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Calabrese et al. [35,36] monitored the stress-corrosion cracking (SCC) process for 17-4PH MSS in a hot MgCl2 solution by electrochemical noise and acoustic emission techniques; they showed that the corrosion mechanisms evolved from localised pitting to SCC. This was in good agreement with largely accepted conclusions that assumed pits were preferential sites for SCC initiation for SS [37,38,39,40,41,42]. Therefore, it could be assumed that differences in the austenite to martensite ratio, which have been related to various pitting behaviours, were likely to lead to changes in the EAC susceptibility of 17-4PH MSS; thus, differences in EAC susceptibility between L-PBF MSS and its conventional counterpart could be observed. Furthermore, the film-rupture depassivation-repassivation (FRDR) model clearly correlated the rupture of the passive film due to mechanical loading and the EAC susceptibility [43,44,45,46,47]. In that sense, the passive film properties constituted a major parameter to explain EAC susceptibility; therefore, any microstructural changes leading to modifications in the passive film properties could lead to an evolution in the EAC susceptibility.
Film Impact Transitions Crack Mac
Nevertheless, even though pitting was identified as a major process in the EAC susceptibility of 17-4PH MSS, the influence of hydrogen could not be disregarded. Protons produced by anodic reactions (formation of the passive film as well as pitting) and subsequent cation hydrolysis may lead to the formation of hydrogen. Hydrogen may contaminate the passive film, making it more susceptible to initiation and propagation of stable pits in potential ranges where the material was supposed to be passive. Once the pits had formed, they constituted a confined environment for the intensification of anodic dissolution processes, and thus, for the production of hydrogen. Then, the hydrogen could interact with the dislocations and be transported, leading to the formation of hydrogen-rich zones where corrosion processes are promoted, resulting in the establishment of a self-sustaining system. In other words, this meant that an autocatalytic process had to be considered. This scenario was in agreement with the work of Thomas et al.  who showed that, on the basis of potentiodynamic tests carried out on steels not charged with hydrogen and cathodically precharged with hydrogen, the corrosion potential moved towards more negative values while the corrosion current increased for steels precharged with hydrogen. These authors also showed that the anodic dissolution of the Fe matrix increased considerably for hydrogen precharged steels. They also noted that, due to its reducing power, hydrogen could reduce the passive film and modify its properties when it was absorbed in the first atomic layers of the matrix, in agreement with other works [58,59]. In the present work, the higher hydrogen content measured for MSS produced by AM as compared with MSS obtained by CM, for identical exposure conditions, therefore, would be consistent with a higher growth of stable pits for the AM samples than for the CM samples, and perhaps even with more intense metastable pits for the AM samples than for the CM samples. Furthermore, the influence of reversed austenite on hydrogen uptake was a major parameter, and the ratio austenite to martensite had to be considered to understand the susceptibility to EAC of the 17-4PH MSS. Indeed, it is well-known that the solubility limit of hydrogen in austenite is much higher than in martensite [60,61,62,63,64], which could contribute to explain that AM samples of MSS were more enriched in hydrogen after the EAC tests than the CM samples. Then, the influence of hydrogen on the mechanical properties should be considered. Here, the austenite to martensite ratio was also a crucial parameter. Indeed, we have shown, in previous work, significant differences in the relaxation properties of MSSs produced by AM and CM due to the higher amount of reversed austenite in AM samples . Furthermore, hydrogen was shown to increase the mobility of dislocations in austenite. This could contribute to explain the significant decrease in YS0.2 and flow stress values observed on the tensile curves plotted for the AM samples of MSS after the EAC tests (Figure 6). Finally, as previously mentioned, pit propagation was associated with the austenite grains. Hydrogen/dislocation interactions that occur inside the austenite grains might impact pit propagation, and thus, pit morphology and stress distribution at the bottom of the pits. The contribution of hydrogen to the EAC susceptibility of 17-4PH MSS will be evaluated in future work. 350c69d7ab