The hydrogen induced cracking of steels takes place according to different processes. Both the environmental conditions and other influences such as stresses and the microstructure of the steels have an effect. In technical applications, a number of technical terms have become established for this, which are explained in the following table.
Table with terms for hydrogen induced cracking processes of steels
| Term | Abbreviation for | Explanation | Remarks |
|---|---|---|---|
| HIC | Hydrogen Induced Cracking | Planar cracking that occurs in carbon and low alloy steels when atomic hydrogen diffuses into the steel and then combines to form molecular hydrogen at trap sites | Cracking results from the pressurization of trap sites by hydrogen. No externally applied stress is required for the formation of hydrogen-induced cracks. Trap sites capable of causing HIC are commonly found in steels with high impurity levels that have a high density of planar inclusions and/or regions of anomalous microstructure e.g. manganese-sulfide |
| HSC | Hydrogen Stress Cracking | Cracking that results from the presence of hydrogen in a metal and tensile stress (residual and/or applied) | HSC describes cracking in metals that are not sensitive to SSC but which can be embrittled by hydrogen when galvanically coupled, as the cathode, to another metal that is corroding actively as an anode. The term “galvanically induced HSC” has been used for this mechanism of cracking. |
| HTHA | High Temperature Hydrogen Attack | HTHA is the result of hydrogen dissociating and dissolving in the steel, and then reacting with the carbon in solution in the steel to form methane. | High temperature hydrogen attack (HTHA), also called hot hydrogen attack, is a problem which can impact steel operating at elevated temperatures (typically above 400°C) in hydrogen environments, in refinery, petrochemical and other chemical facilities and, possibly, high pressure steam boilers. The main factors influencing HTHA are the hydrogen partial pressure, the temperature of the steel and the duration of the exposure |
| SOHIC | Stress-Orientated Hydrogen Induced Cracking | staggered small cracks formed approximately perpendicular to the principal stress (residual or applied) resulting in a “ladder-like” crack array linking (sometimes small) pre-existing HIC cracks | The mode of cracking can be categorized as SSC caused by a combination of external stress and the local strain around hydrogen-induced cracks. SOHIC is related to SSC and HIC. It has been observed in parent material of longitudinally welded pipe and in the heat-affected zone (HAZ) of welds in pressure vessels. SOHIC is a relatively uncommon phenomenon usually associated with low-strength ferritic pipe and pressure vessel steels. |
| SSC | Sulfide Stress Cracking | cracking of metal involving corrosion and tensile stress (residual and/or applied) in the presence of water and H2S | SSC is a form of hydrogen stress cracking (HSC)and involves the embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. Hydrogen uptake is promoted in the presence of sulfides. The atomic hydrogen can diffuse into the metal, reduce ductility, and increase susceptibility to cracking. High-strength metallic materials and hard weld zones are prone to SSC. |
| SCC | Stress Corrosion Cracking | cracking of metal involving anodic processes of localised corrosion and tensile stress (residual and/or applied) in the presence of water and H2S or chloride-containing water | Chlorides and/or oxidants and elevated temperature can increase the susceptibility of metals to this mechanism of attack. |
Read More
Standards and specifications for steel materials in hydrogen systems
