Effect Of Complexant On The Properties Of Trivalent Chromium Solution

The film of hexavalent chromium surface treatment technology is beautiful,good performance,and wide application.But highly toxic of Cr?VI?,the environment and human health have an enormous threat due to the extensive used.Trivalent chromium surface treatment technology can overcome the weakness of the hexavalent chromium,therefore,the research on trivalent chromium is more and more.Different complexant has enormous influence on the electrochemical performance and coordination properties of trivalent chromium solution,thus affecting the quality of trivalent chromium coating.Effects of complexant on trivalent chromium solution were studied by the method of chronoamperometric plot,cathodic polarization curve,UV-vis spectrum,time potential curve,AC impedance curve,tafel curve and NSS test in this paper.Research results are as follows:Five kinds of nitrogen-containing trivalent chromium complex solution were studied,the results of chronoamperometric plot,cathodic polarization curve and experimental phenomena show that three reactions occurred during the process of trivalent chromium plating,they are Hydrogen evolution,Valence change and Chromium precipitation.The chromium will be precipitated from the second inflection point;The polarizability of trivalent chromium coordination solution was reduced and the deposition potential shift positively by the addition of 0.2mol/L urea;The results of UV-vis spectrum show that,with the addition of glycine,the maximum improve of the absorbance of trivalent chromium complex solution was get,and coordination ability is better.Five kinds of carboxylic trivalent chromium complex solution were studied,the results of cathodic polarization curve and UV-vis spectrum show that coordination solutions of sodium citrate,sodium acetate and sodium tartrate have smaller polarizability,the absorbance of sodium tartrate complex solution improve maximum,oxalate are followed and the sodium citrate in the third.The sodium citrate complex solution has largest blue shift among them;The stability constant and apparentactivation energy of sodium tartrate,oxalate and sodium citrate were measured,the size of stability constant: K_tartrate> K_oxalate> K_citrate,the size of apparent activation energy:Ea_oxalate> Ea_tartrate> Ea_citrate.The results illustrate the stability constant of sodium citrate is minimum,reaction rates faster and conducive electrodeposition of metallic chromium.According to previous researches,sodium citrate as main complexant,sodium tartaric,oxalate,urea and triethanolamine as auxiliary compleant,study by time potential curve,cathodic polarization curve and AC impedance curve.Experiment show that with the addition of NaCl and other components,the curves showed a regular change.The polarizability of complex plating bath of sodium citrate and oxalate is smaller.The results of Hull cell show that coatings can be get from plating solutions and a small range of bright coating can be obtained form the combined plating bath of sodium citrate and oxalate.Trivalent chromium passivatin solution of six different complexants were studied by time potential curve,AC impedance curve,tafel curve and NSS test.It is considered that the film forming velocity of acetic acid,lactic acid and sodium gluconate is faster,the passivation film of lactic acid and citric acid thinning faster with the extension of passivation time;The corrosion resistance of urea passivation film is better,passivation film of tartaric acid and citric acid are followed and acetic acid are worst;By laser confocal microscope,the 2-dimensional,3-dimensional and the roughness of the citric acid passivation film shows: the more rough of the passivation film,the more colorful of the surface.