Comparative Study of Weathering Resistance of Two USAF Coating Systems 85285 vs APC Topcoat Contract

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Comparative Study of Weathering Resistance of Two
USAF Coating Systems (85285 vs APC
Topcoat)(Contract No. F33615-02-C-5619)

• S. Koka, A. Shi and J.S. Ullett
• S K Technologies
• Dayton, OH
• May 12, 2004

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Outline of the Presentation

• Introduction of USAF Aircraft Coatings
• Coatings degradation
• Scope and Goals of SBIR project
• Corrosion prevention by coatings
• Materials and Experimental Procedures
• Coating system
• Laboratory aging (Weathering Cycles)
• Different methods used to evaluate the
  performance of coatings
• Electrochemical Impedance Spectroscopy(EIS)
• EIS instrumentation
• Equivalent circuit modeling (ECM)
• Results and Discussion
• Conclusions and Future work

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Scope and Goals of SBIR Project

• Assess the effect of UV light, moisture and
  temperature on the performance of two typical
  USAF coating systems using EIS.
• Development , tuning and validation of coating
  life predictive model.
• Development of an on-aircraft EIS methodology to
  asses the coating degradation in terms of barrier
  properties.
• Development of a web-accessible coating research
  database which houses coating analysis/degradation
  data of different organic coatings for Air Force
  applications.

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SBIR Project Team

• S K Technologies
• Dr. Jill Ullett, PI
• Dr. Alan Shi, Sr. Scientist
• Dr. Jochen Hoffmann, Sr. Materials Engineer
• Mr. Sateesh Koka, Project Engineer
• Boeing
• Dr. Joseph Osborne, PI
• UDRI / CTIO
• Mr. David Barrington, Consultant
• University of Virginia
• Dr. John Scully, Consultant

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Introduction

• Why do AF aircraft need coatings?
• Survivability
• Corrosion Protection
• Appearance

Corrosion related problems costs AF gt 1000 M
/Yr Paint related costs (stripping/repainting/disp
osal of hazardous materials) are 150M /Yr
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Introduction

• Environmental conditions encountered by the
  USAF Aircraft coatings

• UV light, Temperature, Humidity / Moisture and
  their combination

• Airborne pollutants such as ozone, chlorides,
  sulphates, and nitrates

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BACKGROUNDCOATING FAILURE MECHANISMS
low crosslinking density ? migration of corrosive
species to metal-surface
high crosslinking density ? high brittleness ?
cracking

• temperature
• additional curing at high temperatures
• ? embrittlement

• UV-radiation
• photooxidation
• polymer chain scission
• embrittlement
• additional curing
• ? embrittlement

• humidity
• penetration of water and swelling
• hydrolysis
• ? plasticization

Al-Alloy (2024-T3)
corrosion pit crack front
poor interface condition ? weak bonding

• delamination
• exposure of substrate surface
• corrosion pits on Al-surface
• initial sites for fracture

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Coatings Degradation

• UV Exposure
• Photo Oxidation occurs which involves chain
  initiation, propagation and termination.
• Moisture Exposure (0 to 100)
• Plasticization of polymer , loss of adhesion,
  formation of blisters
• Temperature Exposure (-54 to 177oC)
• Accelerate cross linking, chain scission,
  increases rate of polymeric reactions

The above chemical changes in the coating alters
mechanical properties (hardness, modulus, and
toughness) and physical properties (density and
dielectric properties) which eventually leads to
corrosion of aluminum substrate of the aircraft.
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Corrosion Prevention by Coatings

• Coatings acts as a barrier by limiting the
  passage of current necessary to connect the areas
  of anodic and cathodic activity on the substrate.
• Some coatings releases inhibitor materials that
  passivate the substrate or block corrosion
  reactions.
• The barrier properties important for coatings
  are resistance to transport of chemical species
  of the corrosion reaction like H2O, O2,
  electrolyte ions.

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Coating System
Primer Epoxy-polyamide MIL PRF 23377
(Deft, color-02Y40) Top coat Desothane HS
polyurethane topcoat MIL PRL 85285 (Deft
03GY311) Advanced Performance Coating (APC)
topcoat MIL PRL 85285 (Deft 99GY001)
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Evaluation of performance of the corrosion
protective coatings

• Outdoor exposure
• Reliable but need long testing times, and
  multiple test locations, poor reproducibility.
• Cyclic and Accelerated tests
• Expose the coating / metal system to a
  repeatable, measurable stress in excess of what
  it normally experiences (for same exposure time).
  
• Immersion in electrolyte, exposure to continuous
  salt fog at 35oC, exposure to UV light and
  moisture, and their combinations.
• 85285 MIL Spec requires qualification with salt
  spray and UV exposure.
• Cyclic testing in a lab facilitates degradation
  measurements

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UV Exposures

• UV-B contains short wavelengths not present in
  typical sun exposure.
• Typical Xenon Arc exposure about one half the
  power at 340 nm compared with UV-A bulbs.

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Laboratory Aging
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Different Techniques Used to Evaluate Coatings
Performance

• Analytical Techniques
• Color
• Gloss
• FTIR (Fourier Transform Infrared Spectroscopy)
• Electrochemical Techniques
• Electrochemical Impedance Spectroscopy (EIS)
• Electrochemical Noise Method (ENM)
• Nondestructive Techniques
• Acoustic Reflectivity Technique (SAM)

Need to develop a technique which can be used in
the field (on Aircraft)
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Electrochemical Impedance Spectroscopy (EIS)

• EIS is a rapid and convenient technique which
  can be used to evaluate the barrier properties of
  organic coated metals.
• Can evaluate coating degradation before any
  visible signs of degradation occurs and can be
  used to predict the service life of the coatings.
• Provides quantitative kinetic and mechanistic
  information which is very useful for developing
  improved new coating systems.

