A novel nonelectrolytic process for chromium and nickel coating

Abstract

The use of hexavalent chromium in metal coating operations, as per electrolytic processing, is subject to increasingly restrictive regulations due to its carcinogenic and toxic properties. Yet, these coatings are critical for corrosion resistance in aircraft parts such as hydraulic systems. This has led to many efforts to find high quality non-electrolytic coating processes. The guiding hypothesis of this work was that a version of the Reduction Expansion Synthesis (RES) process, previously used to produce submicron metal particles, could be developed to create metal coating. This study involved the production of coatings by a variety of RES-like protocols, based on mixing metal nitrates, urea, and sometimes other reagents with uncoated metal wires, then heating (ca. 900 C) in an inert atmosphere. The primary tools employed to study the coatings were optical microscopy, x-ray diffraction, scanning electron microscope, and energy dispersive x-ray microprobe. The first protocol created essentially a carbon surface layer. This was mitigated in later protocols, and the coatings contained the desired metals. However, the coating morphology was imperfect and contained impurities. Clearly, metal can be deposited with RES like processes, but further development will be needed to create metal layers of acceptable quality.