The advantage of Galvanizing
The main features
Chromium from the commonly used chrome plating solution, compared with other single metal plating solution, although the composition of chrome plating solution is simple, but the chrome plating process is quite complex, and has the following characteristics.
① The main component of chromium plating solution is not metal chromium salt, but chromium oxygen acid - chromic acid, is a strong acid bath. During the electroplating process, the cathodic process is complicated, and most of the cathode current is consumed in the two side reactions of hydrogen evolution and hexavalent chromium reduction to trivalent chromium. Therefore, the cathodic current efficiency of chrome plating is very low (10% ~ 18%). And there are three anomalies: current efficiency decreases with increasing concentration of chromic anhydride l decreases with increasing temperature; increases with the current density.
② in the chrome plating solution, you must add a certain amount of anions, such as SO42-, SiF62-, F-, in order to achieve the normal deposition of chromium metal.
③ chrome plating liquid dispersion is very low, the shape of complex parts, the need to use a pictographic anode or auxiliary cathode, in order to get a uniform chrome layer. The requirements of the hanger is also more stringent.
④ chrome plating need to use a higher cathode current density, usually more than 20A / dm2, plating than the average more than 10 times. Due to the large amount of gas precipitated from the cathode and the anode, the resistance of the plating solution is large, the cell pressure is increased, and the requirement for plating power is high, and the power larger than 12 V is required.
⑤ chrome-plated anode without metal chromium, and the use of insoluble anode. Commonly used lead, lead-antimony alloy and lead-tin alloy. Chromium in the bath due to deposition or other reasons to be consumed by adding chromium anhydride to supplement.
⑥ chrome-plated operating temperature and cathode current density have a certain dependence, changing the relationship between the two can get different performance chrome plating.
Type and Usage
Many types of chrome plating process, according to their use can be classified as follows.
① a protective decorative chrome protective chromium decorative chrome, commonly known as decorative chrome, thin coating, bright and beautiful, usually as the outermost layer of multi-layer plating, in order to achieve the purpose of protection, zinc-based or steel substrate must be plated thick enough The middle layer is then plated with a thin layer of chromium of 0.25-0.5 μm on the bright interlayer. Commonly used processes are Cu / Ni / Cr, Ni / Cu / Ni / Cr, Cu-Sn / Cr and so on. After polishing the surface of the decorative chrome plated products, you can get a silver-blue mirror gloss. Long-lasting color in the atmosphere. Such coatings are widely used in automobiles, bicycles, sewing machines, watches, instruments, household hardware and other parts of the protection and decoration. The polished decorative chrome layer has a high degree of reflection of light and can be used as a reflector. Plating micro-cracks or micro-cracks on multiple nickel layers is an important way to reduce the total thickness of the coating and achieve a high corrosion resistance protection system. It is also the development direction of the modern electroplating process.
② hard chromium plating (wear Cr) coating with high hardness and wear resistance, extending the service life of the workpiece, such as cutting and drawing tools, a variety of materials and die casting mold, bearings, shafts, gauges, gears, etc. It can also be used to repair the dimensional tolerances of worn parts. The thickness of hard chrome plating is generally 5 ~ 50μm, but also according to need, and some up to 200 ~ 800μm. Chrome plated steel parts do not need intermediate plating, such as the special requirements of corrosion resistance, but also can be used in different intermediate coating.
③ Plating chrome chrome chrome layer was milky white, low gloss, good toughness, low porosity, soft color, low hardness than hard chrome and decorative chrome, but high corrosion resistance, it is commonly used in measuring tools and instrument panels. In order to improve its hardness, milky white coating surface can be re-plated with a layer of hard chrome, the so-called double chrome plating, both milky chrome and hard chrome plating characteristics, and more for both coated wear-resistant and corrosion-resistant Components.
④ plated loose hole Chromium (porous chromium) is the use of chromium layer itself has the characteristics of meticulous cracks in hard chrome plating and then mechanical, chemical or electrochemical loose hole processing, the crack network to further deepen and widen. So that the chrome layer surface is covered with a wide groove, not only has the characteristics of wear-resistant chromium, and can effectively store the lubricating medium, to prevent non-lubricating operation, improve the workpiece surface anti-friction and wear resistance. Commonly used in heavy pressure sliding friction surface plating, such as the internal combustion engine cylinder bore, piston rings.
⑤ plated black chrome black chrome black color with a uniform gloss, good decorative, with good extinction; high hardness (130 ~ 350HV), the same thickness of the wear resistance of bright nickel 2 to 3 times higher; its resistance Etchability and ordinary chrome the same, mainly depends on the thickness of the middle layer. Heat resistance, below 300 ℃ does not change color. Black chrome layer can be directly plated on the surface of iron, copper, nickel and stainless steel, in order to improve corrosion resistance and decorative role, but also with copper, nickel or copper-tin alloy as the bottom layer, coated with black chrome plating on its surface. Black chrome plating is commonly used to coat parts and components of aeronautical instrumentation and optical instruments, protection and decoration of solar energy absorbing panels and commodity products.
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