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Electrochemical Impedance Spectroscopy (EIS)

• In EIS the impedance of an electrochemical
  system is studied as a function of the frequency
  of an applied A.C. signal.
• An A.C. Voltage signal is applied over an
  electrochemical cell and the response of the
  current is measured using a Frequency Response
  Analyzer (FRA).

E (t) Eo (cos (w t) j sin (w t)) I (t) Io
(cos (jw t-?) j sin (w t-?) ) Z E (t) / I
(t) Zo exp (j? t)
Frequency range 100KHz-10mHz Electrolyte 0.05 M
NaCl AC amplitude 100 mV
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EIS Instrumentation
Reference Electrode - Ag / AgCl Working Electrode
- Coating Sample Counter Electrode - Platinum
Mesh
Reference Electrode
Counter Electrode
Potentiostat
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Data Interpretation
Nyquist Plot
Bode Phase Magnitude Plots
Z (W cm2)
Reference S.J.Mabbutt, J. Li, G.P. Bierwagen and
D.E. Tallamn, 2002 Tri-service corrosion
conference, TX.
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Equivalent Circuit Modeling
Equivalent Circuit for an intact coated metal
Higher Coating Resistance or lower Coating
Capacitance are desired
Reference J.M. Fildes, P. Chen, and X. Zhan,
North Western University, IL.
Path of the current from the electrolyte to Metal
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Results - outline

• Effect of UVA (no moisture) at 50oC and 70oC on
  85285 and APC topcoat systems
• ECM parameter such as Coating resistance
• Effect of UVA and Moisture at 50oC, 60oC and
  70oC on 85285 APC topcoat systems.
• Microscopic Images

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Impedance data
(b) Bode Plots of 85285 APC exposed to UVA at
50oC, 70oC
(a) Bode Plots for Pristine 85285 APC coupons
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Effect of UVA at 50oC
Coating resistance from replicate coupons of
85285 exposed to UVA at 50oC No degradation was
observed even after 4536 hours of aging
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Effect of UVA at 70oC
APC
85285
Coating resistance from replicate coupons of
85285 APC exposed to UVA at 70oC
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Summary of UV only Exposure

• No degradation was observed on either 85285 or
  APC topcoat systems
• No temperature effect was observed when moisture
  was not part of the weathering cycles.

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Effect of Xenon Arc on APC
Coating resistance from replicate coupons of APC
exposed to Xenon Arc weathering cycle
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Effect of UVA and Moisture at 50oC
APC
85285
Coating resistance from replicate coupons of
85285 and APC exposed to UVA and moisture at 50oC
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Effect of UVA and Moisture at 60oC
APC
85285
Coating resistance from replicate coupons of
85285 and APC exposed to UVA and moisture at 60oC
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Effect of UVA and Moisture at 70oC
APC
85285
Coating resistance from replicate coupons of
85285 and APC exposed to UVA and moisture at 70oC
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Effect temperature on APC Topcoat
60oC
50oC
70oC
Coating resistance of APC topcoat exposed to UVA
and moisture at 50oC, 60oC 70oC After 3024
hours APC shows degradation at 70oC
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Microscopic Images
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Microscopic Images
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Color Data
85285,UVA H2O _at_ 70oC
APC,UVA H2O _at_ 70oC
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Coating Research Database
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Coating Research Database
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Conclusions

• No indication of degradation was detected on
  either the 85285 or APC system if the coating is
  subjected to UVA aging only. The presence of
  water or moisture appears to be a necessary
  ingredient to cause detectable degradation within
  the aging time frame studied.
• The combination of UVA and moisture condensation
  induced a strong adverse effect on the barrier
  properties of the APC coating system, but to a
  much less degree on the barrier properties of the
  85285 coating system
• Under the combination of UVA and moisture
  condensation, the temperature effect appeared to
  be following a descending order of 70o C ? 60oC ?
  50oC the higher the temperature, the earlier the
  indication of degradation detected by EIS
  technique.
• Data analysis up to date seems to support that
  APC topcoat offers better weathering resistance
  than 85285 topcoat initially, however, the long
  term behavior proved to be otherwise.

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Future Work

• The aging process will continue for at least two
  more 504-hours cycles in order to generate more
  failed coupons, thus provide better life data
  for more accurate Weibull analysis.
• An on-aircraft EIS data acquisition methodology
  will be developed to assess the barrier
  properties of the coatings in real service
  environment. (This underway at Boeing)
• A coating life prediction model will be developed
  based on the accelerated laboratory weathering
  data. The data acquired on-aircraft will be
  used to validate the prediction model.
• Development of a web-accessible database
  (http//coating.sktdayton.com) is complete. The
  database will be populated with the laboratory
  data (color, gloss, EIS data) generated at SKT
  and potentially data from CTIO and other sources
  (Boeing), which can be useful for coating
  research and analysis.

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Acknowledgements

• Boeing
• Dr. Joseph Osborne, PI
• UDRI / CTIO
• Mr. David Barrington, Consultant
• University of Virginia
• Dr. John Scully, Consultant

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THANK YOU