Hydrogen Induced Cracking (HIC) is a form of material degradation that occurs in carbon and low-alloy steels exposed to hydrogen-containing environments, typically in the presence of aqueous H2S (hydrogen sulfide) environments, such as those found in oil and gas production.
Hydrogen blistering is another form of degradation caused by the ingress of hydrogen into metals. It typically occurs in high-strength steels, such as those used in pressure vessels and pipelines, when they are exposed to hydrogen-containing environments.
Hydrogen Induced Cracking (HIC) Explanation.
Hydrogen Induced Cracking or hydrogen blistering appears as bulges on the ID surface of a pipe or pressure vessel of the susceptible material in wet H2S service. The ions trapped in the locations of the inclusions combine to form Hydrogen which increases the pressure within the lamination and the localized bulging takes place in form of blistering.
These blisters occur mostly in the base metal along the plate rolling direction in the absence of any stress, however the residual stress from the welding and the cold forming can increase the chances of hydrogen induced cracking damage. The blisters in the susceptible material grow in the size due to accumulation of more hydrogen.
Increasing hydrogen pressure increases the stress on the grain boundaries near the edges of the blisters. As the damage progresses the stress on the grain boundaries increases and appears as intergranular cracking or tears at the edges of the blisters. The intergranular cracking from the blisters at different depths join together, often referred as stepwise cracking. The material strength is considerably reduced due to the stepwise cracking.
Hydrogen induced cracking (HIC) mostly occurs in low‐strength steel (typically < 80 ksi). Most commonly used plate materials for pressure vessels and the tanks is ASTM‐516‐70, and for the piping is ASTM A‐106. Both of these materials are susceptible to Hydrogen induced cracking.
Modern steel making techniques utilize better melting practices, which reduce the concentration of the contaminants in the steel, especially sulfur. HIC resistant steel is manufactured via the electric arc furnace with vacuum degassing techniques, which provides ultra clean and homogeneous steel. Hence the steels manufactured after 80s have better ability to resist the HIC. These steels are graded as HIC resistant steels after the batch testing for HIC.
Following are process conditions which favor initiation and propagation of HIC.
1. pH level of the process stream:
pH plays an important role in determining the aggressiveness of the environment and the likelihood of cracking or blistering in wet H2S service. H2S itself is not the main contributor to the pH of the streams, rather it is controlled by other solutes in streams like HCl, NH4Cl, and alkaline materials.
At low pH values (below 4) the solubility of H2S in water is greatly reduced, hence the amount of atomic hydrogen available to enter the steel is lesser. Therefore the HIC cracking susceptibility is lesser at lower pH values (more acidic) of streams, however lower pH promotes the corrosion (generalized or localized) due to instability of iron sulfide layer in acidic condition. The end result at low pH is typically metal loss due to corrosion but moderate levels of cracking and blistering.
The solubility of H2S in water is high, making it easier for hydrogen to enter the steel if no protective iron sulfide scale exists, however at pH values 8 or above (alkaline conditions) the iron sulfide scale is more stable which can provide effective protective layer to control entry of hydrogen into the steel. Therefore the susceptibility of wet H2S is lesser in alkaline services unless the iron sulfide layer keeps cracking die to high abrasive flow condition.
The wet H2S damage can be expected more prominent in mild acidic conditions with pH of 4 to 8.
2. Temperature:
The concentration of the dissolved hydrogen reduces correspondingly as the temperature of the process streams increase. Hence, along with other condition, the ambient temperature promotes all kind of Wet H2S damages.
3. Partial Pressure of H2S:
Higher partial pressure (concentration) of the H2S provides more atomic hydrogen, increasing the susceptibility of the Wet H2S damages.
To know more about HIC prevention, Materials Susceptibility.
Stress Oriented Hydrogen Induced Cracking (SOHIC).
Stress Oriented Hydrogen Induced Cracking (SOHIC) is a form of HIC associated with the low strength steels, which occurs in the base metal adjacent to the heat affected zones (HAZ) of a weld seams. At the locations of accumulated stress the diffused atomic hydrogen created micro cracks (HIC) oriented parallel to the rolling direction of steel. In case of extended damage these cracks join together with a series of vertical cracks which are oriented perpendicular to the rolling direction of the steel. The progressive damage often ends up with the through wall crack in and around the HAZ.
The stresses, weather applied or residual (from welding and cold forming process) contribute mainly to the susceptibility of SOHIC. Hence the most vulnerable locations for the SOHIC are the welds in the vessels and piping (spiral and butt). SOHIC can also occur at the locations where metal is subjected to cyclic stresses or tensile stress. Post weld heat treatment considerably mitigates the chances of damage due to SOHIC.
Further details of HIC and SOHIC damage can be seen from API‐RP‐571 Para 5.1.2.3.