The difference between seamless steel pipe coatings
1. Galvanizing of seamless steel pipes
Features: Zinc is relatively stable in dry air and does not change color easily. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form an oxide or alkaline zinc carbonate film, which can prevent the zinc from continuing to oxidize and play a protective role.
Zinc is highly susceptible to corrosion in acids, alkalis, and sulfides. The galvanized layer generally undergoes passivation treatment. After passivation in chromic acid or chromate solution, since the formed passivation film is not easy to interact with moist air, 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 must be removed. Copper and copper alloy parts do not need to be dehydrogenated.
Galvanizing has low cost, easy processing, and good effect. The standard potential of zinc is relatively negative, so zinc coating is an anodic coating for many metals.
Application: Galvanizing is commonly used in atmospheric conditions and other favorable environments. But it is not suitable for friction parts
2. Cadmium plating of seamless steel pipes
Features: For parts in contact with marine atmosphere or seawater and in hot water above 70°C, the cadmium coating is relatively stable, has strong corrosion resistance, and good lubricity. It dissolves very slowly in dilute hydrochloric acid, but is extremely soluble in nitric acid. Easily soluble, insoluble in alkali, and its oxides are insoluble in water. Cadmium plating is softer than zinc plating, has less hydrogen embrittlement and stronger adhesion. Under certain electrolysis conditions, the resulting cadmium plating is more beautiful than zinc plating. However, the gas produced when cadmium is melted is toxic, and soluble cadmium salts are also toxic.
Under normal conditions, cadmium acts as a cathodic coating on steel, and acts as an anodic coating in marine and high-temperature atmospheres.
Application: It is mainly used to protect parts from the atmospheric corrosion of seawater or similar salt solutions and saturated seawater vapor. Many aviation, navigation and electronic industry parts, springs, and threaded parts are plated with cadmium. It can be polished, phosphated and used as a base for paint, but it cannot be used as tableware.
3.Chrome plating of seamless steel pipes
Features: Chromium is very stable in humid atmosphere, alkali, nitric acid, sulfide, carbonate solutions and organic acids, and is easily soluble in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if the chromium layer serves as an anode, it is easily soluble in caustic soda solution.
The chromium layer has strong adhesion, high hardness, 800~1000V, good wear resistance, strong light reflectivity, and high heat resistance. It does not discolor below 480°C, starts to oxidize above 500°C, and has a hardness of 700°C. dropped significantly. Its disadvantages are that chromium is hard, brittle and easy to fall off, which is more obvious when subjected to alternating impact loads. And it is porous.
Metal chromium is easily passivated in the air to form a passivation film, thus changing the potential of chromium. Therefore, chromium becomes a cathodic coating on iron.
Application: It is not ideal to directly plating chromium on the surface of steel parts as an anti-corrosion layer. Generally, multi-layer electroplating (i.e. copper plating → nickel → chromium) can be used to achieve the purpose of anti-rust and decoration. It is currently widely used in improving the wear resistance of parts, repairing dimensions, light reflection and decorative lights.
4. Nickel plating of seamless steel pipes
Features: Nickel has good chemical stability in the atmosphere and alkali liquid, is not easy to change color, and is only oxidized when the temperature is above 600°C. It dissolves very slowly in sulfuric acid and hydrochloric acid, but is easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid and has good corrosion resistance.
Nickel plating has high hardness, is easy to polish, has high light reflectivity and can increase the appearance. Its disadvantage is that it is porous. To overcome this shortcoming, multi-layer metal plating can be used, with nickel as the middle layer.
Nickel is a cathodic plating on iron and anode plating on copper.
Application: Usually used to protect decorative coatings in order to prevent corrosion and increase aesthetics. Nickel plating on copper products is ideal for corrosion protection. However, because nickel is relatively expensive, copper-tin alloy plating is often used instead of nickel plating.