The objective of this project is to demonstrate environmentally friendly chrome-free thermal spray coatings on the exterior weather/flight decks of U.S. Navy ships, and validate their performance against thermal spray coatings that are currently qualified to military specification. Aluminum based coatings are a popular choice for protecting steel substrates in severe corrosive environments because of the sacrificial corrosion protection they provide; they are commonly alloyed with other elements, such as chromium, during the manufacturing process to improve material properties, such as toughness. Prior Naval Research Laboratory (NRL) application (which pioneered the use of wire-arc deck coatings aboard Navy ships), has shown the benefit of chromium inclusion translates into greater longevity and in-service performance than chrome-free aluminum wire coatings. Chromium is nontoxic when bound within the grain structure of the bulk material; however, when a wire is drawn and sprayed using a thermal (hot) process, the chromium can be converted to hexavalent chromium, creating toxic gases and fumes from the spray process. For this reason, the California Air Resource Board created the Airborne Toxic Control Measure to Reduce Emissions of Hexavalent Chromium and Nickel from Thermal Spraying, which restricts application of thermal spray coatings that contain chromium. Therefore, to mitigate exposure concerns, commercially available chrome-free thermal spray coatings will be evaluated here against test requirements established in MILPRF-32577 to assess performance. Key tests will include spark/atomic emission spectrometry to determine chemical composition, as well as adhesion, porosity, and corrosion resistance in order to predict in-service performance. Additionally, industrial demonstrations on a land based mock-up will be performed to determine spray-ability and dust collector configuration to support continuous spraying operations (required for coating large surface areas), followed by a shipboard demonstration to complete qualification of the new coating(s).
Thermal spray coatings are utilized in military and commercial applications because of their ability to protect substrates in extreme service conditions. More specifically, the Naval Sea Systems Command, driven by NRL innovation, transitioned the use of thermal spray coatings on exterior decks because they are able to withstand aircraft engine exhaust and abrasion/wear from deck equipment to provide a long life coating solution (10+ years). The thermal spray coatings are qualified to MIL-PRF32577 and have many benefits over other protective coatings, including zero volatile organic compounds, adhesion, durability, and temperature resistance. The coatings are formed by using compressed air to spray molten particles of metal onto a cleaned surface to create splats that build up until the required coating thickness is achieved. The coatings are engineered by fine tuning application parameters (i.e., particle velocity, temperature, etc.) until the desired coating properties are obtained, such as roughness or flexibility. Chrome-free aluminum alloys that contain other elements like silicon or magnesium, as property improvement agents, are commercially available and the variation in processing parameters will enable the demonstration and transition of environmentally friendly thermal spray coating that meets Department of Defense (DoD) needs.
Implementation of chrome-free thermal spray coatings will enable the fleet and DoD to use coatings that are compliant with federal and state regulations related to toxic air emissions from thermal spraying. Implementing coatings that meet these regulations help avoid added requirements and costs associated with permitting, which include air pollution control system design validation, testing to demonstrate compliance, monitoring, inspection, and maintenance of any control system used during operations.
James Tagert, Cameron Miller, and Gregory Smith Center for Corrosion Science and Engineering, Naval Research Laboratory, Washington, DC 20375