How is the corrosion resistance of boiler tubes?

The corrosion resistance of boiler tubes is one of its important performance indicators. Boiler tubes are usually exposed to high temperature, high pressure and corrosive media, such as water vapor, flue gas and chemical media, so the corrosion resistance requirements for boiler tubes are very high.

The corrosion resistance of boiler tubes depends on the material, working environment, medium composition and protective measures. Different types of boiler tubes have significant differences in corrosion resistance under high temperature, high pressure, acid, alkali or oxidizing environments. The following is a detailed analysis:

Common materials for boiler tubes:

Commonly used boiler tube materials include stainless steel, carbon steel, alloy steel, copper alloy, etc. Among them, stainless steel has good corrosion resistance to a certain extent, because its main components are chromium and nickel, which can form a dense oxide film in an oxidizing environment, effectively preventing the erosion of corrosive media.

For some special media and higher corrosion resistance requirements, alloy steel and copper alloy are often used to manufacture boiler tubes. For example, titanium alloy has excellent corrosion resistance and can be used for a long time in harsh environments such as acidic and alkaline media and seawater; nickel alloy has excellent high temperature resistance and corrosion resistance and is widely used in high temperature and high pressure steam boilers.

Key factors affecting the corrosion resistance of boiler tubes:

1. Boiler tube material
Carbon steel (such as 20G, SA210): resistant to general water vapor corrosion, but susceptible to oxygen corrosion and acidic medium erosion.
Low alloy steel (such as 15CrMoG, 12Cr1MoVG): improves oxidation resistance and sulfur corrosion resistance through Cr and Mo elements.
Stainless steel (such as 304H, 321H, 316L): high Cr/Ni content, acid, alkali and chloride corrosion resistance, but high cost.
Duplex steel (such as 2205): resistant to chloride ion stress corrosion cracking (Cl-SCC), suitable for seawater or high salt environment.

2. Working environment

Temperature: high temperature (>400℃) accelerates oxidation and sulfidation corrosion.
Pressure: high pressure may aggravate stress corrosion cracking (SCC).

Medium:
Acidic medium (such as H₂S, CO₂) → prone to hydrogen embrittlement or sulfide corrosion.
Alkaline environment (such as NaOH) → may cause alkali embrittlement (caustic embrittlement).
Containing chloride ions (Cl⁻) → stainless steel is prone to pitting or Cl-SCC.

3. Protective measures

In order to improve the corrosion resistance of boiler tubes, some protective measures can also be taken. For example, the corrosion resistance of boiler tubes can be increased by surface treatment and coating, such as galvanizing, sandblasting, spraying, etc. In addition, regular maintenance and care of boiler tubes are also important means to ensure their corrosion resistance, timely clean up the sediment in the pipeline and prevent the formation of rust.

Inner wall coating/lining: such as enamel, epoxy resin coating, acid and alkali corrosion prevention.
Chemical water treatment: control pH value (8.5~9.2), deoxygenation (<7 ppb) to reduce oxygen corrosion.
Cathode protection: used for seawater or underground pipelines.

Typical corrosion types and solutions:

1. Oxygen corrosion (pitting)
Phenomenon: pits appear on the inner wall of the pipe, commonly seen in water supply systems.
Solution: deaerator + chemical deoxidizer (such as sodium sulfite).

2. Acid corrosion (H₂S/CO₂)
Phenomenon: uniform thinning or hydrogen-induced cracking.
Solution: select Cr-Mo alloy steel (such as T22) and inject corrosion inhibitor.

3. Alkali corrosion (caustic embrittlement)
Phenomenon: cracks near the weld, often seen in high-pressure boilers.
Solution: control NaOH concentration and use phosphate treatment.

4. Chloride ion corrosion (stainless steel)
Phenomenon: pitting or stress corrosion cracking.
Solution: switch to duplex steel or 316L to reduce Cl⁻ concentration (<50 ppm).

Causes and solutions for rust in boiler tubes:

Rust in boiler tubes may be related to the following aspects:

1. Water quality problems: Water contains a large amount of oxygen, carbon dioxide and other substances, which can easily cause corrosion in boiler tubes.
2. Scale accumulation in boiler tubes: Long-term scale accumulation in boiler tubes will form rust spots, and scale accumulation will also affect the normal operation of the boiler.
3. Boiler tube material problems: If inferior materials are used or there are defects on the surface of the tubes, it will also cause comprehensive rust in the boiler tubes.

How to solve the problem of rust in boiler tubes?

1. Improve water quality: Improving the quality of water and reducing impurities in water can effectively reduce the occurrence of rust in boiler tubes.
2. Clean the scale layer in boiler tubes: Regularly cleaning the scale layer in boiler tubes can effectively reduce the occurrence of rust in boiler tubes.
3. Replace pipes: If the overall rust in the boiler tubes is caused by pipe quality problems, high-quality pipes need to be replaced in time.

Boiler tube selection recommendations:
Carbon steel boiler tube: low cost, but requires strict water treatment (anti-oxidation/acid corrosion).
Stainless steel/alloy steel: good corrosion resistance, suitable for harsh working conditions, but high price.

Key measures:
Control medium composition (Cl⁻, O₂, pH value).
Regular testing + protective coating.

In general, the corrosion resistance of boiler tubes is a key factor in ensuring the safety of boiler operation and extending the service life. Selecting suitable materials, taking appropriate treatment measures and regular maintenance can effectively improve the corrosion resistance of boiler tubes and ensure the reliable operation of boiler systems.

Read more: Boiler Tube Size Chart or ASTM standard for Low and Medium Pressure Carbon Steel Boiler Tubes