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What are the electroplating process classifications?
Electroplating is the process of plating a thin layer of other metals or alloys on the surface of some metals using the principle of electrolysis. According to the composition of the plating layer, the electroplating process can be divided into five categories: chromium plating, copper plating, cadmium plating, tin plating, and zinc plating. Next, I will accept the principles and characteristics of the five major categories of electroplating process in detail for everyone.
Chromium is a silver-white metal with a slight blue sky. Although the electrode potential is very negative, it has strong passivation properties. It is quickly passivated in the atmosphere and shows the properties of precious metals. Therefore, the chromium plating of iron parts is the cathode plating. The chromium layer is very stable in the atmosphere and can maintain its gloss for a long time. It is very stable in corrosive media such as alkali, nitric acid, sulfide, carbonate and organic acid, but it is soluble in hydrohalic acid such as hydrochloric acid and hot concentrated sulfuric acid .
The chromium layer has high hardness, good abrasion resistance, strong reflective ability, and good heat resistance. There is no significant change in gloss and hardness below 500 ° C; oxidative discoloration begins at temperatures above 500 ° C; and it begins to soften above 700 ° C. Due to the excellent performance of the chrome plating layer, it is widely used to protect the outer surface layer and functional plating layer of a decorative coating system.
2. Copper plating
The copper-plated layer is pink, soft and has good ductility, electrical conductivity and thermal conductivity. It is easy to polish. After proper chemical treatment, it can obtain decorative colors such as bronze, patina, black and natural colors. Copper plating is apt to lose its luster in the air, and reacts with carbon dioxide or chloride, and a layer of basic copper carbonate or copper chloride is formed on the surface. Brown or black copper sulfide is formed by the action of sulfide. Therefore, it is decorative. The copper plating layer needs to be coated with an organic coating on the surface.
3. Cadmium plating
Cadmium is a silvery white shiny soft metal. Its hardness is harder than tin, softer than zinc, good plasticity, and easy to forge and roll. The chemical properties of cadmium are similar to zinc, but it does not dissolve in lye, dissolves in nitric acid and ammonium nitrate, and dissolves very slowly in dilute sulfuric acid and dilute hydrochloric acid. Cadmium vapor and soluble cadmium salts are toxic, and cadmium pollution must be strictly prevented. Because cadmium pollution is very harmful and expensive, zinc plating or alloy plating is usually used to replace the cadmium plating. The types of cadmium plating solutions used in domestic production are: ammonia carboxylate complex cadmium plating, acid sulfate cadmium plating and cyanide cadmium plating. In addition, there are pyrophosphate cadmium plating, alkaline triethanolamine cadmium plating, and HEDP cadmium plating.
Tin has a silver-white appearance, with an atomic weight of 118.7, a density of 7.3 g / cm ^ 3, a melting point of 231.89 ° C, and atomic valences of divalent and tetravalent, so the electrochemical equivalents are 2.12 g / A.h and 1.107 g / A.h, respectively. Tin has the advantages of corrosion resistance, non-toxicity, easy iron welding, softness and good ductility. Tin plating has the following characteristics and uses:
(1.) High chemical stability;
(2.) The standard potential of tin in the electrification sequence is iron, which is a cathodic coating for steel. It can effectively protect the substrate only when the coating has no pores;
(3.) Tin has good electrical conductivity and is easy to solder;
(4.) The crystallization of tin begins to mutate from -130 ° C, and it will completely change to a crystalline isomer, which is commonly known as "tin pest", and the properties of tin have been completely lost at this time;
(5.) Tin, like zinc and cadmium coatings, can grow into whiskers under high temperature, humidity and closed conditions, called long hair; 6. After tin plating, it can be re-dissolved in hot oil above 231.89 ° C to obtain The shiny pattern tin layer can be used as decorative plating for daily necessities.
Zinc is easily soluble in acid and alkali, so it is called amphoteric metal. Zinc hardly changes in dry air. In humid air, a basic zinc carbonate film is formed on the zinc surface. In the atmosphere containing sulfur dioxide, hydrogen sulfide and ocean, the corrosion resistance of zinc is poor. Especially in the high-temperature and high-humidity atmosphere containing organic acid, the zinc coating is easily corroded. The standard electrode potential of zinc is -0.76V. For the steel substrate, the zinc coating is an anodic coating. It is mainly used to prevent corrosion of steel. Its protective performance has a great relationship with the thickness of the coating.
After the zinc coating is passivated, dyed or coated with a light protectant, it can significantly improve its protection and decoration. With the development of galvanizing technology and the use of high-performance galvanizing brighteners, galvanization has entered protective-decorative applications from pure protective purposes.
There are two types of zinc plating solutions: cyanide bath and cyanide-free bath. The cyanide plating solution is classified into slightly cyanide, low cyanide, medium cyanide, and high cyanide. The cyanide-free plating solution includes alkaline zincate plating solution, ammonium salt plating solution, sulfate plating solution, and ammonia-free chloride plating solution. The cyanide zinc plating solution has good uniform plating ability, and the resulting coating is smooth and fine, and has been used for a long time in production. However, cyanide is highly toxic and severely pollutes the environment, and has tended to use low cyanide, slightly cyanide, and cyanide-free zinc plating solutions.
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