The Department of Defense (DoD) has been investigating alternatives to cadmium (Cd) plating for more than 40 years, with laboratory studies and field trials conducted on coatings such as zinc-nickel (Zn-Ni), tin-zinc (Sn-Zn), ion vapor deposited (IVD) aluminum (Al), and Sermetel. The failure of these or subsequent studies to lead to large-scale replacement of Cd plating is largely due to the inability to match cadmium’s corrosion resistance in marine environments, lubricity, solderability, good adhesion, consistent torque tension, good ductility, and uniform thickness on components with complex geometries. While some of the current processes being considered by DoD include Zn-Ni, Alumiplate, and molten salt aluminum manganese (Al-Mn) coatings, each has drawbacks or limitations related to hydrogen embrittlement (HE), potential emission of toxic vapors, or the elevated temperature of the process. For Cd plating replacement on high-strength (HS) steels, HE due to hydrogen evolution in a corrosive medium and environmentally assisted cracking during service are significant issues. In addition to the environmental and worker safety issues associated with Cd plating, waste streams generated by the extensive cleaning operations required before deposition of the coating are a major concern.
The objective of this project was to perform the research and development necessary to demonstrate the viability of electrolytic plasma processing (EPP) for replacement of both the cleaning and coating operations associated with Cd plating on HS steels. This approach takes advantage of the ability to generate plasmas in electrolytes that can remove oxides and contaminants from surfaces and deposit coatings of pure metals, alloys, or mixtures of metals and oxides.
The EPP technique is similar to conventional electrolytic cleaning or electroplating. However, the applied electrode potential in all of the EPP processes is much higher and leads to the formation of a plasma around the work piece. The plasma causes bubbles to form near the surface that rapidly collapse, producing high localized temperatures and micro-cratering of the surface. Because bulk temperatures remain near ambient, there is a rapid quench near the surface leading to the formation of a nanocrystalline grain structure. If appropriate compounds are added to the electrolyte, the rapid deposition of metal or alloy coatings is possible, with deposition rates for Zn or Zn-Al of more than 10 microns per minute having been demonstrated. In this project, coatings believed to be optimum for replacing Cd such as Al-Zn (with Zn less than 50%), pure Al, and a mixed metal oxide coating of Zn-(Al-O/OH) were investigated. Detailed characterization of the cleaned and coated surfaces on HS steel coupons was conducted together with extensive corrosion, fatigue, HE, and torque tension testing.
When used as a cleaning method, the EPP technique produced a very clean, roughened, practically amorphous surface comprising a thin heat-treated outmost layer. Nevertheless, there was no measurable fatigue debit for EPP-cleaned surfaces. Nor was there any HE, even though copious quantities of hydrogen are evolved in the process.
When used as a coating method, the EPP technique produced nodular coatings with very high porosity. Again there was no HE, but there was a fatigue debit of perhaps a factor of two, with a larger debit for ZnNi at high stress (above 130ksi). This debit is similar to that caused by standard electroplated alkaline ZnNi.
The EPP process has several disadvantages compared with standard cleaning and electroplating processes for the types of complex components that make up the typical depot workload. It does not have sufficient advantages over standard ZnNi and similar electroplates to make further development worthwhile for general DoD operations. However, it may have some niche applications in cleaning and paint adhesion. There may also be some potential uses for the technology for niche original equipment manufacturer (OEM) coatings.