Stress corrosion of pressure vessels caused by several chemicals

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Several chemicals and stress corrosion in pressure vessels: Stress corrosion is a form of degradation that occurs in metal materials under the combined action of a corrosive environment and tensile stress. When metal undergoes stress corrosion, corrosion and stress reinforce each other; on one hand, corrosion reduces the effective cross-sectional area of the metal and creates notches on its surface, leading to stress concentration. If it is intergranular corrosion, it weakens the bonding between the metal grains ； On the other hand, stress accelerates the progression of corrosion, causing surface cracks to extend deeper (or along the grain boundaries), ultimately leading to fracture. In stress corrosion, if the stress is an alternating tensile stress, this type of corrosion is called fatigue corrosion. During fatigue corrosion, corrosion notches first form on the surface, causing stress concentration and serving as the origin of fatigue cracks. Under alternating tensile stress, the damaged protective film cannot be restored, and what remains at the bottom of the corrosion pit is always the anode, which is in an active state and forms part of the corrosion cell. In this way, under the combined effect of corrosion and alternating stress, the cracks continue to spread until the metal material finally breaks. The corrosion-induced failure of pressure vessels is always stress corrosion, as these vessels are generally subjected to significant tensile stresses. Moreover, their structure often contains stress concentration areas to varying degrees, such as openings and welds in the equipment, and the working medium inside the vessels is frequently corrosive. The storage and transportation of liquid ammonia mostly use pressure vessels made of carbon steel or low-alloy steel. In recent years, there have been numerous rupture and explosion incidents of liquid ammonia storage tanks at home and abroad. Accident analyses indicate that many of these were caused by stress corrosion. Furthermore, inspections of the liquid ammonia spheres that had not experienced any accidents revealed a large number of cracks of varying degrees; most of these cracks were found on the Antarctic plates, which are constantly submerged under the liquid level, as well as on the circumferential welds formed by welding the lower temperate zones together. Under normal conditions, anhydrous liquid ammonia causes only mild uniform corrosion to steel. However, liquid ammonia storage tanks are prone to contamination by air during filling, discharging, and maintenance processes. The oxygen and carbon dioxide in the air accelerate the corrosion of steel by ammonia; the reaction is as follows: The ammonium carbamate involved in this reaction has a strong corrosive effect on carbon steel, causing the passivation layer on the surface of the steel to break down, which leads to anodic corrosion. Stress corrosion is severe due to the high residual stress at the welds. 　Many studies indicate that the higher the strength of the steel used in liquid ammonia spheres, the greater the tendency to develop stress corrosion cracks. Furthermore, the higher the operating temperature of the container and the higher the oxygen content in liquid ammonia, the more severe its stress corrosion becomes. Taking the following measures helps to prevent stress corrosion of storage containers caused by liquid ammonia: 1. During welding, take steps to minimize residual stresses as much as possible. Cold-pressed heads must undergo heat treatment ； 2. Use low-carbon steel suitable for low-temperature applications as much as possible for welding liquid ammonia storage tanks ； 3. Try to maintain a low operating temperature ； 4. Reduce air pollution. Add 0.1% to 1% water to liquid ammonia. Experiments and practices have shown that 2% water content in liquid ammonia has a corrosion-inhibiting effect, but this method has little effect on high-strength steel. Under specific conditions of higher temperatures and sodium hydroxide solutions at certain concentrations, hot alkaline solutions can cause stress corrosion in carbon steel or alloy steel; this phenomenon is commonly known as alkali embrittlement or caustic embrittlement. There is currently no unified understanding of the mechanism of steel alkaline embrittlement. Generally, carbon steel undergoes the following chemical reaction with water vapor at high temperatures: In this reaction, sodium hydroxide acts as a catalyst, and the resulting Fe3O4 covers the surface of the steel, forming a protective layer. However, excessive local tensile stress may cause the protective film in that area to be damaged ； It may also be due to the enrichment of sodium hydroxide on the surface, which causes Fe3O4 to dissolve ； Or due to the combined effect of these two factors, initial corrosion cracks form on the metal surface; sodium hydroxide accumulates in these cracks, leading to electrochemical corrosion. The tip area of the crack acts as the anode, while the protective layer surrounding the crack functions as the cathode; combined with the effect of tensile stress, this causes the crack to expand rapidly, ultimately leading to fracture. Alkali embrittlement of steel generally requires three conditions to occur simultaneously: high temperature, high concentration of alkali, and tensile stress. Tests have shown that a sodium hydroxide solution with a concentration of 10% can cause alkali embrittlement, whereas a 5% concentration does not. However, in pressure vessels and boilers, sodium hydroxide often accumulates in certain areas; phenomena such as salt deposits or the evaporation of water at high temperatures can increase the local alkali concentration. Alkali embrittlement often occurs in the pressure-bearing components of boilers. Boiler water, after treatment, may contain excess alkali; in areas such as deposits or beneath porous scale, at rivets or welds, and at flange connections, the alkali concentration can increase. Combined with uneven tensile stresses, this can lead to alkali embrittlement and cracking of the boiler. In recent years, there have been numerous explosion incidents involving containers (gas cylinders) filled with a mixture of carbon monoxide and carbon dioxide, both of which are caused by stress corrosion. Under normal conditions, when carbon monoxide is absorbed by iron, it forms a protective layer on the metal surface; however, carbon monoxide used in industrial applications contains carbon dioxide and moisture. Due to the repeated inflation of the container or cylinder, the alternating stresses on the vessel walls cause localized damage to this protective layer, thereby accelerating the corrosion of the container by wet carbon dioxide. In equipment used in the refining, petrochemical, and coal gas industries, which rely on crude oil, natural gas, or coal as raw materials, hydrogen sulfide corrosion is a fairly common problem; among these, stress corrosion caused by wet hydrogen sulfide on carbon steel and low-alloy steel deserves particular attention. The mechanism of stress corrosion caused by hydrogen sulfide is not yet fully understood. Some studies suggest that wet hydrogen sulfide reacts with iron to produce hydrogen atoms, which then diffuse into the metal, accumulate there, and cause the metal to become brittle; under the influence of these hydrogen atoms, bubbles and cracks form. In terms of stress factors, it is mainly the residual stress from welding. In petrochemical production, there are some containers whose working medium is hydrogen under high temperature and pressure, such as the reactors in plants for ammonia synthesis, thermal cracking, alcohol production, and hydrogenation. If these devices are not properly designed, manufactured, or operated, they may be damaged due to hydrogen corrosion. This type of hydrogen corrosion is a form of chemical corrosion, as sampling and analysis at the rupture sites of the ammonia synthesis towers where hydrogen embrittlement occurred confirmed that the microstructure of the steel was decarburized ferrite. 1. Whether hydrogen embrittlement occurs in steel is primarily determined by the hydrogen pressure, temperature, exposure time, and the chemical composition of the steel. The higher the hydrogen pressure and temperature, the deeper the decarburized layer in carbon steel, and the faster hydrogen embrittlement fracture occurs; among these factors, temperature has the greatest impact. At higher temperatures (e.g., >700°C), carbon steel will suffer from hydrogen embrittlement even when the hydrogen pressure is only 0.1 MPa ； If the temperature is low (for example