What are the classifications of electroplating processes?

Electroplating is the process of using the principle of electrolysis to deposit a thin layer of other metals or alloys on the surface of certain metals.

Electroplating is the process of using the principle of electrolysis to deposit a thin layer of other metals or alloys on the surface of certain metals. According to the composition of the coating, electroplating processes can be divided into five categories: chrome plating, copper plating, cadmium plating, tin plating, and zinc plating. Next, I will provide you with a detailed explanation of the principles and characteristics of the five major categories of electroplating processes.

1. Chrome plating

Chromium is a silver white metal with a slight sky blue tint. Although the electrode potential is very negative, it has strong passivation performance and quickly passivates in the atmosphere, showing the properties of precious metals. Therefore, the chrome plating layer on iron parts is a cathode coating. The chromium layer is very stable in the atmosphere and can maintain its luster for a long time. It is also very stable in corrosive media such as alkali, nitric acid, sulfides, carbonates, and organic acids, but soluble in hydrochloric acid and other hydrogen halide acids, as well as hot concentrated sulfuric acid.

The chromium layer has high hardness, good wear resistance, strong reflective ability, and good heat resistance. There is no significant change in gloss and hardness below 500 ° C; Oxidation discoloration begins when the temperature exceeds 500 ° C; Softening only begins when the temperature exceeds 700 ° C. Due to the excellent performance of chrome plating, it is widely used as an outer layer and functional coating for protective and decorative coating systems.

2. Copper plating

The copper plating layer is pink in color, soft in texture, and has good ductility, conductivity, and thermal conductivity. It is easy to polish and can be decorated with colors such as antique copper, copper green, black, and natural color after appropriate chemical treatment. Copper plating is prone to losing its luster in the air and reacts with carbon dioxide or chloride to form a layer of basic copper carbonate or chloride film on the surface. When exposed to sulfides, it produces brown or black copper sulfide. Therefore, as a decorative copper plating layer, it needs to be coated with an organic coating on the surface.

3. Cadmium plating

Cadmium is a soft metal with a silver white luster, harder than tin and softer than zinc, with good plasticity and easy forging and rolling. Cadmium has chemical properties similar to zinc, but it does not dissolve in alkaline solutions. It is soluble in nitric acid and ammonium nitrate, and dissolves slowly in dilute sulfuric acid and dilute hydrochloric acid. Cadmium vapor and soluble cadmium salts are both toxic, and strict prevention of cadmium pollution is necessary. Due to the significant harm and high cost of cadmium pollution, galvanized or alloy coatings are commonly used to replace cadmium plating. The types of cadmium plating solutions commonly used in domestic production include ammonium carboxylate complex cadmium plating, acidic sulfate cadmium plating, and cyanide cadmium plating. In addition, there are cadmium plating with pyrophosphate, alkaline triethanolamine, and HEDP.

4. Tin plating

Tin has a silver white appearance, with an atomic weight of 118.7, a density of 7.3g/cm ^ 3, a melting point of 231.89 ℃, and atomic valences of divalent and tetravalent. Therefore, the electrochemical equivalents are 2.12g/A · h and 1.107g/A · h, respectively. Tin has the advantages of corrosion resistance, non toxicity, easy soldering, softness, and good ductility. Tin plating has the following characteristics and uses:

(1) High chemical stability;

(2.) In the electrochemical process, the standard potential of tin is opposite to that of iron, which is a cathodic coating for steel. Only when the coating has no pores can it effectively protect the substrate;

(3.) Tin has good conductivity and is easy to solder;

(4.) Tin begins to undergo variation during crystallization from -130 ℃, and by -300 ℃, it will completely transform into a crystalline isomer, commonly known as "tin plague", at which point it has completely lost its properties as tin;

(5.) Tin, like zinc and cadmium coatings, can grow into whiskers under high temperature, humidity, and sealed conditions, known as long hair; 6. After tin plating, re dissolving treatment in hot oil above 231.89 ℃ can obtain a glossy patterned tin layer, which can be used as a decorative coating for daily necessities.

5. Galvanization

Zinc is soluble in both acids and bases, hence it is called an amphoteric metal. Zinc hardly changes in dry air. In humid air, a basic zinc carbonate film will form on the surface of zinc. Zinc has poor corrosion resistance in atmospheres containing sulfur dioxide, hydrogen sulfide, and marine environments, especially in high-temperature, high humidity atmospheres containing organic acids, where zinc coatings are highly susceptible to corrosion. The standard electrode potential of zinc is -0.76V. For steel substrates, zinc coating belongs to anodic coating, which is mainly used to prevent corrosion of steel. The quality of its protective performance is closely related to the thickness of the coating.

After passivation treatment, dyeing or coating with light protectants, the protective and decorative properties of zinc coatings can be significantly improved. With the development of galvanizing technology and the use of high-performance galvanizing brighteners, galvanizing has shifted from simple protective purposes to protective decorative applications.

There are two types of galvanizing solutions: cyanide plating solution and cyanide free plating solution. Cyanide plating solutions can be classified into micro cyanide, low cyanide, medium cyanide, and high cyanide types. Cyanide free plating solutions include alkaline zincate plating solution, ammonium salt plating solution, sulfate plating solution, and ammonia chloride free plating solution. Cyanide zinc plating solution has good plating ability, and the obtained coating is smooth and fine, which has been widely used in production for a long time. However, due to the high toxicity of cyanide and its serious environmental pollution, low cyanide, micro cyanide, and cyanide free zinc plating solutions have been adopted.