What is the chrome plating process?

  Chromium plating is a silvery-white metal with a slight bluish tint. Metallic chromium is very easy to passivate in the air, and a very thin passivation film is formed on the surface, thus showing the properties of precious metals. The chromium plating layer has good chemical stability and does not react in alkali, sulfide, nitric acid and most organic acids, but is soluble in hydrochloric acid (such as hydrochloric acid) and hot sulfuric acid. In the visible light range, the reflectivity of chromium is about 65%, which is between silver (88%) and nickel (55%). In addition, because chromium does not change color, it can maintain its reflectivity for a long time during use, which is better than silver and nickel.

 

There is a mutual dependence between cathode current density and temperature in the chromium plating process. When chromium is plated in the same solution, by adjusting the temperature and current density and controlling them within an appropriate range, three chromium plating layers with different properties, bright chromium, hard chromium and milky white chromium, can be obtained. In the low temperature and high current density area, the chromium plating layer is gray or burnt, and this plating layer has a network of cracks, high hardness and high brittleness; in the high temperature and low current density area, the chromium layer is milky white, this structure is fine, with few pores, no cracks, good protective performance, but low hardness and poor wear resistance: in the medium temperature and medium current density area or when the two are well matched, a bright high chromium plating layer can be obtained, this chromium layer has a high hardness and fine and dense network cracks.

When the current density is constant, the current efficiency decreases with increasing temperature; if the temperature is fixed, the current efficiency increases with increasing current density. However, when the ratio of chromic anhydride to sulfate ions decreases, the change becomes smaller accordingly. Therefore, when hard chromium is plated, a lower temperature and a higher cathode current density are usually used to obtain a higher deposition rate of the coating, provided that the coating performance is met. When the temperature is constant, the dispersibility of the plating solution improves slightly with increasing current density; on the contrary, when the current density is constant, the dispersibility of the plating solution decreases to a certain extent with increasing temperature of the plating solution. In production, medium temperature (45~60℃) and medium current density (30~45A/dm2) are generally used to obtain a bright and hard chromium coating. Although the process conditions for plating bright coatings are quite wide, considering that the dispersibility of the chromium plating solution is particularly poor, when plating decorative chromium or hard chromium on parts with complex shapes, in order to plate a uniform thickness of chromium layer on different parts, the temperature and current density must be strictly controlled. When the chromium plating process conditions are determined, the temperature change of the plating solution is preferably controlled between ±(1~2)C.

1. Protective chromium: Protective-decorative chrome plating is commonly known as decorative chrome. The coating is thin, bright and beautiful. It is usually used as the outermost layer of multi-layer electroplating. In order to achieve the purpose of protection, a sufficiently thick intermediate layer must be plated on the zinc base or steel substrate, and then a thin layer of chromium of 0.25~0.5um is plated on the bright intermediate layer. Commonly used processes include Cu/Ni/CrNi/Cu/Ni/Cr, Cu-Sn/Cr, etc. After the surface of the polished product is plated with decorative chrome, a silver-blue mirror luster can be obtained. It does not change color for a long time in the atmosphere. This type of coating is widely used in the protection and decoration of parts such as automobiles, bicycles, sewing machines, clocks and watches, instruments and meters, and daily hardware. The polished decorative chrome layer has a high reflective ability to light and can be used as a reflector. Plating microporous or microcrack chromium on multi-layer nickel is an important way to reduce the total thickness of the coating and obtain a highly corrosion-resistant protective-decorative system. It is also the development direction of modern electroplating technology.

2. Hard chrome plating (wear-resistant chrome) plating: It has extremely high hardness and wear resistance, which can extend the service life of workpieces, such as cutting and drawing tools, pressing molds and casting molds of various materials, bearings, shafts, gauges, gears, etc. It can also be used to repair the dimensional tolerance of worn parts. The thickness of hard chrome plating is generally 5~50um, and can also be determined according to needs, and some are as high as 200~800um. Hard chrome plating of steel parts does not require an intermediate plating layer. If there are special requirements for corrosion resistance, different intermediate plating layers can also be used.

3. Milky white chrome plating: Milky white, low gloss, good toughness, low porosity, soft color, lower hardness than hard chrome and decorative chrome, but high corrosion resistance, so it is often used for measuring tools and instrument panels. In order to improve its hardness, a layer of hard chrome can be plated on the surface of the milky white coating, the so-called double-layer chrome plating, which has the characteristics of milky white chrome plating and hard chrome plating, and is mostly used for plating parts that require both wear resistance and corrosion resistance.

4. Porous chromium plating (porous chromium): It takes advantage of the fine cracks in the chromium layer itself. After hard chromium plating, mechanical, chemical or electrochemical porous treatment is performed to further deepen and widen the crack network. The chromium layer surface is covered with wider grooves, which not only has the characteristics of wear-resistant chromium, but also can effectively store lubricating media, prevent unlubricated operation, and improve the workpiece surface's ability to resist friction and wear. It is often used for plating the surface of sliding friction parts under heavy pressure, such as the inner cavity of the cylinder barrel of an internal combustion engine, piston rings, etc.

5. Black chrome plating Black chrome plating: black color, uniform luster, good decorative effect, good matte; high hardness (130~350HV), wear resistance is 2~3 times higher than bright nickel at the same thickness: its corrosion resistance is the same as ordinary chrome plating, mainly depending on the thickness of the intermediate layer. It has good heat resistance and will not change color below 300℃. The black chrome layer can be directly plated on the surface of iron, copper, nickel and stainless steel. In order to improve the corrosion resistance and decorative effect, copper, nickel or copper-tin alloy can also be used as the base layer, and black chrome plating can be plated on its surface. Black chrome plating is often used for plating parts of aviation instruments and optical instruments, solar absorption panels and protection and decoration of daily necessities.