USING SIMULATION TO HELP UNDERSTAND DIFFERENCES BETWEEN CORROSION IN ATMO-SPHERIC ENVIRONMENTS AND ACCELERATED CHAMBER TESTS

Dr. Alan Rose, Corrdesa LLC (arose@corrdesa.com; 770-683-3960)

Dr. Siva Palani, Dr. Keith Legg, Dr Julio Mendez, Corrdesa LLC

Keywords: computational corrosion analysis, galvanic corrosion prediction, FEA, corrosion prediction, polarization data, potential model, fluid shell elements, CFD

ABSTRACT

Corrosion in the field occurs over a large timescale, so when considering material choices in the design of aero-space systems and subsystems, use is often made of accelerated tests such as ASTM B117 chamber test to rank the possible materials. Even these ‘accelerated’ tests take more than 1000 hours, and despite their widespread use are often criticized as a design trade tool due to discrepancies between the test and field environments.

The corrosion community has expended substantial effort in trying to make the tests ‘more realistic’ but in doing so, there is considerable debate about whether the tools employed to accelerate the corrosion (temperature, salt concentrations, UV exposure, etc) activate the failure mechanisms found in service, or actually introduce other corrosion processes that are not even present in the targeted field of operation.

Computational techniques hold a great deal of promise as a way to understand the effects of different service en -vironments, but if the simulations cannot even discern between, say, an ASTM B117 test and an atmospheric ex-posure then the simulation results would be of questionable value.

Corrosion is affected by factors such as the chemistry of the environment, the geometric shape of the corroding component, and the fluid dynamics that dictate the electrolyte thickness around the component geometry. In a ‘re-alistic’ marine environment, near the beach, it might be appropriate to consider electrolyte concentrations with 3.5% NaCl; however, in a salt fog chamber test, different concentration of NaCl are used in order to accelerate the testing process. Unfortunately, this creates difficulty in correlating chamber tests to atmospheric, fielded environ -ments.

In this paper, different environments are characterized with the use of appropriate polarization curves (accounting for chloride concentration and electrolyte thickness), and this information is incorporated into a CFD (Computa-tional Fluid Dynamics) framework in order to further take into account the distributed electrolyte thickness over the geometries. In this way, it is possible to distinguish from and simulate fielded environments and chamber test en-vironments, thereby providing more insight to help the designer make more appropriate material choices.

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2019 Department of De-fense – Allied Nations

Technical Corrosion Con-Paper No. 0218_0314_000071

INTRODUCTION

The ‘best’ and unfortunately the most expensive and longest corrosion test is the actual fielded component or plat-form that is to say, “real life”. However, even environments in the field can vary widely. So, materials engineers and designers are faced with a huge challenge. On the one hand corrosion can take many years but data is needed upfront for design, so there is clearly a need for “accelerated” assessment methods. However, when envi-ronments are accelerated, how far do they stray from the environments endured in the field? The objective that should always be kept in mind is that the design must survive and resist corrosion in the eventual fielded environ -ment. The accelerated test results are important but are essentially secondary, since it is possible to survive a test and fail in the field and vice versa. Understanding the impact of the particular corrosion environments is key.

ENVIRONMENTS

The ASTM B117 [1] salt spray (fog) apparatus provides a controlled corrosive environment which has been uti -lized for many years to produce relative corrosion resistant information for specimens of metals and coated metals exposed in test chambers. Since the test was originally developed around chromated coating systems there has been much criticism questioning whether the relevant corrosion mechanisms are activated in more recently devel-oped chrome-free coating systems versus the chromated baseline.

When it comes to accelerated test methods the following three aspects have a major impact on the results and therefore any correlation with the reality natural environments

Chemistry Test piece geometry Fluid dynamics

ASTM B117 essentially comprises 1000-2000 hours of continuous exposure to a salt solution of 5% NaCl at 35°C, conditions that are vastly different from typical maritime environments where seawater concentrations of NaCl are in the region 3.5% with vastly varying temperatures due to diurnal and weather effects [2,3,4]. ASTM B117 condi-tions have clearly been chosen with a more aggressive chemistry to accelerate conditions, not replicate natural environments. So, it is little surprise that correlations with natural environments are not possible, or are certainly limited in application. Recent testing by Dante [5] has shown that corrosion is actually most rapid during the drying cycle, as an increasingly concentrated NaCl solution forms a thinner and thinner layer until it breaks up into droplets on the surface.

