Test you: Classification of metal coatings

This post was last edited by mopeizhi on 2010-2-7 at 11:23. Metal coatings can be classified into the following two categories based on their chemical properties: 1) Anodic coatings. In a certain medium, when the electrode potential of the coated metal is lower than that of the base metal, such a coating is referred to as a positive-type coating. When in use, even if the integrity of such a coating is compromised, electrochemical processes can still be utilized to continue protecting the base metal from corrosion. The protective capacity of the positive polarity coating depends mainly on its thickness; the thicker the coating, the stronger its protective effect. 2) Cathodic coating layer. In a certain medium, when the electrode potential of the coated metal is higher than that of the base metal, such a coating is referred to as a cathodic coating. When in use, such coatings can only provide mechanical protection to prevent the base metal from corroding. If the integrity of the coating is compromised or damaged, it will accelerate the corrosion of the base metal. The protective capacity of the cathodic coating depends on its porosity and thickness.

This post was last edited by mopeizhi on 2010-2-7 11:25. A general term for metal coatings made using various methods. Among them, electroplating, metal spraying, and surface alloying methods are widely used, while hot-dip metal coating, cladding, and vapor deposition methods are also applied to some extent. Based on chemical properties, they are mainly chemical plating and electrochemical plating

This post was last edited by mopeizhi on 2010-2-7 at 11:26. Currently, metal coatings are usually classified according to their intended use or the electrochemical relationship between the coating and the base metal. Based on the purpose of the coating, coatings can be divided into three main categories: protective coatings, protective-decorative coatings, and functional coatings. (1) Protective coatings: These types of coatings are primarily used to prevent corrosion in metal parts. Galvanized coatings, cadmium-plated coatings, tin-plated coatings, and zinc-based alloy (Zn—Fe, Zn—Co, Zn—Ni) coatings all fall into this category of coatings. Black metal parts are commonly protected with a zinc coating under normal atmospheric conditions, while a cadmium coating is typically used for protection in marine climates ； When a thin coating with strong corrosion resistance is required, a tin-cadmium alloy can be used to replace the cadmium coating ； Nautical instruments made of copper alloys can be protected using silver-cadmium alloys ； For ferrous metal parts that come into contact with organic acids, such as food containers, a tin plating is used for protection; it not only provides strong corrosion resistance but also ensures that the corrosion products are harmless to the human body. (2) Protective-decorative coating: For many metal parts, there is a need for both corrosion protection and a durable appearance, which requires the application of protective-decorative coatings. This type of coating is often achieved through multi-layer electroplating, that is, a “base” layer is first plated on the substrate, followed by a “surface” layer, and sometimes an “intermediate” layer as well. For example, common multi-layer plating processes such as Cu-Ni-Cr plating are typical protective and decorative coatings, often used on exposed parts of bicycles, sewing machines, and cars. The currently popular colored plating, black plating, and gold-like coatings also fall into this category of platings. (3) Functional coatings: Coatings deposited to meet the requirements of special physical properties such as light, electrical, magnetic, thermal, and wear resistance are known as functional coatings, and there are many types available today. ①Wear-resistant and anti-friction coatings: Wear-resistant coatings involve applying a layer of highly hard metal to a part in order to enhance its resistance to wear. With hard chromium plating, the hardness can reach 1000–1200 HV; it is used for the journal surfaces of straight or crankshafts, the surfaces of impression rollers, the internal cavities of stamping dies, as well as the internal cavities of guns and cannons. For the plug-in components of some instruments, high electrical conductivity as well as wear resistance are required; therefore, plating with hard silver, hard gold, rhodium, etc., is often employed. Anti-friction coatings are commonly used on sliding contact surfaces to provide lubrication, reduce the coefficient of sliding friction, and extend the service life of components. Metals used as anti-friction coatings include tin, lead-tin alloys, lead-indium alloys, lead-tin-copper, and lead-antimony-tin ternary alloys. ②Coatings for hot working are used to improve the surface physical properties of mechanical parts and similar items, and often require heat treatment. However, for a single component, in order to modify only certain properties, it is necessary to protect those parts whose properties do not need to be changed before heat treatment. In industrial production, copper plating is used to prevent localized carburization, while tin plating is used to prevent localized nitridation; this is achieved by taking advantage of the difficulty with which carbon or nitrogen diffuses in these metals. ③Conductive coatings are widely used in electrical, radio, and communication equipment to improve the conductivity of the surfaces