**4. Discussion**

These results confirmed the higher corrosion susceptibility of the TMAZ/HAZ of the AA 7075 FSW joints with respect to the nugge<sup>t</sup> and the base material, but there is no agreemen<sup>t</sup> between the different geometries of the specimens.

No SCC occurrence was noticed after 4PBB tests, but severe localized attack along the rolling direction at TMAZ/HAZ. Shallow defects at TMAZ/HAZ were observed after CL and SSR tests, which direction was perpendicular respect to the applied load.

The higher corrosion susceptibility of the TMAZ/HAZ can be attributed to the microstructure modification produced by the FSW process. The presence of precipitates at the grain boundaries less noble than the matrix promotes the intergranular corrosion of the alloy [37]. Di fferent mechanisms were proposed to explain their e ffect, Maitra and English a ffirmed that the grain boundaries have a more active breakdown than the matrix and preferentially initiated the pitting [38], while other authors consider the e ffect of acidification generated by the preferential dissolution of the anodic particles as the cause of the continuous grain-boundary attack [11,14]. The intergranular attack through thickness direction (short transverse—S) is much lower than that in either longitudinal (L) and transverse (T) directions for rolled plates owing to the microstructural anisotropy while the breakdown potentials were independent of sample orientations [39].

Similar mechanisms have been formulated to explain the SCC mechanism of aluminum 7XXX alloys, mainly hypothesizing hydrogen embrittlement [18,40], but also slip-dissolution mechanism [41]. Naijar et al. [42] proposed a mixed mechanism, in which the SCC phenomena were initiated from localized attacks at macro-precipitates present on the alloy surface (i.e., Al7Cu2Fe or Al2CuMg) or on grain boundaries. The hydrolysis of the produced cations induces the local acidification of the occluded cell and, as consequence, the possibility to electrochemical evolution of hydrogen. These authors suggested that localized dissolution at precipitates or at the grain boundary correspond to the formation of critical defects, which is the first step of the SCC. The deterioration of the passive film and the anodic dissolution of the metal promotes hydrogen uptake and di ffusion inside the metal lattice. As the defects act as local stress intensifier, intergranular fracture are promoted, especially in the short-transverse direction for which grain boundaries are favorably oriented to the applied tensile stress [43,44]. Vasudevan and Sadananda [45] reported the decrease in the crack propagation rate in NaCl solution with the increasing of the precipitates size at the grain boundaries, and the depletion in copper of the aluminum matrix. The positive e ffect of precipitates size was also underlined by other authors [46].

On the contrary, the results of Ashai et al. [47] on smooth specimens of Al-4.3Zn-1.7Mg alloys in 3.5% NaCl + 0.2% H2O2 solution, demonstrated an inverse correlation between the time to failure (failure rate) and the size of precipitates at the grain boundaries. This can be ascribed to the preferential dissolution of the MgZn2 precipitates at the grain boundaries [48].

Talianker and Cina demonstrated a clear relationship between the presence of dislocations aside to grain boundaries and the susceptibility to stress corrosion of AA7XXX. [48].

The fractographic analysis of the specimens demonstrated the microcracks initiation preferentially at larger precipitates. The nugge<sup>t</sup> and the TMAZ/HAZ showed lower OCP, thus indicating higher susceptibility to corrosion than the base material. No SCC microcracks propagation was noticed at the nugge<sup>t</sup> due to the large size of the strengthening MgZn2 micro-precipitates—that decreased the SCC susceptibility—and the higher tensile strength compared to the TMAZ/HAZ zones. Under constant deformation tests, such as 4PBB tests, the applied stress naturally decreases during the exposure as localized attack propagate toward the specimen thickness. In addition, the tensile stress acts on these specimens in the direction of the elongated grains, and therefore the SCC cracks should propagate in the perpendicular direction. For these reasons, corrosion takes place in form of localized attacks at the elongated grain boundaries along the rolling direction (Figure 9).

In SSR and CL tests, the localized attacks and the plastic strain mainly occurred at TMAZ/HAZ, thus triggering the formation of SCC crack embryos. Based on the abovementioned mechanism, hydrogen embrittlement should promote SCC microcracks propagation. Hydrogen supplied by both the cathodic reaction and the anodic dissolution of the matrix/precipitate interphase can di ffuse in the high stressed zone and promotes brittle crack propagation. The SEM analysis confirms the presence of several submicrometric precipitates in the correspondence of the brittle areas of the fracture surface. The iron-rich phases can consequently enhance the hydrogen evolution mechanism, because the hydrogen overpotential on these phases is lower than that on aluminum. Conversely, the applied plastic strain enhanced the corrosion rate by means of the chemomechanical e ffect underlined by Gutman. This e ffect is higher at the interface between the matrix and second phases, which acts as an obstacle to the movement of the dislocations [28,49]. In these zones, the very high dissolution rate of the alloy is prevalent, and hydrogen cannot reach the critical concentration to produce cracking. In the CL tests the attack becomes enlarged and no failure of the specimens were observed. On the other hand, the only small propagation of microcracks noticed after SSR tests suggests that the final rupture quickly occurred with a shear mechanism because of the high strain rate at the TMAZ/HAZ. For this reason, no evident stress-corrosion cracking phenomena were observed.
