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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

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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

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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

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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 B-117 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 2024-T3 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.).

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 long-term empirical testing.

Figure 1. Non-chromate primer paint on sulfuric acid anodized 7075-T6 (left) after salt spray test compared to anodized 2024-T3 (right).1 The reason for the improved corrosion resistance on the 2024-T3 is unknown but not 7075 is unknown.

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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

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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 with water at either microscopic coating defects or through the intact polymer (Figure 1). Depassivated iron corrodes at exposed defects such as the physical scratch shown in Figure 1, where it is exposed to a high Cl- concentration. Cathodic delamination of the organic coating on the steel ahead of the scratch occurs due to alkaline attack of the polymer/metal bonds by OH- produced from ORR or by its reaction intermediates (e.g., H2O2).17 The adhesive bond between the polymer and metal is disrupted near the head of the delaminated zone (orange). This type of attack is called disbondment, delamination or scribe creep. The delaminated zone of length, l, 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

Figure 2. Coating delamination in the case of an organic coating on steel by a mechanism of cathodic disbondment. A galvanic corrosion cell is established between the prevailing anodic site at the scratch (blue) and the cathodic site (orange) under the O2 permeable coating. Cations such as Na+ must migrate/diffuse along the interface to maintain charge neutrality. The anodic reaction at the physical defect (blue) is coupled to the cathodic reaction at the delamination site.

O2

NaCl

O2 O2

OH-

e-

Fe2+Polymer

Steel

Defect Delaminated zone

Na+

Delaminated zone length, l

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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 and cathodes formed between the alloy matrix and chemical non-uniformities such as intermetallic compound-based constituent particles20 transition into an anodic head (orange zone) and a cathodic tail (blue) on an mm-length scale. Some argue that the cathode is distributed symmetrically about the anode head as well as in the tail, but sealing of the defect tends to arrest under paint corrosion while 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.

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

Figure 3. Coating delamination in the case on an organic coating on Al by a mechanism of anodic undercutting similar to crevice corrosion except that reactants can be transported through the coating perpendicular to the coating interface. The defect is shown in blue.

O2

NaCl

O2 O2

OH-

e-

Al+3

Polymer

Aluminum

Defect Delaminated zone

Cl-

O2

NaCl

O2 O2

OH-

e-

Al+3

Polymer

Aluminum

Defect Delaminated zone

Cl-

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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-Cu-containing 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

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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 #F49620-02-1-0301),45 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

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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 scribe-creep 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.

∑=∑ ccaa AiAi = I galvanic [1]

Ω++=∆ RIE galvaniccagalvanic ηη [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].

( ) coupleIdtdl ν∝

[3]

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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 scribe-creep 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]).

HERi +

+×

=ORRactL

actLcathode ii

iii [4]

nscribe

cathodecathodecouplescribe

AiIdtdw

dtdA

1 l αα= [5]

dtw

Aid

scribe

cathodecathodenscribe ll α [6]

scribe

cathodecathode1n

scribe

wtAi

1n

lα

+

+

[7]

[ ] ( ) ( )( )111

1 1 l +

+

+ nn

scribe

cathodecathodescribe t

wAinνα [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).

( )RTExtl

−== expkkt o

x [9]

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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, scribe-creep 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-

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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.

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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

15

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 Alahar, Ogle, and Orazem44. In this case, there is a distribution of damaged polymer due to the distribution of exposure times to the solution within the delaminated zone. The coating near the defect has been substantially degraded whereas that in the semi-intact zone is 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

Figure 4. One-dimensional model of delaminated organic coating on a metal with a defect at the far right.

Delaminated zone

Delaminated Front Semi-intact Fully-intact

Polymer

Metal

Defect

NaCl

Delaminated zone

Delaminated Front Semi-intact Fully-intact

Polymer

Metal

Defect

NaCl

16

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

[ ] ( ) ( )( )111

1 1 l +

+

+ nn

scribe

cathodecathodescribe t

wAinνα [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

17

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 measured

experimentally using UV-vis spectroscopy and capillary electrophoresis to investigate inhibitor release from pigmented paints in both acidic and alkaline solutions representative of various corrosion sites on Al precipitation age hardened alloys. An example of such release detected from 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

(a)

(b)

Figure 5. Example of UV-vis based detection of Ce+3 released (aliquot) from an Al-Co-Ce metallic coating in contact with AA 2024-T3. (b) Predicted E-I relationships for AA 2024-T3 modelled as a heterogeneous electrode containing non-reactive Al oxide and Cu-rich islands simulating S-Al2CuMg particles in the case where the size, spacing of Cu-rich islands are varied and the electrolyte boundary layer thickness was fixed.47

0.0 -0.5 -1.0 -1.5 -2.00.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

ΘC

u0.017

0.030

0.067

0.092

0.1200.227d = 10 µm; h = 10 µm

d = 11 µm; h = 9 µm 1.000

d = 8 µm; h = 12 µmd = 7 µm; h = 13 µmd = 6 µm; h = 14 µmd = 4 µm; h = 16 µmd = 3 µm; h = 17 µm

"ideal", Levich

i (A/

cm2 )

η (V)

200 250 300 350 400

0.0

0.5

1.0

1.5

2.0

2.5 Aliquot

100 ppm Ce3+ standard

Abs

orba

nce

λ (nm)

Ce3+

18

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 µm. Full or alternate immersion in various HCl or NaCl solutions will create a corrosive environment at the artificial defect as shown in Figure 6b. The rate of scribe-creep will be determined and 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.

