Fig. 1. Optical macrographs of the surface of the friction stir weldment of the 2098-T351 alloy showing the welding zones at the retreating side (RS) and advancing side (AS) of the weldment. TMAZ is the thermomechanical affected zone, HAZ is the heated affected zone and WJ is the welding joint which corresponds to the stir zone (SZ) and the TMAZ.

Fig. 2. Schematic diagram of the 2098-T351 alloy welded by FSW showing the regions (red rectangles) analyzed by SVET.

Fig. 3. Optical images of the surface of the 2098-T351 alloy welded by FSW after various immersion times in 0.005 mol L-1 NaCl solution.

Fig. 4. Optical images in high magnification of the squared regions in Fig. 3 of the 2098-T351 alloy welded by FSW after various immersion times in 0.005 mol L-1 NaCl solution.

Fig. 5. EDX analysis of the (a) Cu-rich and (b) Cu, Fe-rich micrometric particles in the 2098-T351 alloy.

Fig. 6. SEM micrographs showing the distribution and sizes of the micrometric particles in the (a) welding joint and (b) base metal of the 2098-T351 alloy.

Fig. 7. TEM bright-field images showing nano-sized phases distributions in the (a) base metal (BM), (b) heated affected zone (HAZ) and (c, d) welding joint (WJ).

Fig. 8. Agar visualization test for different configurations of the 2098-T351 alloy welded by FSW: (a) galvanic coupling between the welding joint and the heat-affected zone (WJ/HAZ) for both the retreating (RS) and advancing (AS) sides; (b) three differently separated regions of the weldment comprising the base metal and HAZ (BM/HAZ) and the thermomechanical affected zone and stir zone (TMAZ/SZ) or WJ.

Fig. 9. High-resolution XPS spectra obtained in the welding joint (WJ) of the 2098-T351 alloy welded by FSW prior to (polished) and after (corroded) 24 h immersion in 0.005 mol L-1 NaCl solution.

Fig. 10. Open circuit potential (OCP) measurements as a function of time of immersion in 0.005 mol L-1 NaCl solution for the different welding zones. Measurements were obtained every 2 h for 24 h.

Fig. 11. SVET maps of the welded 2098-T351 alloy after 2 h of immersion in 0.005 mol L-1 NaCl solution.

Fig. 12. SVET maps (a-c) and in situ optical images (d-f) of the 2098-T351 alloy welded by FSW after 24 h immersion in 0.005 mol L-1 NaCl solution.

Fig. 13. Optical profilometry images of the corroded surfaces of the welding zones of the 2098-T351 alloy after 24 h of immersion in 0.005 mol L-1 NaCl solution: (a) HAZ (RS); (b) WJ; (c) HAZ(AS); (d) depth penetration profile relative to dashed lines in (a-c).

Fig. 14. SECM maps corresponding to the TMAZ/HAZ boundary in the (a) RS and (b) AS of the welding joint in 0.005 mol L-1 NaCl solution; (c) welded 2098-T351 alloy after 8 h of immersion in 0.005 mol L-1 NaCl solution; welding zones features in the (d) RS and (e) AS, revealed after etching (2% HF and 25% HNO3).

Fig. 15. Plots correlating the microhardness across the top surface of the weld with the SECM line scan plot along the welding joint.