Test piece geometry is also important. Many protection systems are tested on flat panels which are sometimes scribed (yet another feature to accelerate corrosion). Indeed, the standards require that the main surface of con-cern be angled 15° to 30° from the vertical in order to enhance runoff and therefore deliver a continuous replen -ishing flow of electrolyte solution. The reality is that real components can comprise a range of protruding and re-cessed areas that could result in little or even excess electrolyte solution. Such geometries will have an impact on the fluid dynamics and the thickness of the electrolyte. In regions where the electrolyte fluid layers are thin there can be a more abundant oxygen supply diffusing down to the substrate, whereas more bulk-like conditions will im-pede oxygen supply, that can significantly affect the oxygen reduction reaction, which in turn will affect the actual metal dissolution and corrosion, since the mechanisms are electrically connected (Red-Ox).

Fluid dynamics clearly plays an important role, as considered by Van den Steen and Deconinck [6], who devel-oped and investigated 1D and 3D models for predicting film characteristics based on condensation and evapora-tion processes. In this paper we will show that with the use of a commercial off-the-shelf (COTS) Computational Fluid Dynamics software (CFD) it is possible to model corrosion rate under both variable electrolyte concentration and variable electrolyte film thickness, and also demonstrate the fidelity of the simulation by comparison with pre-viously published experimental results.

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MODELING AND SIMULATION METHODOLOGY

In the research reported here, the assemblies and tests used by Feng et al. [7] were reproduced using Solid-Works 2016 to create the CAD geometry, and the COTS software, Siemens STAR-CCM+ multi-physics software to simulate the electrochemical aspects and fluid dynamics.

Electrochemical Data acquisition

Accurate and consistent polarization data is essential for accurate corrosion modeling. In order to validate the modeling approach against Feng’s experimental work, we acquired polarization data relevant to the different ex-posure conditions. Polarization data were acquired in 3.5 wt.% NaCl at 25˚C in near-neutral pH using the stan-dard methodology developed by the Naval Air Systems Command [8], we will refer to this as the NAVAIR environ-ment. Polarization data was also acquired in 5 wt.% NaCl at 35˚C to capture the ASTM B117 environment. This involved measuring the anodic and cathodic curves separately from a well-defined surface, again using the NAVAIR approach, but with the higher concentration and temperature appropriate to B117 and then joining the anodic and cathodic portions in a single curve. The data is most commonly measured under static conditions in bulk electrolyte. Polarization data for Al 2024-T3 (anode) was measured under bulk conditions. However, it is known that for a galvanic couple under thin electrolytes, corrosion becomes more pronounced on the anode as more oxygen is made available for the oxygen reduction reaction at the cathode. Since it is difficult to measure polarization data under thin electrolyte films, a Rotating Disk Electrode (RDE) technique was used to generate the experimental potentiodynamic polarization curves for stainless steel (cathode) fastener material. In this method the film thickness is assumed to be represented by the oxygen diffusion layer of a controlled homogenous hydro-dynamic interface. In an RDE the diffusion layer thickness, δ, is calculated based on the rotation speed, ω (165 rpm to represent a 50µm layer). The relationship between δ and ω is given by Levich [9]:

δ=1.61D13 ν

16ω

−12 (1)

where

D : Diffusion coefficient of oxygen at 35˚C, 2.92x10-9 [m².s-1]

: Kinematic viscosity of water at 35˚C, 7.12x10-4 [Pa.s]

The diffusion coefficient of oxygen in water calculated from literature data based on the Stokes–Einstein equation [10], and the kinematic viscosity of water is calculated from literature data [11].

The B117 polarization data (5% NaCl) is shown in Figure 1 and the NAVAIR environment (5% NaCl) polarization data is shown in Figure 2. When the electrolyte is “stagnant”, i.e. the fluid is stationary, 5mm or more thick, and oxygen only diffuses in from the surface to the substrate through the 5 mm electrolyte layer, the polarization curve is considered to be a bulk polarization curve, which is obtained by measuring the curve in a flat cell under several centimeters of electrolyte, this is represented by the orange curves.

