Abstract
On July 1st 2006, European Union (EU) mandated a switch to reduce the chromium production by hexavalent chromium in many assembly electronic processes because of environmental and health considerations. Accordingly, the studies of trivalent chromium process have attracted much attention recently. In chapter 3, this study is to investigate the electrodeposition process of trivalent chromium, including its deposition mechanism and the effects of boric acid. The chromium depositions are developed in the baths of different concentrations with B(OH)3 at a current density of 40 A / dm2 at 35 °C. The electroplating behavior of the deposits are investigated by the linear sweeps voltammetric (LSV) analyses. The chromium layers are characterized by scanning electron microscopic (SEM) and electron probe X-ray micro analyzer (EPMA), Vicker’s microhardness indenter and X-ray diffraction (XRD). In the experimental results indicate that cracks are significantly affected by boric acid concentration. Furthermore, the addition of boric acid can reduce the evolution of hydrogen and improve the surface morphology. And the reduction of cracks also could enhance the ability of anti-corrosion. Moreover, it could find that the addition of boric acid wouldn’t affect the hardness and composition of the deposit. Trivalent chromium electrodeposition have been studied by the applications of the experimental strategy of fractional factorial design (FFD) and path of the steepest ascent (PSA) in the chapter 4. The crack length on chromium deposits which are electroplated under a direct-current (DC) mode is quantified and could be precisely controlled and predicted. The crack length of chromium deposits is used as the response variable since the number of the crack was proportional to the anti-corrosion ability. Compared with the effects of process parameters (the concentration of H3BO3, the concentration of AlCl3 ˙6H2O, the concentration of CrCl3˙6H2O, pH and the plating temperature) on the crack length of chromium deposits, the results reveal that the crack length is determined by the plating temperature. Moreover, chromium deposits with no cracks could be prepared at 45°C. Besides, the top-view and cross-section images have the same results with the crack length. In chapter 5, the anti-corrosion abilities of chromium deposits obtained at the PSA are investigated by the electrochemical methods and EIS spectra. Furthermore, the passive film of chromium deposits is also measured by XPS, SEM and EDS analyses. According to the testing results, the chromium deposit obtained at 40°C have a better corrosion resistance. And the corrosion resistance increases with the positive shift of the polarization potential, due to the formation of chromium hydroxide. This phenomenon can be confirmed in SEM and EDS analyses.