The objective of this Statement of Need was to identify alternative formulations for surface preparation and protection that did not contain chromium. Proposals should have addressed one or more of the topics below. Of particular interest were solutions that were broadly applicable to a number of materials and applications.
- Passivates for aluminum, magnesium and widely-used corrosion control coatings such as cadmium, zinc, zinc-nickel, phosphate, black oxide, etc.
- Corrosion inhibitors in paint primers
- Sealers for anodize coatings
- Adhesives, sealants and gap fillers
- Corrosion inhibitors in paint strippers
Proposals should have included a task to conduct a Sustainability Analysis of appropriate proportion for the proposed research and development. Proposals should have established a lifecycle framework that could mature as the technology or process advanced through the acquisition process. This tiered approach aimed to develop and document a minimum data set at each stage of research and development that could be used to make informed decisions and streamline transition to an acquisition program. The Sustainability Analysis could have included varying depths of data and information that could inform the goal and scope of an analysis; the identity and quantity of relevant inputs and outputs to the system; and the estimation of life cycle impacts and costs.
Funded projects will appear below as project overviews are posted to the website.
The development of non-Cr formulations with equivalent performance to chrome-containing materials for defense applications will make it possible to manufacture and maintain defense assets to protect them from corrosion while also protecting workers and the environment from exposure to toxic materials.
For many years defense and commercial organizations have used hexavalent chromium (Cr6+) formulations for corrosion inhibition in passivates, sealants, wash primers, paint systems and paint strippers. In order to eliminate hexavalent chromium, but still provide the same high level of corrosion performance, many alternative formulations are based on trivalent chromium (Cr3+).
In 2018, based on an analysis of data , the American Conference of Governmental Industrial Hygienists (ACGIH) proposed new threshold limit values (TLVs) for hexavalent and trivalent chromium. These guidelines set the level for inhalable particulates to 0.00002 mg/m3 for Cr6+ and to 0.003 mg/m3 for Cr3+. The ACGIH time weighted average (TWA) for Cr6+ is nearly two orders of magnitude lower than the current Cr6+ OSHA permissible exposure limit (PEL) of 0.005 mg/m3. The Department of the Army Pamphlet 40-503 and Air Force Manual 48-146 both require their respective services to use ACGIH TLVs when there is no PEL, and the other services are considering applying the TLV in select applications.
Below are some examples of applications where non-Cr formulations are required by the Department of Defense (DoD):
- Non-Cr paint systems for aircraft exteriors (outer mold line, landing gear, etc.) have been developed and put into production on both defense and passenger aircraft, because exterior coatings are easy to inspect and replace. Aircraft weapon system interior surfaces are also subject to severe corrosion during the normal operational life of the asset. However, non-Cr coatings have not been put into production on these surfaces because of the difficulty of inspecting them. Inner surfaces are typically coated with a chromated (Cr6+) corrosion-inhibiting primer during manufacture and then the weapon system is assembled with various types of corrosion-inhibiting sealants. The coatings system applied to the inner surfaces is normally expected to last the lifetime of the aircraft, typically 20 to 30 years or even longer, without periodic removal and replacement.
While not directly exposed to the environment, these interior surfaces are exposed to multiple corrosion-inducing factors such as moisture from condensation, trapped fluids and oils from system leaks and repairs, spillage of fluids from cargo and leaking toilets, and normal dirt and atmospheric dust typical of aircraft operating environments. In many cases, these fluids and soils collect in areas not normally accessed for inspection on a routine basis, so that in these areas, severe corrosion can develop and progress for several years before being discovered, sometimes causing severe structural damage requiring extensive aircraft down time for repairs. The DoD therefore needs a non-Cr coating system including surface pre-treatments, primers and sealants that can protect the interior surfaces for periods of 30 years or longer.
On structural components such as aircraft landing gear, chromate-converted Cd is now being replaced by low hydrogen embrittlement ZnNi plating passivated with Cr3+ formulations, usually with a Co inhibitor (e.g. ESTCP Project Nos. WP-201107 and WP-201412). Although this changeover is still in progress, the lower ACGIH limits are now driving a need to develop a completely non-Cr passivate.
- A much more difficult problem is replacing Cd plating that is widely used on electrical connectors for aerospace and defense. The ZnNi approach used for structures cannot be directly used to replace cadmium plating on connectors, because the passivate does not meet requirements for low long-term electrical contact resistance contained in specifications such as MIL-DTL-38999. We must now develop a passivate that is non-Cr and still provides acceptable long-term contact resistance. A similar contact resistance requirement must be met for non-Cr passivation of aluminum electrical boxes and other electrical and electronic equipment.
- Chemical paint strippers are used throughout maintenance and sustainment operations at DoD depots. These materials are used in spray-on, brush-on, and tank immersion applications for aircraft exteriors and off-aircraft parts. More than 3,000 pounds of Cr6+ are used per year by the Air Force in chemical paint strippers.
While chromate-free and methylene chloride-free paint strippers are commercially available and in use at DoD depots, chromated materials continue to be used to strip “difficult to remove” coatings and aged or weathered coatings. Non-chromate inhibited methylene chloride paint strippers are commercially available and qualified to MIL-R-81294D Class 1A, 2A, and 3A. However, Class 1 and 2, chromated products continue to be used by the Air Force. Non-chromated products do not typically offer the same level of corrosion protection as chromated inhibited strippers and methylene chloride-free materials have not efficiently and effectively removed the necessary coating, thus increasing the risk to readiness of the system. New non-Cr inhibited, methylene chloride-free paint strippers need to be formulated that meet all Air Force and DoD requirements.
- Elastomeric adhesives and sealants are used throughout maintenance and sustainment operations at DoD depots. These materials include sealants for integral fuel tanks, pressurized cabins, faying surfaces, and to protect from corrosion on aluminum, magnesium, titanium, steel, and between dissimilar metals.
SERDP and Environmental Security Technology Certification Program (ESTCP) have published a Cadmium and Hexavalent Chromium Alternatives 5-Year Strategy and Roadmap.
The cost and time to meet the requirements of this SON are at the discretion of the proposer. Two options are available:
Standard Proposals: These proposals describe a complete research effort. The proposer should incorporate the appropriate time, schedule, and cost requirements to accomplish the scope of work proposed. SERDP projects normally run from two to five years in length and vary considerably in cost consistent with the scope of the effort. It is expected that most proposals will fall into this category.
Limited Scope Proposals: Proposers with innovative approaches to the SON that entail high technical risk or have minimal supporting data may submit a Limited Scope Proposal for funding up to $250,000 and approximately one year in duration. Such proposals may be eligible for follow-on funding if they result in a successful initial project. The objective of these proposals should be to acquire the data necessary to demonstrate proof-of-concept or reduction of risk that will lead to development of a future Standard Proposal. Proposers should submit Limited Scope Proposals in accordance with the SERDP Core Solicitation instructions and deadlines.