In situations where a condensing film might form, the thickness would be more of the order of 50 µm, which is rep-resented by the blue curves, which were measured using the rotating disk electrode (RDE) technique. Under these conditions oxygen readily diffuses through the film surface to the substrate. Consequently, the polarization curve shows a very marked increase in the oxygen reduction reaction, as shown by the blue curves in the region from OCP to about -1 V. For 3.5% NaCl (Figure 2) this results in a two order of magnitude increase in corrosion, shown by the locations of the curve crossings between the aluminum and the two stainless steel curves. This clearly shows that there will be far higher corrosion under conditions where oxygen can readily be replenished, i.e. where there are thin films of electrolyte or flowing oxygenated electrolyte. Whereas, regions of stagnant water lying in pools on the surface, would result in a much lower galvanic corrosion rate.

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Figure 1 Polarization curves for 2024-T3 Al (gray), versus 316 stainless steel under stagnant (bulk) (orange) and 50 µm thin-film for B117 conditions, i.e., 35oC at 5% NaCl.

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Figure 2 Polarization curves for 2024-T3 Al (gray), versus 316 stainless steel under stagnant (bulk) (orange) and 50 µm thin-film for NAVAIR conditions (blue) i.e. 25oC 3.5% NaCl.

The aggressive nature of the B117 environment is clearly seen by comparing the curve crossing points in Figure 1 and Figure 2. Whether we are considering bulk or thin electrolyte polarization data, the corrosion current densities in both cases are very high, greater than 0.7A/m2. Whereas, in Figure 2 we only see high corrosion current for the case of the thin electrolyte data. So, already we can see that for a complex, shaped component (where electrolyte thickness may vary due to shape and orientation) a B117 test will not replicate the same fidelity of corrosion ob-served in the field, since in the B117 test the corrosion rate will be high (due to chemistry and temperature) whether the electrolyte film is thin or thick. Indeed, for this material combination, the B117 ‘thick’ 5% NaCl/35°C electrolyte data is more aggressive that the NAVAIR ‘thin’ 3.5% NaCl/25°C electrolyte data!

Modeling methodology

The galvanic corrosion modeling executed in support of this project comprises two key aspects;

1. To capture the electrochemical aspects, we used a potential model approach employing a secondary cur-rent distribution assumption. In this way, materials of interest are characterized by their polarization curves acquired in the electrolyte environment of interest.

2. To estimate electrolyte layer thickness, we used a Computational Fluid Dynamics (CFD) tool that could account for the complex multi-phase physics in an accelerated corrosion test chamber where we expect a mixed flow of air, water vapor, liquid water drops and flowing films.

To capture all of the above, complex phenomena we used the COTS software Siemens STAR-CCM+ (v13.02.011). This is a multi-physics, finite-volume software package which includes various electrochemistry modules.

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Electrochemical modeling

When using STAR-CCM+ for electrochemical modeling we essentially take two routes;

Model the full fluid volume/domain in 3D – which would be considered a regular approach to defining and setting up the CFD model.

Model the fluid domain as a thin shell. This enables the easy modeling of the electrochemical potential and current density in thin fluid shell that effectively ‘wraps’ the geometry with a thin film of uniform thick-ness and constant conductivity.

It is assumed that the potential in the thin layer approach does not vary with distance from the surface, since the layer is very thin, but the potential does vary along the surface according to Ohm’s law, expressing current con-servation in the Laplace-equation which is a second order partial differential equation [12]. The electrolyte is treated as a homogeneous ohmic conductor with no ion diffusion or convection. This allows the model to treat it as a two-dimensional problem, reducing the Laplace equation to:

∇⃗2D=(−σ ∇⃗2 DU )=−f (U) (2)

where D2⃗

is the 2-D gradient operator solving for x, y-coordinates and f(U) is the imposed polarization curve, which serves as the boundary condition. This means that the galvanic current at any location is defined by the gal-vanic current for the local potential, which is defined by the polarization curve.

