PSI - Issue 62

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

288

4

microstructure to hydrogen embrittlement increases in the following order: pearlitic grades, quenched and tempered, bainitic steels formed by continuous cooling, and finally martensitic structures. For prestressing steels in a certain steel grade (hot-rolled, quenched and tempered, cold drawn), the influence of hydrogen on crack formation increases with increasing strength (U. Nürnberger, 1997). According to the relationship (FIP, 1996):

3

(1)

=

9

the lifetime L (as estimated on the basis of hydrogen embrittlement tests) decreases with the 3 rd power of the stress ( ) and the 9 th power of the ultimate tensile strength R m (the factor C being a characteristic value for the material stability in the medium concerned). Hydrogen embrittlement can occur even when the steel is not stressed and becomes evident when the load is applied. Hydrogen can also be responsible for other mechanisms which generally fall under the denomination of stress corrosion cracking (SCC) or more specifically hydrogen-induced stress corrosion cracking (HISCC) or hydrogen assisted cracking (HAC). In all cases, the presence of a stress state and a corrosive environment capable of promoting the reduction of adsorbable hydrogen is required. The process of brittle SCC fracture of prestressing wires consists of three stages; (a) the crack initiation stage, (b) the subcritical crack propagation stage and (c) the final fracture due to mechanical overloading. Crack initiation is a possible consequence of corrosion mechanisms acting on steel surface and of localized corrosion forms (for example pitting or crevice, Fig. 1).

Fig. 1. Cracks initiated at the bottom of crevice attack on a prestessing steel wire in a calcium hydroxide saturated solution (pH 12.4) closed to air with 0.2 M NaCl (Proverbio & Bonaccorsi, 2002)

In this case the crack is triggered from the bottom of the pit where the pH value drops significantly due to the hydrolysis of the ferrous ions (U. Nürnberger, 2002). In this stage, the mechanisms of adsorption and diffusion of atomic hydrogen play a decisive role in competition with the recombination phase and therefore the presence of