Plastic plating-How to classify plating layers
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1. Classification by electrochemical properties
It can be divided into anodic coating and cathodic coating.
(1)anodic coating
A coating whose potential of the coating metal is lower than that of the base metal is called an anodic coating. For example, zinc plating on steel parts, cadmium plating on the surface of iron products in the marine environment, tin plating on the surface of iron products in organic acid environment, etc. The characteristic of this type of coating is that when a corrosive microcell is formed, that is, when electrochemical corrosion occurs, the coating metal at the anode is constantly depleted and the base metal is protected. Therefore, the first thing to corrode is the plating metal (such as zinc and cadmium, compared to steel products), which plays an electrochemical role in protecting steel parts.
For anodic coatings, the thickness of the coating has a decisive influence on the protective ability.
(2)cathodic coating
A coating with a high potential of the coating metal relative to the base metal is called a cathodic coating. Such as copper, nickel, chromium, gold, silver plating, etc. on steel parts. The cathode coating protects the base metal only when it is intact. Otherwise, once these coatings form a corrosive microbattery, the first thing that corrodes is the base metal as the anode, which corrodes from the inside out. Not only does it fail to protect the base metal, but it accelerates the corrosion rate. Therefore, it is particularly important for cathodic coatings that the coating must have a certain thickness and the porosity must be as low as possible.
The electrode potential of a metal changes with the medium and working conditions, so whether the coating is an anodic coating or a cathodic coating depends on the medium and environment in which the coating is located. For example, for iron, zinc is a typical anodic coating under general conditions, but the electrode potential of zinc becomes higher than that of iron in hot water at 70 - 80 ° C, thus becoming a cathodic coating. For example, tin is a cathodic coating for iron under normal conditions, but becomes an anodic coating in the medium of organic acids.
2. categorized by use
It can be divided into protective coatings. Protection-decorative coatings, restorative coatings, functional coatings, etc.
(1)protective coating
Coatings that prevent corrosion of the base metal in the atmosphere or other environments are called protective coatings. For example, zinc, cadmium, tin and other coatings on steel parts and zinc-based alloy coatings (zinc-iron, zinc-cobalt, zinc-nickel, etc.) belong to this type of coating. For ferrous metal workpieces, zinc plating is used for protection under general atmospheric conditions. This coating is widely used and accounts for more than 50% of electroplating production. It can be protected by a cadmium plating layer in humid and marine atmospheres. Cadmium or cadmium-tin alloy is used for protection of fasteners. Such as high-pressure gasket sealing and anti-corrosion in chemical production. Tin plating has good corrosion resistance to organic acids. It not only has strong rust resistance, but also produces corrosive compounds that are harmless to the human body. It is widely used in the food processing industry.
(2)Protective-decorative coating
Coatings that can prevent corrosion of the base metal and are aesthetically pleasing are called protective decorative coatings. Such as copper/nickel/chromium plating on steel parts, nickel-iron/chromium plating, copper-tin/chromium, etc. It requires the coating to be both anti-corrosion and decorative. This type of coating is often coated with the "bottom layer" first, then the "surface layer", and sometimes the "middle layer". The reason for this is that it is difficult to find a single metal coating that can meet the dual requirements of protection and decoration at the same time. Although some coated metals have strong anti-corrosion capabilities, they cannot maintain their luster for a long time in the use environment. Although some plated metals have poor anti-corrosion capabilities, they can give the appearance of the product a long-lasting luster and are not easy to wear. Therefore, multi-layer plating is used to match each other's strengths to make up for each other's defects. For example, copper/nickel/chromium plating is first plated with copper as the bottom layer and then plated with nickel as the middle layer to improve corrosion resistance. The outer layer is made of bright chromium with a slight blue color, which has good decorative properties. This combination coating is mostly used in exterior parts of instruments, automobiles, and bicycles.
(3)restorative coating
A coating that can locally or wholly thicken or restore the size of a locally worn workpiece is called a restorative coating. Some major mechanical parts, such as trains and automobiles. Large shafts, crankshafts, gears, etc. on petrochemicals and other machinery can be repaired by electroplating to extend their service life. Deep well pump shafts, etc. can be repaired by electroplating hard chromium and iron plating.
(4)functional coatings
Functional coatings use various mechanical, physical and chemical properties of the coating metal to meet the needs of various occasions. According to the performance of the coating, they can be divided into the following main categories.
① Wear resistant and anti-friction coating. By improving the surface hardness of the product to increase its wear resistance, hard chromium plating is often used in the industry, such as the journals of various shafts and crankshafts, the roller surfaces of printing rollers, the inner walls of cylinders and piston rings of engines, and the inner cavities of pressing molds., etc.; use lead-tin alloy, lead-tin-copper alloy, etc. as anti-friction coating to be used on sliding contact surfaces to lubricate and reduce sliding friction.
② High temperature oxidation resistant coating. Plating a high-temperature oxidation resistant layer on the surface of parts that may be oxidized (high-temperature corrosion) at high temperatures to improve the resistance to high-temperature oxidation of the surface of parts. Some special purpose workpieces made of metal materials with melting points such as jet engine rotors and blades require nickel and chromium plating. Platinum-germanium alloy plating; in some cases, composite coatings such as nickel-germanium oxide are also used. Nickel-alumina, etc. to improve the high temperature oxidation resistance of metal materials.
③ Magnetic coating. In electronic computer equipment, magnetic rings, drums, magnetic disks, and magnetic film magnetic storage components all need to use magnetic materials. Commonly used magnetic alloy coatings include cobalt-nickel, nickel-iron, nickel-phosphorus-cobalt, etc. At present, electroplating and electroless plating methods are mostly used to prepare magnetic materials to meet this requirement. By changing the plating process parameters, the magnetic property parameters of the coating can also be adjusted.
④ Solderable coating. Some electronic components need to be soldered when assembled. In order to improve their soldering properties, they are usually plated with tin, silver or tin-lead alloy. Gold, etc. all belong to this type of coating. If wear resistance is also required, a silver-antimony alloy can be plated. Gold-cobalt alloy, gold-antimony alloy, etc.
There are many other functional coatings used in production practice. For example, in order to enhance the point fit between steel wire and rubber during hot pressing, the steel wire can be plated with brass (copper-zinc alloy) ; in order to improve the surface conductivity, copper or silver can be plated, which requires good conductivity and also
