Plastic-coated steel pipe, in simple terms, is a layer of plastic coating applied on the surface of the steel pipe. This seemingly simple structure combines the dual advantages of steel pipe and plastic. The steel pipe provides strong mechanical strength and rigidity, enabling it to withstand greater pressure and external force; while the plastic coating gives the steel pipe excellent corrosion resistance, wear resistance, and insulation, effectively extending the service life of the pipeline. This complementary advantage makes plastic-coated steel pipe stand out among many pipes and is widely used in construction, municipal administration, petroleum, chemical industry, electricity, mining, and other fields. The performance of plastic-coated steel pipe is not static, and its corrosion resistance depends to a large extent on the thickness of the coating. The thickness of the coating is like a solid line of defense, and the thickness of the line of defense directly affects its ability to resist corrosion. So, how does the coating thickness affect the corrosion resistance of plastic-coated steel pipes? What scientific principles are hidden behind this? Next, let us delve into the mystery between coating thickness and corrosion resistance.

First, the coating thickness of plastic-coated steel pipes is a key factor in corrosion prevention.
When the coating thickness is thin, it is difficult to effectively block the invasion of external corrosive media. Just like in some environments with high humidity, thin coatings cannot prevent the penetration of water for a long time. Once the water penetrates the coating and contacts the surface of the steel pipe, it will trigger an electrochemical reaction, causing the steel pipe to rust and corrode. Moreover, thin coatings are more likely to break when facing external mechanical impact or friction. Once the coating is damaged, the steel pipe will be directly exposed to the corrosive environment, accelerating the corrosion process.

Increasing the coating thickness is equivalent to thickening this protective barrier. Thicker coatings can greatly extend the path for corrosive media to penetrate into the surface of the steel pipe and significantly reduce the penetration rate of corrosive media. Taking industrial environments containing highly corrosive chemicals such as acids and alkalis as an example, thicker coatings can prevent chemicals from contacting steel pipes for a long time, thereby providing more lasting protection for steel pipes. At the same time, thick coatings also have stronger resistance to mechanical damage, and can better maintain integrity and continue to play their anti-corrosion role even when subjected to a certain degree of external impact or friction.

The influence of coating thickness on the corrosion resistance of plastic-coated steel pipes is comprehensive and crucial. Sufficient coating thickness is the cornerstone to ensure the long-term stable operation of plastic-coated steel pipe in various complex environments and to exert its excellent performance. However, the coating thickness is not the thicker the better. It is also necessary to comprehensively consider factors such as cost and construction technology. This requires us to deeply explore the specific quantitative relationship between coating thickness and corrosion resistance to find the best balance point.

