During prolonged operation, industrial boilers inevitably experience some degree of corrosion, which is a major factor affecting the service life of gas-fired boilers. Therefore, operators and maintenance personnel must promptly address boiler inspections and maintenance to avoid disrupting industrial production. This article primarily discusses how to avoid or prevent corrosion in gas-fired boilers.

To prevent corrosion in gas-fired boilers, it is essential to implement comprehensive measures targeting the three main types of corrosion: low-temperature corrosion, oxygen corrosion (chemical corrosion), and out-of-service corrosion:

1. Preventing Low-Temperature Corrosion (Acid Corrosion/Condensation Corrosion)

Water vapor and trace amounts of acidic gases produced by the combustion of natural gas condense into acidic liquid when the metal wall temperature falls below the dew point.

• Properly control the air-fuel ratio: Reduce the excess air coefficient during combustion (maintaining it around 1.01–1.02) to minimize SO₃ formation and lower the flue gas dew point temperature.

• Increase return water and wall temperatures: Through system adjustments or the installation of heat exchangers and pressure differential bypasses, maintain the boiler return water temperature above 70°C to ensure that the heat transfer surface wall temperature remains above the acid dew point, thereby preventing condensation.

• Select corrosion-resistant materials: Use ND steel, enameled tubes, or acid-resistant coated materials in condensation-prone areas of the boiler’s rear section.

2. Prevention and Control of Oxygen Corrosion in Feedwater and on the Water Side

Dissolved oxygen in water and substandard water quality can cause pitting or ulcer-like perforations in metal surfaces.

• Strictly control feedwater treatment and deoxygenation: Soften or demineralize the makeup water, and use thermal deoxygenation or chemical deoxygenating agents to reduce dissolved oxygen in the water to levels within national standards.

• Control boiler water pH and water quality: Maintain a feedwater pH greater than 7 and perform regular blowdown to prevent excessive salt content in the boiler water or severe scaling that could lead to under-scale corrosion.

• Control the makeup water rate: Strictly manage the system to eliminate leaks, spills, drips, and seepage, ensuring the softened water makeup rate complies with furnace standards.

3. Perform Corrosion Prevention Maintenance During Boiler Shutdown

When the boiler is shut down, moisture absorbed from the air by metal surfaces can form a water film that accelerates corrosion.

• Wet preservation (short-term shutdown): Fill the boiler with an alkaline solution containing chemicals such as sodium hydroxide or sodium carbonate, maintaining a pH of 10 or higher to passivate the metal surfaces.

• Dry preservation (long-term shutdown): Drain the boiler water completely, dry the boiler thoroughly, place a sufficient amount of desiccant (such as quicklime or anhydrous calcium chloride) inside, and tightly close all valves.

Therefore, during boiler operation, strictly follow the methods described above to minimize corrosion and extend the boiler’s service life. When purchasing a gas-fired boiler, be sure to select a manufacturer that offers appropriately sized units. The water used in the boiler must also be softened to ensure safe operation and enhance the boiler’s durability.