After decades of investments by the Department of Defense (DoD) and aerospace companies, environmentally friendly alternatives to cadmium (Cd) plating used for corrosion protection of weapons systems components are now moving toward production. In August 2011, SERDP and ESTCP sponsored an ASETSDefense workshop to apprise attendees of the emerging Cd plating alternatives, arrive at a collective understanding of the related issues, define requirements for various applications, and help coordinate future research, development, test and evaluation (RDT&E) efforts to bring the best options to production. Based on the results of this workshop, ESTCP is now soliciting proposals for the demonstration and validation of alternatives to Cd plating in manufacturing and maintenance of weapons systems.

The workshop focused on the state of the science and related issues for high strength steel components such as wheels and landing gear, for fasteners, and for electrical connectors. These topics were tackled by engineers from the Naval Air Systems Command (NAVAIR), Naval Sea Systems Command (NAVSEA), U.S. Army and U.S. Marine Corps Vehicles, U.S. Air Force, Defense Logistics Agency (DLA), and prime contractors such as Boeing, Lockheed, Raytheon, and Honeywell who participated in the workshop. As with many substitutions, a single treatment, Cd with chromate conversion, must be replaced with at least three or four alternatives—many more if different sealer, topcoat, or dry film lubes are used. In many ways, the logistics issues are likely to be at least as difficult as the technical ones. For the workshop briefings and summary report, visit www.asetsdefense.org/PastWorkshops.aspx.

High Strength Steel Components

By far the largest use of Cd in military depots is for corrosion protection of high strength steel components. One of the earliest Cd alternatives for this application was vacuum-deposited ion vapor deposited (IVD) aluminum (Al). IVD has been complemented in recent years by electroplated Al (AlumiPlate), which must be deposited in an airtight plating line. Both of these processes are commercial and fully qualified, but they are only used in a limited way because of their relative cost and complexity.

Recently, a new electroplated alloy coating has emerged based on a zinc-nickel (ZnNi) chemistry developed for the automotive industry. Modified for aerospace use, a low hydrogen embrittlement (LHE) version with trivalent sealer appears to meet, and in some cases exceed, all of the requirements for Cd on landing gear and other high-strength aircraft alloys. This plating chemistry is available from several vendors, although only two have been qualified to date. Boeing Defense and Hill Air Force Base (AFB) have qualified the coating for landing gear, and Hill AFB is moving forward with scale-up and implementation for across-the-board Cd replacement for U.S. Air Force landing gear sustainment.

• Between LHE ZnNi and AlumiPlate, there are now solutions for almost all high strength steel components. Electroplated Al generally performs better than Cd, while LHE ZnNi performs about the same as Cd. But neither is quite as simple as Cd. Electroplated Al requires an organic bath, while LHE ZnNi requires care to keep the Zn/Ni ratio in the correct range.

Fasteners

Almost every weapon system is held together by Cd-plated fasteners—nuts, bolts, and rivets. There is a clear difference between fastener requirements in vehicles and aircraft, however, because of their different construction materials and alloy strengths.

Some vehicles are moving to Zn-plated fasteners, which are typically inferior to Cd, while others are adopting the approach now found in many modern cars—dip spin paints that embed Zn and/or Al flake into a polymer (or sometimes a ceramic) matrix, with a low-friction polymer topcoat to adjust the lubricity. This is important since all manuals and Tech Orders specify a particular torque to apply the correct clamping load (torque-tension), and changing it would be cost prohibitive. However, there are many production chemistries for these types of coatings, and  the logistical issues will become overwhelming unless users agree on a basic set of requirements that can be met by a number of different interchangeable products on the market.

Another alternative for vehicle fasteners is electroplated Al, which is favored by the U.S. Army Tank Automotive Research, Development and Engineering Center as a universal drop-in alternative, particularly for Al-hulled vehicles or Al wheels. These coatings require a dry film lube or polymer coating to meet torque-tension requirements.

In Europe and the United States, the use of ZnNi coated fasteners is growing for commercial vehicles. Legacy aircraft, with their Al skins, are also natural users of Al and ZnNi-coated fasteners to prevent galvanic corrosion, although they now often use Al alloy fasteners. Boeing Commercial is nearing qualification of LHE ZnNi for aerospace threaded fasteners. There is no alternative so far for rivets, except to make them out of Al alloy.

New aircraft, however, are increasingly employing composite skins and titanium structural members, which are more compatible with stainless steel and titanium (Ti) fasteners, while their engines, with all their Ni alloys, now use stainless fasteners almost exclusively.

• For fasteners, the Cd alternative problem appears largely solved, although with too many options. Al, ZnNi, and dip spin coatings are all viable options for vehicles, while LHE ZnNi provides a good option for legacy aircraft. New aircraft have already largely adopted other fastener alloys.

Electrical Connectors

The most difficult Cd application to replace is on electrical connectors, which are found in every weapon system. Besides meeting the corrosion requirements of components and the lubricity requirements of fasteners, connectors must also meet strict electrical conductivity requirements, even after corrosive attack. Several specifications, such as MIL-DTL-38999L, now permit pure Al, ZnNi, or nickel-fluorocarbon in place of Cd on steel and Al alloy connector shells.

Cr³⁺-sealed AlumiPlate and Cr⁶⁺-sealed ZnNi have been qualified for connectors, and one manufacturer offers chromated ZnNi (ZnNi meets resistance requirements only if Cr⁶⁺-sealed). Most connector manufacturers, however, are coalescing around electroless nickel PTFE (EN-PTFE), which is a Ni coating containing PTFE (Teflon) particles. Since Al and Cd are at one end of the galvanic table and Ni is at the other, one would expect such a coating to create a galvanic problem with Al airframes and legacy Cd-plated mating connectors (essentially forming a Ni-Cd battery), especially if the coating is damaged in service. Although it is not clear why, the expected galvanic interactions have not been seen in corrosion tests, perhaps because the PTFE largely covers and "inerts" the surface. However, it is not clear how these systems will behave when damaged. More testing and evaluation is required before users can be certain that any of the options provided in the specifications will meet all of their needs in all of their applications and complex assemblies. NAVAIR and others are now beginning a thorough evaluation.

• Cadmium-free electrical connectors remain the major outstanding challenge. ZnNi may be an option if a way can be found to meet resistance requirements reliably. Alternatively, EN-PTFE may be the overall solution, or it may be limited to certain use environments. Only laboratory and service testing will tell.

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ASETSDefense—Advanced Surface Engineering Technologies for a Sustainable Defense—is a SERDP and ESTCP initiative that aims to facilitate the implementation of new, environmentally friendly technologies for surface engineering by providing ready access to information and data from RDT&E efforts supported by DoD organizations and the private sector.