Hydrogen induced cracking (HIC) is a type of stress corrosion cracking (SCC) that occurs in metals and alloys exposed to hydrogen at high pressure and low temperatures. HIC is often observed in components used for oil and gas production, such as pipelines and pressure vessels, but can occur in any metal exposed to hydrogen. HIC is usually characterized by small cracks that develop in the metal surface, but can be so severe that entire components can fail.
HIC occurs when hydrogen molecules, typically in the form of hydrogen gas or water vapor, are exposed to the metal surface. Once on the metal surface, the hydrogen molecules can enter the metal lattice structure where they can displace other atoms, such as iron and carbon. Once inside the metal, the hydrogen molecules create stress concentrations, which can cause stresses to be concentrated in particular areas. If the stresses become too great, the metal can crack or fail.
HIC is most commonly observed in steels and alloys, such as stainless steel, nickel alloys, and low-alloy steels, but can occur in any metal or alloy exposed to hydrogen. It is most commonly observed at elevated temperatures; generally between 77°F and 400°F (25°C to 200°C). The severity of HIC is also typically greater at the higher end of this temperature range, but the exact temperatures can vary depending on the metal or alloy being evaluated.
In addition to temperature, the severity of HIC can also be influenced by other factors. The presence of exposed grain boundaries and the presence of non-metallic inclusions can increase the susceptibility of a metal or alloy to HIC. The amount of hydrogen on the metal surface and the amount of applied stress can also influence the severity of HIC.
To limit the severity of HIC, it is often necessary to limit the hydrogen exposure, reduce the temperature to minimise the severity of HIC, or to use low-sulfur or low-phosphorus alloys. In some cases, the use of special coatings on the metal surface can limit the amount of hydrogen that can enter the metal lattice structure. Additionally, annealing or tempering the metal can reduce the susceptibility of the metal or alloy to HIC.
HIC can be detected using nondestructive testing techniques, such as ultrasonic testing, eddy current testing, or direct current voltage drop (DCVG) testing. These techniques can detect the presence of small cracks on the metal surface, indicating the presence of HIC.
In conclusion, hydrogen induced cracking (HIC) is a type of stress corrosion cracking (SCC) that can occur in any metal or alloy exposed to hydrogen at elevated temperatures. HIC is characterized by small cracks on the metal surface, and if left untreated, can lead to catastrophic failure. HIC can be prevented by limiting the hydrogen exposure, reducing the temperature, or using low-sulfur or low-phosphorus alloys. HIC can also be detected using nondestructive testing techniques.
