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The anodic dissolution behaviors of TA2 under elastic stress in acidic 3.5 % NaCl solution with or without 5 mM NaF were investigated using in-situ scanning electrochemical microscopy (SECM), along with polarization curve, SEM and AFM. The results indicated that the dissolution of the passive film in 3.5 % NaCl solution was accelerated under 100 MPa, and more resistant to dissolve under 200 MPa. In presence of 5 mM NaF, the stress intensified the dissolution of the matrix. Specifically, 100 MPa stress increased the corrosion difference between different grains, while 200 MPa stress made the corrosion of TA2 more uniform. [Display omitted] • SECM in-situ monitoring was performed on TA2 under elastic stress loading conditions in NaCl solution, with or without F-. • The anodic dissolution of TA2 was accelerated under elastic loading conditions. • The stress enhanced the corrosion difference between different grains. [ABSTRACT FROM AUTHOR]

Metal corrosion exhibits coupled multi-reaction features, making it challenging to quantitatively study the dissolution reaction with variable-valent cations, particularly in valence quantification. This work develops a novel scanning electrochemical microscopy mode and applies it to Ti dissolution study in acidic fluorides. This mode identifies the potential-dependent corrosion reactions and quantifies the valence evolution of Ti from HER to transpassive regions. Results provide direct electrochemical evidence for soluble Ti(z ≤3), for the first time, and revise the reported dissolution equation. The proposed mode overcomes the drawbacks of macro-electrochemical analysis based on coupling information and provides an effective quantitation method for anodic reactions. [Display omitted] • Modified RTG/SC-SECM was proposed to quantitatively study Ti anodic reaction. • Potential-dependent corrosion reaction was identified, implying a coupling feature. • Evolution of dissolution valence was quantified from HER to transpassive regions. • Results provide direct evidence of Ti(z ≤3) products and revise dissolution equation. • This mode is a general mean for dissolution reaction study with multivalent cations. [ABSTRACT FROM AUTHOR]

The behaviors and kinetics of hydrogen evolution reaction (HER) on pure aluminum with passive film in the presence and absence of chloride ion are quantitatively investigated by using the tip generation/substrate collection mode of scanning electrochemical microscopy (SECM) with dual Al/Pt ultramicroelectrode (UME) as tip electrode and Pt UME as substrate electrode. The standard rate constants k 0 and transfer coefficients α H of HER in ClO 4 −- and Cl−-containing solution are 6.9 × 10−7 cm/s and 0.22, 7.1 × 10−6 cm/s and 0.19, respectively. Results show that the kinetic of HER is slow and the destruction of Cl− on passive film can significantly promote the HER on Al surface. Moreover, these α H far less than commonly used 0.5 in corrosion research, can explain the great difference between theoretical Tafel slopes and experimental results. Besides, the existence of current plateau in Al electrode explains the large difference in corrosion potential during parallel testing. The Al tip and Pt substrate current were recorded by scanning the tip potential from −0.967 V to the positive direction until current suddenly drops in 5 mM HCl +0.1 M NaCl solution (graph on the left). Schematic diagram of distance determination by Fc/Fc+ between Pt tip and Pt substrate electrode in 1 mM FcMeOH +0.1 M KNO 3 solution (dotted line), and the quantitative detection of HER generated on Al surface in deaerated acid solution after moving Al tip above the Pt substrate (solid line), as shown in the middle graph. The Al tip current, hydrogen evolution reaction current and Al oxidation current were recorded in the potential from −0.967 V to 1.333 V in 5 mM HClO 4 + 0.1 M NaClO 4 (graph on the right). [Display omitted] • The kinetics of HER on Al surface was studied by the modified TG/SC mode of SECM. • Kinetic parameters k 0 and transfer coefficients α H of HER are obtained. • HER rate in Cl− ion solution is significantly higher than that in ClO 4 − ion solution. • α H far from 0.5 explains the difference of theoretical β c and experimental results. • The current platform can explain the large difference of E corr during parallel testing. [ABSTRACT FROM AUTHOR]

• A glass-ceramic coating incorporated with YSZ and ZrB 2 was prepared. • An electrodeposited SiO 2 interlayer was introduced into the coating system. • The coating bulk and interfacial structures were both reinforced by the particles. • The coated TiAl alloy exhibited satisfactory oxidation resistance at 1000 ℃. With electrodeposited SiO 2 film as the interlayer, 8YSZ (8.0 wt.% Y 2 O 3 partially stabilized ZrO 2) and ZrB 2 particles co-incorporated glass-ceramic coating was fabricated on TiAl alloy. The oxidation performance and interfacial reaction behavior of this coating were studied. The compact glass-ceramic coating inhibited the inward diffusion of oxygen. The reaction between the SiO 2 interlayer and the potassium silicate coating benefited to improve the coating adhesion property and suppress the coating degradation at the coating/substrate interface. Due to the synergy of the incorporated particles and SiO 2 interlayer, both the oxidation performance and interfacial structure of the glass-ceramic coating were improved. [ABSTRACT FROM AUTHOR]

