Main features of chrome plating
Chrome plating from common chromic acid plating baths, compared with other single metal plating solutions, chrome plating liquid is simple in composition, but the chrome plating process is quite complicated and has the following characteristics.
1. The main component of the chrome plating solution is not the metal chromium salt, but the chromic acid, chromic acid, which is a strong acid plating solution. In the electroplating process, the cathode process is complicated, and the cathode current is mostly consumed in the two side reactions of hydrogen evolution and hexavalent chromium reduction to trivalent chromium, so the cathode current efficiency of chrome plating is very low (10%~l8%). Moreover, there are three anomalies: the current efficiency decreases with increasing chromic anhydride concentration, and decreases with increasing temperature; it increases with increasing current density.
2. In the chrome plating solution, a certain amount of anions, such as SO42-, SiF62, F, etc., must be added to achieve the normal deposition of metallic chromium.
3. The chrome plating solution has a low dispersing ability. For complex shapes, a pictographic anode or an auxiliary cathode is required to obtain a uniform chrome plating layer. The requirements for the hangers are also strict.
4. The chrome plating needs to adopt a higher cathode current density, usually above 20A/dm2, which is more than 10 times higher than the general plating. Since the cathode and the anode deposit a large amount of gas, the resistance of the plating solution is large, the cell pressure is increased, and the plating power source is required to be high, and a power source larger than l2V is required, and other plating materials can be used with a power source of 8V or less.
5. The chrome-plated anode does not use metallic chromium but uses an insoluble anode. Lead, lead-bismuth alloys and lead-tin alloys are commonly used. Chromium consumed in the bath due to deposition or other reasons is supplemented by the addition of chromic anhydride.
6. The operating temperature of chrome plating has a certain dependence on the cathode current density, and the relationship between the two can be changed to obtain chrome plating with different properties.