Chromate Conversion Alternatives

 

Current Usage

Chromate conversion coatings and chromated sealers are used to create a self-healing conversion coating on Al and Mg alloys that is resistant to corrosion. They are also used for sealing electroplated and anodized coatings. These treatments are typically used prior to painting and finishing, since they generally improve adhesion of paints and sealants.
F_18_TCP_primer72

 

 

Typical Applications Typical Chromate Conversion Coatings Specifications
  • Aircraft skins
  • Al frames for aircraft and vehicles
  • Mg gearboxes
  • Corrosion-resistant coatings (Cd, Al, ZnNi, etc.)
  • Anodize sealing
  • Fasteners and electrical connectors (Zn or Cd plated)
  • Wash primer for steels, armor
  • Conversion and sealing coatings for Al (e.g., Alodine, Iridite, etc.)
  • Conversion and sealing coatings for Mg (e.g., Dow 7, 17, 19, HAE anodize)
  • MIL-DTL-81706
  • MIL-C-5541
  • MIL-M-45202
  • AMS 3171
  • TO 1-1-8
  • MIL-A-8625
  • MIL-C-3171
  • MIL-C-17711
  • MIL-M-45202
  • DOD-P-15328
  • QQ-P-416

ESOH Issues

Cr6+ (CrVI, hexavalent chromium) is a known carcinogen that is strongly regulated under

  • EPA Clean Air Act rules
  • OSHA Occupational Exposure to Hexavalent Chromium (Cr6+ PEL is currently 5μgm-3)
  • European rules (RoHS, WEEE, ELV, REACH)

Exposure 
Personnel may be exposed during manufacture, depot overhaul, repaint, and operational level touch-up and repair.

  • OEM and depot personnel may be exposed in dip and spray processes for components and for repaint operations.  Emission controls, PPE required.
  • Maintainers are exposed in scuff sand and paint operations.  PPE required.

Alternatives, Applications, Status

Primary alternatives are Cr3+ (CrIII, trivalent chromium) and non-chrome products (Mo, Mn, rare earth, and other inhibitors).  Alternatives are commercially available and some have been authorized and implemented.

Common Cr6+Alternatives Examples of Authorizations and Implementations Specifications
Adhesion promoters, sol gels, silanes
  • Boeing commercial
  • USAF aircraft skins
T.O. 1-1-8
Cr3+ sealers
  • USN, Army helicopter skins
  • Vehicle touch-up
MIL-DTL-81706
Non-chrome sealers
  • NASA Solid rocket boosters
  • Roadwheels and tracks for vehicles
MIL-DTL-81706