of components; common types include copper plating, silver plating, and gold plating. If wear resistance is also required, silver-antimony alloys, gold-cobalt alloys, gold-antimony alloys, etc., can be used for plating. ④In devices such as magnetic-coated recorders and computers, storage devices like tape, magnetic coils, drum magnets, and disks all require magnetic materials; common ones include magnetic alloy coatings made of cobalt-nickel, nickel-iron, cobalt-nickel-phosphorus, etc ； As magneto-optical recording materials, there are samarium-cobalt and others. During production, when the conditions of the electroplating process change, the magnetic properties of the coating also change accordingly; therefore, the processing conditions must be strictly controlled. ⑤After wear occurs on important machine parts, repair can be carried out using electroplating. For example, the crankshafts, camshafts, gears, and splines in cars and tractors, as well as the pressure rollers in textile machines and the shafts of deep well pumps, can be repaired by electroplating with hard chromium or iron (or a composite iron plating) ； Some components in the printing and dyeing, papermaking, and film industries can also be repaired by copper plating or chromium plating ； Printing type or plates can be repaired by iron plating. In addition to the above, with the advancement of technology, electroplating or electrodeposition can also be used to prepare nanomaterials and thin films of high-performance materials, such as superconducting oxide thin films, electrochromic oxide thin films, metal compound semiconductor thin films, shape memory alloy thin films, and gradient material thin films. Electroplating has a wide range of applications in the field of functional materials. Based on the electrochemical relationship between the base metal and the coating, coatings can be divided into two main categories: anodic coatings and cathodic coatings. For example, zinc plating on iron is a common anodic coating, while tin plating on iron is a cathodic coating. The so-called anodic coating refers to a coating that acts as the anode when a corrosion microcell is formed between the coating and the base metal; it is the anode that dissolves first. Such a coating not only provides mechanical protection for the base metal but also offers electrochemical protection. In the case of galvanizing iron, under normal conditions, since the standard electrode potential of zinc is more negative than that of iron, when defects in the coating (such as pores or scratches) expose the base metal, and water vapor condenses at those areas, a corrosion cell between zinc and iron is formed. At this time, zinc dissolves as the anode, while iron acts as the cathode; H+ ions are discharged there, producing hydrogen gas which thus protects the iron from corrosion. Therefore, the zinc coating in this case is called an anodic coating. To prevent metal corrosion, anodic coatings should be used as much as possible, and it is necessary to ensure that the coating has a certain thickness. A cathodic coating is one in which, when the coating and the substrate form a corrosion microcell, the coating acts as the cathode; such a coating can only provide mechanical protection for the base metal. For example, when tin is plated on a steel substrate and defects exist in the coating, iron and tin form a corrosion cell. However, the standard electrode potential of tin is higher than that of iron, so tin acts as the cathode; as a result of this corrosion cell, iron at the anode dissolves, while hydrogen is released at the tin cathode. In this way, the coating remains intact, while the substrate beneath it is gradually corroded, and eventually the coating will also fall off. Therefore, a cathodic coating can provide mechanical protection to the substrate only when it is intact; once the coating is damaged, it fails to protect the substrate and instead accelerates its corrosion. Hence, the porosity of a cathodic coating should be minimized as much as possible. Since the electrode potential of metals changes depending on the medium, it depends on the medium whether the coating is an anodic coating or a cathodic coating. For example, for a steel substrate, a zinc coating is typically an anodic coating under normal conditions; however, in hot water at 70–80°C, the potential of zinc becomes more positive than that of iron, thus it turns into a cathodic coating ； Tin acts as a cathodic coating for iron under normal conditions, but it becomes an anodic coating in organic acids. Not all metals with a more negative potential than the base metal can be used as protective coatings, because if the coating is unstable in its surrounding medium, it will be rapidly corroded by that medium and lose its ability to protect the base metal. Zinc can serve as a protective coating for ferrous metals in the atmosphere because it functions both as an anodic coating and as a protective layer of basic zinc carbonate, which gives it high stability. However, in seawater, although zinc remains an anodic coating for iron, it is unstable in chloride environments and thus loses its protective effect; therefore, instruments on sailing ships cannot be protected solely with a zinc coating, and a cadmium coating or a cadmium substitute coating is preferable. 1# inlegend

This post was last edited by mopeizhi on 2010-2-7 at 11:27. Metal coatings can be classified into anodic coatings and cathodic coatings based on their electrochemical properties.