Third, the corrosion resistance comparison of different thickness coatings of plastic-coated steel pipes
1. Thin coating is initial protection and potential risks: In the early stage of use, the plastic-coated steel pipe with thin coating can play a certain degree of protection on the steel pipe by virtue of its plastic coating characteristics. In some scenes with relatively mild corrosion environments, such as ordinary indoor dry environments, the thin coating can effectively isolate oxygen and a small amount of moisture in the air, prevent oxidation reaction on the surface of steel pipe, and keep good performance of steel pipe for a certain period of time. However, the protective ability of thin coating is limited after all. When facing a more complex or harsh environment, its disadvantages will gradually emerge. Taking the underground environment that has been in a humid environment for a long time as an example, it is difficult for the thin coating to resist the continuous penetration of moisture for a long time. Water will slowly pass through the tiny pores of the coating and contact the surface of the steel pipe. Once a water film is formed on the surface of the steel pipe, it will form a primary battery with the oxygen in the air, causing electrochemical corrosion. Moreover, the thin coating is easily damaged when subjected to external mechanical effects, such as slight collisions during construction or friction during transportation. Once the coating is damaged, the substrate of the steel pipe will be exposed to the corrosive medium, and the corrosion rate will accelerate sharply, resulting in serious rust on the steel pipe in a short period of time, greatly shortening the service life of the pipeline and increasing the subsequent maintenance cost and replacement risk.
2. Moderate thickness coating is a balance between performance and cost: Plastic-coated steel pipes with moderate-thickness coatings have found a more ideal balance between corrosion resistance and cost, and have become a common choice in many engineering projects. From the perspective of corrosion resistance, moderate-thickness coatings can provide more reliable protection for steel pipes. In general industrial and civil environments, such as urban water supply and drainage systems, ordinary chemical workshop internal pipelines, etc., this thickness of coating is sufficient to resist the erosion of common corrosive media such as weak acids and alkalis, water, oxygen, etc. Through reasonable process control, the moderate thickness coating can form a uniform and dense protective layer on the surface of the steel pipe, effectively prolonging the time for the corrosive medium to penetrate into the surface of the steel pipe, thereby significantly slowing down the corrosion rate of the steel pipe. In terms of cost, the moderate thickness coating does not require a large amount of coating and a complex construction process like the thick coating, thereby reducing the material cost and construction cost. Taking the water supply pipeline renovation project in a certain city as an example, plastic-coated steel pipes with moderate-thickness coatings were selected. During the years of use, these pipes have withstood the corrosion of groundwater and the scouring of water flow, and no obvious corrosion damage has occurred. At the same time, due to the use of moderate thickness coatings, compared with thick-coated steel pipes, certain costs have been saved in material procurement and construction costs, so that the entire project has achieved better economic benefits while ensuring the quality of the project. This fully illustrates the good balance between the performance and cost of moderate-thickness coated steel pipes, making them cost-effective in a wide range of application scenarios.
3. Thick coating is super protection and application scenarios: Thick-coated plastic-coated steel pipes have excellent corrosion resistance and can be called the "super protective shield" of steel pipes, especially suitable for extreme environments with high corrosion risks. In the field of marine engineering, the high salinity, high humidity, and complex marine microbial environment of seawater are extremely corrosive to pipelines. Thick-coated plastic-coated steel pipes can effectively block the erosion of seawater, salt, and marine organisms with their thick protective layer. Even when immersed in seawater for a long time, the thick coating can maintain its integrity for a long time, provide reliable protection for the steel pipe, and ensure the stable operation of the pipeline system. In the chemical industry, some factories need to transport media containing strong corrosive chemicals such as strong acids and alkalis. Thick-coated plastic-coated steel pipes can withstand the long-term scouring and erosion of these highly corrosive media, prevent the media from directly contacting the steel pipe, and thus avoid the risk of rapid corrosion of the steel pipe. For example, in some chemical plants that produce strong acids such as sulfuric acid and hydrochloric acid, thick-coated plastic-coated steel pipes are used to transport these strong acid media. They have maintained good performance for many years, greatly reducing the frequency of pipeline replacement and reducing safety hazards and production losses caused by pipeline corrosion and leakage. In some areas with severe pollution or adverse climatic conditions, such as industrial areas with frequent acid rain, and desert edge areas with strong winds and dry climates, thick-coated plastic-coated steel pipes can also play their advantages, effectively resisting the damage of harsh environments to pipes, and ensure the normal operation of the pipeline system.
4. Standards and testing of coating thickness
(1) Standard specifications: In the application field of plastic-coated steel pipes, the coating thickness is not determined arbitrarily, but has strict standards and specifications. Different industry standards and application scenarios have their own clear requirements for coating thickness. Taking the commonly used CJ/T120-2016 urban construction industry standard as an example, for water supply plastic-coated composite steel pipes, when the nominal diameter DN≤65mm, the coating thickness is required to be greater than 0.30mm; when DN≥80mm, the coating thickness is required to be greater than 0.35mm. This standard is formulated based on comprehensive consideration of multiple factors such as water quality safety and pipe service life of the water supply system. In the water supply system, steel pipes need to be in contact with water for a long time. If the coating thickness is insufficient, it is easy to cause corrosion of the steel pipe, which in turn affects the water quality and endangers human health. In the field of drainage, the GB/T23257-2009 national standard stipulates that the minimum thickness of the inner wall plastic coating layer of drainage plastic-coated steel pipes is 150μm and the maximum is 500μm; the minimum thickness of the outer wall plastic coating layer is 250μm and the maximum is 500μm. The environment faced by drainage pipes is relatively complex and may be exposed to various corrosive sewage and chemical substances in the soil, so there are corresponding specifications for coating thickness to ensure that the pipes can operate normally in harsh environments. In some special industrial application scenarios, such as petrochemicals and marine engineering, due to the extremely corrosive environment in which the pipelines are located, the requirements for the coating thickness of plastic-coated steel pipes are more stringent. Often, more detailed and strict internal enterprise standards or industry-recommended standards are formulated based on specific engineering needs and corrosion risk assessments.
