Valve Corrosion & Protection under High-Temperature Conditions

In industrial production, valves, as key components for controlling fluid flow, often need to operate in high-temperature environments. However, high-temperature conditions pose severe challenges to the performance of valve materials. Metal materials are prone to corrosion at high temperatures, which not only reduces their mechanical strength and ductility but can also lead to valve failure, thereby triggering safety accidents. Therefore, understanding the types of high-temperature corrosion and their influencing factors, and taking effective protective measures, is crucial for ensuring the reliability and safety of valves.

Under high-temperature conditions, valve materials face various forms of corrosion. These types of corrosion have different characteristics and varying degrees of impact on the performance and service life of valves. The following are several common types of high-temperature corrosion.

Oxidation is the most common form of high-temperature corrosion. When the metal material of a valve comes into contact with oxygen at high temperatures, an oxide scale forms on the metal surface. This phenomenon typically occurs in the oxygen-rich environment of combustion in furnaces and boilers, where the oxygen content in the air is about 20%. Most alloys, including carbon steel and low-alloy steel, experience general thinning under oxidation. As the temperature rises and exposure time extends, the outer surface of valve components is covered by a layer of oxide scale. 300-series stainless steel and nickel-based alloys also form a thin, dark oxide layer at extremely high temperatures, resulting in excessive metal loss rates.

Sulfidation refers to the reaction of certain elements in valve alloys with sulfides in high-temperature environments, causing corrosion of carbon steel and other alloys. The presence of hydrogen accelerates this corrosion process. All iron-based materials, including carbon steel, low-alloy steel, 300-series stainless steel, 400-series stainless steel, and nickel-based alloys, are susceptible to sulfidation. However, the chemical composition of nickel-based alloys has a significant impact on sulfidation, especially the chromium content. Copper-based alloys form sulfides at lower temperatures than carbon steel. The main influencing factors of sulfidation include alloy composition, valve operating temperature, and the concentration of corrosive sulfides.

When the metal material of a valve comes into contact with carbon-containing materials or a carburizing environment, carbon diffuses into the metal material at high temperatures. Carbon steel, low-alloy steel, 300-series stainless steel, 400-series stainless steel, and nickel-based alloys with high iron content (such as Alloy 600 and 800) can all experience carburization. Carburization requires three conditions: exposure to a carburizing environment or carbon-containing materials; a sufficiently high temperature (generally above 593°C) to allow carbon to diffuse into the metal; and susceptible materials. Carburization leads to reduced high-temperature creep ductility, decreased room-temperature mechanical properties (especially strength and ductility), and reduced weldability and corrosion resistance.

Decarburization refers to the removal of carbon and carbides from valve steel in high-temperature environments, resulting in reduced steel strength. This process typically occurs during heat treatment in high-temperature environments, including exposure to fire or high-temperature gas atmospheres. Carbon steel and low-alloy steel are more prone to decarburization. The key factors for decarburization include time, temperature, and the carbon activity in the process fluid. Metal materials must be exposed to a gas phase with low carbon activity for the carbon in the steel to diffuse to the surface and react with the gas phase components. The extent and depth of decarburization are closely related to temperature and exposure time.

Metal dusting is a special form of carburization that typically occurs in carburizing gases or process fluids containing carbon and hydrogen, leading to accelerated local pitting. Pits usually form on the surface and may contain coal ash or graphite dust. Low-alloy steel, 300-series stainless steel, nickel-based alloys, and heat-resistant alloys can all experience metal dusting. No metal alloy has been found to resist metal dusting under all conditions. Metal dusting usually occurs in the temperature range of 482°C to 816°C, with damage increasing with temperature. Its mechanism includes the saturation of the metal matrix by carburization, the precipitation of metal carbides on the metal surface and grain boundaries, the deposition of graphite on the carbides on the metal surface, the decomposition of metal carbides into metal particles and graphite, and the further deposition of graphite catalyzed by the metal particles on the surface.

