Chromium-Free Conversion Coating for Electroplated Zinc-Nickel

Ogden Air Logistics Complex (OO-ALC) is transitioning zinc-nickel (Zn-Ni) electroplating for high strength steel as a replacement for cadmium coatings.  Currently, a trivalent chromium- [Cr(III)-] based conversion coating (CC) is used with its electroplated Zn-Ni.  The Cr(III) compounds are regulated as toxic/hazardous chromium compounds under the Clean Air Act, included under the Department of Transportation’s Hazardous Materials Regulations, and require monitoring as hazardous substances under both the Comprehensive Environmental Response, Compensation and Liability Act and the Resource Conservation and Recovery Act. The regulation of Cr and Cr compounds as a group is one of the factors driving the USAF preference to avoid the use of any forms of Cr.  There is also concern that Cr(III) compounds could be converted to Cr(VI) during processing, particularly during coating removal operations.  The objective of conducted program is to identify Cr-free conversion coatings applicable to Zn-Ni alloys, and evaluate the Cr-free coating systems (electroplated Zn-Ni + Cr-free CC) that exhibit the most potential for implementation at OO-ALC.

Three conversion coating alternatives (Cr-free CC for electroplated Zn-Ni) were selected for Phase I testing and were subjected to adhesion and corrosion resistance testing in comparison to a Cr(III) baseline.     A fluorozirconate-based conversion coating with post-treatment (i.e., a silicate sealer), performed best of all alternatives during Phase I, while meeting or exceeding the performance of the Cr(III) baseline in all critical requirements.  The successful alternative was selected for Phase II process optimization and performance testing, which is currently in progress.  A summary of the Phase I testing will be presented, along with preliminary results from the Phase II evaluation.