Computational fluid dynamics (CFD) modeling

Whatever the corrosion environment, an electrolyte must always be present, and in the case of atmospheric cor-rosion, the electrolyte film can be a few tens of microns. Before we can start to simulate corrosion, we must know the thickness of the electrolyte, since this is a key parameter which controls the corrosion rate.

A very thin electrolyte allows easier access of oxygen to the substrate beneath, and creates a larger iR (resistive) drop, which impacts the distribution of potential and current density.

The thickness of the film greatly depends on relative humidity, the shape of the surface, presence of contaminants and many other factors such as temperature, sunlight exposure etc. So, what thickness should be used in gal -

vanic corrosion simulations? By balancing the assumed conductive heat transfer from a film surface to the wall with the enthalpy of evaporation for the mass flow down the wall, Nusselt was able to derive an expression for the thickness profile of the condensate film down the vertical wall (Figure 3). This was extended to the case of an in-clined plane;

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Figure 3 Nusselt’s fluid flow down an inclined plane, and wetting of a horizontal plane.

δ (z )=[ 4 μk (T s−T w)ρ ( ρL−ρG ) g sinαhGL

Z]1/4

(3)

Where

δ : Local film thickness [m] T : Temperature (saturated; wall) [˚C] ρ : Mass density (liquid; gas) [kg.m-3] μ : Viscosity (liquid) [kg.m-1.s-1] g : Gravity acceleration [m.s-2] k : Thermal conductivity (liquid) [W.m-1] z : z-coordinate point from top of the plate [m] h : Latent heat [kJ.kg-1] α : Angle relative to horizontal

Using a transient solver, the STAR-CCM+ implementation of the fluid film model accounts for transport of con-served quantities within the film and its interaction with surroundings, considering conservation of mass, momen-tum, energy and species. A key assumption is that the film is thin enough for laminar boundary layer approxima-tion to apply, resulting in a parabolic velocity profile across the film. This enables the simulation of evaporation and condensation, leading to film thickness predictions. However, in ASTM B117 tests, an assembly will experi -ence dropwise film condensation on a pristine surface, at least during the initial stage of exposure, and more film-wise condensation as the surface corrodes.

As a verification exercise, a model was constructed for a simple, inclined flat plate which showed a close match with the use of Nusselt’s equation (Figure 4).

3D CORROSION SIMULATIONS

Earlier we saw that quite a lot of information could be gleaned from the crossing points of the respective polariza-tion curves. However, the curve crossing point is really a statement of the situation where we have a cathode:an -ode ratio of 1, which is rarely the case. The only way to properly account for non-unity anode:cathode ratios, par -ticularly with thin, or less conductive electrolytes, is to explicitly capture the geometry in a 3D model and solve the potential equation presented earlier. This will now be done for the scribed coupon reported by Feng et al. [7], where the cathode:anode ratio is in fact 19. 3D CCM+ simulations were therefore conducted for the situations of ‘physically’ thin (and uniform) film electrolytes of 50 µm, and also bulk electrolytes, using respectively the B117 environment and NAVAIR environment.

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Figure 4: Thickness of a flowing film down an inclined plane comparing the Nusselt and CCM results.

Figure 5 shows the CAD model and the computational domain and mesh used for modeling the exact geometry used in the work of Feng et al. The domain comprises a 5 mm deep pool of 3.5% NaCl electrolyte. Figure 6 shows the meshing details around the point where the scribe runs beneath the washer. It was necessary to refine the computational mesh around the fasteners, and especially in this area to allow the scribe to traverse beneath the fasteners and hold electrolyte.

The only galvanically active areas in this model are the scribe on the Al 2024 and the four SS 316 fasteners. The rest of the Al 2024 plate is considered a perfect insulator since it is painted. In Figure 7 the highest corrosion cur-rents are observed in the vicinity of the fasteners and fall off with distance. This is a result of the iR drop in the thin film electrolyte. This is very similar behavior to that usually seen in this type of experiment, and also reported by Feng et al. Note that corrosion is predicted to be more extensive next to the fasteners and diminishes with dis -

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75 mm

100

mm

Figure 5: CAD drawing of corrosion test assembly (left). Computational domain and mesh (right).

