Tri-chrome is a nickname for Chromium(III), or Chromium in it's +3 oxidation state. Chromium's other ion is hex-chrome (Chromium(VI)). The traditional method for electrodeposition of chromium involves a highly acidic, concentrated bath composed mostly of chromic acid (CrO2). The hex-chrome method of plating produces a deep bluish-white deposit over a wide current density. Tri-chrome usually produces a darker, thinner deposit, over a much smaller range of current densities.
Despite their limitations, tri-chrome baths are worth investigating because of their reduced waste disposal costs, their ongoing maintenance, and their compositional similarities with bright nickel baths. Prior tri-chrome baths would achieve 40-70% coverage on a standard hull cell test plate, compared to about 97% coverage on hex-chrome. They were either based on a chloride salt system, or a sulfate system. The chloride system produced brighter deposits, but with only 40% coverage. Sulfates produced 70% coverage with a noticeably bluer, darker deposit.
Our goal was to devise a tri-chrome system that combined the best traits of each of these conventional systems and therefore approached the utility of a hex-chrome. By experimenting with various salt combinations, wetters, brighteners and carriers, we eventually developed a tri-chrome bath that had the desired bright bluish-white chrome deposit, while achieving 90-95% coverage. It also worked with carbon anodes in place of the standard lead anode/acid bag combination. The bath was a hybrid, with a combination of chloride and sulfate salts. The downside was that the bath was highly sensitive to copper contamination. The copper would initially cause a skip in the high-current density range, followed with black streaking as the copper contamination increased. At about 40 ppm Cu, the bath became unusable.
The next step was to construct a 25 gallon pilot tank at our plant. The tank was air-agitated and used carbon anodes. Nickel-plated brass hangers were used to hold the test panel. Tests were successful, although copper contamination was a problem throughout, as copper frames were used to support both the anodes and the cathode.
At the last I heard, the process was being put into production on a trial basis at Toastmaster, and Quin-Tec was applying for a patent.
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Last updated on 11/05/96.