Electroplating is the process of plating a thin layer of other metals or alloys on the surface of certain metals using the principle of electrolysis. According to the composition of the coating, the electroplating process can be divided into five categories: chrome plating, copper plating, cadmium plating, tin plating, and zinc plating.
1. Chrome plating
Chromium is a silver-white metal with a slight sky blue color. Although the electrode potential is very negative, it has a strong passivation performance. It is quickly passivated in the atmosphere, showing the properties of precious metals, so the chromium plating layer of iron parts is a cathode plating layer. The chromium layer is stable in the atmosphere and can maintain its luster 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 hydrochloric acid and other hydrohalic acid and hot concentrated sulfuric acid. .
The chromium layer has high hardness, good wear resistance, strong light reflecting ability and good heat resistance. There is no obvious change in gloss and hardness below 500°C; it starts to oxidize and discolor when the temperature is higher than 500°C; it starts to soften when it exceeds 700°C. Due to the excellent performance of the chromium plating layer, it is widely used as the outer surface layer and functional coating layer of a protective decorative coating system.
2. Copper plating
The copper-plated layer is pink, soft, has good ductility, electrical conductivity and thermal conductivity, and is easy to polish. After proper chemical treatment, decorative colors such as bronze, copper green, black and natural colors can be obtained. Copper plating is easy to lose its luster in the air and react with carbon dioxide or chloride. A layer of basic copper carbonate or copper chloride film is formed on the surface. Under the action of sulfide, brown or black copper sulfide is formed. Therefore, it is used as a decorative The copper plating layer needs to be coated with an organic coating on the surface.
3. Cadmium plating
Cadmium is a silver-white shiny soft metal. Its hardness is harder than tin and softer than zinc. It has better plasticity and is easy to forge and roll. The chemical properties of cadmium are similar to zinc, but it does not dissolve in lye. It dissolves in nitric acid and ammonium nitrate, and dissolves very slowly in dilute sulfuric acid and dilute hydrochloric acid. Cadmium vapor and soluble cadmium salt are both poisonous, and cadmium pollution must be strictly prevented. Because cadmium pollution is very harmful and expensive, zinc plating or alloy plating is usually used instead of cadmium plating. The types of cadmium plating solutions that are frequently used in domestic production are: ammonia carboxy 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.
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°C, and an atomic valence of divalent and tetravalent, so the electrochemical equivalents are 2.12g/A.h and 1.107g/A.h, respectively. Tin has the advantages of corrosion resistance, non-toxicity, easy iron welding, softness and good ductility. The tin plating has the following characteristics and uses:
(1.) High chemical stability;
(2.) In the electrochemical sequence, the standard potential of tin is positive, which is a cathodic coating for steel, and the substrate can be effectively protected only when the coating has no porosity;
(3.) Tin has good conductivity and is easy to solder;
(4.) Tin begins to crystallize from -130°C to mutate, and to -300°C it will be completely transformed into a crystal form of allotrope, commonly known as «tin plague», which has completely lost the properties of tin at this time;
(5.) Like zinc and cadmium coatings, tin can grow into whiskers under high temperature, humidity and airtight conditions, which is called long hair; after tin plating, it can be re-dissolved in hot oil above 231.89°C to obtain gloss The patterned tin layer can be used as a decorative coating for daily necessities.
Zinc is soluble in acid and alkali, so it is called amphoteric metal. Zinc hardly changes in dry air. In humid air, basic zinc carbonate film will be formed on the surface of zinc. In atmospheres containing sulfur dioxide, hydrogen sulfide, and marine atmospheres, zinc has poor corrosion resistance, especially in high-temperature and high-humidity atmospheres containing organic acids, zinc coatings are 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 the corrosion of steel. The quality of its protective performance is closely related to the thickness of the coating.
After the zinc coating is passivated, dyed or coated with a light protectant, its protective and decorative properties can be significantly improved. With the development of the galvanizing process and the use of high-performance galvanizing brighteners, galvanizing has moved from purely protective purposes to protective-decorative applications.
There are two types of zinc plating solution: cyanide plating solution and non-cyanide plating solution. Cyanide plating solutions are divided into micro cyanide, low cyanide, medium cyanide, and high cyanide. Cyanide-free baths include alkaline zincate baths, ammonium salt baths, sulfate baths and ammonia-free chloride baths. The cyanide zinc plating solution has good coating ability and the obtained coating is smooth and meticulous, which has been used for a long time in production. However, due to the high toxicity of cyanide and serious environmental pollution, low-cyanide, micro-cyanide, and non-cyanide zinc plating solutions have tended to be used.