What are the differences between zinc plating, cadmium plating, chromium plating and nickel plating?
What is electroplating?
Electroplating is the process of plating a thin layer of other metals or alloys on the surface of some metals based on the principle of electrolysis. It is the process of attaching a layer of metal film on the surface of metal or other material parts by electrolysis, so as to prevent metal oxidation (such as corrosion), improve wear resistance, conductivity, reflective property, corrosion resistance (copper sulfate, etc.) and enhance beauty.
Electroplating is divided into copper plating, gold plating, silver plating, chromium plating, nickel plating and zinc plating and other specific processes, especially in the field of manufacturing, zinc plating, cadmium plating, nickel plating and chromium plating are most widely used. And there must be some difference between the four?
Features: zinc is relatively stable in dry air and not easy to change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form oxide or alkaline zinc carbonate film, which can prevent zinc from further oxidation and play a protective role.
Zinc is easily corroded in acid, alkali and sulfide. After passivation in chromic acid or chromate solution, the passivation film formed is not easy to interact with humid air, so the anti-corrosion ability is greatly enhanced. For spring parts, thin-walled parts (wall thickness < 0.5m) and steel parts requiring high mechanical strength, hydrogen removal must be carried out, while copper and copper alloy parts may not.
The standard potential of zinc is negative, so the zinc coating is anodic to many metals.
Application: zinc plating is widely used in atmospheric conditions and other good environments. But it is not suitable for friction parts.
The main purpose of galvanizing is to prevent rust. Zn is an active metal and can react with acid, so it has poor corrosion resistance and is the cheapest of the four electroplating methods.
Features: for parts in contact with marine atmosphere or sea water and in hot water above 70 ℃, cadmium coating is relatively stable, with strong corrosion resistance and good lubricity. It dissolves very slowly in dilute hydrochloric acid, but it is very easy to dissolve in nitric acid. It is insoluble in alkali and its oxides are insoluble in water. The cadmium coating is softer than the zinc coating, the hydrogen brittleness of the coating is smaller, and the adhesion is stronger. Moreover, under certain electrolytic conditions, the cadmium coating is more beautiful than the zinc coating. However, the gas produced by the melting of cadmium is toxic, and the soluble cadmium salt is also toxic.
In general conditions, cadmium is a cathodic coating on steel, and an anodic coating in marine and high temperature atmosphere.
Application: it is mainly used to protect the parts from the atmospheric corrosion of seawater or similar salt solution and saturated seawater vapor. Cadmium plating is widely used in aviation, navigation and electronic industrial parts, springs and threaded parts. It can be polished, phosphated and used as paint base, but it can not be used as tableware
Features: chromium is very stable in humid atmosphere, alkali, nitric acid, sulfide, carbonate solution and organic acid, and easily soluble in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if chromium layer is used as anode, it is easy to dissolve in caustic soda solution.
The chromium layer has strong adhesion, high hardness, 800 ~ 1000V, good wear resistance, strong light reflection, and high heat resistance. It does not change color below 480 ℃, starts to oxidize above 500 ℃, and the hardness decreases significantly at 700 ℃. Its disadvantage is that chromium is hard, brittle and easy to fall off, especially under alternating impact load. It is porous.
Chromium is easy to be passivated in air to form a passive film, which changes the potential of chromium. Therefore, chromium becomes a cathodic coating on iron.
Application: it is not ideal to use chromium plating directly on the surface of iron and steel parts as anti-corrosion coating. Generally, the purpose of anti rust and decoration can be achieved only by multi-layer electroplating (copper plating → nickel plating → chromium plating). At present, it is widely used to improve the wear resistance of parts, repair size, light reflection and decorative lamp.
Chromium plating mainly improves surface hardness, aesthetics and rust prevention. Chromium coating has good chemical stability, and does not work in alkali, sulfide, nitric acid and most organic acids, but can be dissolved in hydrohalic acid (such as hydrochloric acid) and hot sulfuric acid. Because chromium does not change color, it can keep its reflection ability for a long time, and is better than silver and nickel. The process is generally electroplated.
Plate with nickel
Characteristics: nickel has good chemical stability in the atmosphere and alkaline solution, and is not easy to change color. It is oxidized only when the temperature is above 600 ° C. It dissolves slowly in sulfuric acid and hydrochloric acid, but dissolves easily in dilute nitric acid. It is easy to passivate in concentrated nitric acid, so it has good corrosion resistance.
The nickel coating has the advantages of high hardness, easy polishing, high light reflection and beautiful appearance. In order to overcome this shortcoming, multi-layer metal coating can be used, and nickel is the intermediate layer.
Nickel is cathodic to iron and anodic to copper
Application: usually in order to prevent corrosion and increase aesthetic use, so it is generally used to protect decorative coating. Nickel plating on copper products is ideal for corrosion protection, but because nickel is more valuable, copper tin alloy is often used instead of nickel plating.
Nickel plating is mainly wear-resistant, anti-corrosion, rust, general thickness is thin, the process is divided into electroplating and chemical two kinds.
In terms of cost, chromium plating is the most expensive, followed by nickel, and zinc is the cheapest. Among them, hanging plating and barrel plating should also be distinguished. Hanging plating is expensive and barrel plating is cheap.
Chromium plating is bright white, nickel plating is a little yellowish, zinc plating is silvery white (in fact, there are color zinc, gray zinc, matte chromium, bright chromium, white nickel, black nickel, etc., the more you say, the more confused you are)
- 1. Electroplating production is mainly polluted by sewage and heavy metals in sewage. The state has strictly controlled the expansion of electroplating industry and reduced it year by year.
- 2. The main electroplating processes in China are galvanizing, copper plating, nickel plating and chromium plating, of which 50% are galvanizing and 30% are copper plating, chromium plating and nickel plating.
- 3. if the purpose is to prevent rust, galvanizing or cadmium plating can be used; if the focus is on preventing abrasion, nickel plating or chromium plating is the best choice.
Source: China Flanges Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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