Cadmium (Cd) plating is used as a coating on High Strength Steel (HSS) landing gear alloys to prevent corrosion. The cadmium plating process uses three chemicals currently listed as hazardous materials by the Environmental Protection Agency, the Occupational Safety and Health Administration, and the Agency for Toxic Substances and Disease Registry: cadmium, sodium cyanide, and hexavalent chrome. Cd is a known human carcinogen and a toxic heavy metal that can easily be leached, therefore it poses both environmental and health concerns. Due to these concerns and other limitations of Cd plating, Low Hydrogen Embrittlement Zinc-Nickel (LHE Zn-Ni) has been tested and qualified for use as a sacrificial coating to prevent corrosion to HSS alloys.
LHE Zn-Ni is applied through an electroplating process similar to that of Cd plating, except LHE Zn-Ni does not use sodium cyanide as a chelator in the process, or hexavalent chrome conversion coat, reducing the use of hazardous chemicals and waste. The main objective of this project was to implement a full scale prototype LHE Zn-Ni plating line and demonstrate and validate that LHE Zn-Ni plating performs as good or better than Cd plating in corrosion prevention, resulting from environmental conditions and operational use.
The objectives of the project were met by installation of the full scale plating line at Hill Air Force Base (AFB) and validation of the performance of LHE Zn-Ni. After qualifying the plating tanks, landing gear were plated to perform a Performance Tracking Program (PTP) where LHE Zn-Ni plated components were installed and tested on aircrafts at select bases around the country. Bases were selected with corrosive environmental conditions, to simulate the most aggressive natural environment that would be experienced at any location worldwide. The selected bases include: McEntire Joint National Guard Base, Barbers Point US Coast Guard, New Orleans Naval Air Station Louisiana, and Hickam AFB.
During the installation of the prototype full-scale production line, existing equipment located at Hill AFB building 505, was demolished, removed, relocated and rebuilt to accommodate the prototype 3200 gallon LHE alkaline LHE Zn-Ni production plating line. Upon final installation, the LHE Zn-Ni plating line chemical plating baths and processes were qualified by the following tests in accordance with United States Air Force (USAF) drawing 201027456: corrosion (ASTM B117), hydrogen embrittlement (ASTM F519) and adhesion (ASTM B571). Additional studies involving de-zincification, hydrogen reembrittlement, and LHE Zn-Ni stripping were also conducted. The installation of the 3200-gallon LHE zinc-nickel plating line in bldg. 505 at Hill AFB was successful and the 309 CMXG Production Group is currently LHE Zn-Ni plating landing gear components.
De-zincification was studied with Boeing Research & Technology. It was found that accelerated or forced corrosion may start to cause de-zincification, however corrosion rates closer to natural corrosion does not appear to have any impact on de-zincification. Additional studies are suggested to verify these results and investigate true corrosion scenarios in the field.
Hydrogen re-embrittlement was performed using anodes for plating in both the LHE ZnNi solution and Cd solution. LHE Zn-Ni passed the salt water immersion test while Cd failed in less than one hour. Inspection of the Cd plated coupon under a microscope suggests that the notch may have not plated thick enough to protect the base material. Additional testing where the thickness of the notch plating is verified before testing is suggested.
Stripping chemicals and procedures were experimented with for removing LHE Zn-Ni plating. Increasing the concentration of ammonium nitrate to 32 oz/gallon ammonium nitrate solution, which is currently used for stripping Cd, proved to be an acceptable method to remove the LHE Zn-Ni.
Landing gear components were plated with LHE Zn-Ni and sent to the select bases mentioned above where PTP was performed over a 24-month period. Two LHE Zn-Ni plated components were installed, and two Cd plated components were installed opposite (LH/RH) to be used as a control during the PTP, on select aircraft. During the PTP three inspections occur, first at time of installation, and two more during the following 24 months. All components selected for the PTP passed visual inspections.
Installation of the LHE Zn-Ni plating line at Hill AFB was successful and is now fully operational. The LHE Zn-Ni process has corrosion resistance that meets or exceeds that of Cd plating, and it can be used on all components that Cd plating is currently applied to. Test results throughout the project show that LHE Zn-Ni plated landing gear comply with USAF performance objectives and meet current environmental objectives to reduce the purchase and use of hazardous chemicals, thereby reducing hazardous waste and pollution generation.