Galvanized pipe, the full name of galvanized steel pipe, is a metal pipe that is anti-corrosion reinforced by applying a galvanized layer on the surface of carbon steel pipes. It not only retains the high strength, high rigidity and easy processability of carbon steel pipes, but also greatly improves corrosion resistance with the sacrificial anode protection of the galvanized layer. It has become a traditional core pipe in the fields of municipal engineering, building water supply and drainage, agricultural irrigation and other fields. From water supply pipelines in old communities to outdoor steel structure supports, from farmland irrigation pipe networks to fire protection system pipelines, the galvanizing process and material specifications of galvanized pipes directly determine their applicable scenarios and service life. This article will provide an in-depth analysis of the anti-corrosion principles, classification system, core characteristics and key points of selection of galvanized pipes, and provide professional guidance for practical applications and quality control.
1. Core anti-corrosion principles and process differences of galvanized pipes

The core of the anti-corrosion performance of galvanized pipes comes from the synergistic effect of the galvanized layer and the substrate. Different galvanizing processes determine the coating thickness, adhesion and anti-corrosion effect, adapting to differentiated use environments.

(1) Core anti-corrosion principles

The anti-corrosion mechanism of galvanized pipes is mainly divided into two layers of protection: one is physical isolation protection, a dense galvanized layer covers the surface of the carbon steel pipe, isolating air, moisture and corrosive media from contact with the substrate, preventing oxidation and rust of the steel; the other is sacrificial anode protection, the electrode potential of zinc is lower than that of iron. When the galvanized layer is damaged, zinc will preferentially undergo oxidation reactions to provide cathodic protection for the steel substrate and delay substrate corrosion. Even if the coating is partially damaged, it can still maintain a certain anti-corrosion capability. This dual protection feature increases the service life of galvanized pipes by 3-5 times compared with ordinary carbon steel pipes.

(2) Mainstream galvanizing processes and characteristics

At present, industrial mass production of galvanized pipes mainly uses two processes: hot-dip galvanizing and cold-dip galvanizing. The two processes have significant differences in production process, coating performance and cost, and each has its own focus on applicable scenarios:

1. Hot-dip galvanizing (hot-dip galvanizing) process: The carbon steel pipe after pickling and rust removal is immersed in molten zinc liquid (temperature is about 450°C), so that a zinc alloy layer is evenly attached to the surface of the steel pipe, and a dense galvanized layer is formed after cooling. The advantages of this process are the uniform thickness of the galvanized layer (usually above 85 μm), extremely strong adhesion, excellent anti-corrosion performance, ability to withstand outdoor moisture and slightly corrosive environments, and a service life of up to 15-20 years. Hot-dip galvanized pipes are currently the most widely used type and are suitable for municipal water supply and drainage, outdoor steel structures, fire pipes, agricultural irrigation and other scenarios. However, the production process is complex, the cost is slightly higher, and the appearance surface is relatively rough.

2. Cold galvanizing (electro-galvanizing) process: Zinc ions are deposited on the surface of pre-treated carbon steel pipes through electrolysis to form a galvanized layer. The thickness of the cold galvanized layer is thin (usually 10-30μm), the adhesion is weak, and the anti-corrosion performance is far inferior to that of hot-dip galvanized pipes. It can only be used in indoor dry environments or temporary protection scenes, such as indoor electrical wiring pipes, temporary pipeline supports, etc. Its advantages include short production cycle, low cost, and high surface finish. However, if used outdoors or in humid environments, it is very prone to coating peeling and matrix corrosion problems. It has been gradually replaced by hot-dip galvanized pipes.

2. Classification system and core characteristics of galvanized pipes

The classification of galvanized pipes can be divided according to galvanizing process, material specifications, and connection methods. There are obvious differences in the mechanical properties and application scope of different categories of products, forming a complete product matrix.

