PSI - Issue 62

Edoardo Proverbio et al. / Procedia Structural Integrity 62 (2024) 285–298 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2. Corrosion, embrittlement and stress corrosion cracking

Wet corrosion of metallic materials is the consequence of two reactions that occur simultaneously on the surface of the metal itself: – an anodic reaction, which involves the oxidation of constituents of the metal alloy and makes electrons available in the metallic matrix. – a cathodic reaction involving these electrons which is linked to the reduction of one or more species present in the electrolyte and adsorbed on the metal surface. In natural environments, for metal structures in contact with an aqueous electrolyte (surface or ground water, sea water, rainwater, condensed water, etc..) or solids assimilable to an electrolyte due to their porous nature (soil, concrete, mortars, etc.. ) the main cathodic reactions are those of oxygen reduction and hydrogen reduction (hydrogen cation H + always present in water in variable concentrations depending on the pH and in equilibrium with the water itself) (Revie, 2011). Reduction reactions can also occur simultaneously. However, the reduction of hydrogen does not always occur, depending on the electrochemical potential of the metal surface (the so-called "corrosion" potential) which is linked to the balance of all the electrochemical reactions that occur on the metal surface. If the corrosion potential is higher than the equilibrium potential of hydrogen reduction, this reaction cannot occur. In the presence of oxygen, the corrosion potential can be relatively high (there are however many factors that can influence the corrosion potential in addition to the equilibrium potential of the species involved, e.g., oxygen, hydrogen and metal, not least the possibility of the metal passivation), in this case the hydrogen reduction reaction can be inhibited. Even in acidic environments, the presence of a cathodic reaction with a high equilibrium potential could be able to prevent the reduction of hydrogen. The reduced hydrogen is initially in atomic form and is unstable as it tends to recombine to give molecular hydrogen gas. Indeed, if there are no substances present in the environment that inhibit this reaction, recombination occurs quickly, so the quantity of hydrogen that remains on the metal surface, and is therefore available to penetrate inside the material, is modest. The presence of oxygen can also contribute to further reducing its concentration because it can react with atomic hydrogen and form water. On the other hand, in the presence of substances such as compounds based on sulphur, phosphorus, arsenic, antimony, selenium, tellurium, or cyanides or other inhibitory species, the reaction that brings atomic hydrogen to molecular hydrogen is slowed down or even blocked, whereby atomic hydrogen accumulates at the surface of the metal and is therefore available to penetrate the material. Once the hydrogen enters the crystalline lattice of the steel it diffuses and positions itself in areas of greater deformation which allows it to accommodate more easily the, albeit small, hydrogen atom. Such areas are for example near dislocations or accumulations of dislocations or grain boundaries. In this case we refer to the so-called hydrogen embrittlement (HE). Indeed, hydrogen embrittlement is a complex process involving several distinct micro mechanisms that contribute to the behaviour of the material, although not all may be present at the same time. Mechanisms include the formation of brittle hydrides, the creation of voids that can lead to the formation of high pressure bubbles, decohesion mechanisms on internal surfaces (postulated in the hydrogen enhanced decohesion mechanism, HEDE) or localized microplasticity at the apex of defects or cracks (considered in the hydrogen enhanced plasticity model, HELP) (Robertson et al., 2015) (W. Gerberich, 2012). The amount of hydrogen necessary to cause embrittlement is closely linked to the mechanical properties of the steel (as well as to the microstructure and the presence of segregation products of phosphorus, antimony, tin, sulphur as sulphides and arsenic) and decreases as they increase. Microstructure as a great influence too. Sensitivity of steel