| Chromium plating is one of the most widely used technologies in electroplating industry. Chrome layer has some good performances, such as high hardness, good wear resistance, good corrosion resistance and good appearance capability. Chrome layer is widely used in many fields, not only in decorative plating but also in functional plating. The traditional chromium plating uses hexavalent chromium as plating solution, but the toxicity of hexavalent chrome is relatively strong; it has bad effects on environment, and is a strong carcinogen. So hexavalent chromium plating has caused widely attention. Governments have gradually constituted regulations to limit and phase out the usage of hexavalent chromium.The toxicity of the trivalent chromium is less; it is only one percent of the one of hexavalent chromium, so it has less pollution to the environmrnt. The trivalent chromium plating is the trend of the chrome plating. Recently, some governments accelerate the research on the trivalent chromium plating and get a better development, some trivalent chromium plating technologies are even used in production, but it is not perfect and need more studies.The main results of this thesis are:(1) The choice of the trivalent chromium plating system The chromium plating solutions are prepared using different complexing agent and additives. According to the results of our chromium plating, we selected the appropriate trivalent chromium plating additives, and identified the appropriate composition of the solution. At last, we determined the formate and urea as the best complexing agent and additive of our plating system.(2) Study of the technology of trivalent chromium platingWith two powers of DC and pulse current, the plating experiments were done by the orthographical test in the formate-urea system. According to the effects of various factors on the chromium plating, the optimum combination of factors was determined. Then according to the influence of the factors on the chromium plating, we made the single experiments of the main factors, and selected the optimized conditions for each factor. Ultimately, we chose the most suitable composition of the formate-urea system of the chromium electroplating as: the main salt concentration is 0.4 mol/L,the mole ratio of the complex is 6:3, namely HCOONa·2H2O: CO(NH2)2=0.6mol/L: 0.3mol/L , the conduction salt is 130g/L, H3BO3 as buffering agent is 38g/L and the wetting agent is 38 drops per litre; and the best process conditions are: the process condition of the DC plating is: the current density is 17A/dm2, the bath pH is 1.7 and the plating temperature is 25℃, we can get 68.11microns chrome coating at this condition; And the process condition of pluse plating is: the average current density is 14 A/dm2, the pulse frequency is 1000Hz and the pulse rate factor is 30%, we can get 56.78 microns chrome coating at this condition.(3) The performances testing of the plating solution and the chrome platingThe quality of plating can be proved by the performances testing of the solution and the plating layer. With some practical testing methods, the conductivity of the plating is 62.96mS/cm, the current efficiency is about 20% and the capability of deep-plating is 95%. The shine, appearance and combinability of the chrome layers attained by DC and pulse plating are satisfactory. The pore rates are 4～5 dots per square centimetres and 3～4 ones with DC and pulse plating, respectively; And the hardness are 940 HV and 1100 HV, respectively. The results indicate that the performances of both the coating and the solution are nicer as expected.(4) Investigation of the polarization curves of the plating solutionThe relationship between the polarization curve and the scanning rates, the main salt concentration, the solution acidity, the additives and the bath temperature are investigated respectively. The results show that the separation of the chromium from solution is completely irreversible. From the polarization curves, the reasonability of the technology we chosed are also explained.
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