Plastic plating-Classification of plating is classified according to coating composition
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Chrome
Chrome
Chromium is a silver-white metal with a slight blue color. Although the electrode potential is very negative, it has strong passivation properties and is quickly passivated in the atmosphere, showing the properties of precious metals. Therefore, the chromium plating layer of iron parts is a cathode coating. 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 acids, but it is soluble in hydrohalic acids such as hydrochloric acid and 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 and discoloration begin when temperatures are above 500°C; softness begins when temperatures are above 700°C.
Due to its excellent properties, chromium plating is widely used as an exterior layer and functional coating for protective and decorative plating systems.
copper plating
copper plating
The copper plating layer is pink, soft, has good ductility, electrical and thermal conductivity, and is easy to polish. After appropriate chemical treatment, decorative colors such as bronze, copper green, black and natural colors can be obtained. Copper plating easily tarnishes in the air. It reacts with carbon dioxide or chloride to form a basic copper carbonate or copper chloride film on the surface. Under the action of sulfides, brown or black copper sulfide will be formed. Therefore, as a decorative copper plating layer, an organic coating layer needs to be applied to the surface.
cadmium plated
Cadmium is a silvery white shiny soft metal. Its hardness is harder than tin and softer than zinc. It has good plasticity and is easy to forge and roll. Cadmium has chemical properties similar to zinc, but it does not dissolve in alkali liquor, dissolves in nitric acid and ammonium nitrate, and dissolves 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 instead of cadmium plating. At present, the types of cadmium plating solutions that are widely used in domestic production include: ammonium 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.
tinned
Tin has a silvery white appearance, with an atomic weight of 118.7, a density of 7.3 g/cm^3, a melting point of 231.89℃, and an atomic valence of two and four, so the electrochemical equivalent is 2.12 g/A.h and 1.107 g/A.h respectively. Tin has the advantages of corrosion resistance, non-toxicity, easy iron welding, flexibility and good ductility. Tin coating has the following characteristics and uses: 1. High chemical stability;2. The standard potential of tin in electrochemical sequence is a cathodic coating for steel. It can only effectively protect the substrate when the coating has no pores;3. Tin has good conductivity and is easy to weld;4. Tin begins to crystallize from-130℃, and will completely transform into a crystalline allotrope at-300℃, commonly known as "tin plague". At this time, the nature of tin has been completely lost; 5. Tin, like zinc and cadmium coatings, can grow into whiskers under high temperature, humidity and airtight conditions, called long hairs;6. After tin plating, it can be re-dissolved in hot oil above 231.89 ° C to obtain a shiny pattern tin layer, which can be used as a decorative coating for daily necessities.
galvanized
Zinc is easily soluble in acids and also soluble in alkali, so it is called an amphoteric metal. Zinc hardly changes in dry air. In humid air, a basic zinc carbonate film will form on the zinc surface. In atmospheres containing sulfur dioxide, hydrogen sulfide and marine nature, zinc has poor corrosion resistance, especially in atmospheres containing organic acids at high temperatures and high humidity, zinc coatings are easily corroded.
galvanized
The standard electrode potential of zinc is-0.76V. For steel substrates, zinc coating is an anodic coating. It is mainly used to prevent corrosion of steel. Its protective performance is closely related to the thickness of the coating.
After the zinc coating is passivated, dyed or coated with light protectors, its protective and decorative properties can be significantly improved. With the development of galvanizing process and the adoption of high-performance galvanizing brighteners, galvanizing has moved from purely protective purposes to protection-decorative applications.
There are two types of zinc plating solutions: cyanide plating solutions and non-cyanide plating solutions. Cyanide plating baths are divided into micro-cyanide, low-cyanide, medium-cyanide, and high-cyanide. Cyanide-free plating solutions include alkaline zincate plating solutions, ammonium salt plating solutions, sulfate plating solutions and non-ammonia chloride plating solutions. Cyanide zinc plating solution has good plating ability, and the resulting coating is smooth and meticulous, and has been used in production for a long time. However, due to the extremely toxic toxicity of cyanide and serious environmental pollution, low-cyanide, micro-cyanide and non-cyanide zinc plating solutions have tended to be used.
