For steel fasteners used in the aerospace and transportation industries, cadmium (Cd) coatings are the choice of most major manufacturers due to their superior corrosion resistance, lubricity, and fatigue resistance. When electroplated onto steel fasteners and parts, Cd provides cathodic protection in the form of an impermeable sacrificial coating. High lubricity allows Cd-coated fasteners to be repetitively installed and reduces the torque required for tightening. Resistance to fatigue extends the life of the plated fastener, which, combined with the corrosion resistance and ability to reinstall, results in a long-lasting reliable product. Electrodeposition is the most common method of coating high-strength steel fasteners with Cd. Typically Cd is post-treated with chromium conversion coatings to increase the durability of the part before installation.
Cadmium, however, is a class B1 carcinogen that has been banned from most industries. Cd electroplating produces toxic fumes that affect bath operators. Cd waste disposal and emissions are heavily regulated. Risks are not limited to plating shops as Cd dissolves in rainwater, thus extending the danger of exposure throughout the entire life of the coated part, from plating to installation and usage and finally disposal.
The objective of this project was to develop and investigate the benefits of nanostructured zinc (Zn)-based alloys over current and emerging sacrificial protective coatings, focusing in particular on the relevant properties for high-strength steel fasteners.
In Phase I of this project, a number of nanostructured Zn-based alloy coatings were synthesized (Zn-Co, Zn-Fe, Zn-Ni, and Zn-Ni-Co, in particular) based on simple modifications to currently available, off-the-shelf commercial bath chemistries. The various nanostructured Zn-alloys were then subjected to comprehensive characterization and performance tests, including grain size, crystallographic texture, microhardness, ductility, torque/tension friction, salt-spray corrosion, and hydrogen embrittlement performance. Based on the Phase I results, the alkaline Zn-Ni, acid Zn-Ni, and acid Zn-Ni-Co plating systems were selected to carry forward for further development in Phase II.
Phase II involved five additional tasks: (1) optimization of a selected nanocrystalline Zn-based alloy; (2) evaluation of trivalent (Cr3+) and hexavalent chromium (Cr6+) conversion coatings to enhance corrosion resistance; (3) test sample production for the testing of hydrogen re-embrittlement (HRE); (4) further optimization of plating conditions specific for fasteners; and (5) evaluation of scaled-up production plating, focusing on bath stability and barrel plating.
The fine-grained structures produced via pulse plating from modified commercial Zn-alloy plating solutions were found to have a number of benefits over conventional direct current (DC) plating, including bright, uniform, dense microstructure; uniform, equiaxed grain size throughout the thickness of the coating; increased microhardness; single γ-phase crystallographic microstructure; increased corrosion resistance compared to other Zn-Ni alloys; and decreased friction. They also passed the ASTM F519 hydrogen embrittlement (HE) test. ZnNi coatings synthesized from alkaline bath chemistries had greater compositional uniformity (less fluctuation with variations in current density) compared to those from acid chemistries, resulting in more uniform peak to valley composition in threaded fasteners. In addition to providing dense structures, pulse plated deposits typically possessed higher nickel content than DC plated deposits from the same plating bath. Both the Cr6+ and Cr3+ conversion coating were successfully applied to the pulse plated ZnNi structures, providing similar enhancement to salt spray corrosion performance. The ability to pass the HRE test was highly dependent on the following factors: pre-plating treatment methods, porosity of coating, nickel content, and the open circuit potential. Finally, bulk processing of large numbers of fasteners was successfully accomplished using pulsed electrical parameters while barrel plating.
Dense, fine-grained Zn-alloy structures synthesized via pulse electrodeposition using a commercial Zn-Ni alkaline system can lead to the retention of numerous benefits associated with Cd-coating technology, including non-line-of-sight application, excellent corrosion resistance, low coefficient of friction, excellent coating adhesion, high dimensional consistency, and superior surface finish. The use of pulse plating was also found to have benefits over conventional ZnNi plating, including superior corrosion protection and improved HRE performance. Pulse electrodeposition can be implemented within the existing Cd-plating infrastructure in the defense sector for conventional rack plating as well as bulk plating of fasteners, e.g., barrel plating.