(1) Classification according to material specifications

The material of galvanized pipes is mainly low carbon steel, and the specifications are divided by outer diameter, wall thickness and nominal diameter, which directly affects the pressure-bearing capacity and applicable scenarios of the pipe:

1. Material classification: The core uses low-carbon steel such as Q235 and Q355. This type of steel has good plasticity, toughness, excellent welding performance, and is suitable for galvanizing and subsequent processing. Among them, Q235 galvanized pipe is cost-effective and suitable for low-pressure working conditions (such as water supply and drainage, irrigation); Q355 material is stronger and suitable for medium-pressure pipelines, steel structure supports and other scenarios that require high mechanical properties.

2. Specification classification: According to the nominal diameter (DN), it can be divided into small diameter (DN15-DN50), medium diameter (DN65-DN150), and large diameter (DN200 and above). Small-diameter galvanized pipes are often used for indoor water supply and drainage and electrical wiring; medium-diameter pipes are suitable for municipal pipe networks and fire protection systems; large-diameter pipes are mostly used for industrial pipes and farmland irrigation main pipes. In terms of wall thickness, it is divided into ordinary wall thickness and thickened wall thickness. The thickened type can withstand higher pressure and is suitable for high-pressure working conditions.

(2) Classification by connection method

The connection method directly affects the construction efficiency and sealing performance. It is mainly divided into three categories: threaded connection, flange connection and welded connection:

1. Threaded connection: Connected to pipe fittings (joints, valves) through the thread at the end of the pipe. It is suitable for small-diameter galvanized pipes (DN15-DN100). It is easy to construct and flexible to disassemble. It is widely used in indoor water supply and drainage, fire branch pipes and other scenarios. Sealing packing (such as raw material tape, sealant) must be used when connecting to ensure sealing performance and avoid leakage.

2. Flange connection: The flange is welded to the end of the pipe, and the connection is achieved by tightening the flange with bolts. It has strong sealing and high pressure-bearing capacity. It is suitable for medium and large diameter pipes and high-pressure working conditions (such as industrial pipelines and municipal main pipes). Its advantages are firm connection and easy maintenance, but the construction cost is high and it takes up a lot of space.

3. Welding connection: Pipes and fittings are directly connected by welding. The connection strength is extremely high and is suitable for large diameter, high pressure and highly corrosive scenarios. However, welding will destroy the galvanized layer at the interface, and the interface needs to be re-galvanized or anti-corrosion treated after welding. Otherwise, the interface will easily corrode preferentially, affecting the overall service life.

3. Key points for selection and quality control of galvanized pipes

The selection of galvanized pipes needs to be comprehensively judged based on the use environment, pressure requirements, and construction conditions. At the same time, quality inspection is done to avoid hidden engineering risks caused by inferior products.

(1) Scientific selection principles

1. Selection according to environment: In outdoor humid environments, coastal areas, and mild corrosion scenarios, hot-dip galvanized pipes are preferred, and products with thick zinc layers must be selected (zinc layer thickness ≥ 100 μm); in indoor dry environments and temporary pipelines, cold-dip galvanized pipes can be used to reduce costs; in drinking water transportation scenarios, food-grade hot-dip galvanized pipes must be selected to ensure that no harmful impurities are precipitated from the galvanized layer, in line with the GB/T 3091-2015 standard.

2. Selection according to pressure: For low-pressure working conditions (≤1.0MPa), such as indoor water supply and drainage, choose ordinary wall thickness Q235 galvanized pipes; for medium and high-pressure working conditions (1.0-2.5MPa), such as fire protection systems and industrial pipelines, choose thickened wall thickness Q355 hot-dip galvanized pipes, along with pipe fittings that match the pressure level.

3. Selection according to construction: In scenarios where indoor space is small and frequent maintenance is required, small-diameter galvanized pipes should be used for threaded connections; for outdoor large-diameter, high-pressure pipelines, flange connections or welded connections should be used. After welding, the interfaces must be anti-corrosion treated.

