Industrial Corrosion and Protection--Hydrogen Damage in 13 Hydrogenation Units

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Hydrogen damage is a typical and highly harmful form of damage that equipment materials in hydrogenation units (including hydrocracking and hydrorefining) face under harsh operating conditions of high temperature, high pressure, and exposure to hydrogen. It refers to the deterioration of material properties (especially toughness, ductility, and strength) resulting from the diffusion of hydrogen atoms into the metal matrix or from reactions between hydrogen and the metal. The feed oils used in hydrogenation units usually contain impurities such as sulfur and nitrogen, which react with hydrogen to produce corrosive substances like hydrogen sulfide and ammonia, further increasing the risk of damage to the equipment. Hydrogen damage denotes the degradation of mechanical properties in metal materials due to the presence of hydrogen or interactions with hydrogen. Based on the temperature conditions under which hydrogen damage occurs, it can be classified into hydrogen embrittlement and hydrogen corrosion. Depending on whether the original mechanical properties of the material can be restored through hydrogen removal treatment, it can be categorized as reversible or irreversible hydrogen damage. Hydrogen embrittlement can include hydrogen pressure cracking (white spots in steel, H2S-induced cracks, welding cold cracks, and cracks caused by hydrogen charging or pickling), hydride embrittlement, hydrogen-induced martensitic transformation, and hydrogen-induced delayed fracture; hydrogen corrosion refers primarily to the phenomenon in which, at moderate and high temperatures, hydrogen reacts with carbon in steel to form methane, resulting in the formation of hydrogen bubbles and cracks that degrade the mechanical properties of the material. Conditions for hydrogen-induced damage (I) Hydrogen pressure cracks 1. White spots in steel. After pickling the steel section, thin, hair-like cracks can sometimes be seen; their width is generally around 1 μm, which is why they are also referred to as \"hair cracks.\" If the specimen is broken along these cracks, oval spots with a silvery-white luster can be observed on the fracture surface, and hence they are called \"white spots.\" These are actually coin-shaped cracks filled with H2. The cause of white spots is generally considered to be the effect of hydrogen pressure; if internal stresses are present in the steel, they can add to this hydrogen pressure, making it easier for white spots to appear. The nuclei of the white spots are defects that were originally present in the steel. The hydrogen content in steel is a fundamental factor determining whether white spots can form. The critical hydrogen content at which white spots appear depends on both the type of steel and the size of the component; for larger components, hydrogen has difficulty diffusing out, making it more likely for white spots to form. The chemical composition and microstructure of steel have a great influence on the formation of white spots. Austenitic steel, ferritic steel, and ledeburitic steel are insensitive to white spot formation; whereas rail steel, alloy structural steel, and some alloy tool steels are prone to forming white spots. The internal stresses generated in steel during cooling, namely thermal stresses and phase transformation stresses (high internal stresses can be produced during processes such as martensite transformation), are important factors that contribute to the formation of white spots. 2. H2S-induced cracks. When carbon steel or low-alloy pipeline steel is immersed in an H2S solution, microcracks form within the specimen even in the absence of external stress. These cracks are step-shaped; if they are located near the surface of the specimen, they can easily cause bubbling on the surface. It is generally believed that this is caused by hydrogen pressure. H2S reacts at the interface of steel to form H; once it enters the specimen, it accumulates around inclusions (especially elongated MnS particles), which act as hydrogen traps. Thereafter, it combines to form H2, generating hydrogen pressure. If the pipe material contains segregation, S, P, and Mn are significantly enriched in the segregation zones; in such cases, H2S-induced cracks can occur in these zones even though the average S content is low. H2S-induced cracks primarily nucleate on elongated MnS inclusions, and the greater the total length of these inclusions per unit area, the more likely hydrogen-induced cracks will occur. 3. Welding cold cracks. The welding process is a form of localized melting; the moisture in the welding electrode and in the atmosphere enters the molten pool and turns into hydrogen. When the amount of hydrogen that enters is high, hydrogen-induced microcracks may occur during the cooling phase after welding (similar to white spots in steel). 4. Microcracks generated during the hydrogen charging (pickling) process. Hydrogen pressure cracks may also occur during pickling or electroplating, as well as during electrolytic hydrogen charging; no traces of slip lines can be observed on the surface of the polished specimens. Even low-carbon steel or pure iron can develop hydrogen-induced cracks under severe electrolytic hydrogen charging. Cracks often nucleate at grain boundaries, dislocation entanglements, and carbides. (II) Hydrogen embrittlement (hydrogen corrosion) caused by hydrogen-induced chemical changes: Under high temperature and pressure, hydrogen enters the steel and reacts with carbides to form methane. The resulting CH₄ molecules cannot diffuse out of the steel; instead, they form bubbles at grain boundary inclusions, generating significant pressure. As CH4 continues to form, the bubbles grow larger. When the pressure of CH4 within the bubbles exceeds the strength of the material at that temperature, the bubbles transform into cracks. The higher the pressure and temperature in environment H, the greater the pressure within the methane bubbles. When the pressure in these bubbles equals the material’s fracture strength, it leads to the nucleation of microcracks. Meanwhile, the methane-generation reaction causes decarburization of the steel, reducing its strength. (III) Hydrogen embrittlement caused by hydrogen-induced phase transformation 1. Hydride embrittlement. Many metals or alloys (such as Ti, Zr, Hf, V, Nb, Ta, REs, etc.) can form stable hydrides. Hydrides are brittle interphases; once they form, the plasticity and toughness of the material decrease, that is, the formation of hydrides makes the material more brittle. This is a type of hydrogen embrittlement caused by a hydrogen-induced phase transition. 2. Hydrogen-induced martensitic transformation. For unstable austenitic stainless steels (such as 18-8 type stainless steels like 304, 321, and 316 steel with relatively low Ni and Cr contents), they are in the austenitic state at room temperature, exhibiting good plasticity and toughness. However, if subjected to deep cryogenic quenching (below -60°C) or cold working at room temperature, part of the austenite transforms into martensite. Once the martensite phase appears, the plasticity and toughness of the material decrease significantly. (IV) Hydrogen-induced delayed fracture: Under constant load (or constant displacement) conditions, when atomic hydrogen is enriched to a critical level through stress-induced diffusion, it leads to the nucleation and propagation of hydrogen-induced cracks, thereby causing low-stress fracture; this phenomenon is known as hydrogen-induced delayed fracture. The so-called lag refers to the fact that it takes some time for hydrogen to diffuse and reach a critical concentration; therefore, after loading, it takes a certain period of time before hydrogen-induced cracks can nucleate and propagate. If atomic hydrogen is removed, delayed fracture does not occur; therefore, it is also reversible.