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In August 2006, when the Qingyang Special Equipment Inspection Institute conducted a comprehensive inspection of pressure vessels at a petrochemical subsidiary of CNPC, it was found that there were multiple hydrogen-induced bulges on the inner wall surface of the liquid hydrocarbon sedimentation tank (made of 16MnR material) that had been in use since 2003; the maximum diameter of these bulges was approximately 200 mm. In addition, bulging, microcracks, and delamination were also found in heat exchangers, isomerization units, liquefied petroleum gas storage tanks, and rich gas water washing tanks. The defects present in some of these devices will continue to arise and spread under the original process conditions. Given that these defects are similar and numerous, analyzing them and proposing corresponding measures is of great significance for the safe operation of equipment with such defects. Cause analysis: The crude oil used by this petrochemical subsidiary sometimes has a high sulfur content. According to the analysis data from the equipment operators, the medium within the faulty equipment contained sulfur to varying degrees. Although crude oil is desulfurized during refining, the desulfurization effect is not stable. As a result, a wet hydrogen sulfide environment (a corrosion environment of type H2S+H2O) is widely present in the first stage of crude oil processing in refineries, the light oil sections of refineries’ secondary processing units, petrochemical catalytic units, and liquefied petroleum gas storage tanks. Based on relevant literature and taking into account the actual conditions of this petrochemical branch, the presence of hydrogen sulfide in the equipment medium is the fundamental cause of the corrosion and damage to the aforementioned pressure vessels. Furthermore, there are also many factors that cause localized high stress during use, including mechanical or chemical damage such as wear, erosion, pitting, intergranular corrosion, concentration cell corrosion, crevice corrosion, and various other forms of localized corrosion ； Thermal stress generated due to different temperatures in various parts of the component ； Gaseous carbide media such as carbon, hydrogen, nitrogen, and oxygen diffuse into the interior of metal components, as well as residual stresses are present. When stress and corrosive environments combine, it can easily lead to corrosion and damage of equipment. Corrosion mechanism: In a wet hydrogen sulfide environment, hydrogen-induced degradation is caused by the formation of atomic hydrogen. Atomic hydrogen is a byproduct of the corrosion reaction, and subsequently, it diffuses into the steel. As shown below, when steel reacts with hydrogen sulfide in aqueous form, atomic hydrogen (H) and molecular hydrogen (H2) are produced: Fe + H2S → FeS + 2 H; subsequently, 2H → H2. This leads to uniform corrosion and wet hydrogen sulfide stress corrosion cracking in carbon steel equipment. The forms of cracking include the following. (1) During the corrosion of sulfur-containing compounds by hydrogen bubbling, hydrogen atoms released penetrate into the steel, accumulate at cracks, inclusions, defects, and other areas to form molecules, thereby generating a large expansion force. As the number of hydrogen molecules increases, the pressure on the lattice interfaces keeps rising, causing the interfaces to crack and hydrogen bubbles to form. It is mainly distributed on the shallow surfaces of the inner walls of the equipment. (2) In areas within steel where hydrogen-induced blistering occurs, as the hydrogen pressure continues to rise, small blister cracks tend to connect with each other, forming hydrogen-induced cracking with a stepped pattern that runs parallel to the surface. The presence of banded MnS inclusions in steel increases its susceptibility to hydrogen-induced cracking. (3) Sulfide stress corrosion cracking: In a wet hydrogen sulfide environment, hydrogen atoms penetrate into the steel and dissolve in the crystal lattice, causing hydrogen embrittlement; cracks then form under the effect of applied stress or residual stress. It usually occurs in high-hardness areas such as welds and heat-affected zones. (4) Stress-guided hydrogen-induced cracking: Under stress guidance, rows of small cracks form at inclusions and defects due to hydrogen accumulation, and these cracks develop in a direction perpendicular to the stress. It usually occurs in the heat-affected zone of welded joints and areas with high stress concentration, such as at nozzles, points where the geometry changes abruptly, locations of crack-like defects, or sites of stress corrosion cracking. Wet hydrogen sulfide cracking is a form of hydrogen-induced damage that occurs in a wet hydrogen sulfide environment. In a wet hydrogen sulfide environment, other forms of damage caused by the presence of hydrogen include: • Hydrogen blistering • Sulfide stress cracking (SSC) • Hydrogen-induced cracking (HIC) • Stress-oriented hydrogen-induced cracking (SOHIC). I. Hydrogen-induced cracking (HIC) and hydrogen blistering Hydrogen-induced cracking occurs when parallel layers of hydrogen combine to form through-wall cracks; there is no significant interaction between these cracks and applied or residual stresses. At the bubbling sites, the stress generated by the accumulation of hydrogen inside exacerbates hydrogen-induced cracking. Hydrogen-induced cracking is closely related to the purity of the steel, as well as to the method of steel manufacturing, the impurities present, and their shape. In accordance with the laboratory execution standards GB/T8650-2015 and NACE TM0284-2015, when heterogeneous, elongated sulfide or oxide inclusions occur parallel to the rolling direction of the steel plate, hydrogen-induced cracking generally takes place. These inclusions serve as sites for the formation of microscopic hydrogen bubbles, which grow and eventually connect together through step-like cracks. In fact, hydrogen-induced cracking is sometimes referred to as step-like cracking. Since hydrogen-induced cracking has no dependence on stress and does not occur alongside the microstructure hardening, post-weld heat treatment is completely ineffective. Restricting trace elements such as sulfur and controlling the variables in steel manufacturing are necessary to endow steel with resistance to hydrogen-induced cracking. Hydrogen embrittlement occurs when atomic hydrogen diffuses into the steel and gets trapped in voids, interlayers, or non-metallic inclusions. As mentioned above, hydrogen atoms that enter these areas combine to form molecular hydrogen, and molecular hydrogen cannot diffuse outward and escape. The expansion pressure of the gathered hydrogen eventually causes wall separation of the components, resulting in noticeable bubbling on the metal surface. Hydrogen bubbling can occur on both sides of a plate, or the bubbling can appear on top of another bubble, depending on the location of the interlayer. They vary in size, from small bumps to swellings several feet in diameter. The increasing bubbles can cause the surface to crack, rendering the equipment unable to withstand pressure.