Technology Of Effective Trivalent Chromium Plating

With high abrasion resistance, good corrosion resistance, high hardness and decorative appearance, chromium coating has been one of the most extensively used as coatings. For resolve the serious environmental pollution and healthy hazard of hexavalent chromium plating process, electrodeposition chromium from trivalent chromium electrolyte now has been extensive investigated. The plating technique from trivalent chromium has many disadvantages, such as low stability of solution, high cost of raw materials and difficulty to obtain hard thick deposits. While high quality deposition from trivalent chromium electrolyte could only be sustained for short periods and thin deposits obtained of decorative value only, it is difficult to substitute hexavalent chromium plating in thick deposits by trivalent chromium plating. In order to resolve the problems, a high deposition-speed process from chromium (Ⅲ) sulfate electrolytes and a thick chromium plating process from chromium (Ⅲ) chloride electrolytes were invented after a large number of experiments on the base of our primary work.The optimum compositions and operating conditions for DC electrodepositing chromium from sulfate system and chloride system were obtained through orthogonal experiments and single-factor tests. The throwing power of baths is over 95%, and the cover power both reaches 100%. Both baths worked at normal temperature and have good stability, 41 and 64 Ah/L. The coatings achieved from both systems had good adherence and good corrosion resistance. Under the optimum conditions, covering powers are 1²5 A/dm² and 1.25²5 A/dm², plating rates 0.25μm/min and 0.33μm/min respectively. The thickest both qualified coatings are more than 10μm. The influences of the compositions and the operating conditions on the covering power of Hull test and the electrodeposition rate were investigated systemically. NH4COOH content is a primary factor both in the sulfate system and in the chloride system. The effects of temperature, pH value, current density are also great on the performance of baths and coatings.Pulse plating was researched on the base of chloride system. The optimum condition: frequency 2000 Hz, duty cycle 0.3, and average current density 50 A/dm². Under the optimum condition, the mean deposition rate is 0.53μm/min. It is found that high plating rate and thicker coating can be gotten by PC plating under lower average current density than DC plating. The structures of the Cr by DC plating or PC plating are confirmed amorphous by XRD. SEM reveals that the deposits by PC have uniform and delieate grain, and the metallic appearances are smooth, which exhibit free-pitting and cracking, while the deposits by DC exhibit microcrack structure.The effects of compositions and operating conditions on the cathodic and anodic progresses in chloride system trivalent chromium electroplating were studied with polarization curves and EIS curves. Cathodic progresses were simulated by equivalent circuits. They proved that trivalent chromium reduced through two steps. The first step which chromium (Ⅲ) receives one electron is irreversible; while the second step receives two electrons. Only the first step can be finished without NH4COOH in bath. So Cr coating can not be gotten. Different components of plating solution system show different electrical inductance. Interface capacitive is weak because it related with that Cr³⁺ reduction process of the middle of the state on the adsorption membrane.