Hydrogen, which can degrade the performance of most alloys, is nearly always present in some form. Sour gas and crude oil contain hydrogen sulphide (H2S); hydrogen gas (H2) is an integral part of key industrial processes, such as petroleum refining and ammonia production; virtually all corrosion processes generate atomic hydrogen or H2 gas; and of course, water (H2O) contains hydrogen.
Hydrogen damage and embrittlement can take a variety of forms. Hydrogen degradation mechanisms that occur at or near ambient temperature are summarized below. High-temperature hydrogen attack (HTHA) falls into a special category and is described elsewhere.
Hydrogen blisters. This phenomenon is typically observed in carbon steel with high sulfur content. Atomic hydrogen diffuses through the steel until it reaches a sulfide inclusion or a lamination in the plate, at which point the hydrogen atoms recombine into H2 molecules. These molecules become trapped because they are too large to diffuse through the steel. As additional H2 molecules become trapped within the inclusion or lamination, the hydrogen gas pressure builds up and creates a bulge on the free surface of the plate. Hydrogen blisters can form in the absence of an applied stress.
Hydrogen-induced cracking (HIC). The mechanism for the formation of HIC damage is essentially identical to blister formation, except that bulging is not normally observed in the former. Trapped H2 molecules cause internal cracking that is parallel to the plate surface. Neighboring cracks on slightly different planes link up, resulting in a step-wise cracking pattern. If the material remains in a hydrogen-charging environment, the cracks increase in size and number over time.
Stress-oriented hydrogen-induced cracking (SOHIC). When HIC damage approaches a weld, cracks often change direction and propagate along the fusion boundary. SOHIC poses a significantly higher risk of brittle fracture than standard HIC damage because the former results in cracks that are nominally perpendicular to the applied stress.
Delayed hydrogen cracking in the weld heat affected zone (HAZ). Cracks can form in the weld HAZ if three factors are present: dissolved atomic hydrogen in sufficient concentration, high hardness, and tensile residual stresses. This cracking is delayed, in that it typically occurs several hours after welding. Atomic hydrogen can be absorbed during welding if sufficient moisture is present in the weld consumable or the ambient environment. If the component operates in hydrogen-charging service, dissolved hydrogen can migrate to the weld unless the material is outgassed prior to welding. Dissolved hydrogen tends to concentrate in the HAZ because the hydrogen solubility is higher than in either the weld metal or base metal. The most effective way to prevent delayed hydrogen cracking is to perform a post-weld heat treatment immediately after welding, before the weldment cools to ambient temperature.
Hydrogen-assisted cracking under applied stress. Some materials perform well in a hydrogen environment under normal circumstances, but exhibit severe embrittlement when a crack or planar flaw is present. The solubility of atomic hydrogen at ambient temperature is very low in most alloys, but can increase by several orders of magnitude at the tip of a crack under stress. The high local concentration of hydrogen causes the material at the crack tip to become embrittled, which results in crack propagation. This crack propagation can be slow and stable initially, but catastrophic failure can occur if the crack progresses and reaches a critical size. The susceptibility of hydrogen-assisted cracking tends to increase with increasing tensile strength of the material.
The API 579-1/ASME FFS-1 standard includes assessments for blisters and HIC damage. The standard does not provide any specific guidelines for SOHIC and hydrogen-assisted cracking, other than to state that Level 3 assessment is required. Delayed hydrogen cracking can be addressed with a Level 2 crack assessment.
Quest Reliability’s relevant expertise and technology for addressing hydrogen embrittlement and damage includes:
- Practical experience with hydrogen damage and embrittlement in a wide variety of applications
- Laboratory facilities for material testing and failure analysis
- Finite element modeling of welding, including hydrogen diffusion to the HAZ
- Theoretical modeling of hydrogen-assisted cracking and hydrogen diffusion to the crack tip
- Proprietary software for predicting hydrogen-assisted cracking in hydroprocessing reactors in refinery service
For further information about Hydrogen Embrittlement, please contact us.