Fig. 1. In-situ ex-environment TEM observation showing localization of inhomogeneous dissolution of MnS in a stainless steel: (a) SEM image of as-received 316F stainless steel showing distribution of needle-like MnS inclusions (in black); (b) HAADF image showing a MnS inclusion section, in which several nano-particles embedded in MnS are arrowed and labelled; (c) same section as that in (b) but suffered the corrosion in 1 mol/L NaCl solution for 45 min. The localized dissolution of MnS happened around the particles; (d) zoom-in images of nano-particles in (b) labelled with I, II, III, IV, and V; (e) zoom-in images of local dissolution around nano-particles in (c); (f) dissolution mode visualized by digitizing contrast in experimental images in (e) [24].

Fig. 2. Composition analysis on a nano-MnCr2O4 around which MnS dissolution occurs in the presence of salt water: (a) HAADF image showing a pit in MnS around a particle; (b) EDS results of a scan made along the red line in (a). The pit contributes little to MnS signals which provide a clear imprint of MnS dissolution [24].

Fig. 3. In-situ ex-environmental TEM showing a continuous development of a pit versus immersion duration in a fixed MnS inclusion where two MnCr2O4 particles are marked with arrows. The formation of pit results from the dissolution of MnS around the MnCr2O4 particle. The micrographs are recorded in the HAADF mode: (a) before immersion, the contrast difference between MnCr2O4 particles and the MnS medium is little, which is due to the similar high-angle scattering of the elements in these two compounds; (b) after 15 min immersion in 1 mol/L NaCl solution; (c) after 30 min immersion; (d) after 60 min immersion. It is seen that the pits around the MnCr2O4 particles spread with the increasing of immersion duration [24].

Fig. 4. Identification of an octahedron by means of large-angle tilting experiments and 3D tomography. The octahedron is enclosed by eight triangles labelled with I, II, III…..VIII, respectively [24].

Fig. 5. Crystal structure of spinel MnCr2O4: (a) 3D atomic configuration in a unit cell of spinel MnCr2O4; (b) atomic projection of structure along [110] direction. Four (111) sub-layers are highlighted; (c) atomic configurations where oxygen ions are located at the (111) surface layer, and Cr ions are underneath oxygen (This is designated as O-Cr configuration); (d) atomic configurations where Cr ions are located at the terminal layer (Cr-O configuration); (e) atomic configurations where O-Mn puckered layer is at the terminal (O-Mn configuration); (f) atomic configurations where Mn-Cr puckered layer is at the terminal [24].

Fig. 6. Structure models of an octahedron with variant terminal layers: (a) Cr-terminated octahedron; (b) Mn-terminated octahedron; (c) O-terminated octahedron. The reactivity of an octahedron strongly depends on the terminal ions at the surface [24].

Fig. 7. In-situ ex-environment TEM observations showing that the localized dissolution of MnS in a stainless steel preferentially initiated at the dislocation emergences: (a) HAADF-STEM image of a section of MnS, which shows a homogeneous contrast without any visible precipitates inside; (b) the same MnS as that in (a) but suffered 1 h immersion in NaCl solution; (c) 3D visualized image of dissolution topography, digitizing from the contrast of the image in (b); (d-f) bright field TEM images taken under g = (200), g = (220) and g = (020) two-beam conditions, respectively, showing the dissolution taking place at both the edge and screw dislocation emergences; (g) schematic illustration of the dislocation characteristics and relative dissolution pits [25].

Fig. 8. SEP variations around dislocation. The values drop sharply in the region with $\widetilde{1}$ nm radius around the dislocation core [25].

Fig. 9. Preferential dissolution of Cu-rich phase induced evolution in local chemistry: (a) HAADF-STEM image showing nanometer Cu-rich phase particles with brighter contrast dispersedly distributed in the matrix; (b) zoom-in image of the zone enclosed within the white box in (a), showing two Cu-rich particles; (c-g) EDS mapping analysis focusing on the two particles; (h) the same area as that in (a) after immersion in 3.5 wt% NaCl for 30 min. Preferential dissolution occurred in the Cu-rich phase particles; (i) the same two particles as that in (b) yield darker contrast after dissolution; (j-n) EDS mapping analysis focusing on the evolution in chemistry of the two particles; (o-r) composite graphs obtained by superimposing the O map (red) on the Cr map (green) with variant opacity of 10% (o), 30% (p), 50% (q) and 100% (r); (s-v) composite image obtained by superimposing the O map (red) on the Fe map (yellow) with variant opacity of 10% (s), 30% (t), 50% (u) and 100% (v). The ring enriched in O just overlapped with the ring of Cr and Fe [28].

