[Haichuan Corrosion Prevention Knowledge] The temperature range where boilers suffer the fastest high-temperature corrosion | It’s not the highest temperature, but this one

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In a certain thermal power plant, the water wall tubes were exposed to a furnace center temperature of over 1500°C, yet the tubes were not damaged. On the contrary, in the range of around 600°C, the corrosion-induced thinning reached 2 mm in less than two years. The confusion is: \"It’s fine in places with high temperatures, but why does it rot in places with low temperatures?\" ”In fact, the answer lies in the number 580°C. The area where high-temperature corrosion in boilers occurs fastest is never at the center of the flame, but rather within a specific \"corrosion temperature range\". 01 The temperature range with the fastest corrosion rate: 550–700°C; there is a \"golden corrosion temperature range\" for peak high-temperature corrosion at 580–590°C. Studies show that this temperature threshold generally lies between 550°C and 700°C, with the corrosion rate being highest at temperatures of 580°C–590°C. The same is true for the high-temperature superheater in waste incinerators; the corrosion rate reaches its maximum at temperatures of 600–700°C. Composite sulfate corrosion also occurs as a liquid phase that condenses on the tube walls in the range of 550–710°C, destroying the protective oxide film on the surface of the tube walls. Why are temperatures in the high tens of degrees the \"most deadly\"? Because in this range, the corrosion products are exactly in a liquid or semi-molten state. The complex sulfates formed by alkali metals and SO3 (such as Na3Fe(SO4)3, K3Fe(SO4)3) begin to melt at temperatures above 550°C, taking on the form of viscous molten salts that adhere to the surface of the tube walls. Its basic reaction is: alkali metal oxides react with SO3 to form M2SO4 (M=Na/K), which then reacts with Fe2O3 and SO3 to produce low-melting-point composite sulfates. Too low: it’s a solid, doesn’t stick well and reacts slowly ； If it’s too high, it’s gas, and it gets blown away directly. It is within this liquid range of several dozen degrees that the corrosive substances stick to the pipe walls and undergo repeated reactions; as a result, this condition can persist for several years. 02 Different furnace types have different temperature ranges; the corrosion temperature range varies slightly depending on the fuel and furnace type: for coal-fired boiler water walls, it is 550–700°C, with a peak value of 580–590°C. This is the most common area where high-temperature corrosion occurs, with a large number of empirical cases concentrated in this range. Waste incinerator superheater: 600-700°C. Waste incineration involves high chlorine content, with KCl/NaCl being the main agents causing corrosion. Compound sulfates melt into a liquid state at 550–570°C, giving a temperature range that is slightly higher than that of coal-fired boilers. Reducing atmosphere (H₂S corrosion): 300-500°C. Studies have shown that in a reducing atmosphere, when the concentration of H₂S in the flue gas exceeds 0.01%, it causes severe corrosion to steel, with the most intense corrosive effect occurring in the range of 300°C to 500°C. For every 50°C increase in the external surface temperature of the pipe wall, the degree of corrosion doubles. Biomass boiler superheater: The temperature range for alkali metal sulfide corrosion in stainless steel tube walls is 566–732°C. Biomass has a high content of alkali metals (potassium, sodium), as well as a high chlorine content. KCl volatilizes at high temperatures, and the resulting FeCl3 has a melting point of only 282°C, making it highly volatile and causing severe damage to the protective film. 03 Why is the high-temperature area actually “safe”? Although the temperature at the center of the furnace exceeds 1500°C, there are two reasons why that area is relatively safe: first, the corrosive substances are \"blown away\". At excessively high temperatures, the corrosion products are in gaseous form and cannot remain and accumulate on the tube wall surface; instead, they are carried away by the fast-flowing flue gas. Corrosion is a chemical reaction that requires the reactants to remain on the surface for a sufficient amount of time; too high a temperature prevents this. Second, the oxide film repairs itself quickly. At high temperatures, the oxide film on metal surfaces forms rapidly; as long as it remains intact, it can provide dynamic protection. In the range of 550–700°C, the oxide film is precisely dissolved and damaged by the corrosive molten salt, and its formation rate cannot keep up, resulting in a cycle of rapid destruction and slow repair. 04 Preventive measures: Knowing that the \"culprit lies at 580°C\" allows for appropriate countermeasures to be taken. Operational adjustments: Control the wall temperature so that it does not fall within the range where corrosion is most severe, and avoid operating at temperatures between 550–700°C for extended periods. Ensure proper air distribution to prevent the flame from scouring the wall surface. Control the excess air coefficient to reduce the reductive atmosphere. Material upgrades: Use corrosion-resistant alloy steels, such as TP347H austenitic stainless steel, which can be used at temperatures between 625–649°C. Apply anti-corrosion coatings to the water walls and superheaters. Although the initial investment is high, it avoids frequent shutdowns for tube replacement due to corrosion thinning, making it cost-effective for long-term operation. Additives: Kaolin can be added to biomass boilers to prevent the deposition of alkali metal chlorides. Burning coal with a high ash melting point reduces the risk of coking and corrosion. Finally, remember these three points: For high-temperature corrosion, temperature is key – 580–590°C is the most critical range. Liquid molten salts can cause tube wall fouling, leading to problems that persist for years. Avoid operating in peak temperature zones; material coating is crucial. The tube may not be damaged at high temperatures, but it can be severely corroded at moderate temperatures – don’t blame the highest temperatures for corrosion issues. Forward this to the guy who still thinks it was damaged due to \"too high furnace temperature\" – the area where corrosion occurs fastest at high temperatures is not the center of the flame at 1500°C, but rather the pipe walls at 580°C. Don’t pick the wrong enemy.