PSI - Issue 17

Garan Martin et al. / Procedia Structural Integrity 17 (2019) 514–519 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Brittleness

The susceptibility of the material to the brittle fracture may cause its prolonged contact with some chemical element or chemical substance. Hydrogen fragility (hydrogen cracking) and corrosion cracking are well-known cases in practice. Hydrogen embrittlement (sometimes called as Hydrogen cracking) is a phenomenon caused by the interaction of the material with hydrogen, resulting in local cracks under external loadings meeting operational safety conditions. Hydrogen is a highly chemically active element and, under appropriate conditions, can dissociate to atoms or ionize, thereby increasing its ability to penetrate the metal. Dissolution of hydrogen refers to a process whereby hydrogen gas adsorbed on a metal surface decomposes into atoms that are already capable of diffusing in the crystallographic lattice of the metal. Adsorption of hydrogen by the metal surface has a physical nature, i.e. hydrogen is bound to the metal surface by weak inter-molecular forces. It follows from the chemical-physical properties and the size of the hydrogen atoms that its high absorption and diffusion ability to penetrate the depth of the crystal structure of the steel is only in the atomic and dissociated state. Chemical reactions are as follows: H 2 → 2H ads 2Fe + 2 H ads → 2FeH abs (1) where H ads is a hydrogen atom adsorbed on the steel surface and FeH abs is hydrogen already absorbed into the steel grid, thus the total reaction is 2Fe + H 2 → 2FeH abs (2) Hydrogen can enter the steel The atomic hydrogen has, from a thermodynamic point of view, an effort to exit the iron lattice and recreate the hydrogen molecular. The accumulation by the following hydrogen molecular recombination in disorders, pores, cracks, inclusions and the area of grain boundaries and dislocations accumulated results in a local increase in pressure (a function of the hydrogen concentration to hundreds or thousands MPa - Podhodnya, Shvachko 2001, AINSI/API 2008). Hydrogen brittleness is a problem associated with defects in material. For each volume of material there is a local concentration of hydrogen c H and a critical value of this concentration of c K . Hydrogen cracking can occur in places where the critical hydrogen concentration of the c K is exceeded. At each point of the material, the resulting mechanical stress is the superposition of the internal stresses (e.g., around the pads) and the external tension. The sum of these two stresses is normally less than the cohesive strength of the material, that means σ internal +σ external  σ c (3) The presence of hydrogen may cause a decrease in the cohesive strength of the material to a value of σ HC and cause a local pressure of pH2. At that time, the condition for the formation of local hydrogen cracking takes shape σ internal +σ external +p H2  σ HC (4) For such critical concentration of hydrogen c H and c K have influence many factors like mechanical properties, chemical composition, and micro-structure. Corrosion cracking is a phenomenon requiring interplay of metallurgical, mechanical and chemical factors, in which the progressive propagation of a corrosive crack is closely linked to both corrosive and mechanical factors. Highly solid metals with more than enough ductility, used at relatively low loads, undergo fatal cracking under  at higher pressures (above 25MPa) and at ambient temperature  at higher temperatures (190-250) ° C even at lower pressures