Coating System Specification Approach

to Non-Chromium CoatingsD I A N E B U H R M A S T E R , S E N I O R M A T E R I A L S E N G I N E E R

M A T E R I A L S A N D M A N U F A C T U R I N G D I R E C T O R A T E

O C T O B E R 2 0 1 9

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Outline• BLUF: Hexavalent Chromium Issue• Hierarchy of Controls• Non-Cr Coatings Are…• Salt Spray Conundrum• Path Forward• Summary

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Bottom Line Up-Front: The Hexavalent Chromium IssueCoatings systems contain hexavalent chromium (Cr6+)

Cr6+ is highly carcinogenic

Standard Polyurethane TopcoatHexavalent Chromium Pigmented Epoxy Primer

Standard Cr‐Free Pretreatment

Aerospace Aluminum Substrate

Old Way of Looking At Coatings

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Most Effective

LessEffectiveStandard Method

Reduce HazMat

Improved Facilities

Shift Rotation

Technology Development

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Technology Development: Non-Chromium Coatings

Replacement

Standard Polyurethane TopcoatNon‐Chromium Pigmented Epoxy Primer

Standard or Altenrate Cr‐Free Pretreatment

Aerospace Aluminum SubstrateBaCrO4

SrCrO4

H2CrO4New Way of Looking At Coatings

Non-Cr = Elements that are not Cr BaSO4

Cr3+Al3+Li+Mg2+

Y+/‐

ZrOPr6O11

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Military Specifications - Aircraft Standard 

Polyurethane Topcoat

Class C: Hexavalent Chromium Pigmented Epoxy Primer

Class N: Engineering Approval Only, Tested over Cr Pretreatments

Standard PretreatmentUsually Cr6+, Some Cr3+,

*One Rare Earth Non‐Cr (2018)

Mix & Match

MIL‐PRF‐85285

MIL‐PRF‐23377

MIL‐DTL‐81706

H2CrO4

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Key Materials Properties – Previous DoD ApproachStandard 

Polyurethane TopcoatClass C: Hexavalent Chromium 

Pigmented Epoxy Primer

Class N: Engineering Approval Only, Tested over Cr Pretreatments

Standard PretreatmentUsually Cr6+, Some Cr3+,

*One Rare Earth Non‐Cr (2018)

Mix & Match

H2CrO4

Ultraviolet Light

Solvents

Aircraft Fluids

Humidity

Flexibility SoilingAdhesion

Aircraft FluidsAdhesion Solvents

FlexibilityCorrosion Resistance

Adhesion

Corrosion Resistance

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Key Materials Properties – AFRL ApproachStandard 

Polyurethane TopcoatHexavalent Chromium Free PrimerChromium‐Free Pretreatment

Paired

Ultraviolet Light

Solvents

Aircraft Fluids

Humidity

Flexibility SoilingAdhesion

Aircraft FluidsAdhesion Solvents

FlexibilityCorrosion Resistance

Adhesion

Corrosion Resistance

Cr3+Al3+ Li+Mg2+

Y+/‐ZrOPr6O11

BaSO4Y+/‐Pr6O11

Glycidyl 3‐(trimethoxysilyl)propyl ether

Zirconium N‐propoxide

3M AC‐131

Henkel Bonderite 5200

AkzoNobelAluminum Rich Primer

HentzenPPG (Deft)

Effective as a SystemNot Mix & Match

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

The “Salt Spray” ConundrumClass C: Hexavalent Chromium 

Pigmented Epoxy Primer

Class N: Engineering Approval Only, Tested over Cr Pretreatments

Standard PretreatmentUsually Cr6+, Some Cr3+,

*One Rare Earth Non‐Cr (2018)

H2CrO4

Corrosion ResistanceCorrosion Resistance

MIL‐PRF‐23377MIL‐DTL‐81706

336 hours 2000 hours

Pretreatment only With and Without Topcoat

Exposure Cabinet

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

The “Salt Spray” Conundrum

Zero Cr

Corrosion ResistanceCorrosion Resistance

MIL‐PRF‐23377MIL‐DTL‐81706

336 hours 2000 hours

Pretreatment only With and Without Topcoat

Chromium Free PrimerChromium‐Free Pretreatment

Exposure Cabinet“Passed” Outdoor 

Exposure Testing “Failed”

Outdoor Exposure Testing

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

The “Salt Spray” Conundrum

Corrosion Mechanism in Salt Spray Cabinet Testing Corrosion Mechanism in Outdoor Exposure Testing

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

The “Salt Spray” Conundrum

Accelerated aging should not change the target aging mechanism!

Accelerated aging @ 100°F

21 days

Long‐term aging of an egg @ 80°F

1 year

Slide courtesy Dr. Greg SchoeppnerAFLCMC/EZFS.

Accelerated aging @ 212°F

8 min

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Path Forward to Non-Chromium Coatings

Replacement

Standard Polyurethane TopcoatNon‐Chromium Pigmented Epoxy Primer

Standard or Altenrate Cr‐Free Pretreatment

Aerospace Aluminum Substrate

BaCrO4

SrCrO4

H2CrO4

New Way of Looking At Coatings

Non-Cr = Elements that are not Cr

BaSO4

Cr3+Al3+Li+Mg2+

Y+/‐

ZrOPr6O11

Military System Specification with Outdoor Exposure TestingCorrosion Community Seeking Improvements to Corrosion Resistance Performance Prediction

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Path Forward to Non-Chromium Coatings

Goal is Cr Replacement

Standard Polyurethane TopcoatNon‐Chromium Pigmented Epoxy Primer

Standard or Altenrate Cr‐Free Pretreatment

Aerospace Aluminum Substrate

Non-Cr = Elements that are not Cr

BaSO4

Cr3+Al3+Li+Mg2+

Y+/‐

ZrOPr6O11

New Technology + Qualification + Authorization = Transition

MIL‐PRF‐32239

QPL‐32239

6 Systems

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Summary• BLUF: Hexavalent Chromium Issue ---- Well Understood• Hierarchy of Controls ---- Replacement• Non-Cr Coatings Are ---- Everything that isn’t Cr• Salt Spray Conundrum ---- Acceleration does not always give the right answer• Path Forward ---- Military System Specification

Distribution A: Approved for public release: Distribution Unlimited. 88ABW‐2019‐4680, 25 Sep 2019

Questions?