Fuel Tank Topcoat Peeling

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Fuel Tank Topcoat Peeling

• Rex Cash
• OC-ALC/LHRH
• DSN 336-5401
• rex.cash_at_tinker.af.mil

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FTTPOverview

• Plan
• Status
• Topcoat
• Sealant
• Corrosion
• Potential solutions
• Strip and re-paint
• Summary

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FTTPPlan

• Determine scope of issues in order to determine
  most advantageous solution
• Topcoat
• Sealant
• Corrosion
• Identify potential solutions
• Brief results to OC-ALC/LH and ACC in May 04.
  Decide most advantageous solution.

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FTTPPlan

• Proceed with development of procedures to remove
  topcoat and provide corrosion control through
  concept demonstration phase.
• Once program decision is made, complete solution
  development.
• Seek to identify and involve potential production
  source(s) at earliest date
• Begin production in FY08.

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FTTPStatus

• Investigations of cause and concept
  demonstrations of potential solutions near
  completion.
• Completing documentation.
• Obtaining engineering coordination.
• Preparing to brief OC-ALC/LH and ACC

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FTTPTopcoat

• Cause of FTTP
• Two independent investigations identified the
  Fuel System Icing Inhibitor (FSII) additive
  DIEGME as the cause.
• PPG (formerly Pittsburg Paint and Glass)
• Wichita State University
• Concentrated DIEGME softens and swells BMS10-39
  more than typical paint strippers such as
  chloroform (23.7 vs 9.5).
• No affect at concentrations specified by fuel
  specifications.

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FTTPTopcoat

• Cause of FTTP (continued)
• DIEGME concentrates in water up to 50.
• Settles on lower skins
• Lower volatility of JP-8 allows higher
  concentration of vaporous DIEGME in vapor areas
  than was possible with JP-4.
• Preliminary conclusion(s)
• Concentrated DIEGME is attacking the paint.
• Conversion to JP-8 triggered FTTP.
• Affects of DIEGME should be minimal in areas
  usual immersed in fuel where water cannot settle.

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FTTPTopcoat

• DIEGME (FSII)
• FSII introduced into military jet fuels in 1963
  as a result of a B-52 crash in 1957 due to fuel
  starvation due to ice blockage.
• EGME (Ethylene Glycol Monomethyl Ether) was
  initial FSSI additive.
• Replaced with DIEGME (Di Ethylene Monomethyl
  Ether) in 1993 due to toxicity of EGME.
• DIEGME was introduced into military jet fuels
  (1993) without proper material compatibility
  testing with fielded systems.

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FTTPTopcoat

• DIEGME (Continued)
• DIEGME is a strong solvent and corrosive.
• DIEGME conclusively linked to material
  compatibility issues on various weapon systems.
• B-52 FTTP.
• ALCM fuel tank corrosion.
• Fuel probe protective coating.
• Fuel bladders.
• DIEGME issues have potential to affect all
  DoD/NATO AC.

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FTTPTopcoat

• Scope of FTTP
• 20 AC inspected in PDM from Jun01 to Jun03
• Affected areas range from 0 to 5.79
• Average is 1.7
• 16 AC (80) less than 3
• Worst AC had 50 more FTTP than 2nd worst AC
  (5.79 vs 3.84)
• Preliminary conclusion(s)
• FTTP is limited in scope
• Primarily confined to vapor areas and lower skin

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FTTPTopcoat
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FTTPTopcoat
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FTTPTopcoat

• Condition of remaining paint
• Adhesion test on 3 PDM AC in Mar 04
• Measured adhesion at incremental spanwise
  locations of front spar, rear spar, forward and
  aft locations of upper and lower skins
• Adhesion at one third of test points exceeded
  limits of tester!
• Majority of main tank paint above 1000psi
• Majority of outboard tank paint below 1000psi

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FTTPTopcoat
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FTTPTopcoat

• Condition of remaining paint (continued)
• 1500 hour salt fog test conducted on original
  BMS10-39 from G and H model AC
• Passed No corrosion
• Original requirement was 500 hours
• Preliminary conclusion(s)
• Majority of remaining paint is intact with strong
  adhesion and still providing adequate corrosion
  protection.
• Outboard tank adhesion questionable

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FTTPSealant

• Condition of Sealant
• AFRL compared chemical and mechanical tests
  results of sealant removed from five G and H
  model aircraft with results from new sealant to
  determine condition and remaining service-life.
• Minor surface imperfections (cracks, blisters,
  etc)
• No significant chemical differences
• Some degradation of mechanical properties but
  well within acceptable limits.

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FTTPSealant

• Condition of Sealant (continued)
• 95 reduction in lap joint shear strength after
  30 day exposure to 50 water-50 DIEGME.
• Loss of adhesion of fillet sealant.
• AFRL planning additional DIEGME compatibility
  testing.
• Preliminary conclusion(s)
• AFRL expected to conclude existing sealant will
  remain serviceable for remainder of B-52
  service-life.

