What is Chromium?
1. Chromium is a silver-white metal with a microstrip of blue, with an atomic weight of 51.99, density 6.98-7. The melting point was 1875 ~ 1920℃. The standard electrode potential was yush /c, -0.74V, old PI /c, minus 0.41V, and P very e+/Cr hidden-1.33V, the metal chromium is easily passivated in air, forming a very thin passivating film on the surface, thus showing the properties of the precious metal.
The chromium plating layer has a very high hardness, according to the composition and process conditions, the hardness can vary in a wide range of 400 ~ 1200HV. The chrome plating layer has good heat resistance, heating under 500℃, its luster, hardness has no obvious change, temperature more than 500℃ began to oxidation color, more than 700℃ hardness began to reduce. The friction coefficient of chrome plating, especially the dry friction coefficient, is the lowest of all metals. So the chromium plating layer has good wear resistance.
Chromium plating layer has good chemical stability, in alkali, sulfide, nitric acid and most organic acids do not occur but can be dissolved in hydrochloric acid (such as hydrochloric acid) and hot sulfuric acid.
In visible light, chromium's reflectivity is about 65 percent, between silver (88 percent) and nickel (55 percent), and better than silver and nickel because chromium does not change color and can be used for a long time.
Type and use
There are many kinds of chrome-plating processes, which can be classified as follows according to their USES.
(1) protection of decorative chromium protection of a decorative chromium plating commonly known as decorative chromium, the coating is thin, bright and beautiful, usually as a multilayer
The outermost layer of electroplating, in order to achieve the purpose of protection, the zinc base or steel matrix must be plated with a thick enough intermediate layer, and then coated with 0.25 ~0.5-micron thin layer of chromium. Common processes include Cu/Ni/Cr, Ni/Cu/Ni/Cr, Cu -- Sn/Cr, etc. After polishing the surface of products decorated with chromium, you can get a silver-blue mirror luster. It does not change color over time in the atmosphere. This kind of coating is widely used in automobile, bicycle, sewing machine, clock, instrument and meter, daily hardware and other parts protection and decoration. The polished decorative chrome layer is highly reflective and can be used as a reflector.
Microhole or microcrack chromium plating on multi-layer nickel is an important way to reduce the overall thickness of the coating and obtain a decorative system with high corrosion resistance, and it is also the development direction of modern electroplating technology.
(2) hard chromium (wear-resistant chromium) coating with high hardness and wear resistance, can extend the service life of the workpiece, such as cutting and drawing tools, all kinds of materials die and die, bearing, shaft, gauge, gear, etc., can also be used to repair the dimensional tolerance of worn parts. The thickness of hard chromium plating is usually 5 to 50 microns or as much as 200 to 800 microns. Hard chromium plating on steel parts does not require intermediate coating. The different intermediate coating can be used if corrosion resistance is required.
(3) the milky white chrome plating layer is milky white, low gloss, good toughness, low porosity, soft color, hardness than hard chromium and decorative chromium low, but high corrosion resistance, so often used for measuring tools and instrument panels. In order to improve its hardness, a layer of hard chromium can be plated on the surface of the opalescent coating, that is, the double layer of chromium coating has the characteristics of opalescent and hard chromium coating, which is used for coating parts that require both wear resistance and corrosion resistance.
(4) plating porous chromium (porous chromium) is the use of chromium layer itself with fine cracks in the characteristics of hard chromium plating after mechanical, chemical or electrochemical hole treatment so that the crack network is further deepened and widened. The chromium layer surface is covered with wide grooves, which not only has the characteristics of wear-resistant chromium, but also can effectively store the lubricating medium, prevent the operation without lubrication, and improve the anti-friction and wearability of the workpiece surface. It is often used for plating the surface of sliding friction parts under heavy pressure, such as the internal cavity of the internal combustion engine cylinder, piston ring, etc.
(5) black chrome coating with uniform luster, good decorative, with good extinction; 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 chromium plating, mainly depends on the thickness of the intermediate layer. Good heat resistance, no discoloration under 300℃.The black chromium layer can be directly plated on the surface of iron, copper, nickel and stainless steel, in order to improve corrosion resistance and decorative effect, it can also use copper, nickel or copper-tin alloy as the bottom layer, on its surface plating black chromium coating. The black chrome coating is often used for the protection and decoration of the parts, solar absorption panels and daily necessities of the aviation instruments and optical instruments.