Aircraft Corrosion During Depaint Operations

1
Aircraft Corrosion During Depaint Operations

• Presented By
• C.S. Brossia, N. Sridhar, J.F. Dante, M. Dammann
• Southwest Research Institute
• San Antonio, TX

2
Background

• Pitting noted on wings and fuselage during
  routine depaint operations
• Root cause not obvious based on visual inspection
• no apparent order (random)
• found on clad and unlcad material

• What is the cause?

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Possible Causes of Pitting

• 1. Pits existed prior to paint stripping from
  prior exposure/damage to coating
• 2. Excessive stripper dwell time or by prior
  surface contamination by salts/corrosive agents
• 3. High chloride and/or sulfate in rinse/dilution
  waters
• 4. Galvanic interaction between wing skin and
  steel rivets

4
Approach - Panel Preparation

• AA2014-T6, Clad AA2024-T3
• Environmentally Friendly Oxidizing Acidic
  Stripper, Standard Alkaline Wash (301), Standard
  Brightener (31)
• Replicate panels to gage variability
• Painting performed at Kenedy Aircraft Inc
• Alodine
• Primer
• Polyurethane Primer
• Top Coat (White)
• Aging Air dry for 1 week, 4 day bake at 210 F

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Detectability of Pre-Existing Pits

• Determine if pits (if present) could be detected
  during pre-strip inspection
• Approach
• Clad and unclad panels
• Some with pinholes/scribes in coating (damage)
• Exposure cyclic salt spray environment

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Detectability of Pre-Existing Pits

• No significant coating delamination/creep back at
  pinhole/scribe
• Conclusion pits unlikely to have been present
  prior to stripping without detection during
  pre-depaint inspection

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Excessive Stripper Dwell Time

• Determine if long surface dwell time for acid
  stripper led to pitting
• Approach
• Subject panels to recommended 4 hr stripper time
  and for longest time experienced by aircraft (24
  hr)
• Examine panels after stripping

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Excessive Stripper Dwell Time

• Conclusions
• 4 h stripper time inadequate
• 24 h better
• No effect on corrosion/pitting noted

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Dwell Time Contamination

• Determine if long surface dwell time for acid
  stripper in combination with pre-existing surface
  contamination led to pitting

• Approach
• Prior to stripping, apply salt spray to painted
  panels
• Subject panels to 4 and 24 hr stripper time
• Examine panels after stripping

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Dwell Time Contamination

• No massive corrosion noted
• 2-4 micron 0.08 - 0.16 mil pits noted after 24
  h stripping w/ prior salt contamination

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Dwell Time Contamination

• Pitting also noted with contaminated stripper on
  unpainted panels
• Known to be a problem (NACE/SSPC committee)

• Conclusion prior surface contaminants may have
  caused pitting
• Recommendation washdown prior to stripping

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Rinse/Dilution Water Chemistry

• Determine if rinse/dilution water chemistry led
  to pitting
• Approach
• Examine two main water chemistries DI and
  simulated tap water (330 ppm chloride 180 ppm
  sulfate)
• Look for differences in surface appearance
  resulting from different water chemistries used
  for rinse and/or dilution
• Interactions between water chemistry and
  brightener and alkaline wash also examined

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Rinse/Dilution Water Chemistry

• 4 hr stripper dwell time

Alkaline Wash DI Brightener DI Rinse Water DI
Alkaline Wash ST Brightener DI Rinse Water DI
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Rinse/Dilution Water Chemistry

• 4 hr stripper dwell time

Alkaline Wash DI Brightener ST Rinse Water DI
Alkaline Wash DI Brightener DI Rinse Water ST
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Rinse/Dilution Water Chemistry
22 4 hr stripper dwell time w/ 4 day delay
prior to alkaline wash (effective stripping
operation)

• 4 hr stripper dwell time

Alkaline Wash ST Brightener ST Rinse Water ST
Alkaline Wash ST Brightener ST Rinse Water ST
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Rinse/Dilution Water Chemistry
clad
unclad

• With long stripper time and tap water, 3-7 micron
  0.12 - 0.28 mils pits noted on both clad and
  unclad
• Conclusion
• long stripper time along with high chloride and
  sulfate in tap water likely cause

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Effect of Stripper Sequence

• 22 4 hr stripper dwell time with and without
  delay

Alkaline Wash DI Brightener DI Rinse Water
DI 4-Day Delay Yes
Alkaline Wash DI Brightener DI Rinse Water
DI 4-Day Delay No

• Conclusion delay in sequence does not cause
  pitting long stripper time along with high
  chloride and sulfate in tap water most likely
  cause

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Rinse/Dilution Water Chemistry

• Conclusions
• Long stripper time along with high chloride and
  sulfate in tap water may have caused pitting
• Dwell time along not sufficient to induce pitting
• Recommendation use rinse waters low in
  dissolved salts or include additive

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Summary and Conclusions

• Based on Work Conducted
• Pits unlikely to have already existed prior to
  stripping
• Contamination prior to stripper application may
  have caused pitting
• Long stripper times in combination with delay
  between stripping and alkaline wash along with
  high chloride and sulfate in tap water most
  likely to have caused pitting

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Recommendations