High-temperature corrosion treatment for valves in refining equipment

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The valves in oil refining plants operate under severe conditions, with high working temperatures. As these valves function continuously at such high temperatures, the metal materials suffer from corrosion, which results in a significant reduction in the mechanical strength and ductility of the materials, and may even lead to their destruction. The types of high-temperature corrosion include oxidation, sulfidation, carburization, decarburization, metal ashing, and nitridation. Below, the damage conditions of these corrosion types, the materials affected, the key factors in the damage mechanism, as well as the measures for prevention and mitigation are described separately. I. Oxidation (1) Damage condition: Oxygen reacts with carbon steel and other alloy steels at high temperatures, resulting in the formation of an oxide scale on the metal surface. Oxidation usually occurs in the oxygen-rich environment of heating furnaces and boiler combustion (about 20% of air). Most alloys, including carbon steel and low-alloy steel, experience overall thinning when subjected to oxidation. Typically, depending on temperature and exposure time, an oxide layer forms on the outer surface of the components. 300 series stainless steels and nickel-based alloys exposed to extremely high temperatures resulting in a high metal loss rate generally develop a thin layer of dark-colored oxide. (2) Affected materials: Iron-based materials, whether cast or forged, including carbon steel and low-alloy steel, are susceptible to oxidation. In addition, all 300-series stainless steels, 400-series stainless steels, and nickel-based alloys also undergo oxidation to varying degrees depending on their alloy composition and operating temperature. (3) Key factors The main factors affecting high-temperature oxidation are the operating temperature of the valve and the alloy composition. Carbon steel undergoes oxidation at temperatures above 538°C, and the rate of metal loss increases as the temperature rises. Generally, the oxidation resistance of carbon steel and other alloys is determined by the chromium content in the material. Increasing the chromium content results in a more protective oxide scale. The oxidation resistance of 300 series stainless steel reaches 816°C. The predicted corrosion rates for oxidation of common materials used in refinery plant valves are shown in Table 1 (below). Valves operating in high-temperature environments tend to undergo oxidation when the operating temperature exceeds about 538°C. (4) Prevention and mitigation: To counter oxidation, when selecting valve materials, they are usually upgraded to a more resistant alloy in order to achieve the best oxidation resistance. Furthermore, slowing down this process can also be achieved by controlling the chemical composition of the alloy. Chromium is the main alloying element that affects oxidation resistance; other alloying elements such as silicon and aluminum are equally effective, but their content must be limited depending on the actual conditions due to their adverse effects on mechanical properties. II. Sulfidation (1) Damage conditions: Corrosion of carbon steel and other alloys caused by the reaction of certain elements in the alloy with sulfides in high-temperature environments; the presence of hydrogen accelerates this corrosion. (2) Affected materials All iron-based materials, including carbon steel and low-alloy steel, as well as 300 series stainless steels and 400 series stainless steels, will be affected ； Nickel-based alloys are also affected to varying degrees, but their chemical composition has a significant impact on sulfidation, especially the chromium content. Copper-based alloys form sulfides at a lower temperature than carbon steel. (3) Key factors The main factors affecting sulfidation are alloy composition, valve operating temperature, and the concentration of corrosive sulfides. The sensitivity of an alloy to sulfidation is determined by its ability to form protective sulfide scales; the sulfidation temperature for iron-based alloys generally begins above 260°C. Figures 1 and 2 (as shown below) illustrate the effect of increasing temperature, chromium content, and sulfur content on sulfidation. Generally, the resistance of iron- and nickel-based alloys to sulfidation is determined by the chromium content of the material. Increasing the chromium content can significantly improve resistance to sulfidation. 300 series stainless steels, such as 304, 316, 321, and 347 grades, possess high resistance to sulfidation in most petroleum refining environments. Nickel-based alloys are similar to stainless steels, as a similar chromium content provides similar resistance to sulfidation. Crude oil, coal, and other hydrocarbons contain sulfur in varying concentrations; the total sulfur content is composed of many different sulfur-containing compounds. Sulfidation is primarily caused by H2S and other reactive sulfides produced by the thermal decomposition of sulfur compounds at high temperatures. Some sulfides are more prone to reacting to produce H2S. Therefore, predicting the corrosion rate in engineering based solely on the weight percentage of sulfur often leads to errors. (4) Prevention and mitigation: Pipeline valves operating in high-temperature, sulfur-containing fluid environments are subject to sulfidation, a phenomenon that typically occurs in the valves of catalytic cracking units, coking units, and hydrogenation units. High-temperature valves exposed to sulfur-containing gases are also affected. To prevent sulfidation in valves, higher chromium alloys are typically chosen as valve materials to achieve sulfidation resistance. Valves made of 300 or 400 series stainless steel can significantly improve resistance to high-temperature sulfurization corrosion. For valves manufactured from low-alloy steel materials, aluminum diffusion treatment is commonly applied to the valve components in order to reduce the sulfidation rate and thereby minimize scale formation; however, it should be noted that this method does not provide complete protection. III. Carburization (1) Damage condition: When in contact with carbon-containing materials or a carburizing environment, carbon diffuses into the