One of the major issues that causes deterioration of aircraft structures is corrosion. Chemical conversion coatings (CCCs) are used to convert the surface of aluminum alloys to more passivated states that are substantially less susceptible to corrosion. Chromated compounds, such as trivalent chromium and hexavalent chromium, have been traditionally used to convert the surface to a less active state. The governing specifications for this process in the Department of Defense (DoD) have been MIL-DTL-81706 and MIL-DTL-5541. Alternative conversion coatings with no chrome are commercially available and widely used. However, compounds that do not contain chromium have historically not been able to pass the rigorous test requirements in these specifications. Use of chromated compounds has been targeted for phase out for several decades now. Chromated compounds, while effective at preventing corrosion, are known carcinogens. Maintainers can be exposed to chromated compounds during the application of chemicals, such as CCCs or epoxy primers. As exposure levels have become increasingly stringent, research has prioritized alternatives that provide similar corrosion protection to chromated compounds without the environmental and occupational safety and health adverse effects. To this end, ES3 is working with the DoD to qualify Zr-based pretreatments to the newly revised MIL-DTL-81706.
