High-speeHigh-speed Chromium And Thick Chromium Platings From Environmental Trivalent Chromium Sulfate Solutions

Chromium coatings have been widely used in plating industry for its excellent properties ofhardness, corrosion resistance, wear resistance and low coefficient of friction . However, thetraditional hexavalent chromium plating, which does harm to the environment and the human,is restricted more and more in a variety of industries. Consequently, it is very urgent to studyand develop the technics for replacing hexavalent chromium plating such as trivalentchromium plating process, chromium plating process of lower concentration and chromiumalternative process. Among them, trivalent chromium plating process develops quicker and isthe most likely to be the replacement for hexavalent chromium plating process.There always have been three solution systems on trivalent chromium plating, which aresulfate solution, chloride solution and sulfate-chloride solution systems. And the sulfatesolution system is the most friendly to environment. A new trivalent chromiumelectrodeposition process in sulfate system, which was used for high-speed chromium platingand thick chromium plating, had been studied in the paper. By orthogonal experiments andsingle-factor experiments, the optimal compositions of trivalent chromium electrolyte wereobtained as follows: 0.5 mol/L Cr³⁺, 0.7mol/L HCOOH, 60⁸0g/L H₃BO₃, 144g/L Na₂SO₄,50g/L K₂SO₄, 1.0ml/L wetting agent; and with the value of pH at about 2.50. The chromiumcoating which was consecutive and compact was obtained at the optimal current density of10A/dm² and bath temperature of 30⁴0℃, by means of high-speed plating in sulfateelectrolyte above, and the current efficiency of trivalent chromium plating reaches 25.0%and above. The average thickness reaches over 0.40μm on the copper at the current density of10A/dm², after plating 10s, with the average plating rate of 2.50μm/min. However, theaverage thickness on mild steel and nickel is about 0.10μm and the plating rate is only0.70μm/min on the same condition. The bath volume of trivalent chromium plating willdecrease, the pH lower after long-time electroplating.A thick chromium coating could be electroplated on substrates of different shapes intrivalent chromium electrolyte above. The chromium deposit with thickness of 30μm couldbe obtained on planar workpieces by circulation flow method with average plating rate at above 25μm/hr, and the optimal bath temperature was 30 40℃, and current density was 8 10 A/dm². For cylindrical workpiece, above 50μm white and bright chromium coating couldbe deposited by cathode revolver method, and it gained its highest plating rate when thecurrent density was 8¹1A/dm², rotary speed 20r/min and pole distance 8cm. Moreover,above 50μm white chromium could be deposited on shock absorber by cathode-protectingmethod, and the best current density and bath temperature were 15²0A/dm² and 35⁴0℃.The performances of thick chromium deposit obtained as above were tested in the paper,and the result indicated that the average friction coefficient of trivalent chromium was close tothat of hexavalent chromium, and the abrasion loss of trivalent chromium was much less thanthat of hexavalent chromium, so the abrasion performance of trivalent chromium was betterthan that of hexavalent chromium. The deposit hardness of trivalent chromium beforeheat-treatment was 730HV, and increased with increasing the heat treatment temperature andreached 1090 HV at 500℃. The micro morphology of trivalent chromium coating was nodule,and the diameter of nodule became small as the heat-treatment temperature rise. The depositchanged color when the heat-treatment temperature reached 600℃.The crystal structure of trivalent chromium deposit was characterized by XRD test andDSC test, and the result showed that the trivalent chromium coating before heat-treatment wasnon-crystalline, and transformed to crystalline after heat-treatment at 293℃. And thechromium deposit after heat-treatment at 400℃was typical Cr crystalline, with crystallinegrain of nanoscale. The burning chromium deposit at high density current density wasnon-crystalline, which was as same as chromium deposit before heat-treatment. Thedifference between them might be that the carbon content in burning chromium deposit werehigher and there were impurity elements such as H, O and so on in buring deposit, whichworsened deposit quality.