Costs associated with the prevention and correction of corrosion-generated failures are more than $1 billion per year for the military. Most of these costs are associated with compliance for environmental regulations regarding worker safety and hazardous waste disposal. Current corrosion control coating systems for alloys, especially aluminum alloys, rely on extensive use of hexavalent chromium (Cr6+) in conversion coatings and primers. Considering the toxic and carcinogenic properties of Cr6+, a new chromium-free coating system is critically needed to meet corrosion protection performance requirements in all Department of Defense (DoD) applications.
The objective of this project was to develop an environmentally benign coating system for metal alloy structural components in DoD systems.
With support from the U.S. Environmental Protection Agency, previous work demonstrated an in situ crystallization process for depositing high-zeolite coatings onto aluminum alloy AA-2024-T3 coupons. The as-synthesized coatings were corrosion-resistant in strong acid, base, and pitting aggressive environments and had good thermal stability, thermal shock stability, and adhesion under mechanical stresses.
This project extended work on the coating process to other DoD relevant alloys. A seeded growth method and a non-aluminosilicate molecular sieve coating were developed to aid in the scale-up and synthesis process. The zeolite coating system was then tested for compatibility with Army and Air Force water-dispersible polyurethane topcoats. Standard testing procedures were used to characterize the zeolite and zeolite/topcoat systems and compare them to the current military Cr6+-containing coating systems.
The high-silica zeolite coating (ZSM-5) was successfully extended from AA 2024-T3 to AA-6061-T6, AA7075-T6, and steel SS1008 using 2x3-cm coupons. A universal solution composition and a universal deposition procedure were developed that produce high quality coatings on all of the aluminum alloys tested (2000, 5000, 6000, and 7000 series). Researchers then scaled up the coating process from 2x3-cm coupons to 3x6-inch panels (ZSM-5 on AA-2024-T3). The scale up and application of Generation - 3 zeolite coatings was successfully expanded to other DoD aluminum alloys (AA-5052-H32, AA-6061-T6, and 7075-T6) and to several steels (SS304-2b, S4130, and C1008).
The environmentally compliant, novel, zeolite coating system can provide the DoD community with an attractive, cost saving alternative to the current pretreatment and primer layer used in military coating systems. The successful development of the zeolite coating system for effective corrosion control can eliminate exposure of the environment and personnel to heavy metals, while reducing hazardous waste disposal costs.