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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The possibility of achieving a depolarization of steel by contact with zinc or galvanized steel surfaces, such as to allow the reduction of hydrogen even in non-acid environments, is a debated issue in the technical-scientific literature. Zinc is characterized by a significantly lower equilibrium potential than that of steel in water and the contact between the two metals can locally lead to a condition favourable to the reduction of hydrogen. Zinc, however, in an alkaline environment (for example in contact with cement paste) corrodes giving rise to the formation of a passivating calcium hydroxyzincate layer and hydrogen which could subsequently diffuse into the crystalline lattice of the steel. This aspect has raised numerous doubts in the application of galvanized steel wires and strands in contact with concrete or cement mortars, even if, in principle, the surface zinc layer, although reduced by the aggression of the alkaline environment, should constitute a barrier to the diffusion of hydrogen in steel, at least for short exposure times (Recio et al., 2011). The use of galvanized prestressing strands is not common in North America and is prohibited by the Federal Highway Administration (FHWA) for use in bridges (American Concrete Institute. ACI Committee 222., 2014). The use of galvanizing in prestressing applications and cable-stayed structures, however, is popular in Europe and regulated in specific standards (CEN/TC 459/SC 4 Concrete reinforcing and prestressing steels, 2022). However, the main concern is not so much that of the use of galvanized elements but rather the consequences of the coupling between black steel wires or strands and galvanized elements. In this regard, it is interesting to note that in the DIN 1045 standard, part 1, 2008 edition (Deutsches Institut für Normung, 2008), it is required that there is at least 20 mm of concrete between prestressing cables and galvanized elements or galvanized reinforcement and furthermore there must be no direct metallic connection. This aspect was taken up by J. Mietz, et al. (Mietz et al., 2008) who starting from the consideration that different cases of damage have been observed in practice as a consequence of direct contact between galvanized elements and prestressing steel, carried out a series of experimental tests in different conditions. The authors concluded that a risk for hydrogen-induced stress corrosion cracking of prestressing steels can be excluded for indirect contact between galvanized steel and prestressing steel. Notwithstanding, the demanded minimum distance of 2 cm should still be kept, avoiding direct contact in practice. The fib bulletin 64 (Ganz et al., 2012) addresses the issue in detail starting from the critical aspects raised in the technical-scientific literature when referring to the following two applications: a) The use of bare (non-coated) prestressing steels in direct or indirect contact with galvanized components, such as galvanized ducts for post-tensioning tendons, galvanized inserts or galvanized reinforcing steel near prestressing steel in pretensioned members. b) The use of galvanized prestressing steel in grouted post-tensioning tendons or in pretensioned concrete members. The problem of zinc-steel systems exposed to the atmosphere was also examined with particular attention. This condition is mostly related to the use of high-resistance steel cables and ropes, for example in suspension bridges or cable-stayed bridges but can also be extended to other situations (for example post-tensioned cables in ungrouted ducts, tie rods with galvanized steel, fastening elements, etc.). In detail it is highlighted that in metal to metal contacts where crevice conditions can be established, because of the lack of oxygen in the electrolyte, the corrosion potential of zinc is in the range of the decomposition of water leading to zinc corrosion and hydrogen reduction. Since the latter reaction is unfavored on zinc surface, it occurs directly on the high-strength steel. So, hydrogen induced embrittlement can occur. In the presence of zinc-coated high-strength steel for applications under atmospheric corrosion conditions, design details favoring almost permanently wet surfaces with limited oxygen supply represent unfavorable corrosion conditions with active zinc corrosion and hydrogen evolution which may last for extended periods of time. Therefore, a considerable risk of damage due to hydrogen exists. Similarly, when considering bare prestressing steel embedded in fresh cementitious grout or concrete in contact with galvanized components embedded in the grout or