This post was last edited by mopeizhi on 2010-2-7 11:27. 1# inlegend electroplating and hot-dip plating

This post was last edited by mopeizhi on 2010-2-7 at 11:28. Metal coatings can be classified into the following two categories based on their chemical properties: 1) Anodic coatings. In a certain medium, when the electrode potential of the coated metal is lower than that of the base metal, such a coating is referred to as a positive-type coating. When in use, even if the integrity of such a coating is compromised, electrochemical processes can still be utilized to continue protecting the base metal from corrosion. The protective capacity of the positive polarity coating depends mainly on its thickness; the thicker the coating, the stronger its protective effect. 2) Cathodic coating layer. In a certain medium, when the electrode potential of the coated metal is higher than that of the base metal, such a coating is referred to as a cathodic coating. When in use, such coatings can only provide mechanical protection to prevent the base metal from corroding. If the integrity of the coating is compromised or damaged, it will accelerate the corrosion of the base metal. The protective capacity of the cathodic coating depends on its porosity and thickness. 1# inlegend

This post was last edited by mopeizhi on 2010-2-7 at 11:29. 1. Anodic coating: In a certain medium, when the electrode potential of the coated metal is lower than that of the base metal, such a coating is referred to as an anodic coating. When its integrity is compromised during use, it can still continue to protect the base metal from corrosion through electrochemical processes. The protective capacity of an anodic coating depends mainly on the thickness of the coating; the thicker the coating, the stronger its protective effect. 2. A cathodic coating is one in which, within a certain medium, the electrode potential of the coated metal is higher than that of the base metal. Such coatings can only provide mechanical protection against corrosion of the base metal; if their integrity is compromised or they are damaged, it will accelerate the corrosion of the base metal. The protective capacity of a cathodic coating depends on its porosity and thickness

This post was last edited by mopeizhi on 2010-2-7 11:30. Into which two categories can metal coatings be divided based on their chemical properties? 1. Anodic coating: In a certain medium, when the electrode potential of the coated metal is lower than that of the base metal, such a coating is referred to as an anodic coating. When its integrity is compromised during use, it can still continue to protect the base metal from corrosion through electrochemical processes. The protective capacity of an anodic coating depends primarily on the thickness of the coating; the thicker the coating, the stronger its protective effect. 2. A cathodic coating is one in which, within a certain medium, the electrode potential of the coated metal is higher than that of the base metal. Such coatings can only provide mechanical protection against corrosion of the base metal; if their integrity is compromised or they are damaged, it will accelerate the corrosion of the base metal. The protective capacity of a cathodic coating depends on its porosity and thickness.

