What is cryogenic corrosion? How does low-temperature corrosion occur?

Low-temperature corrosion: It refers to the sulfuric acid corrosion that occurs in the heat-exchanging surfaces at the rear of the boiler (economizer, air preheater). It is called low-temperature corrosion because the temperature of the flue gas and the tube walls in these rear heat-exchanging areas is low. 　　 Formation of low-temperature corrosion: Sulfur in the fuel burns to produce sulfur dioxide (S+O2=SO2). Under the action of a catalyst, sulfur dioxide is further oxidized to form sulfur trioxide (2SO2+O2=2SO3). SO3 reacts with water vapor in the flue gases to produce sulfuric acid vapor (SO3+H2O=H2SO4). The presence of sulfuric acid vapor significantly raises the dew point of the flue gas. Due to the low temperature of the air in the air preheater, the flue gas temperature in the preheater section is not high, and the wall temperature is often below the flue gas dew point. As a result, sulfuric acid vapor condenses on the heated surfaces of the air preheater, causing sulfuric acid corrosion. 　　 Low-temperature corrosion often occurs on air preheaters, but the economizer tubes can also be subject to low-temperature corrosion when the fuel contains high levels of sulfur, the excess air coefficient is high, the SO3 content in the flue gases is high, the dew point rises, and the feedwater temperature is low (with the high-pressure steam generator shut down).

Sulfuric acid corrosion of the boiler’s rear heating surfaces (economizer, air preheater) occurs; since the temperature of the flue gas and the tube walls in this area is low, it is referred to as low-temperature corrosion. Sulfur in the fuel burns to produce sulfur dioxide (S+O2→SO2); under the action of a catalyst, sulfur dioxide is further oxidized to form sulfur trioxide (2SO2+O2→2SO3). SO3 then reacts with water vapor in the flue gases to produce sulfuric acid vapor. The presence of sulfuric acid vapor significantly raises the dew point of the flue gas. For example, the water vapor partial pressure in the flue gas from fuel burners is approximately 0.008–0.014 Mpa, with a corresponding thermodynamic dew point of 41–52°C; when the concentration of sulfuric acid vapor in the flue gas is 10%, the dew point rises to 190°C. The temperature of the preheater tube wall is related to the flow rates and temperatures of the flue gas and air, and is approximately equal to the average temperature of the flue gas and air. Due to the lower temperature of the air at the bottom of the air preheater, the temperature of the flue gas there is not high, so the wall temperature is often below the dew point of the flue gas. Sulfuric acid vapor condenses on the heated surfaces of the preheater, causing sulfuric acid corrosion. Low-temperature corrosion often occurs in air preheaters, but the economizer tubes can also suffer from low-temperature corrosion when the fuel contains a high amount of sulfur, the excess air coefficient is high, resulting in a high level of SO3 in the flue gases and a high dew point, as well as when the feedwater temperature is low (for example, when the high-pressure feedwater heater is not in use). Solution: ND steel is currently the most ideal material for resisting corrosion caused by low-temperature sulfuric acid dew points, both domestically and internationally. For seamless pipes and plates made of 09CrCuSb (ND steel), the key performance criterion is that the corrosion rate after 24 hours of immersion in a 70°50% H2SO4 solution should not exceed 14 mg/cm·h. Compared to carbon steel, similar imported steels from Japan, and stainless steels, its corrosion resistance is 1.84 times that of Japanese CR1R steel, 2.97 times that of 1Cr18Ni9 steel, 8.63 times that of Corten steel, and 14.11 times that of Q235B steel. 09CrCuSb (ND steel) is a type of steel used in boilers ； Hot-rolled steel plates and steel pipes with excellent resistance to sulfuric acid dew point corrosion, developed for use in electric furnace heat exchange, flue pipes, chimneys, and similar applications. Its excellent resistance to sulfuric acid dew point corrosion, along with its very high cost-performance ratio, makes it fully capable of replacing stainless steel; it is the best material that can outperform stainless steel in terms of resistance to sulfuric acid dew point corrosion. ND steel holds great economic significance and aligns with today’s concepts of efficiency, longevity, energy savings, and environmental protection – as well as with the guidelines for sustainable development.

This post was last edited by dongjiang1001 on 2011-6-23 08:36; it should be a decrease in dew point.

Some of the end heating surface tubes of our company’s products suffer from pitted penetrative corrosion； The holes are irregularly distributed, far apart from each other, sometimes up to 2 meters ； Have any colleagues seen similar corrosion phenomena?

What was said on the second floor is correct – it’s low-temperature dew point corrosion, which causes severe damage to metals within a certain temperature range. Common solutions include raising the temperature at the exhaust outlet of the flue or using ND steel. However, raising the flue temperature reduces thermal efficiency. ND steel is not yet fully developed. To improve thermal efficiency, coatings resistant to dew point corrosion are applied to the inner surface of the furnace shell, as well as acid-resistant cast materials being used; but for the furnace tubes, only the first method can be employed.