metal material of the valve under high temperatures. (2) Affected materials include carbon steel and low-alloy steel, 300 series stainless steels and 400 series stainless steels, nickel-based alloys with a relatively high iron content (such as alloys 600 and 800), as well as HK/HP alloys. (3) Key factors: Three conditions must be met for carburization to occur: ① Exposure to a carburizing environment or carbon-containing materials ； ②A sufficiently high temperature that allows carbon to diffuse into the metal (usually above 593°C) ； ③Sensitive materials. Conditions favorable for carburization include high gas-phase carbon activity (gases such as hydrocarbons, coke, CO-rich gases, CO2, methane, and ethane) and low oxygen content (minimum O2 or steam). At the onset of carburization, carbon diffuses into the component at a high rate, and then this rate gradually decreases as the depth of carburization increases. Due to their higher chromium and nickel content, 300-series stainless steels have greater resistance to carburization than carbon steel and low-alloy steels. Carburization leads to a decrease in high-temperature creep ductility, reduced mechanical properties at room temperature (especially strength and ductility), as well as diminished weldability and corrosion resistance. (4) Prevention and mitigation: To prevent carburization, alloys with sufficient resistance to carburization are generally chosen; these include alloys containing elements (such as silicon and aluminum) that can form strong surface oxide or sulfide films. IV. Decarburization (1) Damage condition: Decarburization causes the removal of carbon and carbides from steel, resulting in a decrease in its strength. Decarburization occurs during heat treatment in high-temperature environments, including exposure to fire or in high-temperature gaseous environments. (2) Affected materials: carbon steel and low-alloy steel. (3) Key factors The key factors for decarburization are time, temperature, and carbon activity in the process fluid. Metallic materials must be exposed to a gas phase with low carbon activity; generally, the carbon in steel diffuses to the surface where it reacts with the components of the gas phase. The decarburization range and depth are related to temperature and exposure time. Generally, shallow decarburization reduces the strength of steel, but it has no adverse effect on the overall performance of the component. However, when steel is overheated, there are other effects, such as decarburization caused by high-temperature hydrogen attack on the valves of hydrocracking units. (4) Prevention and mitigation: Almost all valves exposed to high-temperature environments are susceptible to decarburization, and the valves in hydrocracking units and catalytic reforming units may also be affected. For preventing decarburization of valves under hydrogen service, appropriate alloys are generally selected in accordance with API RP 941 to prevent high-temperature hydrogen-induced decarburization. Alloy steels containing chromium and molybdenum form more stable carbides with carbon during smelting, thereby possessing greater resistance to decarburization. V. Metal Ashing (1) Damage Conditions Metal ashing is a form of carburization that occurs in carburizing gases or process fluids containing carbon and hydrogen, leading to an acceleration of localized pitting. Erosion pits usually form on the surface and may contain coal ash or graphite dust. (2) Affected materials: low-alloy steel, 300 series stainless steels, nickel-based alloys, and heat-resistant alloys. To date, no known metal alloys resistant to metal graying under all conditions have been found. (3) Key factors The key factors for metal graying are the process fluid composition, operating temperature, and alloy composition. Metal dusting occurs after carburizing, and is characterized by the rapid loss of metal. Metal ash formation involves a series of complex reactions that involve reducing gases such as hydrogen, methane, propane, or carbon monoxide. Metal ashing typically occurs in a temperature range of 482°C to 816°C. The damage increases with rising temperature. In high-nickel alloys, it is believed that no metal carbides are formed during metal ashing. Metal ashing also occurs under alternating reducing and oxidizing conditions. The mechanism behind metal graying is believed to be: ① the metal matrix becomes saturated through carburization ； ②Metal carbides precipitate on metal surfaces and grain boundaries ； ③Graphite deposits on the carbides on the metal surface ； ④Metal carbides decompose into metal particles and graphite ； ⑤Further deposition of graphite catalyzed by the surface metal particles. (4) Prevention and mitigation: Currently, no metal can withstand metal ashing under all conditions; material selection must be based on the specific application environment. Aluminum diffusion treatment of the valve component substrate may be beneficial in certain applications. VI. Nitriding (1) Damage condition: Some alloys exposed to high-temperature process fluids containing high levels of nitrogen compounds such as ammonia or cyanides will form a hard and brittle surface layer, especially under reducing conditions. (2) Affected materials: carbon steel, low-alloy steel, 300 series stainless steel, and 400 series stainless steel. (3) Key factors Nitriding is related to temperature, time, the partial pressure of nitrogen, and the metal composition; it is a process that enables nitrogen to diffuse into the metal matrix. The temperature must be high enough to allow nitrogen to thermally decompose from ammonia or other nitrides, while at the same time nitrogen can diffuse into the metal. Nitriding begins above 316°C, and becomes more severe above 482°C. High gas-phase nitrogen activity (high partial pressure of nitrogen) promotes nitriding. The corrosion resistance of metal materials may be adversely affected by nitriding. Nitriding may lead to a decrease in high-temperature creep strength, mechanical properties at ambient temperature (especially toughness and ductility), weldability, and corrosion resistance. (4) Prevention and mitigation: Nickel-based alloys possess resistance to nitriding; therefore, nickel-based alloy materials containing 30% to 80% nickel are typically used to manufacture valves in order to prevent nitriding. jsm86.com