The replacement of hard chrome plating in aircraft manufacturing activities and maintenance depots is a high priority for the U.S. Department of Defense (DoD). Chromium plating baths contain chromic acid, in which the chromium is present in the hexavalent state (Cr+6), a known carcinogen having a high level of toxicity. During operation, chrome plating tanks emit a Cr+6 mist into the air, which must be ducted away and be removed by scrubbers or mist eliminators. Wastes generated from plating operations must be disposed of as hazardous waste and plating operations must abide by U.S. Environmental Protection Agency (EPA) emissions standards and Occupational Safety and Health Administration (OSHA) permissible exposure limits (PEL).
Nanocrystalline cobalt-phosphorus (nCoP) is an environmentally benign alternative to engineering hard chrome (EHC) plating. The nCoP coatings are applied to both line-of-sight and non-line-of-sight surfaces using similar electroplating processes. Previous Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) projects demonstrated nCoP possessed excellent corrosion and sliding wear resistance. The process was demonstrated to readily integrate within existing electroplating infrastructure present in DoD repair and overhaul facilities. While the nCoP technology has been in use for industrial applications for several years, its use for aerospace applications necessitates further investigation prior to implementation for DoD manufacturing repair and overhaul.
This project presents the results of the demonstration and validation (Dem/Val) program to evaluate nCoP, which included extensive material characterization and testing. The primary objective of the work was to evaluate coatings through coupon testing, functional rig testing, and field testing selected components on relevant platforms. The nCoP process was operated primarily at Naval Air Systems Command (NAVAIR) Fleet Readiness Center Southeast (FRCSE) in Jacksonville, FL to prepare test coupons and for manufacturing of Dem/Val components.
A Joint Test Protocol (JTP) was devised by stakeholders to validate nCoP coatings for use in relevant applications such as landing gear, arresting gear, hydraulic cylinders, actuators, and dimensional restoration of damaged components. The stakeholders included aerospace original equipment manufacturers (OEM; Pratt & Whitney Canada, Heroux-Devtek, Messier-Bugatti-Dowty, and Boeing), the DoD maintenance depots (FRCSE, NAVAIR Fleet Readiness Center East [FRCE]), DoD engineering authority (NAVAIR Patuxent River [PAX], NAVAIR Lakehurst), and Naval Sea Systems Command (NAVSEA; through leveraged support funded by Navy’s Environmental Sustainability Development to Integration [NESDI]), as well as the Integran Technologies, Inc. and Rowan Technology Group.
Performance testing conducted per the JTP included: coating quality, adhesion, fatigue, corrosion, hydrogen embrittlement, fluid compatibility, wear, and impact testing. The test matrix aimed to provide engineers and scientists with sufficient information for evaluation of the coating as a suitable alternative for hard chrome. The results showed that nCoP exceeded EHC performance in corrosion and sliding wear tests. The abrasive wear performance of nCoP by Taber wear was less than EHC, as previously determined, while gravelometry testing showed equivalent performance relative to hard chrome. Fatigue and hydrogen embrittlement testing showed nCoP met or exceeded performance of EHC.
Naval Surface Warfare Center, Carderock Division (NSWCCD)/NAVSEA, with leveraged support from NESDI, conducted an evaluation of nCoP and other hard chrome replacement candidate coatings for use on hydraulic cylinders for U.S. Marine Corps (USMC) vehicles. The testing demonstrated nCoP performed equivalent to EHC in adhesion, impact resistance, and wear resistance testing, and significantly outperformed EHC in corrosion testing. A hydraulic cycling test resulted in nCoP samples lasting over three times longer than any other candidate coating including EHC. On the basis of these results, NAVSEA conducted a Dem/Val on a hydraulic actuator on the M9 Armored Combat Earthmover (ACE) at Panama City, Florida.
Successful field testing on demonstration components coated with nCoP were completed:
A cost benefit analysis was performed to determine the expected payback period if nCoP replaces the current FRCSE workload. A payback period of 4.7 years was determined. A substantial improvement in payback is expected by increasing workload as a result of the increased throughput obtained due to the high plating rate and lower energy consumption. Furthermore, a reduced plating shop infrastructure is possible to support an equivalent workload of EHC plating and elimination of hazardous materials (e.g., chromic acid volume reduction, lead anodes, etc.)
Further work is required to address implementation issues in order to facilitate widespread adoption of the nCoP technology, including: identification of appropriate masking materials that can survive the elevated operating temperature of the process; modification of the process to allow use of conventional direct current rectifiers; improvement of Taber wear performance; and additional characterization data on fatigue performance to study the variability present in the data set.
Overall, nCoP met the majority of acceptance criteria, as defined in the JTP, and showed excellent performance in Dem/Val field testing. While further testing may be required to support “General” authorization, it is anticipated that nCoP may be widely specified per MIL‑DTL‑32502, “Coating, Cobalt-Phosphorus Alloy, Nanocrystalline (Electrodeposited)”, as a hard chrome alternative on the basis of testing completed to date.