How to protect boiler tube from corrosion?

As the most critical pressure-bearing and heat-transfer components in a boiler system, boiler tubes operate for long periods under high temperature, high pressure, corrosive media, and complex combustion atmospheres. Their corrosion condition directly determines boiler safety, thermal efficiency, and overall service life. Engineering practice indicates that boiler tube corrosion is rarely caused by a single factor; instead, it results from the combined effects of material selection, working medium, water quality, combustion conditions, and operation and maintenance practices.

Boiler tubes are a specialized category of steel tubes and are predominantly manufactured as seamless steel pipes. Consequently, corrosion protection for boiler tubes must be implemented as a full-life-cycle strategy, covering design and material selection, surface protection, chemical water treatment, operational control, and shutdown maintenance.

I. Overview of Boiler Tube Corrosion Mechanisms

Boiler tube corrosion can generally be divided into internal corrosion and external corrosion.

Internal corrosion is mainly caused by steam-water oxidation, alkaline corrosion, acid corrosion, electrochemical corrosion induced by dissolved oxygen and carbon dioxide, and hydrogen damage. Among these mechanisms, the breakdown of protective films under alkaline conditions, chloride ion penetration of passivation layers, and hydrogen atom diffusion along grain boundaries can significantly degrade mechanical properties and lead to premature tube failure.

External corrosion is dominated by high-temperature oxidation, sulfidation corrosion, reducing-atmosphere corrosion, and sulfuric acid dew point corrosion. These forms of corrosion are particularly severe when burning high-sulfur fuels or operating under SNCR denitrification conditions, where sulfates, sulfides, and complex chemical reactions accelerate metal substrate degradation.

II. Surface Treatment and Coating-Based Corrosion Protection

Surface treatment and protective coatings represent the first line of defense against boiler tube corrosion. Field experience shows that surface preparation quality can influence coating service life by more than 50%.

Prior to coating application, sandblasting, shot blasting, or pickling is typically employed to remove oxide scale, rust, and surface contaminants. Achieving a surface cleanliness level of Sa 2.5 or St 3.0 ensures uniform roughness and provides a reliable mechanical bonding base for coatings.

Commonly used protective coatings include high-temperature ceramic coatings, graphene-based anti-corrosion coatings, and high-temperature sealing or far-infrared radiation coatings. These coatings offer high density, strong resistance to acids and alkalis, good wear resistance, and favorable thermal conductivity. In addition to isolating corrosive media, they can improve heat transfer efficiency and reduce ash deposition and acid condensation risks.

During application, strict control of mixing ratios, spray layers, coating thickness, and curing conditions is essential. High-pressure airless spraying with multiple thin coats is typically adopted to achieve uniform, defect-free films. In addition, arc spraying technology, known for its high bonding strength, high deposition efficiency, and excellent on-site adaptability, is increasingly used for corrosion protection on boiler heating surfaces.

III. Chemical Corrosion Prevention and Water Quality Control

Chemical treatment is the core measure for mitigating internal corrosion of boiler tubes. The use of dedicated scale and corrosion inhibitors promotes the formation of dense, stable protective films on metal surfaces while simultaneously inhibiting scale deposition and dispersing corrosion products. The mechanisms involved include complexation solubilization, lattice distortion, dispersion stabilization, and passivation film reinforcement, all of which significantly reduce corrosion and scaling rates.

Equally important is rigorous water quality management. Boiler feedwater must be effectively deoxygenated and maintained within an appropriate pH range. Key parameters such as alkalinity, phosphate concentration, and chloride ion content should be strictly controlled. Continuous blowdown and routine water quality monitoring are essential to prevent salt concentration and localized corrosion.

IV. Corrosion Control During Boiler Operation

Operating conditions have a decisive impact on boiler tube corrosion rates. Optimizing combustion organization and accurately controlling furnace oxygen content and air distribution ratios can effectively prevent reducing atmospheres and high-temperature sulfidation corrosion. Proper burner maintenance and minimizing combustion deviation are critical to reducing localized overheating and corrosion.

From a fuel management perspective, controlled blending of high-sulfur and low-sulfur fuels, combined with real-time coal quality monitoring, can significantly reduce corrosion risks associated with sulfur fluctuations.

Additionally, installing online gas monitoring systems in corrosion-prone zones to track indicators such as CO and H₂S, and integrating them with air distribution control systems, can greatly enhance early warning and response capabilities. Rational soot-blowing strategies and controlled load change rates are also important to avoid sharp temperature fluctuations on heating surfaces.

V. Corrosion Prevention During Boiler Shutdown

Corrosion rates during boiler shutdown can exceed those during normal operation, making shutdown maintenance particularly critical.

External corrosion risks can be reduced by thoroughly cleaning ash deposits, maintaining dry and well-ventilated furnace conditions, and periodically operating fans to prevent moisture accumulation. For internal protection, wet preservation or dry preservation methods should be selected based on the expected shutdown duration to prevent oxygen- and moisture-induced corrosion.

Regular inspection and drainage of safety valves and discharge systems are also necessary to eliminate localized water accumulation and freezing hazards.

VI. Conclusion

Boiler tube corrosion prevention is not a single-technology solution but a systematic engineering approach encompassing material selection, surface protection, chemical water treatment, operational optimization, and shutdown management. Only through coordinated application of multiple technologies and comprehensive process control can corrosion rates be effectively reduced, boiler tube service life extended, and long-term safe and stable boiler operation ensured.

FAQ – Boiler Tube Corrosion Prevention

1. Which boiler tube materials offer better corrosion resistance?

Boiler tubes with higher chromium and molybdenum content generally provide improved resistance to high-temperature oxidation and sulfidation corrosion. For severe service conditions, alloy steel boiler tubes specified under standards such as ASTM A213 or EN 10216 are often preferred over conventional carbon steel tubes.

2. How can early-stage boiler tube corrosion be detected during operation?

Early corrosion can be identified through a combination of wall thickness monitoring, flue gas composition analysis (CO, SO₂, H₂S), temperature deviation tracking, and periodic non-destructive testing (UT, ECT). Online monitoring systems significantly improve early warning capability and help prevent unexpected tube failures.

3. How often should boiler tubes be inspected for corrosion?

Inspection frequency depends on operating conditions, fuel type, and water quality. Under normal conditions, comprehensive inspections are typically conducted annually or during scheduled shutdowns, while high-sulfur fuel or high-load operations may require more frequent localized inspections.