(a)

Figure 6. (a) Method of Strattmann and co-workers18 used to create an artificial scratch by inserting tape along the right side of the panel prior to coating. (b) Method of Ogle, et al.,15 where the galvanic current between the head and tail can be inventoried by electrical separation of the artificial scratch from the delaminated region. In the UVA proposal, the delaminated region would be instrumented with separately addressable electrodes buried under the coating and accessible for AC and DC electrochemistry.

(b)

19

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

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

5.08

mm

1.91

cm

1.91

cm

2.54 cm

10 holes (~ 411 m dia)µ

10 holes (~ 263 m dia)µ

8 holes drilled ~ 762 m apart (center to center) and then a 9th hole ~ 9.208 mmfrom first hole and a 10th hole~ 13.08 mm from first hole (center to center)

µ

Scribe

(a)

Instrumented and Non-instrumented panels Exposed in 80% RH at 40oC and 50oC

Exposure Time (days)0 20 40 60 80

Ave

rage

Scr

ibe-

Cre

ep L

engt

h (m

m)

0

1

2

3

4

5

2024-T3 unpretreated instrumented panel in 80% RHat 40oC and retreated every 3 days with 16 wt% HCl (near 115 µm dia wires)2024-T3 unpretreated instrumented panel in 80% RHat 40oC and retreated every 3 days with 16 wt% HCl (near 254 µm dia wires)2024-T3 unpretreated non-instrumented panel in80%RH at 40oC2024-T3 unpretreated non-instrumented panel in80%RH at 50oCPure Al (99.99%) in 80% RH at 40oC

(b)

(c)

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.

i (A/cm2)

1e-10 1e-9 1e-8 1e-7 1e-6 1e-5

E (V

vs

Hg/

Hg 2

SO4)

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0wire #1 scratcwire #2wire #3wire #4wire #5wire #7wire #8wire #9

Pulse Potential

20

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 Hexavalent Cr” (Unified Agenda No. 1979, June, (2004). 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 J. Sinko, Progress in Organic Coatings, 42, 267, (2001). 8 M.W. Kendig, M. Hon, Corrosion J. (60), pp. 1024-1030, (2004). 9 O. Schneider, G. O. Ilevbare, J.R. Scully, R.G. Kelly, “Confocal Laser Scanning Microscopy as a Tool for In Situ Monitoring of Corrosion underneath Organic Coatings,” J. Electrochem. And Solid State Letters, 4(12), B35-B38, (2001).

