Replacement for Hexavalent Chromium in Surface Finishing Processes
NCMS Project #: 140377
Problem: Tightening workplace regulations and mounting liability concerns will make the hexavalent chrome plating process increasingly costly and risky over the next few years. Because of an increasing level of concern with hexavalent chromium processes, several of the DoD Maintenance and Logistics facilities were keenly interested in potential replacement technologies. Prudence dictates that we begin without delay to work toward qualifying alternative processes such as the trivalent process for commercial and military applications.
Pulse plating technology has been extensively developed by Faraday Technologies of Dayton, OH for a number of electroplating applications, including trivalent chromium. Two DoD facilities, Cherry Point Naval Air Depot and the Oklahoma City Air Logistics Center joined with Faraday to launch a project to develop and evaluate the Faraday trivalent chromium process for DoD applications. The Faraday trivalent chromium coating was, in the experience of all participants, the closest available candidate to an acceptable replacement for hexavalent hard chrome.
Benefit: Potential readiness benefits include:
- Availability: If confirmed by further testing, better wear resistance will mean longer time in the field between required maintenance for parts currently plated with hexavalent hard chromium.
- Cost Savings: Compliance costs associated with the new federal worker exposure rule for hexavalent chromium for one DoD facility alone was estimated to exceed $14M over a two year period. Several other DoD facilities that also carry out extensive hexavalent chromium electroplating operations may face comparable costs.
- Applications: Could potentially apply to every application with a rotating shaft or mating surface parts needing improved wear resistance properties.
Solution/Approach: NCMS with CTMA support designed a relatively low budget study of the Faraday trivalent chromium coating process to optimize this new method for electroplating hard chromium coatings using a pulse plating process (which is safer to work with than the traditional hexavalent chromium process) and evaluate the wear resistance characteristics of a hard chrome coating. However, at the start of this project there were still some properties that needed improvement; specifically targeted in this project were:
Cracks – the ability to plate a crystalline deposit from a trivalent bath was a significant breakthrough, but crystalline materials, in addition to being hard, tend also to be brittle allowing propagation of cracks along well-defined directions in the crystal lattice. Under a microscope, cross sections of the Faraday trivalent coating exhibited these lattice-type cracks, many of which extended from the surface of the coating through to the underlying substrate. Cracks in the coating that expose the base material are unacceptable. Hexavalent coatings have relatively few through-cracks owing to its crystalline grain structure being such that propagation of cracks is blocked.
Hardness – the hardness of Faraday trivalent coating at the outset of the project was substantially better than that of decorative trivalent coatings, but fell short of the values achieved by standard hexavalent coatings. Hardness itself is not a crucial property for a wear resistant surface. Its relevance lies in its ability to predict wear resistance. (Hardness, or resistance to plastic deformation, was measured in this project according to the Vickers method, which gauges the resistance of the coating to penetration by the point of a pyramid-shaped diamond indenter).
Impact on Warfighter: If this trivalent chromium process were to produce better wear resistant parts translating into increased durability and availability of Warfighter/Weapons Systems, plus the safety and cost savings associated with process conversion from hexavalent to trivalent chromium, the trivalent chromium process could yield enormous savings for DoD.
DOD Participation:
- U.S. Navy – NADEP, Cherry Point
- U.S. Air Force – Oklahoma City Air Logistics Center
Industry Participation:
- Faraday Technologies
- NCMS