Galvanizing requirements for seamless steel pipes

Talks about the characteristics and requirements of zinc-plating of seamless steel pipe. The appearance of zinc-plated layer of smls pipe is bluish white, and the standard electrode potential is -0.76V. It is easily soluble in acid and soluble in alkali. It is typical Amphoteric metal. Zinc hardly changes in dry air, and the critical humidity of zinc corrosion is greater than 70%. Therefore, it can interact with carbon dioxide and oxygen in humid atmosphere to form a film mainly composed of basic zinc carbonate, which has a certain slowness. Zinc reacts with sulfides such as hydrogen sulfide to form zinc sulfide; zinc is easily corroded by chloride ions, so it is unstable in seawater.

When the zinc of seamless steel pipe contacts with volatiles of non-metallic materials (low molecular carboxylic acid, aldehyde, phenol, ammonia, etc.) under airtight or poor ventilation and humid air conditions, zinc is easily corroded and white loose Corrosion products, commonly known as "hoarfrost". When zinc comes into contact with volatiles released from bakelite, paint and wood under high temperature, high humidity and airtight conditions, zinc tends to grow filamentous single crystals, commonly known as "long hair". Zinc has a negative standard potential than iron and is an anodic coating for steel, providing reliable electrochemical protection.

Zinc can be selected as a protective layer for steel products used in industrial atmosphere, rural atmosphere and marine atmosphere, and its protective life is almost proportional to the thickness of the zinc layer. In the atmosphere with industrial waste gas pollution, the corrosion resistance of zinc coating of seamless steel pipe is better than that of cadmium coating, and the opposite is true in sea or sea water. After the zinc coating is passivated by chromate, the corrosion resistance can be improved by 6-8 times. Therefore, galvanizing passivation treatment is an essential step.

Organic dyeing on passivation film can be used as protection for low-profile products---decorative coating. In order to reduce the thickness of the zinc coating and improve the corrosion resistance, the following two measures are adopted at home and abroad: one is to improve the quality of the passivation film, such as blue, green, military green, black passivation instead of the conventional iridescent passivation film , especially the military greening passivation film has good corrosion resistance; the second is to replace the pure zinc coating with a thinner Zn-Fe alloy containing 0.3%-0.6% iron, or a Zn-Ni alloy containing 6%-10% nickel, The corrosion resistance is improved by more than three times, and it is widely used in automobile steel plate instead of galvanized. The use temperature of the zinc coating of the seamless steel pipe shall not exceed 2500 degrees Celsius. At this temperature, the zinc crystal structure is deformed, brittle, and the corrosion resistance is reduced. Parts with corrosion resistance and heat resistance requirements should use Zn-Ni alloy layer.

Galvanizing electrolyte can be divided into two categories: alkaline and weak acid. Commonly used at home and abroad are cyanide zincate, cyanide zinc, ammonium salt, chloride zinc, sulfate zinc and so on. Domestically, cyanide-free zincate galvanizing is still the mainstream. In recent years, due to the improvement of the performance of galvanizing additives, chloride galvanizing and low-cyanide bright galvanizing have developed rapidly, while ammonium salt galvanizing continues to decline.

The selection of the plating solution depends on the material, shape and appearance requirements of the plated parts, such as small stamping and complex parts, especially for deep holes or tubular parts that require auxiliary anodes. Alkaline plating solution should be used; parts sensitive to hydrogen embrittlement should be used. Weak acid zinc plating solution. When two kinds of baths with different properties are used, it is very beneficial to the production of galvanized seamless pipes.