21

10 O. Schneider, G.O. Ilevbare, J.R. Scully, R.G. Kelly, “Surface Metrology Studies of Corrosion on AA2024-T3 Using In-situ Confocal Laser Scanning Microscopy: Part 2. Influence of electrolyte composition on trench formation around particles,” J. Electrochem. Soc., B151(8), pp. B465-B472, (2004). 11 G. Grundmeyer, W. Schmidt, M. Stratmann, Electrochem. Acta 45, p. 2515, (2005). 12 W. J. van Ooij, A. Sabata, D. Loison, T. Jossic, J. C. Charbonnier, J. Adhes. Sci. Technol. Vol. 3, p. 1, (1989). 13 A. Leng, H. Streckel, M. Stratmann, “The delamination of polymeric coatings from steel. Part 1: Calibration of the Kelvinprobe and basic delamination mechanism”, Corrosion Science, Vol. 41, pp. 547-578, (1999). 14 C.R. Shastry, H.E. Townsend, Corrosion 45(2), p. 103, (1989). 15 K. Ogle, S. Morel, N. Meddahi, Corrosion Science, 47, pp. 2034-2052, (2005). 16 G. Grundmeyer, A. Simoes, in: A.J. Bard, M. Stratmann (Eds.), Encyclopedia of the Electrochemistry, vol. 5, Wiley-VCH, Weinham, p. 500, (2003). 17 G. Grundmeier, M. Stratmann, Werkst. Korros., vol. 49, pg. 150, (1998). 18 W. Furbeth, M. Strattmann, Corros. Science, 43, pp. 207-227, (2001). 19 W. Furbeth, M. Strattmann, Corros. Science, 43, pp. 229-241, (2001). 20 G.O. Ilevbare, O. Schneider, R.G. Kelly, J.R. Scully, “Surface Metrology Studies of Corrosion on AA2024-T3 Using In-situ Confocal Laser Scanning Microscopy: Part 1. Influence of electrolyte composition on the localized corrosion of constituent particles,” J. Electrochem. Soc., B151(8), pp. B453-B464 (2004). 21 R.T. Ruggeri, T.R. Beck, Corrosion, 39, p. 452, (1983). 22 G.M. Hoch, In Localized Corrosion, NACE, Williamsburg, Ed. Staehle, Kruger, Agarwal, p. 134 (1982). 23 T. P. Hoar, Filiform Corrosion and Related Phenomena meeting, 1126, London, Chem. Ind., (1952). 24 A. Bautista, Prog. Org. Coatings, vol. 28, p. 49-58, (1996). 25 H. Kaesche, Werkst. Korros., vol. 11, p. 668-681, (1959). 26 H. J. W. Lenderink, "Filiform Corrosion of Coated Aluminium Alloys - a study of mechanisms" (PhD Dissertation, Technische Universiteit Delft, (1995). 27 R. T. Ruggeri and T. R. Beck, Corrosion, vol. 39, p. 452-465, (1983). 28 G. Williams and H. N. McMurray, J. Electrochem. Soc., vol. 150, p. B380-B388, (2003). 29 G. Williams and H. N. McMurray, Electrochem. Comm., p. 871-877 (2003). 30 W. Schmidt and M. Stratmann, Corros. Sci., vol. 40, , p. 1441-1443 (1998). 31 . V. Kloet, W. Schmidt, A. W. Hassel and M. Stratmann, Investigations into the Role of Copper in AA2024-T3 Aluminum Alloys on Filiform Corrosion Advancement and the Role of Chromium in Corrosion Inhibition, 2001 AFOSR Corrosion Review, Duck Key, FL, AFOSR, (2001). 32 H. Leth-Olsen and K. Nisancioglu, Corrosion, vol. 53, p. 705, (1997). 33 L. F. Vega, F. Bovard, T. Nakayama, K. Ikeda, H. Shige and E. L. Colvin, Filiform Corrosion of Aluminum Alloys: Influence of Alloy Composition, NACE Topical Research Symposium Proceedings, 149, NACE, March (2000). 34 F. Mansfeld, M. W. Kendig and S. Tsai, Corrosion, vol. 38, 1982. 35 M. W. Kendig, A. T. Allen, S. L. Jeanjaquet and F. Mansfeld, Corrosion/85, (Houston, NACE, 1985). 36 A. Afseth, J. H. Nordlien, G. M. Scamans and K. Nisancioglu, Corros. Sci., vol. 43, 2001, p. 2359-2377. 37 X. Zhou, G. E. Thompson and G. M. Scamans, Corros. Sci., vol. 45, 2003, p. 1767-1777. 38 D. Little, M Jakab, J.R. Scully, Corrosion J., 62(4), pp. 300-315, (2006). 39 J. M. C. Mol, B. R. W. Hinton, D. H. Van Der Weijde, J. H. W. De Wit and S. van der Zwaag, J. Mater. Sci., vol. 35, p. 1629-1639, (2000). 40 J. M. C. Mol, A. E. Hughes, B. R. W. Hinton and S. van der Zwaag, Corros. Sci., vol. 46, p. 1201-1224 (2004). 41 A. E. Hughes, J. M. C. Mol, B. R. W. Hinton and S. van der Zwaag, Corros. Sci., vol. 47, p. 107-124 (2005). 42 G. Williams and H. N. McMurray, Electrochemical and Solid State Ltrs, 7(5), B13-B15, (2004). 43 W. Furbeth, M. Stratmann, Corrosion Science, 43, pp. 243-254 (2001). 44 K.N. Allahar, K. Ogle and M.E. Orazem, ECS Proceedings Vol. 2002-24, p. 475 (2002). 45 John R. Scully and Daryl A. Little, Identification of the Factors Governing the Origin & Propagation of Corrosion Failure of Organically Coated Aluminum Aerospace Alloys, Final report to AFOSR under F49620-02-1-0301, 14-9-2005, (2005). 46 W. Furbeth, M. Stratmann, Corros. Sci., vol. 43, pg. 243, (2001). 47 M.A. Jakab, F. Presuel, J.R. Scully, “The Oxygen Reduction Reaction on AA 2024-T3; Experiment and Modeling using the Theory of a Heterogeneous Electrode” J. Electrochem. Soc., 152(8), B311-B320, (2005).

22

48 M. A. Jakab, F. Presuel-Moreno and J. R. Scully, “Effect of Molybdate, Cerium and Cobalt Ions on the Oxygen Reduction Reaction on AA2024-T3 and Selected Intermetallics: Experimental and Modeling Studies,” J. Electrochem. Soc., 153(7), pp. 244-252, (2006). 49 M.A. Jakab, F. Presuel, J.R. Scully, “Critical Concentrations Associated with Cobalt, Cerium and Molybdenum Inhibition of AA 2024-T3 Corrosion, Delivery from Al-Co-Ce-(Mo) Alloys,” Corrosion Journal, 61(3), pp. 246-263, (2005). 50 W. Funke, et al., J. Of Coatings Technolgy, 58(741), p70, (1986).