The primary objective of this effort is to leverage previous work performed under ESTCP project WP-201318, to demonstrate zirconium-based pretreatment as a viable alternative to trivalent and hexavalent chromium (Cr3+ and Cr6+) pretreatments. This will result in an updated version of MIL-DTL-81706 with qualified pretreatments containing no form of chromium.
Zirconium-based pretreatments are commercially available and have been used extensively in the automobile industry as a replacement for zinc phosphate and hexavalent chromium. Zirconium is typically applied via a diluted solution of fluorozirconic acid at ambient temperature for 30 – 120 seconds. The bath has a pH of 4.5 and slightly etches the surface, which results in a pH increase at the substrate/solution interface. This increase in pH results in the precipitation and subsequent bonding of zirconium oxide, which forms the protective layer and promotes adhesion. MIL-DTL-81706 is a specification governing chromate conversion coating used for pretreatment of aluminum substrates. Currently all products qualified to MIL-DTL-81706 use Cr3+ or Cr6+ in their formulations to protect against corrosion and promote adhesion. The pretreatments qualified to MIL-DTL-81706 come in two (2) Classes: Class 1A and Class 3. Class 1A is for maximum corrosion protection, painted or unpainted. Class 3 requires low electrical resistance, such as for use in electrical grounding applications. This typically requires corrosion protection without the use of a primer or topcoat, which has historically been very difficult to meet without the use of chromates. This effort intends to demonstrate mature, commercial zirconium-based technologies on aerospace assets after performing testing in accordance with MIL-DTL-81706. Successful laboratory testing followed by an update to the specification and tri-service demonstration on aerospace assets to promote technology readiness level will constitute success of this program.
Qualifying zirconium-based pretreatments to MIL-DTL-81706 would be a major milestone toward reducing Cr3+ and Cr6+ usage throughout the Department of Defense. Exposure and disposal regulations to Cr3+ and Cr6+ by the Environmental Protection Agency, Occupational Safety and Health Administration, and American Conference of Government Industrial Hygienists have become increasingly stringent. Qualification of zirconium-based pretreatments to MIL-DTL-81706 offers a suitable substitute that would dramatically reduce exposure to Cr3+ and Cr6+ while providing confidence that weapons systems are protected against exposure to environmental degradation.