Figure 6: Mesh refinement around the scribe area (Left). View using transparent washer (right) show-ing how the scribe runs beneath the bolt/water assembly.

tance – this was also observed by Feng. In fact, the corrosion even appears to be somewhat higher outside the fasteners than between them, again as Figure 7 leads us to expect. The reason for this is that the area exposed by the scribe is shorter on the outside than the inside, but since the cathode is more or less symmetric, a roughly equal total current travels in each direction, but occupies a smaller area on the outside, leading to a higher current density.

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Figure 7. Galvanic corrosion currents under thin-film, NAVAIR conditions (left), with magnified view (right).

Figure 8 Comparison of corrosion currents near fasteners under thick film stagnant and thin-film electrolyte.

All of the above information tells us about corrosion on the surface outside the fasteners. Figure 8 shows the dis-tribution of current density for the NAVAIR environment in the scribe near the fastener for the two cases of bulk polarization data and 50 µm thin film polarization data for the SS 316. As discussed, the thin film polarization data accounts for the greater diffusion of oxygen through the thin electrolyte layer. Consequently, the corrosion current density is higher under thin-film conditions – in this case, by a factor of about x100.

In the model, as described earlier, we included the scribe traversing underneath the fastener and the corrosion current outside and beneath the fastener shown in Figure 8. (Note the large difference in scales between the stagnant and 50 µm calculations.) There is clearly some corrosion beneath the fastener, and the current density from the exterior of the fastener rapidly decreases under the fastener due to the iR drop along the thin electrolyte film. This is most clearly visible for the 50 µm electrolyte layer. Beneath the fastener the galvanic corrosion current is derived only from the surface of the washer immediately above it.

It is difficult to envisage what is actually happening in the very narrow scribe, but we see that models using thin and thick layers really do not by themselves represent what is happening. What matters is not strictly the physical thickness, but the availability of oxygen at the cathode. For a thin film bounded by the anode below and the air above it is thickness that defines oxygen availability. But beneath the fastener the electrolyte film is bounded by the anode below and the cathode above. Corrosion can only occur if oxygen can reach the cathode, and it can only do so in this case by diffusion from outside the fastener, and by limited flow of electrolyte between the sur -

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faces, whose gap is unsealed. Thus, the next step in our model should be to correctly treat the anode and cath-ode under the fastener as “stagnant”, which is characterized as low-oxygen. This can be done by treating the un -der-fastener area as bulk (low oxygen supply). In this case the under-fastener area on the right-hand side of Fig-ure 8 would have a current density of about 0.06 Am-2, corresponding to the current density on the left-hand side, rather than the 3-4 Am-2 shown in the figure. In fact, of course, the situation is more complex still, since the chem-istry during corrosion inside crevices is considerably more complex, and largely unknown.

COMPARISON OF SIMULATION RESULTS WITH FENG’S MEASURED DATA

To understand how the modeling corresponds with real measurements we will leverage previously published work by Feng et al [7Error: Reference source not found] who measured galvanic corrosion currents in an assembly comprising a primed, painted and scribed Al 2024-T3 plate with 316 stainless steel fasteners, tested by atmo-spheric exposure, and in ASTM B117 accelerated testing.

The fasteners were inserted in such a way as to electrically isolate them from the panel and permit measurement of the galvanic corrosion current between the cathodic fasteners and the anodic scribed areas in the panel as a function of time.

Feng observed that fastener corrosion current in the B117 environment started at 50 µA and peaked towards 125 µA. The simulations predict corrosion currents of a similar magnitude, that is 117 µA to 149 µA, depending on whether we use bulk or thin film conditions.

Table 1 Comparison of curve crossing points for NAVAIR and ASTM B117 Conditions

Quite a lot of information can be gleaned from the respective polarization curves reported by Djinn. However, the curve crossing point is really a statement of the situation where we have a cathode:anode ratio of 1, but the scribe and fasteners in the Feng coupon result in a cathode:anode (stainless:aluminum) ratio of 19. In this circumstance it is therefore necessary to take into account the impact of explicit geometry, electrical conductivity and conse-quently iR drop. This can be achieved with the 3D CCM+ model. 3D simulations were therefore conducted for the situations of ‘physically’ thin (and uniform) film electrolytes of 50 µm and also bulk electrolytes, using respectively the B117 environment and NAVAIR environment.

