Industrial Corrosion and Protection--06 Corrosion by High-Temperature and High-Pressure Water

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Industrial Corrosion and Protection--06 Corrosion by High-Temperature and High-Pressure Water. In industrial production, there are many devices that use high-temperature and high-pressure water as their working medium; for example, high-pressure boilers. These boilers utilize water of high purity as the feedwater, which evaporates at temperatures of around 300°C, with the resulting steam being superheated to temperatures above 600°C. To improve thermal efficiency, the operating pressure of these devices is increasing steadily; the operating pressure of many boilers has reached over 10 MPa, while those classified as supercritical boilers have pressures exceeding 30 MPa. Clearly, for these devices operating under high temperature and pressure conditions, corrosion represents a significant threat, and corrosion-induced damage to the devices can have serious consequences. The corrosivity of completely pure high-temperature, high-pressure water is not severe, but industrial high-temperature water often contains impurities such as oxygen and salts, which significantly increases its corrosivity. Many materials that are resistant to corrosion in low-temperature industrial water will corrode and deteriorate rapidly in high-temperature, high-pressure water. The final corrosion product of Fe in hot water is Fe3O4. Under oxidizing conditions and with high water vapor pressure, the corrosion product FeOOH forms. Fe3O4 generally forms a good protective film on steel surfaces. Whether a dense Fe3O4 film can be formed is influenced by external factors such as dissolved oxygen levels, CI concentration, and pH value, and it is also related to the metal composition and structure. As the chromium content in steel increases, corrosion resistance improves significantly. The addition of aluminum is also effective in improving corrosion resistance. The surface oxide of type 18-8 stainless steel in high-temperature water is similar to Fe3O4. a-Fe2O3 is a reddish-brown oxide that does not possess protective properties like FeO does, in the case of a-Fe2O3. Its presence indicates the existence of dissolved oxygen in the boiler water. The corrosion product formed on the surface of copper alloys in cold water is micro-red cuprous oxide, Cu2O, whereas at high temperatures black copper oxide, CuO, is produced. Typically, CuO forms on the surface of Cu2O and has a grayish-black powdery appearance. In hot water, the oxide layer formed on the aluminum surface consists of two layers: the inner layer is amorphous Al2O3 along with mixed-oriented AIOOH, while the outer layer is AIOOH with aligned orientation. At the temperature of boiler water, the oxide film on nickel is dark gray and slightly powdery. In hot water, spinel-type oxides M3O are formed on the surface of nickel alloys containing Cr or Fe, where M are metal atoms of Cr, Fe, and Ni. 1. Dissolved oxygen. Dissolved oxygen in water is the primary factor affecting corrosion in high-temperature water. In the presence of dissolved oxygen, local corrosion such as pitting and crevice corrosion often occurs. Therefore, in practical operations, it is necessary to reduce the oxygen content in water as much as possible. As a general rule, the oxygen content in the feedwater of power generation boilers operating at pressures above 6 MPa should be below 7 ug/L; for power generation boilers with pressures between 3.8 and 5.8 MPa, the oxygen content in the feedwater should be less than 15 ug/L. In the case of industrial boilers operating at pressures between 1.6 and 2.5 MPa, the standard for oxygen content in the feedwater is less than 50 ug/L, in order to keep the corrosion rate within acceptable limits. There are various methods for deoxygenating boiler water, with thermal deoxygenation being the primary approach. Chemical deoxidation is commonly used in industrial boilers with small capacity, and it is also often employed as an auxiliary method in boilers with high operating parameters. Chemical deoxygenation methods include sulfite deoxygenation, hydrazine deoxygenation, and deoxygenation using steel scrap or tannins. 2. pH value. At room temperature, the corrosion rate of steel decreases significantly as the pH value of the water increases; similarly, at high temperatures, raising the pH value of the aqueous solution reduces corrosion in steel due to the formation of a stable Fe3O4 surface protective layer. Changes in pH value are adjusted by adding HCl and NaOH. When the pH value is maintained between 10 and 11, the amount of iron corrosion is minimal. Among the measures to prevent corrosion by hot water, in addition to deoxygenation and degassing mentioned above, controlling the pH value is the most important. In heating devices such as boilers that use high-temperature and high-pressure water as the working medium, alkaline substances such as sodium hydroxide are typically added to adjust the pH value of the water. Under normal conditions, boiler water containing a small amount of NaOH has little corrosive effect on steel. However, in certain localized areas (such as gaps in the boiler, beneath layers of corrosion products or deposits, and in areas with local overheating due to surface boiling), repeated evaporation and condensation cause NaOH to become locally concentrated. In these areas, the metal is rapidly corroded by the concentrated alkaline solution, resulting in alkali corrosion. When the alkali content increases to 25%, the corrosion rate increases sharply. Metal surface deposits and corrosion products are important factors that trigger alkaline corrosion. 3. Overheating. On heat transfer surfaces such as evaporation tubes, as water boils on the surface and a large number of vapor bubbles are formed, the presence of these vapor bubbles on the metal surface reduces its thermal conductivity, thereby increasing the risk of localized overheating. As the bubbles leave, the influx of solution causes the temperature in that area to drop. The rapid formation and destruction of bubbles result in sharp fluctuations in temperature on the metal surface (with temperature differences of about 10–15°C), which damages the oxide layer on the metal surface and accelerates corrosion. Another important cause of overheating is the accumulation of scale or corrosion product layers on the metal surface, which deteriorates heat transfer, raises the wall temperature of the pipes, and may even lead to pipe explosions. These situations occur mostly on the flame-heated side of the tube wall. Local overheating can also concentrate salts and alkalis in the water, leading to alkaline corrosion. 4. CO2 content. When carbon dioxide dissolves in water, the pH value decreases. The free CO2 content in pure water at room temperature is generally 1 ug/g, corresponding to a pH value of 5.5; at this point, the corrosion rate of iron begins to increase sharply. Carbonates in hot water also produce carbon dioxide upon thermal decomposition; therefore, the corrosion problem caused by carbon dioxide in hot water cannot be ignored. The corrosion caused by carbon dioxide is mostly uniform corrosion; the corrosion products are carried away by water flow or air currents, thereby reducing the quality of water vapor. In steam power equipment, the main source of carbon dioxide is the thermal decomposition of carbonates. Therefore, the main measure to prevent carbon dioxide corrosion is to reduce the alkalinity in water. Based on the characteristics of the water quality, methods such as neutralization and ion exchange resin treatment can be employed. The main materials used for pipelines and steam generators are austenitic stainless steel or Inconel 600 alloy (Ni-15Cr-7Fe), with a corrosion rate of approximately 1.55X10-3 mm/a. Carbon steel is also used in some areas, with a corrosion rate of approximately 1.27X102 mm/a. In pressurized water reactors, the primary water is under pressure; it does not boil, which prevents the concentration of chlorides and alkalis as well as stress corrosion cracking. The operating conditions for the primary water in pressurized water reactors are: temperature of 260–288°C, pH value of 1.0–11, dissolved oxygen level of less than 0.1 pg/g, and flow rate of 6.1–9.2 m/s. In secondary water environments, devices such as stainless steel tubular heat exchangers have experienced alkali cracking and chloride stress corrosion cracking.