(2) Detection method: At present, the commonly used coating thickness detection methods are mainly magnetic method, electromagnetic induction method and ultrasonic method.
Magnetic method: The magnetic method uses a magnetic thickness gauge for detection. Its principle is based on the change in magnetic flux between the probe of the magnetic thickness gauge and the coating on the surface of the steel pipe. When the probe is close to the coated steel pipe, the non-magnetic properties of the coating will change the magnetic flux between the probe and the steel pipe substrate. By measuring the change in this magnetic flux, the coating thickness can be accurately calculated. This method is simple and fast to operate and is suitable for non-magnetic coating thickness detection on most metal substrates. It is one of the most widely used coating thickness detection methods. There are many brands of magnetic thickness gauges on the market, such as the magnetic thickness gauge produced by Defelsko in the United States, which has a variety of probes with different measuring ranges to choose from, from 0-1500μm general-purpose probes to 0-625μm micro probes specifically used for measuring aluminum-based oxide films, and even 0-6mm and 0-13mm ultra-thick probes, which can meet the detection needs of different coating thicknesses.
Electromagnetic induction method: The electromagnetic induction method uses the principle of electromagnetic induction to measure the coating thickness. When the alternating magnetic field generated by the detection probe acts on the coated metal substrate, an induced current will be generated in the metal substrate, and the magnitude of the induced current is closely related to the coating thickness. By detecting the change in the induced current, the thickness of the coating can be obtained through circuit conversion and calculation inside the instrument. This method has high accuracy and reliability for the detection of some metal substrates with good conductivity and non-magnetic coatings.
Ultrasonic method: This method uses the difference in the propagation speed of ultrasonic waves in different media to measure the coating thickness. When the ultrasonic wave is emitted from the probe, propagates through the coating to the steel pipe substrate, and then reflects back to the probe, the coating thickness can be calculated based on the propagation time of the ultrasonic wave in the coating and substrate and the propagation speed in different media. The ultrasonic method is suitable for occasions with high requirements for coating thickness and strict precision, especially for some multi-layer coating structures or complex shapes of steel pipe coating thickness detection.
In the actual detection process, in order to ensure the accuracy and reliability of the test results, the operator needs to strictly follow the test standards and operating procedures and also needs to calibrate and maintain the test instrument regularly to ensure that the performance of the instrument is in good condition.
(3) How to choose a plastic-coated steel pipe with a suitable coating thickness
In actual applications, it is very important to choose a plastic-coated steel pipe with a suitable coating thickness, which requires comprehensive consideration of many factors.
First, the use environment is the key determining factor. If it is in a dry and less corrosive environment indoors, such as the water supply and drainage system of an ordinary residential building, a plastic-coated steel pipe with a thin coating or a moderate thickness coating may be sufficient to meet the needs. The indoor environment is relatively stable, without corrosive media such as strong acids and alkalis. The thin coating can protect the steel pipe to a certain extent, and the cost is relatively low.
Secondly, the cost budget is also a factor that cannot be ignored. The material cost and processing cost of thin-coated plastic-coated steel pipes are relatively low, which is suitable for small projects or short-term projects that are more cost-sensitive and have a less harsh use environment. For example, for some temporary water pipes for construction, the use of thin-coated plastic-coated steel pipes can reduce the initial investment cost while meeting basic needs.
Finally, the expected life also affects the choice of coating thickness. If you want the pipeline to be used for a long time, such as the permanent water supply and drainage network in the city, the expected service life may be as long as several decades, then you need to choose a plastic-coated steel pipe with sufficient coating thickness to ensure that it can effectively resist corrosion for a long time and reduce the high cost and inconvenience caused by frequent replacement of pipes in the later stage.
When choosing the coating thickness of plastic-coated steel pipes, it is necessary to comprehensively weigh factors such as the use environment, cost budget, and expected life span, so as to make reasonable decisions based on local conditions to achieve the best economic benefits and use effects.

Summary: Plastic-coated steel pipes, with their characteristics of integrating the advantages of steel pipes and plastics, occupy an important position in pipeline systems in many fields, and the coating thickness, as a key factor affecting its corrosion resistance, plays an important role that cannot be ignored. From basic principles to practical applications, from the performance comparison of coatings of different thicknesses to the standards and testing of coating thickness, to how to choose plastic-coated steel pipes with suitable coating thickness, each link is closely linked to jointly build the anti-corrosion system of plastic-coated steel pipes.
Although thin coatings can provide certain protection in the early stage, there are many risks in the face of complex environments; moderate thickness coatings achieve a good balance between performance and cost, and are suitable for most conventional projects; thick coatings show super strong protection in extremely high-corrosion environments. In practical applications, we must scientifically and reasonably select the coating thickness of plastic-coated steel pipes based on multiple factors such as the specific use environment, cost budget, and expected life span. Only in this way can we give full play to the advantages of plastic-coated steel pipes, ensure the safe, stable, and efficient operation of the pipeline system during long-term use, and provide a solid and reliable foundation for the development of various industries. In future engineering construction and industrial production, I hope everyone can pay attention to the selection of coating thickness of plastic-coated steel pipes so that this high-quality pipe can better serve our lives and work.