The objectives of this SERDP Exploratory Development (SEED) were to: (1) demonstrate environmentally-benign coating processes using conductive polymers (CP), as an alternative to hexavalent chromium (Cr(VI)) containing passivates; (2) provide passivation and acceptable shell-to-shell contact resistance that would meet the requirements detailed in MIL-DTL-38999L, Class Z for CP coatings; and (3) show compatibility with, strong adhesion to, and corrosion-inhibition of the existing Low Hydrogen Embrittlement (LHE) alkaline Zn-Ni coatings.

Technical Approach

Conductive Polymer (CPs) coatings were synthesized, characterized and coated onto Zn-Ni plated 4130 high-strength steel substrates (supplied by Wright Paterson Air Force Base (WPAFB)) by researchers at the Naval Air Warfare Center Weapons Division (NAWCWD), China Lake, California.  The evaluation of novel conductive polymer coatings developed by the Polymer Science & Engineering Branch of the NAWCWD was performed both by the NAWCWD Engineering Branch and the University of Dayton Research Institute (UDRI) Coatings Corrosion and Erosion Group. The coating samples that were supplied by the NAWCWD as part of the SEED/SERDP-supported research effort on developing environmentally-benign corrosion protective chromate-free coatings that offer low contact resistance on zinc-nickel electroplated steel substrates and can meet the requirements for use in military electrical connectors and assemblies. The project was coordinated by the NAWCWD and the Air Force Coatings, Corrosion & Erosion Laboratory (CCEL) with the goal of demonstrating the feasibility of this novel type of CP coating materials. 

Two batches of the CP coating samples were tested for adhesion (including wet/dry tape adhesion and tensile PATTI adhesion), corrosion resistance properties via electrochemical impedance spectroscopy (EIS), a neutral salt spray exposure, and hydrogen embrittlement. EIS of coated Zn-Ni coated high strength steel and contact resistance of CP coatings before and after neutral salt fog experiments were determined. In addition to corrosion and adhesion properties testing, the morphology of the coatings was examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).

Interim Results

Comparative evaluation of coating samples demonstrated reasonably good corrosion performance, and strong adhesion to zinc-nickel electroplated steel substrate were achieved with the second batch of the coating samples when the coatings were applied in a two-layer fashion for a thicker buildup on Zn-Ni plated high-strength steel substrate. These coatings showed no hydrogen embrittlement on 4340 high strength steel notched bars but did not meet the mil spec requirements for contact resistance before and after NSS. For these two-layer coating samples, the study of the film morphology revealed uniformly dense and defect-free coating. This overall improvement in performance based on the second batch of coating samples brings these coating materials to a promising performance level that would justify further and more detailed investigation of these coatings.


The benefits of this SEED will allow for the development of: a). innovative CP coatings that can be deposited via environmentally green deposition techniques; b). demonstrate that these CP coatings are compatible with and adhere to Zn-Ni coated steel substrates, and eliminate all heavy metals (Cr(VI) and Cd) from the passivating coatings; c). verify that these CPs meet contact resistance requirements spelled out in MIL-DTL-38999L, Class Z and d). provide evidence to show that these CPs offer corrosion protection. The implementation of these promising results will require additional research to meet the contact resistance requirements.  Listed below are the specific issues that must be addressed in order for this technology to meet the requirements spelled out in in MIL-DTL-38999L, Class Z:

  • The poly(diethyl ((2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl)phosphonate) was dissolved in furfuryl alcohol but the process to obtain CP films resulted in polymerization of the furfuryl alcohol. This caused the films to be insulating.
  • In order to overcome this problem further synthesis of the diethyl ((2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl)phosphonate monomer containing either longer chain n-alkyl groups or oligoether groups must be investigated.
  • Both these types of functional groups can improve the solubility of the polymer in a VOC-exempt solvent or in water-dispersible formulations.
  • The resulting polymer can therefore be highly conducting meeting the requirements of contact resistance before and after NSS and still maintain the acceptable properties previously described in the results section.
  • Coating,

  • Manufacturing,

  • Corrosion Mitigation,