Trivalent chromium is one of the main compositions of colloidal membrane which generates on the surface of cathode in chromium plating process. The certain amount of trivalent chromium in the chromium plating solution can improve the throwing power of the solution. Only when moderate amount of trivalent chromium exist in the plating solution can possibly get the bright and perfect chromium layer, or chromium plating will not achieve.
When the content of trivalent chromium in the plating solution is too high (the range is between7g/L and l0g/L), the surface of cathode will be covered by a alkaline Dichromiumtris(chromate) mucosa layer, which is composed by trivalent chromium and hexavalent chromium. With the sulfate radical dissolve mucosa layer slowing down, which will control the reduction reaction of hexavalent chromium on cathode. If the mucosa layer is dissolved off and the matrix exposes, the actual current density of metal reduction will higher than that the best region needs, which will cause hard spots, turn gray or white, or even rough layer on the surface.
When the trivalent chromium reaches a value higher than l0g/L, the conductivity of solution will decline, the resistance will increase, the solution will turn black, and the throwing power and covering power will get bad obviously.
Trivalent chromium mainly increases for the following reasons:
1. When the cathodal area is larger than anodal area, trivalent chromium will increase gradually. This is because the oxidation reaction is less than the reduction reaction.
2. As the chromium trioxide is strong oxidant, all polishing factice and other organic impurities in the plating tank can be oxidized. The hexavalent chromium will be deacidized to trivalent chromium soon.
3. When the content of sulfuric acid is too high in the solution, the trivalent chromium will combine with sulfuric acid into Cr2(S04) 3, making the trivalent chromium difficult to be oxidized in anode, and the trivalent chromium will gather and increase.
4. When the temperature of solution is too high, the generating speed of colloidal mucosa layer will decrease, and the trend that hexavalent chromium deacidizes to trivalent chromium in cathodal area will increase.
5. When the content of iron impurity in the solution exceeds4g/L, the trivalent chromium will be very hard to be oxidized to hexavalent chromium.
To regulate the content of trivalent chromium in solution, the following methods can be adopted:
1. Energizing, when the iron impurity content in solution is not high, energizing can be adopted to make superabundant trivalent chromium be oxidized to hexavalent chromium. During the energizing process, it need to adopt rustless iron plate in cathode, making the anodal area be 5 times larger than the cathodal area, keep the anodic current density to be 1～2A/d㎡ and keep the temperature to be 50～60℃. It need about4 A·h when the trivalent chromium declines1g, and it can be regulated to a normative level according to the content of trivalent chromium.
2. Oxidizing with hydrogen peroxide. When the content of iron impurity is relatively high and the trivalent chromium is not easy to be oxidized in anode, hydrogen peroxide can be used to oxidize it first, and then use the electrolytic treatment together. Before the above processes, the temperature of solution can not higher than45℃and the sulfuric acid need to be subsided in advance. During the process, hydrogen peroxide should be added slowly, in case of that the solution spill out because of adding too fast.
3. Dilution, dilute and regulate it according to the calculated amount. To prevent the increases of the trivalent chromium, eliminate the greasy dirt and organics polluted solution, the key is that we should always keep an eye on regulating the current density of cathode and anode, and keep the area of cathode and anode maintain the proportion on 2:3.
1. HN-82 High speed decorative chromium plating process:
Adopting the easy-controlled soluble and mixed catalyst, it has high tolerance against impurities and easy to operate; has higher current efficiency of cathode and deposition velocity, can operate under high current density; components can be activated well and has wide brightening range; with excellent covering power and dispersive ability.
2. HN-25 Hard chromium plating process:
Adopting the unique formula, no fluoride, it won’t corrode the low current area of the component; it won’t corrode tin-lead anode and is not require special anode materials; the current efficiency of cathode is high, can reach 22~26%; has even, fine and bright deposit; has high micro hardness of deposit and good corrosion resistance.
3. HN-09 Hexavalent black chromium plating process:
Adopting the easy-controlled soluble and mixed catalyst, it is easy to operate; use pb-sn-sb anode to improve the quality of deposit and the stability of plating solution; the coating color is uniform and bright with good covering power and distribution ability; has good corrosion resistance and high-temperature resistance, the black chromium layer won’t change color and get crisp even when the temperature rises to about500℃, has good thermostability; be widely used in protective decorative chromium layer which work for deceasing reflectance or functional layer which has special demand, such as the applications in the area of light industry, instrument, aerospace.
4. HN-701 Chromium fog inhibitor:
It is a chromium fog inhibitor specially designed for chromium plating. It not only can eliminate the environmental pollution coming from chromium fog, reduce the maintenance costs of exhaust equipment and the dosage of chromic acid, but also can decrease the microcrack degree, improve the chromium layer hardness and promote the adhesion.
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