Corrosion protection of high-strength fasteners is of utmost importance because of the critical nature of structural loads transferred through them and their potential for galvanic corrosion. Adequate corrosion protection is essential for sustaining the structural integrity of fastener connections and preventing preferential corrosion of connected members at the expense of galvanically coupled fasteners. Traditionally, electroplated cadmium layers and hexavalent chromium rinses have been employed for these applications due to their excellent corrosion performance and lubricity for threaded applications; however, cadmium and chromates are hazardous substances (known human carcinogens) and subsequently have high handling and disposal costs. These environmental, health, and safety issues have resulted in more stringent Occupational Safety and Health Administration (OSHA) and Environmental Protection Agency (EPA) regulations. Alternative deposition options for protective coatings have been considered; however, the desired combination of corrosion protection, tribological properties, and production rates has been difficult to achieve with an environmentally benign system.
The objective of this project was to demonstrate the feasibility of using a directed vapor deposition (DVD) process to deposit zinc-nickel-cobalt (Zn-Ni-Co) ternary alloy coatings onto high-strength steel substrates for cadmium plating replacement.
DVD is an advanced approach for depositing high quality coatings from vapor. It provides the technical basis for a flexible, high quality coating process capable of atomistically depositing dense, compositionally controlled coatings onto line-of-sight and non-line-of-sight regions of high strength fastener components. Unlike other physical vapor deposition approaches, DVD is specifically designed to enable the transport of vapor atoms from a source to a substrate to be highly controlled. Additionally, this novel vapor deposition solution serves to eliminate hazardous cadmium waste streams while improving fastener corrosion performance and should offer a competitive production throughput as compared to traditional fastener cadmium plating production lines.
In this project, the DVD technology was used to produce several alloy coatings with good corrosion properties. Specifically, coatings were deposited onto 4340 steel substrates and evaluated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD) techniques. Analyses revealed that both Zn-Ni and Zn-Ni-Co alloys can be deposited onto substrates with uniform composition and microstructure that provide corrosion resistance. By controlling the substrate temperature during the deposition process and other parameters, including electron beam current, scanning frequency, source material feed rate, and carrier gas flow behavior, the ability to deposit a unique set of Zn-Ni and Zn-Ni-Co alloys that appear to have good density coupled with good adhesion has been shown. These coatings deposited by the DVD process outperform traditional electroplated Zn-Ni coatings.
The cadmium coating replacement technology developed will result in reduced life-cycle costs. In particular, the DVD solution will help eliminate hazardous cadmium and chromium waste streams while improving fastener corrosion performance and extending the service life of protected components. Further work is needed to optimize these coatings for hydrogen embrittlement resistance, excellent adhesion to a variety of substrates and geometries, mechanical durability, and lubricity.