Hexavalent chromium is the main corrosion inhibiting pigment in primers for most Department of Defense aviation and aerospace applications over aluminum substrates. It is a known carcinogen and, as such, is targeted for removal by thorough adoption of the Defense Federal Acquisition Regulation Supplement Subpart 223.73 “Minimizing the Use of Materials Containing Hexavalent Chromium.” For military weapon systems to adopt new coating materials, there must be prototype demonstration testing in an operation environment (Technology Readiness Assessment Level 7) and then actual system testing completed and qualified through test and demonstration (Technology Readiness Assessment Level 8). These tests require 12 to 18 months of outdoor environmental exposure. Rapid development and iteration of new coating materials requires fast, yet reliable and predictive, accelerated test methods. This project focused on two large tasks: (Technology 1) the development of a novel fully chromium-free pretreatment, inhibited primer, and inhibited topcoat coating system that would meet or exceed the corrosion protection requirements of existing chromium-containing primer systems that exist on Department of the Air Force aircraft; and (Technology 2) define and develop accelerated combined-effects corrosion testing protocols that subject a test article to diurnal cycles (sunlight, heat, and wind followed by nighttime cooling and moisture condensation), dynamic loading, and environmental contaminants that correlate more accurately to real-world environments than existing laboratory accelerated methods.

For Technology 1, PPG Industries, Inc. (PPG) has developed a hexavalent chromium-free coating system where both the primer and the topcoat contain corrosion inhibitor(s) of distinctly different chemistries. The synergy between them results in corrosion protection comparable to chromium-containing coatings without known carcinogenicity concerns. For Technology 2, the Naval Research Laboratory at Key West (NRL-Key West) has developed the Center for Corrosion and Atmospheric Structural Testing (C-CoAST) facility. This facility combines dynamic mechanical actuation in operationally relevant high-ultraviolet (UV) light and high-corrosivity environments. This system can run various load spectra on large, complex mechanical specimens and provide real-world combined effects testing in the open air and approximately 200 ft from the beach to provide materials characterization under conditions that closely mirror those experienced by assets in service. Additionally, the Air Force Research Laboratory (AFRL) has developed the Accelerated Combined-Effects Simulation (ACES) chamber, which can simulate key environmental variables related to corrosion and corrosion-induced failures including electrolyte levels, high and low temperatures, background gases, UV/solar radiation, and dynamic mechanical loading in an accelerated laboratory environment.

The leading PPG prototype system of inhibited primer with inhibited topcoat provided excellent corrosion protection in both 2000-hour salt-spray cabinet testing and in 18-month Florida outdoor exposure, in fact surpassing the chromium-containing controls in the latter case. This system also passed all the key laboratory testing requirements in MIL-PRF-32239B (Performance Specification, Coating System Advanced Performance, for Aerospace Applications). However, the inhibited topcoat demonstrated excessive color change (∆E between 7 and 10 units) after 18 months of outdoor exposure at the three outdoor locations (Battelle’s Daytona, Florida Materials Research Facility; National Aeronautics and Space Administration-Kennedy Space Center Corrosion Engineering Laboratory; and NRL-Key West). Although this does not fail an explicit performance specification requirement, stakeholder review indicated that this level of color shift would be unacceptable by legacy aircraft corrosion managers. Initial evaluation by PPG indicated that resolving the color shift issue would require formulation changes which would then trigger the requirement to fully retest the system to MIL-PRF-32239B requirements. Therefore, PPG has halted further performance testing on these novel coatings under this project but continues to evaluate formulation options for this technology in other research efforts.

NRL has observed highly promising results in dynamic load testing using the C-CoAST Facility, which showed the accelerated development of coating and substrate cracks at fasteners under cyclic loading, thereby creating points for corrosion and fracture propagation. Additionally, testing of specific outdoor exposure environments was duplicated in the accelerated combined effects simulation chamber and then subsequently successfully accelerated to a much shorter test cycle.

Implementation requires authorization for use by the Cognizant Engineering Authority for a weapon system. Typically, the first requirement is to have a material qualified to specification, and this is to ensure that the material properties are stable and reproducible from batch to batch. For this program, qualification to specification MIL-PRF-32239 was selected to meet that implementation requirement. The PPG novel corrosion inhibiting primer and corrosion inhibiting topcoat system did meet the performance requirements. However, the color shift observed post-beach-exposure generated concerns with the user community. Although this is not a performance requirement, it does interfere with the adoption of this coating system by an end user. PPG will continue to evaluate formulation options to further develop this technology outside of this program.

The accelerated testing methods developed by AFRL and NRL could be incorporated into new versions of the military specifications they control if greater correlation to real-world performance is observed relative to current testing. However, the capital cost of replicating these capabilities at additional sites would be high. Instead, a better result would be to use the data from this and prior efforts to design smaller chambers with more discrete capabilities that would be necessary for specific exposure situations and incorporate those methods into an American Society for Testing and Materials.