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Hydrogen-induced cracking in HF environments and stress-guided hydrogen-induced cracking HIC/SOHIC-HF1 – Overview. Hydrogen-induced cracking is defined as stepped cracks that form between adjacent hydrogen bubbles on different planes within a metal, either internally or on the metal surface. Hydrogen-induced cracking does not require external applied stress. The driving force for hydrogen-induced cracking is the high gas pressure formed within the hydrogen bubbles surrounding them. Internal reactions occur between those high-stress areas, which tend to promote crack growth and then connect hydrogen bubbles on different planes within the steel. The source of hydrogen in steel is wet H2S or HF acid. HF is used in alkylation units, with the concentration of HF aqueous solutions ranging from 96–99+%; carbon steel exposed to HF, whether aqueous or anhydrous, can suffer from HIC/SOHIC. Hydrogen bubbling is the formation of hydrogen-filled bubbles inside or on the surface, parallel to the steel surface. In steel, their formation is not continuous (for example: voids, impurities, interlayers, sulfur impurity sites). Most foaming occurs in rolled steel products, which contain microscopic strip-like inclusions, such as elongated strip-shaped sulfides formed during rolling. Therefore, for HIC, the sensitivity to hydrogen embrittlement is of primary importance for the quality of steel (e.g., quantity, size, and shape of discontinuities). With this in mind, the sulfur content in steel is a key parameter. Reducing the sulfur content in steel lowers its susceptibility to bubbling and HIC; adding calcium to steel helps control the microstructure of sulfides. SOHIC is defined as many tiny bubbles piled up together in an array, with the hydrogen-induced cracks running parallel to the direction of the array. Due to the highly concentrated tensile stress, the crack direction is perpendicular to the thickness of the base material. SOHIC is a special form of HIC; HIC often occurs at the junction between the base metal and the weld, due to the high residual stresses present in that area. For HIC, the quality of the steel plate is a very important parameter affecting SOHIC sensitivity. In addition, post-weld heat treatment can reduce the residual stress in the equipment, but it may still not be able to avoid or eliminate SOHIC sensitivity. 2. Basic data: The data listed in Table H-24 are used to analyze the sensitivity of carbon steel equipment to HIC/SOHIC in HF environments. If the sulfur content in the steel is unknown, it is necessary to consult an experienced materials engineer for an assessment of the quality of the carbon steel. 3. Determine the sensitivity of HIC/SOHIC in HF environments. If any concentration of HF is present in the equipment, then such equipment and pipelines are susceptible to HIC/SOHIC caused by HF. The basic data in Table H-24 is used to obtain sensitive evaluations. Table H-25 is for carbon steel. Commonly used pipelines are forged steel pipes and fittings (such as A153, A106, API5L, A234, A105, etc.), which are considered to have low sensitivity to HIC/SOHIC in HF. The equipment and large-diameter steel pipes are made by rolling and welding plate steel, with H-25 being used to determine sensitivity. Figure H-11 lists the steps for the process in which sensitivity needs to be determined. Hydrogen bubbling in steel is related to the purity of the steel, primarily the content of sulfur in it. It should be recognized that steel bubbling is not dangerous and does not lead to leakage or damage, unless it is accompanied by hydrogen-induced cracks that extend to the surface. Bubbling indeed poses a threat to the integrity of steel structures; when there are welds with sufficient residual stress in its vicinity, these stresses can cause hydrogen-induced cracks to extend to the surface. In this last case, the condition considered to be the most severe will determine the sensitivity of HIC/SOHIC in the HF environment.