Underground corrosion monitoring

Underground corrosion monitoring is a key technology for evaluating the corrosion status of underground metal pipelines and facilities in complex soil environments. Its core purpose is to prevent leaks, environmental pollution, and safety accidents caused by corrosion, and to ensure the long-term safe operation of infrastructure.

Underground corrosion monitoring is a key technology for evaluating the corrosion status of underground metal pipelines and facilities in complex soil environments. Its core purpose is to prevent leaks, environmental pollution, and safety accidents caused by corrosion, and to ensure the long-term safe operation of infrastructure. The following is an introduction from five aspects: monitoring principles, methods, influencing factors, application scenarios, and development trends:

1、 Monitoring principle

Underground corrosion monitoring is based on the principles of electrochemistry and electromagnetics, which indirectly calculates the corrosion rate of metal materials by measuring electrochemical parameters (such as current, potential, resistivity) or changes in electromagnetic signals in the soil environment. For example:

Current attenuation method: Apply a specific frequency of AC signal to the pipeline, and the rate of current attenuation along the pipeline is related to the quality of the anti-corrosion coating. When the anti-corrosion layer is damaged, the current leaks at the damaged point, causing a change in the surrounding magnetic field gradient. By detecting the sudden change in magnetic field, the damaged point can be located.

Potential difference method: By measuring the difference between the energized and de energized potentials at different positions of the pipeline, the resistivity of the anti-corrosion layer is calculated to evaluate the overall protective performance.

Resistance method: Monitor the cross-sectional area changes of metal specimens caused by corrosion, and indirectly calculate the corrosion rate through resistance changes.

2、 Monitoring methods

The monitoring methods for buried corrosion can be divided into two categories: non-destructive testing and destructive testing, as follows:

Non destructive testing:

Multi frequency tube current method (PCM): using a transmitter to apply an AC signal to the pipeline, measuring the current attenuation rate through a receiver to determine the integrity of the anti-corrosion coating. Suitable for rapid screening of long-distance pipelines, with a positioning accuracy of ± 2.5%.

AC ground potential difference method: Insert a probe into the ground above the pipeline to measure the change in potential gradient. When the anti-corrosion layer is damaged, the potential gradient is the smallest directly above the damaged point, and the direction indicated by the arrow is reversed, which can accurately locate the damaged point.

Ultrasonic testing method: By using ultrasonic waves to detect and locate cracks, pores, etc. caused by corrosion inside the pipeline wall, risks are evaluated and diagnosed.

Magnetic leakage detection method: using magnetic principles to detect changes in pipeline wall thickness, suitable for detecting wall thinning and dents caused by internal and external corrosion.

Destructive testing:

Sampling analysis method: Sampling samples from pipelines for laboratory analysis to obtain data on corrosion product composition, corrosion rate, etc. Suitable for precise assessment of local corrosion conditions, but requiring damage to pipeline structure.

3、 Influencing factors

The corrosion state of buried metal pipelines is influenced by multiple factors such as soil environment, stray current, microbial activity, and material characteristics, as follows:

Soil environment: Soil porosity, moisture content, electrical resistivity, acidity, and salt content are key factors affecting corrosion. For example, soils with high salt content have higher electrical conductivity and stronger corrosiveness; Acidic soil (pH<6) accelerates the process of hydrogen depolarization and enhances corrosion.

Stray current: DC stray current from electrified railways, electrolytic cells, and other equipment may cause lead-acid salt corrosion at the inlet of the pipeline (cathode area), while directly causing iron dissolution at the outlet (anode area). A current of 1A can cause electrochemical dissolution of approximately 9kg of iron per year.

Microbial activity: Sulfate reducing bacteria and other microorganisms decompose organic matter in anaerobic environments to produce hydrogen sulfide, which reacts with metals to produce sulfide corrosion products, accelerating the local corrosion process.

Material characteristics: Pipeline materials (such as carbon steel, stainless steel), surface conditions (such as coating integrity), and stress distribution all affect the corrosion rate. For example, the connection between new and old pipelines can easily form corrosive macrocells due to differences in surface conditions.

4、 Application scenarios

Underground corrosion monitoring technology is widely used in fields such as petroleum, natural gas, chemical, and municipal engineering, with specific scenarios including:

Long distance oil and gas pipelines: Regularly inspect the integrity of the anti-corrosion coating to prevent leakage accidents caused by corrosion and ensure the safety of energy supply. For example, using PCM technology for segmented detection of cross regional gas pipelines, locating damaged points, and evaluating repair priorities.

Urban gas pipelines: For densely distributed underground gas pipelines in urban areas, the AC ground potential difference method is used to detect damage to the anti-corrosion layer and avoid the risk of explosion caused by gas leaks.

Subsea pipelines and underwater facilities: In the marine environment, resistance probes are used to monitor the corrosion rate of subsea pipelines in real time, providing data support for maintenance decisions.

Complex structure of industrial pipelines: Install corrosion monitoring sensors on parts of pipelines such as bends and tees that are susceptible to fluid erosion, assess local corrosion risks, and take targeted maintenance measures.