When some alloys are exposed to high-nitrogen-containing process fluids (such as ammonia or cyanides) at high temperatures, especially under reducing conditions, a hard and brittle surface layer forms on the surface. Carbon steel, low-alloy steel, 300-series stainless steel, and 400-series stainless steel can all be affected by nitriding. Nitriding is related to temperature, time, the partial pressure of nitrogen, and the metal composition, and is a process that allows nitrogen to diffuse into the metal matrix. Nitriding begins above 316°C and becomes more severe above 482°C. High gas-phase nitrogen activity (high partial pressure of nitrogen) promotes nitriding. Nitriding can lead to reduced high-temperature creep strength, room-temperature mechanical properties (especially toughness and ductility), weldability, and corrosion resistance.

The rate and extent of high-temperature corrosion are influenced by various factors, which collectively determine the corrosion resistance of valve materials in high-temperature environments. Understanding these factors helps make more rational decisions in the design and material selection stages, thereby improving the service life and reliability of valves.

The operating temperature of valves is one of the key factors affecting high-temperature corrosion. Generally, the higher the temperature, the faster the corrosion rate. For example, carbon steel undergoes oxidation at temperatures above 538°C, with the metal loss rate increasing as the temperature rises. The sulfidation of iron-based alloys usually begins at temperatures above 260°C, while metal dusting typically occurs in the temperature range of 482°C to 816°C. Therefore, when selecting valve materials, their corrosion resistance at the actual operating temperature must be considered.

The alloy composition is crucial for high-temperature corrosion. Chromium is the main alloy element affecting oxidation and sulfidation resistance. Increasing the chromium content can produce more protective oxide or sulfide scales, thereby enhancing the material's corrosion resistance. For example, 300-series stainless steel, with its higher chromium content, has strong oxidation and sulfidation resistance. The sulfidation and nitriding resistance of nickel-based alloys are also closely related to their chromium content. In addition, other alloy elements such as silicon and aluminum can also improve the material's corrosion resistance, but their content needs to be limited according to the actual situation as they may have adverse effects on mechanical properties.

The concentration of corrosive media is also an important factor affecting high-temperature corrosion. For example, the higher the concentration of sulfides, the faster the sulfidation rate. Valves in pipelines operating in high-temperature sulfur-containing fluid environments are more prone to sulfidation. Similarly, the concentration of nitrogen compounds also affects the nitriding rate. Therefore, when designing and selecting valves, the concentration and type of corrosive media should be fully considered to choose the appropriate materials and protective measures.

In high-temperature conditions, selecting suitable valve materials and taking effective protective measures are key to ensuring valve performance and extending service life. The following are some specific selection and protection strategies.

Selecting appropriate valve materials is crucial for preventing corrosion in high-temperature conditions. For oxidation and sulfidation resistance, alloys with higher chromium content are usually chosen. 300-series stainless steel and nickel-based alloys, with their higher chromium content, have good oxidation and sulfidation resistance. For carburization resistance, alloys with elements that form strong surface oxide or sulfide films (such as silicon and aluminum) should be selected. Nickel-based alloys, with their higher nickel content, have good nitriding resistance, and nickel-based alloy materials containing 30% to 80% nickel are commonly used to manufacture valves to prevent nitriding. In addition, other suitable alloy materials can be chosen according to the specific application environment and corrosion conditions.

In addition to selecting the right materials, surface treatment is also an effective way to improve the corrosion resistance of valves. For example, aluminum diffusion treatment of valve components can reduce sulfidation rates and metal dusting in some applications. This treatment method forms an aluminum-rich protective layer on the metal surface, thereby enhancing the material's corrosion resistance. However, it should be noted that this treatment method does not provide complete protection, so it is necessary to weigh the specific application environment when selecting it.

In actual production, controlling process conditions can also effectively slow down high-temperature corrosion. For example, controlling the concentration of oxygen, sulfides, and nitrogen compounds in the process fluid can reduce the corrosion rate. In addition, designing the valve structure reasonably to avoid stress concentration and local overheating also helps improve the valve's corrosion resistance.

High-temperature corrosion is a common problem for valves in high-temperature conditions, which can lead to the degradation of valve material mechanical properties and even cause safety accidents. Understanding the types of high-temperature corrosion and their influencing factors, and taking effective protective measures, is crucial for ensuring the reliability and safety of valves. By selecting appropriate materials, performing surface treatments, and controlling process conditions, the corrosion resistance of valves can be effectively improved, and their service life can be extended. In practical applications, it is necessary to consider various factors comprehensively according to the specific working environment and corrosion conditions, and choose the most suitable protection scheme to ensure the safe operation of valves.