(2) Key points of quality inspection

1. Appearance quality inspection: The galvanized layer on the surface of high-quality galvanized pipes is uniform, without defects such as missing plating, bottom exposure, peeling, bulging, color difference, etc., and the color is uniform silver-white or off-white; there are no scratches, pits, or deformations on the surface of the pipe, the end cuts are flat, and the threads are clear and undamaged.

2. Galvanized layer performance testing: Tested by a zinc layer thickness meter, the zinc layer thickness of hot-dip galvanized pipes must be ≥ 85 μm, and cold-dip galvanized pipes ≥ 10 μm; the adhesion can be verified by a tapping test, and the coating does not fall off or peel after tapping; the corrosion resistance can be tested by a salt spray test, and the hot-dip galvanized pipe has no obvious rust after 48 hours of salt spray testing.

3. Material and standard verification: The supplier is required to provide a material certificate and test report to confirm that the material is Q235/Q355 and complies with national standards such as GB/T 3091 "Welded Steel Pipe for Low-Pressure Fluid Transport", GB/T 13912 "Metal Covering Technical Requirements and Test Methods for Hot-Dip Galvanized Layers of Steel Parts"; galvanized pipes for drinking water need to provide an additional health test report to ensure that they meet the safety requirements for food contact materials.

4. Typical applications and industry development trends of galvanized pipes

Galvanized pipes have long been dominant in many fields due to their high cost performance and reliable anti-corrosion performance. At the same time, with the upgrade of environmental protection requirements and technological innovation, they are gradually transforming towards high efficiency and environmental protection.

(1) Core application areas

1. Municipal and construction fields: accounting for more than 50% of galvanized pipe consumption, used in building indoor water supply and drainage, fire protection system pipes, gas branch pipes, outdoor street light brackets, community fences, etc. Among them, hot-dip galvanized pipes are the first choice for outdoor and humid environments due to their excellent anti-corrosion properties, while cold-dip galvanized pipes are mostly used for indoor electrical wiring pipes.

2. Agriculture and water conservancy fields: used in farmland irrigation pipe networks, reservoir water diversion pipes, drainage canal linings, etc., which need to withstand outdoor sun, rain, and soil corrosion. Hot-dip galvanized pipes are all used. In some scenes, additional anti-corrosion coatings are applied to strengthen protection.

3. Industrial and transportation fields: used for industrial low-pressure fluid transportation, steel structure support, truck carriage frames, rail transit contact network brackets, etc. Thickened hot-dip galvanized pipes are used for medium and high-pressure working conditions, and the welded interfaces are repaired and anti-corrosion treated.

(2) Industry development trends

Environmentally friendly upgrade and substitution: In the fields of drinking water transportation and high-end construction, galvanized pipes are gradually being replaced by stainless steel pipes and PE pipes to avoid the problem of impurity precipitation that may be caused by the aging of the zinc layer; however, in outdoor anti-corrosion, steel structures and other scenarios, hot-dip galvanized pipes are still difficult to replace due to their cost-effective advantages. Process optimization and innovation: develop cyanide-free galvanizing and low-energy hot-dip galvanizing process to reduce pollutant emissions during the production process while improving the uniformity and adhesion of the galvanized layer; promote zinc-aluminum alloy coating technology to further improve anti-corrosion performance and extend service life. Diversified functions: Develop composite galvanized pipes, such as galvanized + anti-corrosion coating, galvanized + anti-bacterial layer, to adapt to the needs of special scenarios and expand the application boundaries.

As a representative of traditional anti-corrosion pipes, galvanized pipes still play an irreplaceable role in industrial production and people's livelihood projects. In the future, as process upgrades and environmental protection requirements increase, galvanized pipes will continue to optimize performance, focus on advantageous scenarios, maintain market vitality in differentiated competition, and provide reliable pipe solutions for various fields.