The influences of SO 4 2−, Cl−, ClO 4 − and BF 4 − on the corrosion behavior of Fe in acidic solutions were quantitatively investigated through macro- and micro-electrochemical techniques, combined with DFT calculation. The electrochemical results showed that the corrosion rate increased in the order of BF 4 −, ClO 4 −, Cl− and SO 4 2−, confirmed by the negative shift of OCP and E corr , the decrease of R ct and the increase of k 0 eff of Fe oxidation. DFT calculation showed that the adsorption of SO 4 2− on Fe(100) surface was easier and stronger than Cl−, and caused more significant Fe surface structure changes, resulting in more serious corrosion of Fe. [Display omitted] • The corrosion behavior and mechanisms of Fe in acidic solution under different anions were investigated. • The increase of Fe corrosion rate in the following order of SO 4 2− > Cl− > ClO 4 −> BF 4 −. • The k 0 eff of SO 4 2− was about two orders of magnitude greater than BF 4 −, but α Fe was similar about 0.76. • SO 4 2− caused more significant surface structure changes and electron losses of Fe surface than Cl−. [ABSTRACT FROM AUTHOR]

The current research on the corrosion resistance of high entropy alloys (HEAs), mainly focuses on the influence of element composition and percentage difference. Considering the almost consistent passivation range with different corrosion resistance contributed by element addition, it inevitably exists obvious differences the passive film properties. The effect of passive film on corrosion behavior of HEA is worthy to be investigated in detail. Considering the microstructure transformation detected by electron probe X-ray microanalysis (EPMA) and surface feedback effect measured by approach curve of scanning electrochemical microscopy (SECM), the passivity of Al x (CoCrFeNi) 100-x (x = 0–20) HEAs in 0.5 M H 2 SO 4 solution has been investigated by cyclic potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis and X-ray photoelectron spectroscopy (XPS). The results demonstrate that the passive film is thick and dense at low Al content (5%), which shows excellent corrosion resistance. However, with the increase of Al content, the corrosion resistance of passive film deteriorates gradually. XPS are adopted to detect film composition to explain the reason of film property change. The results suggest that there are more oxidized states of Al and less oxidized states of Cr in the passive film of HEAs with Al addition. The dominate semiconductor characteristics of Al x (CoCrFeNi) 100-x with Al addition change from p-type to n-type. All results indicate that the addition of Al has a significantly influence on the composition and properties of passive film on the Al x (CoCrFeNi) 100-x alloys surface. • The corrosion resistance and semiconductor characterization of passive film on Al x (CoCrFeNi) 100-x was studied. • The phase constituents of Al x (CoCrFeNi) 100-x change from single FCC phase to BCC phases as more Al is being added. • A small amount of Al without phase transformation is beneficial to stabilize a thick and compact passive film. • The passive film semiconductor characteristic of Al x (CoCrFeNi) 100-x with Al addition is predominant from p-type to n-type. [ABSTRACT FROM AUTHOR]

SiOC coating was electrodeposited on TiAl alloy to improve the high temperature oxidation resistance. The mass gain of the compact SiOC coating is 0.06 ± 0.026 mg cm-2, which is one order of magnitude lower than that of TiAl alloy. The SiOC coating exhibits good thermal stability and partial amorphous SiOC transforms into crystal SiO 2 or SiC. After oxidation, no obvious pore can be found in the coating. Moreover, an Al 2 O 3 layer is generated due to the enrichment of Al and the low oxygen partial pressure at the coating/substrate interface, therefore improving the barrier property of the oxide scale. [Display omitted] • SiOC coating is prepared on TiAl alloy by electrodeposition. • The high temperature oxidation resistance of the TiAl alloy with the SiOC coating is dramatically improved. • Thermal exposure results in partial amorphous SiOC transforms into crystal SiO 2 or SiC. • The outward diffused Al can strengthen the network structure of the SiOC coating and establish an Al 2 O 3 intermediate layer. [ABSTRACT FROM AUTHOR]