This post was last edited by mopeizhi on 2010-2-7 at 11:31. Based on the electrochemical relationship between the base metal and the coating, coatings can be divided into two main categories: anodic coatings and cathodic coatings. For example, galvanizing iron is a common example of an anodic coating, while tin plating on iron is a cathodic coating. The so-called anodic coating refers to a coating that acts as the anode when a corrosion microcell is formed between the coating and the base metal; it is the anode that dissolves first. Such a coating not only provides mechanical protection for the base metal but also offers electrochemical protection. In the case of galvanizing iron, under normal conditions, since the standard electrode potential of zinc is more negative than that of iron, when defects in the coating (such as pores or scratches) expose the base metal, and water vapor condenses at those areas, a corrosion cell between zinc and iron is formed. At this time, zinc dissolves as the anode, while iron acts as the cathode; H+ ions are discharged there, producing hydrogen gas which thus protects the iron from corrosion. Therefore, the zinc coating in this case is called an anodic coating. To prevent metal corrosion, anodic coatings should be used as much as possible, and it is necessary to ensure that the coating has a certain thickness. A cathodic coating is one in which, when the coating and the substrate form a corrosion microcell, the coating acts as the cathode; such a coating can only provide mechanical protection for the base metal. For example, when tin is plated on a steel substrate and defects exist in the coating, iron and tin form a corrosion cell. However, the standard electrode potential of tin is higher than that of iron, so tin acts as the cathode; as a result of this corrosion cell, iron at the anode dissolves, while hydrogen is released at the tin cathode. In this way, the coating remains intact, while the substrate beneath it is gradually corroded, and eventually the coating will also fall off. Therefore, a cathodic coating can provide mechanical protection to the substrate only when it is intact; once the coating is damaged, it fails to protect the substrate and instead accelerates its corrosion. Hence, the porosity of a cathodic coating should be minimized as much as possible. Since the electrode potential of metals changes depending on the medium, it depends on the medium whether the coating is an anodic coating or a cathodic coating. For example, for a steel substrate, a zinc coating is typically an anodic coating under normal conditions; however, in hot water at 70–80°C, the potential of zinc becomes more positive than that of iron, thus it turns into a cathodic coating ； Tin acts as a cathodic coating for iron under normal conditions, but it becomes an anodic coating in organic acids. Not all metals with a more negative potential than the base metal can be used as protective coatings, because if the coating is unstable in its surrounding medium, it will be rapidly corroded by that medium and lose its ability to protect the base metal. Zinc can serve as a protective coating for ferrous metals in the atmosphere because it functions both as an anodic coating and as a protective layer of basic zinc carbonate, which gives it high stability. However, in seawater, although zinc remains an anodic coating for iron, it is unstable in chloride environments and thus loses its protective effect; therefore, instruments on sailing ships cannot be protected solely with a zinc coating, and a cadmium coating or a cadmium substitute coating is preferable.

　　Electroplating includes methods such as rack plating, barrel plating, continuous plating, and brush plating, and it is primarily determined by the size and quantity of the parts to be plated. Electroplating is suitable for products of standard sizes, such as car bumpers and bicycle handles. Roll plating is suitable for small parts such as fasteners, washers, pins, etc. Continuous plating is suitable for wire and strip produced in batches. Brush plating is suitable for local plating or repair. Plating solutions can be acidic, alkaline, or acidic and neutral solutions containing chromium complexes. Regardless of the plating method used, the plating tanks, hanging fixtures, etc., that come into contact with the items to be plated and the plating solution should have a certain degree of versatility. 　　Coating classifications Coatings are divided into two categories: decorative and protective coatings, and functional coatings. 　　Decorative protective coatings mainly refer to chromium plating on ferrous and non-ferrous metals as well as plastics; in particular, the copper-nickel-chromium coating on steel, and the nickel-chromium coating on zinc and steel. To save nickel, copper-nickel/iron-high-sulfur nickel-nickel/iron-low-solid-content nickel-chromium coatings can now be applied to steel. A tin/nickel plating similar to the chromium plating can be used on analytical balances, chemical pumps, valves, and flow measurement instruments. 　　Functional coatings: There are many types of such coatings; for example: ① Coatings for bearing housings that enhance compatibility and interpenetration with the shaft journals, including composite coatings such as lead-tin, lead-copper-tin, and lead-indium ； ②A hard chromium coating used on piston rings of medium and high-speed diesel engines to improve wear resistance; this coating can also be applied to plastic molds, offering non-stick properties and a long service life ； ③Copper plating on the sliding surface of large herringbone gears can prevent premature scuffing of that surface ； ④Galvanizing, used to prevent the steel substrate from suffering atmospheric corrosion; ⑤ Copper-tin plating, to prevent nitriding; ⑥ Tin-zinc plating, used in the manufacturing of radios and televisions for soldering and to prevent galvanic corrosion between steel and aluminum. Engineering coatings suitable for repair and manufacturing include chromium, silver, copper, etc.; their thicknesses are relatively large, with hard chromium layers reaching up to 300 microns in thickness.