The objective of this effort is to develop a chromate-free, molybdenum-based alkaline surface passivation treatment for the protection of zinc-nickel (ZnNi) surfaces. Performance goals for the surface treatment will include:
- Surface resistivity of < 5 milliohms per square inch (mΩ/in2)
- Surface resistivity of < 10 mΩ/in2 after 1000 hours of American Society for Testing and Materials (ASTM) B117 exposure testing, and
- Electrochemical, corrosion, coating adhesion, and hydrogen embrittlement (HE) properties similar to a tri-chrome pretreatment.
New environmental regulations have effectively eliminated the use of chrome-based chemistries from military use. For zinc (Zn) and ZnNi, there is currently no alternative to a tri-chrome surface seal. Molybdenum (Mo)-based systems have been shown to be a promising alternative to chromium (Cr)-based systems for numerous alloys. Recent work has demonstrated the ability to incorporate molybdenum-vanadium species into oxide films on Zn and nickel-phosphorous alloys passivated under alkaline conditions, which has shown performance improvements over chromate passivation treatment. The goal is to develop a Mo-based surface passivation for ZnNi alloys used in aerospace applications. In the first year of this effort, a Mo-based alkaline passivation chemistry will be developed to reduce the risk of HE of the ZnNi coated steel substrate. Electrochemical polarization data will first be acquired for ZnNi-plated steel. Of interest are the electrochemical oxidation potentials for Zn and Ni. Measurements will be made as a function of hydrogen-ion concentration (pH) and molybdate content. Once the critical potentials are identified, oxidizers can be added to the bath to specifically target potentials where Zn or Ni oxidation will occur thus allowing control of Zn, Ni, and Mo content in the resultant passivation film. Surface film characterization including scanning electron microscopy/energy-dispersive spectrometry, X-Ray diffraction, Raman, and X-ray photoelectron spectroscopy will be performed to identify the oxide film composition and morphology. Electrochemical screening of the resultant passivated alloys in neutral salt solutions will be used to understand the corrosion performance. Finally, surface resistance will be quantified and related to the film composition. At the end of the first year, a go/no-go decision will be made based on the relative performance of the Mo-based films compared with tri-chrome films.
If successful, the team will optimize the surface passivation performance by examining the effect of different oxidizing agents, surface modifiers, and other additives during year 2. The team will also determine the effect of the surface treatment on corrosion and coating adhesion at Air Force Research Laboratory (AFRL). In the third and final year of the effort, the team will demonstrate HE resistance of the alloy. Additionally, as part of the technology transition, a limited sustainability analysis will be performed to help determine a path forward for technology implementation. To assist with this analysis, team members from both AFRL and Naval Air Systems Command (NAVAIR) Fleet Readiness Center Southeast will be engaged.
If successful, the primary benefit of this work will be the elimination of chrome from the ZnNi processing lines thus reducing environmental burden and cost while maintaining the performance of the alloy. As a secondary benefit, an alkaline bath chemistry will reduce part processing times and improve safety and reliability by eliminating hydrogen cracking of steel resulting from improper processing. A sustainability analysis will provide guidance for rapid technology transition into original equipment manufacturer (OEM) and depot processes. Finally, the use of electrochemical tools to probe the process-structure-properties relationships during passivation treatment development provides a novel way of developing improved Cr-free systems that can be exploited for other surfaces and chemistries.