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FTTPCorrosion

• Requirement for corrosion protection
• Coating fuel tanks is standard military /
  commercial practice
• Various TOs require coating
• AFRL and AFCPCO recommend coating
• Inspection of B-52 tanks with FTTP as far back as
  1995 found no systemic corrosion
• Three independent investigations conclude DIEGME
  is very corrosive

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FTTPCorrosion

• Requirement for corrosion protection (continued)
• Water from fuel and condensation routinely found
  in tanks.
• Cad plated fasteners reacting with sulphur in
  fuel. Similar condition on other airframes has
  resulted in corroded fasteners.
• Preliminary conclusion(s)
• Coating of integral fuel tanks is standard
  policy.
• Corrosion potential exist in B-52 integral tanks.
• Corrosion prevention system required for B-52.

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FTTPPotential Solutions

• Remove / replace DIEGME
• Will stop FTTP.
• Need to protect bare areas from corrosion.
• Strip and re-paint
• Corrosion preventing fuel additives.
• Fuel already contains additive to prevent
  corrosion of steel.
• Boeing concept test demonstrated possibility with
  aluminum.

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FTTPPotential Solutions

• Sacrificial materials
• Place materials (zinc) that will corrode
  preferentially to aluminum in strategic
  locations.
• Same principle as chromated primers/paints.
• Used on some hot waters heaters.
• Under development to preserve historic shipwreaks
  (USS Arizona).
• Water scavenge system
• Removes settled water from tanks.
• In use on other airframes

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FTTPPotential Solutions

• New wing
• Potential to solve multiple issues
• FTTP
• Reduce un-usable fuel
• Safer and more efficient fuel system
• Re-engine
• More efficient airfoil
• Extend B-52 service-life
• Preliminary conclusion(s)
• Strip and re-paint most conventional and readily
  available option

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FTTPStrip and Re-paint

• Issues
• Long term paint adhesion
• Dependent on quality of clean-up, surface
  preparation and application
• Potential for fuel /propulsion contamination.
• All stripping technologies have potential to
  damage sealant by exceeding dwell time.
• Labor intensive

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FTTPStrip and Re-paint

• Issues (continued)
• Human factors
• Confined, limited access work area
• Protective clothing
• Respirators
• Visibility
• Bulky equipment
• No suitable coatings qualified for DIEGME

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FTTPStrip and Re-paint

• Concept Demonstrations
• Demonstrated top level requirements of various
  stripping technologies in Aug 03
• Most promising technologies
• Laser
• Pro No clean up
• Con High equipment cost

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FTTPStrip and Re-paint

• Concept Demonstrations (continued)
• SpongeJet with glass beads
• Pro More maneuverable in fuel tank
• Con Residual contaminates
• Chemical
• Pro Fast
• Con Potential for chemical residue

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FTTPStrip and Re-paint

• Concept Demonstrations
• Bulkheads at BBL55 and WS492 strip with Sponge
  Jet and re-painted with BMS10-39 on AC 60-057 in
  PDM in 1999. Re-examined in Mar 04.
• Paint failing due to DIEGME
• Adhesion excellent in unaffected areas
• Fastener heads corroded
• Residual Sponge Jet material found

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FTTPStrip and Re-paint
60-057Corrosion colored fasteners 4 years after
re-paint
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FTTPStrip and Re-paint
60-057 Paint failing 4 years after application
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FTTPStrip and Re-paint
60-057 Residual SpongeJet materials 4 years
after application
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FTTPStrip and Re-paint

• Concept Demonstrations
• Over painted original BMS10-39 coupons soaked in
  JP-8 for 40 days. Over-painted coupons soaked in
  JP-8 or 50DIEGME-50water for 40 days.
  Conducted adhesion, tape and pencil hardness
  test.
• BMS10-39 passed
• Sprayable sealant passed
• Soft easily damaged
• Dark color reduces visibility
• MIL-C-27725 failed
• Coating currently TO approved for tank use.

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FTTPStrip and Re-paint

• Strip and re-paint affected areas only
• Strip with Laser or SpongeJet paint with
  BMS10-39
• Eliminate/replace DIEGME
• Advantages
• Least manpower intensive
• Least risk of residual contamination
• In case of adhesion failure, no worse than
  current issue.
• Dis-advantages
• May miss some damaged areas
• Remaining paint may age-out at later date

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FTTPStrip and Re-paint

• Strip affected areas and complete re-paint
• Strip with Laser or SpongeJet paint with DIEGME
  compatible coating
• Advantages
• DIEGME compatible
• Coat fasteners
• Disadvantages
• In case of adhesion failure, peeling could be
  massive.
• No DIEGME coating currently available.
• Remaining paint may age-out at later date

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FTTPStrip and Re-paint

• Complete strip and repaint
• Strip with Laser, SpongeJet or chemical paint
  with DIEGME compatible paint
• Advantages
• No adhesion issues with old coating
• DIEGME compatible
• Coat fasteners
• Disadvantages
• In case of adhesion failure, peeling could be
  massive.
• No DIEGME coating currently available.
• Most risk of residual contamination
• Most labor intensive.

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FTTPSummary

• Investigation of cause and concept demonstration
  of potential solutions near completion.
• Corrosion protection in integral tanks required.
• Majority of paint in tanks still has excellent
  adhesion and providing adequate corrosion
  protection.
• Strip and re-paint affected areas only is an
  attractive option if DIEGME can be eliminated or
  replaced.
• Many reasons besides FTTP to replace DIEGME.
• Other options available but require greater
  developmental effort.
• Brief OC-ALC/LH and HQ ACC in May 04 for decision
  on most advantageous solution.

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