Zirconium-based pretreatments have been developed and are currently being supplied to the automotive and general industrial markets. The objective of this project was to evaluate and adapt existing zirconium oxide-based pretreatment systems for Department of Defense (DoD) use in original equipment manufacturer (OEM) production, depot application, and field application, in order to eliminate the use of regulated heavy metals, such as chrome (VI) or nickel, and replace phosphate-based pretreatments, which generate byproducts that are environmental hazards.
In this project, composition and process variations of zirconium oxide-based pretreatment systems were compared for corrosion and adhesion on aluminum and steel substrates. Additionally, the pretreatment was tested with carbon fiber epoxy panels to determine if there was any detrimental effect. Commercial immersion and spray pretreatments were evaluated on DoD substrates and coated with water-borne and solvent-borne chemical agent resistant coating (CARC) systems. Experimental immersion and spray formulas were tested by the U.S. Army Research Laboratory (ARL). These formulations consisted of the standard formulas, along with process modifications that previous testing had shown to increase the zirconium coating thickness and uniformity of the treatment film, promote the adhesion of the primer to the metal surface, or in general improve the corrosion performance in accelerated corrosion testing.
The evaluation of zirconium immersion pretreatments intended for OEM application over CRS and aluminum with water-borne and solvent-borne Mil-Spec primers gave results in corrosion performance similar to the chrome (VI) and zinc phosphate pretreatment controls. In conjunction with ARL, modified versions of the commercial zirconium pretreatment were developed that showed corrosion resistance equal to or better than zinc phosphate in both spray-applied and immersion-applied processes. The work showed that the current commercial zirconium pretreatment technology with some modifications is suited for use in DoD OEM processes.
Versions of zirconium pretreatments based on the current commercial systems were tested as a Spray-On/Rinse-Off treatment or as a Dried-In-Place (DIP) treatment. By adding a rheology modifier to a spray version of the OEM product to hold the pretreatment on the intended part during reaction, a Spray-On/Rinse-Off product was developed that demonstrated good corrosion and adhesion performance under CARC primers. In addition, several versions of a DIP zirconium treatment were developed that gave improved corrosion performance by the addition of either an organic-modified inorganic metal salt compound or by using an alternative source of zirconium ions.
The results suggest that zirconium oxide-based pretreatments can be successfully used as a replacement for zinc phosphate and chrome conversion coating pretreatments and chrome (VI)-based wash primer in either an OEM application or in field settings, enabling DoD to comply with current and future environmental regulations, enhancing worker safety, reducing waste generation, and potentially reducing overall cost.