The objective of this Statement of Need (SON) was to develop a model that accurately characterized and could potentially predict the cumulative damage state of a military platform’s coating system. Such information allowed for better informed decisions regarding maintenance intervals, reducing worker exposure to hazardous materials such as methylene chloride in paint strippers as well as hexavalent chromium found in most primers. In order to be considered useful and effective, the model should have accounted for the impact that both static and dynamic influences have on the integrity of the topcoat and the system as a whole entity or stack-up. This included, but was not necessarily limited to, UV degradation, vibrational fatigue experienced during operation, wind and rain erosion, temperature, relative humidity, and age of the coating stack-up. The most prevalent coating stack-ups of interest for this research were as follows:
- PreKote adhesion promoter on 2024-T3 aluminum, primed with MIL-PRF-23377, Type I, Class C2, and top coated with MIL-PRF-85285, Type IV, or MIL DTL 64159/53039.
- 2024-T3 aluminum, pretreated per MIL-DTL-81706/5541 – primed with MIL-PRF- 23377 or MIL-PRF-85582 – and top coated with MIL-PRF-85285. Aluminum for aviation is MIL-DTL-5541, type I or II pretreatment, MIL-PRF-23377 or MIL-PRF- 85582 primer and CARC topcoat, MIL-DTL-64159/55039. The new non-chrome system will be zirconium oxide pretreatment and the same primers and topcoats. The zirconium pretreatment will be qualified for a new type under the 5541 specification.
- For steel associated with ground vehicles the standard pretreatment is either TT-C-490, Types I or III, with 53022 primer and CARC topcoat MIL-DTL-64159/55039. The new system will be TT-C-490, type IV with the same primer and topcoats.
- For aluminum, the pretreatment for ground support equipment is either 490, type III or 5541 types I or II. Same primers and topcoats. The new systems will include zirconium pretreatment complying with 490, type IV or the new type under 5541.
For detailed guidance, reference to MIL-DTL-53072 F; 31 May 2017, Chemical Agent Resistant Coating (CARC) System Application Procedures and Quality Control Inspection is recommended.
Inputs to the model should have been readily available environmental data and flight vibrational patterns of common aircraft within the Department of Defense (DoD) and/or relevant commercial aerospace data. The output of the model would have provided an assessment of the condition of the coatings at locations on the platform that were vulnerable to degradation from sunlight and atmospheric compounds. Development and validation of the model should have included a laboratory-based approach where environmental conditions and operational conditions were simulated and compared with a laboratory study with actual coating integrity on selected platforms within the DoD. This would provide relevant data on the coating system’s response to multiple combinations of environmental exposures in static as well as dynamic states.
Any effort in response to this SON was required to leverage previous work in the research and study of corrosion and environmental degradation of coatings accomplished by SERDP and/or the DoD services.
The ideal model should have predicted with reasonable accuracy the cumulative degradation effect of static and dynamic exposures on polyurethane and epoxy coating systems stack ups.
Funded projects will appear below as project overviews are posted to the website.
An understanding of the integrity of the coatings provides Condition Based Maintenance Plus (CBM+) information that can be used to make intelligent decisions on the corrosion and coating inspection processes. Current corrosion inspection processes take place on time-based intervals, irrespective of whether sufficient environmental exposure or flight time warrants the inspection action. Knowledge of the integrity of the topcoat lends itself to understanding when chromates in the primer will begin to perform the function of inhibiting corrosion of the aluminum substrate. Furthermore, knowledge of the condition of the topcoat and primer assists in scheduling for programmed depot maintenance (PDM). For example, knowing that a topcoat is still intact on a C-130 can facilitate prioritization of C-130 aircraft for scheduled PDM. This would prevent over 1,600 man hours of de-paint work from being accomplished unnecessarily, as well as reduce exposure to chromate primers by reducing the hazardous material waste stream. Furthermore, successful development of this model will likely reduce the frequency of repainting, thereby reducing HAPs and VOCs from the painting process as well as the man-hours for repainting and downtime of the weapon or platform.
Corrosion is the natural process that takes place when elemental materials degrade to their lowest energy equilibrium states. Raw materials, such as bauxite or iron, are refined into aluminums and steels to improve their structural properties. If not adequately protected through coatings, anodization, plating, or other measures, these refined materials will begin to degrade from their higher energy refined states back to their naturally occurring oxidized states.
Corrosion adds a multibillion cost to the DoD each year, comprising approximately 25% of all maintenance costs and totaling nearly $20 billion annually. Corrosion initiates when a coating system fails, leaving a surface exposed to the environment. For the example of an aircraft, corrosion most commonly occurs at locations on the exterior surface where non-aluminum structural fasteners (rivets) are used to attach aluminum skins (typically 2024-T3) to the aircraft structural frame. After many operational hours, fatigue cycles can lead to cracking of the UV- embrittled coating, enabling water and atmospheric contaminants to permeate the damaged topcoat and porous primer, making the ingredients for corrosion available between the two dissimilar metals.
Once the inhibitors in the primer are fully expensed and the pretreatments are no longer providing further protection, the substrates are at the mercy of the environment and galvanic corrosion of the active substrate rapidly commences. This and other forms of corrosion result in costly repairs that must take place in order to maintain military readiness and ensure safety of crew members during operations.
Because of corrosion’s cost, prevention of corrosion results in millions of man-hours of labor each year, resulting in decreased military readiness due to platform downtime and extensive taxpayer expenditures. The use of a CBM+ approach to predicting corrosion based on extended levels of environmental exposure could eliminate many of the extraneous preventative maintenance actions performed by military maintainers, reducing environmental footprint and helping military leadership to better allocate resources to assets that are more likely to corrode than others.
Degradation of coatings is not necessarily just a function of time, but rather the sum total of all exposures that adversely impact the coating. The use of models can help quantify the degradation of a coating’s integrity as a function of the cumulative environmental and dynamic exposure that a coating has observed. This enables corrosion maintenance and inspection intervals to exit an interval-based approach that is a function of operational hours or elapsed time and enter a condition-based approach where maintenance actions are performed on the basis of need.
SERDP and ESTCP have a portfolio of projects that focus on environmental aspects of corrosion mitigation for weapons and platforms. For example, work undertaken in ESTCP project WP-201710 seeks to identify the point at which chromates in MIL-PRF-23377 are fully exhausted. Additionally, many DoD corrosion activities, service-specific activities, commercial, and academic activities focus on related technical aspects of this problem and should be utilized as much as possible to establish a DoD-relevant solution. This SON will aid in identifying the starting point at which chromate leaching begins during initial topcoat failure, and thus enables a data-driven approach to making smarter corrosion inspection actions by leveraging work in WP-201710.
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 $200,000 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.