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Hard Chromium Plating Alternatives 


Current Usage

Hard chrome plating is used by OEMs to provide wear protection to a wide variety of components for aircraft, vehicles, vessels, and gun barrels.  It is used by OEMs and depots to rebuild worn components back to print.  Almost all hydraulic actuators on vehicles, vessels, and aircraft are chrome plated, as are almost all aircraft landing gear and most components that are subject to wear.

Typical Applications

Typical Chrome Plates

Specifications
  • Aircraft landing gear
  • Hydraulic actuators, aircraft
  • Hydraulic actuators, vehicles
  • Steering rams, ships
  • Wear components
  • Rebuild of worn surfaces
  • Internals of gun tubes
  • Hard chrome (OEM wear coating, rebuild coating)
  • Thin dense chrome (OEM only)
  • Flash chrome (thin chrome plate sometimes used in place of thin dense chrome)
  • MIL-STD-1501
  • QQ-C-320
  • MIL-C-20218

ESOH Issues
Cr6+ (CrVI, hexavalent chromium) is a known carcinogen that is strongly regulated.  Chromium electroplating liberates chromic acid (Cr6+) mist that must be ducted away to meet the workplace OSHA Cr6+ PEL and scrubbed to meet the EPA Clean Air Act emission limits.

  • EPA Clean Air Act rules


  • OSHA Occupational Exposure to Hexavalent Chromium (Cr6+ PEL is currently 5μgm-3)

Exposure
Personnel may be exposed at the OEM level and during depot overhaul.

  • OEM and depot personnel may be exposed to Cr6+ mist during chrome plating operations, and to chromic acid solution during masking, demasking.  Emission controls required.

Alternatives, Applications, Status
A number of hard coatings are used in place of hard chrome, both for OEM use and for overhaul. The primary alternative in use today for aircraft is HVOF (High Velocity Oxy-Fuel) thermal spray of tungsten carbide coatings (WC-Co and WC-CoCr). Off-road mining vehicle hydraulics and other wear components use HVOF Cr3C2-NiCr. HVOF is a dry coating technology that can be used both for initial production and for rebuilding worn components. All new-design aircraft landing gear and many hydraulic actuators use HVOF in place of hard chrome, and the same coatings are frequently used by DoD depots and commercial shops for aircraft component MRO. This coating is also used for many other wear-resistant and rebuild applications, such as repair of flap and slat tracks.

For internal diameters and complex parts electroless nickel is often used. Other electroplates are being developed or evaluated, particularly for internal diameters, such as pulse-plated nanophase Co-P (nCo-P) and Ni-W, and Co composite with chrome carbide or SiC.

Examples of Alternatives

Examples of Authorizations and Implementations

Specifications

HVOF WC-Co, WC-CoCr,  Cr3C2-NiCr implementations
  • All new-design military and civilian aircraft landing gear, various actuator components – piston rods, pins
  •  F-35 landing gear, various actuator rods
  • C-17 nose landing gear post shelf
  • Numerous landing gear components repairs at OO-ALC
  • Airline and aircraft MRO – landing gear, slat/flap tracks
  • Various turbine engine components – FRC-E, FRC-SE, OC-ALC
  • Ships, submarines: large hydraulic rams,  shafts for pumps, generators, turbines
  • OEM and MRO of large vehicle hydraulic rods
  • Numerous industrial valves, rolls, etc
  • AMS 2447
  • AMS 2448
  • AMS 7881
  • AMS 7882
  • BAC 5851 / BMS 10-67

HVOF WC-Co, WC-CoCr approvals
  • Approved by NAVAIR for P-3 landing gear
  • Approved by Boeing for commercial landing gear MRO
  • AMS 2447
  • AMS 2448
  • AMS 7881
  • AMS 7882

Electroless Ni-P implementations
  • Various hydraulic cylinder internal diameters (aircraft landing gear)
  • ASTM B733
  • ASTM B656
  • AMS 2404
  • AMS 2405

Ion nitride (plasma nitride)
  • Corrosion service on some commercial actuators

 

Ferritic nitrocarburizing
  • Commercial hydraulics, including corrosion service

 

PVD CrN
  • Diesel engine piston rings (Japan)

 

PVD WC-C (diamond-like carbon with tungsten)
  • Diesel engine fuel injector rods
  • Molds, dies