Fig. 10. Structural evolution with various degree of dissolution at the atomic scale: (a) high resolution HAADF-STEM image along the [110] direction showing the Cu-rich particle well orientated with the matrix. The inset is a zoom-in image of the zone enclosed within the white box in (a). The Cu-rich phase has a cube-on-cube crystallographic orientation with the Matrix. The (111) plane spacing of the Cu-rich phase and the matrix is approximately 2.1 ?; (b) high resolution HAADF-STEM image along the [110] direction showing the Cu-rich phase after slight dissolution; (c) zoom-in image of the zone enclosed in square in (b) showing that the center zone had become amorphous due to dissolution, the immediate adjacent zone became the transition region wherein lattice fringes are still obvious; (d) high resolution HAADF-STEM image along the [110] direction showing the Cu-rich phase after severe dissolution. A spinel oxide ring was formed [28].

Fig. 11. In situ ex-environmental TEM observation showing the local dissolution of S phase: (a) SEM image of the as-received 2024Al showing coarse intermetallic particles: S (Al2CuMg), θ (Al2Cu) and Al-Cu-Mn-Fe phases; (b) HAADF image showing an S phase particle embedded by some nanoparticles; (c) the same particle as that in (b) but suffering an immersion in 0.5 mol/L NaCl for 15 min. It shows the local dissolution around these nanoparticles [48].

Fig. 12. Identification of nanoparticle: (a) bright-field TEM image of nanoparticle; (b) electron diffraction pattern (EDP) obtained from the nanoparticle in (a); (c) EDS analysis of the particle which is composed of Al, Mn and Cu; (d) HRTEM image along the same axis in (b) [48].

Fig. 13. Observation on the continuous development of local dissolution of S phase and the resulting dissolution of Al martrix. The Al20Cu2Mn3 particles are arrowed: (a) HAADF image showing slightly local dissolution occurs and fine pits form at the Al20Cu2Mn3; (b) pits enlarge to the S phase at the periphery of dissolved Al20Cu2Mn3; (c) when the pits are extended to reach the boundary of S phase/matrix, the dissolution develops along the boundary quickly and results in the dissolution of the adjacent Al matrix; (d) cathodic S-phase remnant, on the one hand causing the Al matrix dissolving, and on the other hand continue accelerating the S-phase dissolution to extend into the interior [48].

Fig. 14. (a) HAADF image showing an S phase particle in which some Al20Cu2Mn3 particles dissolve severely (marked with black arrows) and S phase, even Al matrix, also dissolves locally, while the other four particles labeled as I, II, III and IV have no change. The specimen was immersed in 0.5 mol/L NaCl for 25 min and (b) bright-field TEM image corresponding to (a); (c) zoom-in TEM images of the particles I, II, III and IV [48].

Fig. 15. Twins in Al20Cu2Mn3 provide the initial dissolution sites: (a) bright-field TEM images of Al20Cu2Mn3 particles showing the feature of twins in Al20Cu2Mn3; (b) HADDF images of the same particles showing the initial site of dissolution in Al20Cu2Mn3. The specimen was immersed in 0.5 mol/L NaCl for 25 min [48].

Fig. 16. Heterogeneous chemistry induced local dissolution at the atomic scale: (a) HAADF-STEM image along the [010] axis showing an Al20Cu2Mn3 particle pre-immersed for about 8 min in 0.5 mol/L NaCl electrolyte. Slight dissolution occurs adjacent to the bright curled streak and a few dissolution sites with darker contrast encircled by the bright curled streak are obvious; (b-e) electron energy loss spectroscopy (EELS) mapping analysis focusing on the four spots enclosed within the box in (a). The darker spots are depleted in the elements Al, Mn, Cu and enriched in element O, while the brighter rings are enriched in Cu; (f) magnified image corresponding to the boxed area in (a), highlighting the lattice and the hexagonal subunits beneath the corroded sites; (g) magnified image of the boxed area in (f) showing a large number of defects within the Cu-rich streak [49].