Referring back to Table 1 we can see that the 3D predicted corrosion current per fastener for the bulk conditions closely matches the Djinn curve-crossing result, that is 117 µA vs 123.7 µA for the B117 environment. Similarly, for the NAVAIR environment the 3D predicted corrosion current per fastener for the bulk conditions closely match the curve-crossing result, that is 0.5 µA vs 0.5 µA. However, the same is not seen when comparing the thin film predictions, due to the iR drops that curve crossing does not take into account. The curve-crossing results do, however, return a ‘conservative’ result for the corrosion rates, in that higher currents are predicted.

The fastener corrosion current in the beach exposure of course depends on the weather conditions at the time. For example, when splashed with seawater the current reaches as high as 40 µA but reduces significantly to-wards just a few µA in the case of heavy dew. When beach-exposed panels dry out, they are often left with a residue of salt, so when it rains again, NaCl corrosion is reactivated but the salt solution is gradually washed away, hence the peak of 40 µA dropping to 10 µA and below, as the chloride washes off and the rain abates. The NAVAIR thin film conditions predict corrosion current of 32 µA which is of the order of current observed in the ini-tial raining condition. The NAVAIR bulk conditions predict currents of only 0.5 µA which would equate more to-

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Condition Film (µm)

Djinn calculated Current Density (A/m2)

Djinn calculated Corrosion Current per fastener (µA)

CCM+ calculated Corrosion Current per fastener (µA)

B117, 5%NaCl, 35o C - 50µm conditions 50 1.84 319.7 149.0B117, 5%NaCl, 35o C - Bulk conditions Bulk 0.712 123.7 117.03.5%NaCl, 25o C - 50µm conditions 50 0.319 55.4 32.03.5%NaCl, 25o C - bulk conditions Bulk 0.00284 0.5 0.5

wards the lower currents found in the heavy dew regime. Indeed, Feng’s beach-exposed coupons were actually horizontal, so the dew might pool and be quite stagnant, which could justify the use of the bulk condition polariza-tion curve to model such a scenario.

Effect of electrolyte flow and accumulation

With use of the multi-physics CCM+ CFD software it is possible to build a 3D multiphase model in order to simu-late the condensation, evaporation and flow of thin electrolyte layers on quite complex surfaces. Figure 9 illus-trates how a film of electrolyte condensing on an inclined surface flows down and around protruding fasteners heads. Note how the electrolyte layer is thickest on the lower side of the fasteners, and how different fastener head shapes affect the flow. Any computation of corrosion for this type of situation must include the fluid dynamics of the electrolyte flow if it is to properly reflect the details of surface corrosion – particularly if the component is subjected to 3.5% NaCl conditions, where we have shown a much larger difference between bulk and thin elec-trolyte conditions.

CONCLUSIONS

In this paper we presented polarization data for ASTM B117 conditions of 5 wt.% NaCl, 35 oC and also polarization data for 3.5 wt.% NaCl, 25oC conditions (those used by a NAVAIR protocol as a proxy for seawater conditions). Using the Djinn software which employs a curve-crossing technique, the predicted corrosion currents between SS 316 fasteners and an Al 2024 panel were much higher when using the B117 polarization data – which is of course expected since the objective of the B117 conditions is to accelerate corrosion. However, the predicted corrosion currents in the B117 environment are relatively insensitive to electrolyte thickness, probably due to a combination of higher electrical conductivity and mobility of dissolved oxygen with a much higher concentration of chloride ions present. Predicted corrosion currents for the NAVAIR environment were quite low for bulk conditions but more than 100 times higher for a thin electrolyte film of 50 µm.

