Hydrogen brittle failure

Cracking mechanisms in which corrosion is implicated include stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement. Purely mechanical forms of cracking such as brittle failure are not considered here. 

Brittle Failure (8). Brittleness is a principal consideration in selecting construction materials for liquid hydrogen service. Brittle fracture can result in the essentially instantaneous release of a vessel s contents, the hazard being a combined one of PV energy release and the possibility of fire and/or explosion. Three conditions must exist for a brittle fracture to occur 1) a stress riser, a crack, notch, or other discontinuity, 2) a section where the actual stress exceeds the yield stress of the material, and 3) a temperature below which failure occurs without appreciable plastic deformation. Metals that are satisfactory for liquid hydrogen service include aluminum, stainless steels, brass, and copper. Carbon steel is not suitable. 

Evolution of atomic hydrogen on the steel surface may provoke the initiation and propagation of cracks starting from the metal surface, especially in the presence of notches or localised corrosion attack. Even in the absence of flaws on the surface, atomic hydrogen may penetrate the steel lattice, accumulate in the areas subjected to the highest tensile stress, above all at points corresponding to lattice defects, and lead to brittle failure beginning at one of these sites. 

Mechanical failure of the containment vessel, piping, or auxiliary components (brittle failure, hydrogen embrittlement, freeze-up). 

Hydrogen bonds between the fibrillar units stabilize the whole structiue and confer its high mechanical strength [6,47]. The BNC behaves like a viscoelastic material brittle failure has previously been reached at approximately 20% strain and 1.5 MPa stress under uniaxial tension [48]. Youngs modulus of BNC single fibrils of 114 GPa have been reported [49]. 

In addition to hydrogen embrittlement, brittle failure can occur as a result of stress corrosion, liquid metal attack, or strain-age hardening. The last mentioned cause is well known and can occur on strained steel of any strength but seldom actually occurs in modem steels it is adequately documented in Appendix E of BS 729 (British Standards Institution, 1971 reaffirmed in 1986) users often erroneously refer to this effect as hydrogen embrittlement. This clearly states that strain-age embrittlement is the only type of embrittlement that can be aggravated by the hot dip galvanizing process. 

Failures have occurred in the field when storage tank roofs have become saturated with hydrogen by corrosion and then subjected to a surge in pressure, resulting in the brittle failure of circumferential welds. In rare instances, even copper and Monel 400 (N04400) have suffered of HIC. More resistant materials, such as Inconels and Hastelloys often employed to combat HIC, can become susceptible under the combined influence of severe cold work, the presence of hydrogen recombination poisons, and their presence in a galvanic couple with a more anodic metal or alloy. 

Brittle failure by cracking under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide. See also environmental cracking. 

In high-pressure boilers, there are three types of on-load corrosion acidic chloride, neutral chloride/dissolved oxygen, and caustic attack. The first and second (once it becomes established) are brittle and thick-walled and are accompanied by hydrogen damage which can lead to failure within a few hundred hours. Caustic attack tends to produce a gouged appearance of the metal due to extensive wastage. The morphology is fairly characteristic of the failure type. Acidic chloride forms hard, laminated oxide, whilst, with caustic attack, the oxide is often soft, and, as it is easily removed, may be absent. 

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

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