A conclusion therefore is that if we wish to simulate ASTM B117 conditions we certainly need the 5% NaCl (35oC) polarization information, but knowledge of the electrolyte thickness is not as critical. However, if we wish to simu -late beach exposure conditions, the 3.5%NaCl polarization data is required, but also knowledge of the electrolyte

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Figure 9. Flow of condensed water vapor down the surface with fasteners, showing variations in thickness.

thickness variation is critical for accurate predictions. And if the assembly contains crevices where the films are physically thin but not oxygenated, then these areas must be treated as “bulk film”.

When compared to experimental data we found that for bulk electrolyte conditions, the curve-crossing technique predicted corrosion currents very close to those measured by Feng et al. [7]. However, for thin electrolyte films the curve-crossing technique overpredicted the corrosion currents for both the B117 and the NAVAIR environments. To improve prediction accuracy, it is necessary to account for the explicit electrolyte thickness, which of course can be done with the 3D modeling approach. In this way, a proper treatment of the iR drop resulted in corrosion currents that were much more in line with those measured by Feng.

FUTURE WORK

Work is presently underway to incorporate the electrochemical modeling directly in the multi-phase-fluid thin film approach. In conjunction with this, we have deconvoluted the polarization curves, so eventually we will be able to access, on the fly, the polarization data appropriate to the locally calculated film thickness on the geometry under investigation.

ACKNOWLEDGEMENTS

This work was sponsored by ONR, William Nickerson. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements either expressed or implied, of the U.S. Government.

REFERENCES

1. ASTM B117-18. Standard Practice for Operating Salt Spray (Fog) Apparatus.

2. E.L. Montgomery, L.M. Calle, J.C. Curran, M.R. Kolody, “Timescale correlation between marine atmo-spheric exposure and accelerated corrosion testing, "Corrosion 2012 (Houston, TX: NACE International, 2011): p. 15.

3. C. A. Matt Stark, C. W. Nickerson, "Assessment of Accelerated Tests Compared to Beachfront Test and Proposed Evaluation Method", DoD Corrosion Conference (Patuxent River, MD: Naval Air Systems Com-mand (NAVAIR), 2009.

4. M.L. Tayler, M. Blanton, C. Kinecki, J. Rawlings, J.R. Scully, ”Scribe Cream and Underpaint Corrosion on Ultra-High Molecular Weight Epoxy Resin-Coated 1018. Steel Part One: Comparison of Field Exposures to Standard Laboratory Accelerated Life Tests. CORROSION-Vol. 71 (January 2015), No. 1 p.71.

5. J. Dante, “Accelerated Dynamic Corrosion Test Method Development”, Final Report to SERDP-ESTCP, available at https://www.serdp-estcp.org/content/download/46955/439875/file/WP-1673 (October 1, 2018).

6. N. Van den Steen, J. Deconinck, “Numerical Prediction of Liquid Film Formation During Accelerated Cor-rosion Tests”. 2014, Chemical Engineering Transactions, 41, 277-282 DOI: 10.3303/CET1441047

7. Z. Feng, G.A. Frankel, W.H. Abbott, C.A. Matzdorf, “Galvanic Attack of Coated Al Alloy Panels Laboratory and Field Exposure”, Corrosion 72 (2016); p. 342.

8. Best Practices for Polarization Data Acquisition: Data Collection Guide for MIL-STD-889C Technical Re-vision. Prepared by Naval Air Systems Command (NAVAIR). Version 4: Final. Point of contact Victor Ro-driguez-Santiago victor.rodriguezsant@navy.mil.

9. V.G. Levich, Physicochemical Hydrodynamics, (Englewood Cliffs, NJ, Prentice Hall 1962).10. C.R. Wilke, P. Chang, “Correlation of Diffusion Coefficients in Dilute Solutions”, A.I.Ch.E. Journal, 1,

(1955); p. 264.11. W.M. Haynes, CRC Handbook of Chemistry and Physics,” CRC Press, (2010).

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12. S. Palani, “Modeling of Galvanic Corrosion on Hybrid Structures in Aircraft – application to CFRP-AA2024 unclad material combination”. PhD Thesis, (Brussels, Belgium: Vrije Universiteit Brussel (VUB) 2013).

13. W. Nusselt, “Die Oberflächenkondesation des Wasserdampfes”, Z. Ver.Dt. Ing., 60 (1916); p. 541.

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