The development of crevice corrosion is closely related to changes in the microchemical environment in crevices. However, it is difficult to detect the microchemical information inside the crevice using conventional electrochemical techniques. To this end, a Pt/IrO x -pH ultramicroelectrode (UME) with a radius of 690 nm was developed to study the local radial pH distribution in the crevice using the potentiometric mode of SECM in situ. This pH UME showed excellent sensitivity and stability over a wide pH range between 1.0 and 13.0. The feasibility of this pH UME was demonstrated in the study of the radial pH distribution within the slit with a slit of 101.30 μm for 201 stainless steel (201- SS). The result showed that a significant radial pH difference was observed within the gap: the largest pH drop was observed at the opening from pH = 4.2 to 1.6, followed by the middle and the bottom. This result agreed well with the images of in-situ corrosion morphology seen in optical microscopy. This combination of SECM and submicron pH UME could be used to explore the mechanisms of crevice corrosion in various metals as well as the early stages of pitting corrosion. [ABSTRACT FROM AUTHOR]

The influence of surface pretreatment on the oxidation performance of the anodized TiAl alloy was investigated. Before anodization in NH 4 F containing ethylene glycol, TiAl alloy was sand blasted or ground by SiC paper. Results showed that the humps on the rough surface serve as crystallographic templates for the nucleation of the isostructural α-Al 2 O 3 at the initial stage of oxidation. The derived Al 2 O 3 act as the crystal nuclei for the formation of continuous Al 2 O 3 oxide scale during thermal exposure, therefore providing good oxidation resistance. The surface roughness acts a key role to improve the oxidation resistance of the anodized TiAl alloy. • Surface roughness has a great influence on the oxidation resistance of anodized TiAl alloy. • Sand blasting results in a deformation zone and prompts the generation of Al 2 O 3. • The humps on the surface serve as templates for the nucleation of isostructural Al 2 O 3. • Surface roughness but not surface deformation acts a key role in the oxidation performance of anodized TiAl alloy. [ABSTRACT FROM AUTHOR]

The galvanic corrosion of titanium/304 stainless steel (TA1/304SS) couple was investigated in neutral and acidic solution with and without fluorine ion (F-) using macro-electrochemical technique and scanning electrochemical microscope (SECM). The results show that the synergistic effect of H+ and F- affects the passive film on the surface of galvanic couple, resulting in a great change in heterogeneous reaction rate constant and surface electrochemical activity characterized by approach curve and surface mapping of SECM in feedback mode. In addition, a typical polarity reversal occurs in pH = 2.0 acidic solution with adding F- which the TA1 is anodic to 304SS. [Display omitted] • Direct measuring the surface reactivity differences of passive film on TA1 and 304SS using multiscale electrochemical techniques. • The corrosion of TA1 is more serious than that of 304SS due to the synergistic effect of H+ and F- to the passive film. • A typical polarity reversal in the TA1/304SS couple was found after adding F- in pH = 2.0 acid solution. [ABSTRACT FROM AUTHOR]

Ni nanoparticle incorporated SiO 2 coating was fabricated on Ti48Al2Nb2Cr alloy by electro-codeposition. Results showed that the Ni-SiO 2 composite coating exhibits good isothermal and cyclic oxidation resistance at 900 ºC. The incorporation of Ni particles can adjust the coating thermal stress. After oxidation, no crack or pore can be found in the Ni-SiO 2 composite coating. The consumption of the inward diffused oxygen could reduce the oxygen partial pressure at the coating/substrate interface, therefore promoting the selective oxidation of Ti48Al2Nb2Cr to form Al 2 O 3. Owing to the good compatibility with SiO 2 , the oxidation product of NiO could strengthen the structure of the coating. [Display omitted] • The Ni-SiO 2 composite coating exhibits good isothermal and cyclic oxidation resistance. • The incorporated Ni particles can adjust the coating thermal stress and prevent the generation of cracks. • The inward diffused oxygen is consumed by the Ni particles, promoting the selected oxidation of Ti48Al2Nb2Cr to form α-Al 2 O 3. • The structure of the oxide scale can be improved by the oxide product of NiO. [ABSTRACT FROM AUTHOR]

The long-term isothermal oxidation and cyclic oxidation performance of the Ti48Al2Nb2Cr alloy anodized in NH 4 F containing electrolyte were investigated. After isothermal oxidation at 1000 °C and cyclic oxidation at 900 °C for 1000 h, the weight gains of the anodized Ti48Al2Nb2Cr alloys are 1.12 mg cm−2 and 0.48 mg cm−2, respectively. The excellent oxidation performance is ascribed to the fluorine effect. Raman and photo-stimulated luminescence spectra revealed that anodization impedes the formation of rutile and promotes the generation of α-Al 2 O 3 at high temperatures. The Al 2 O 3 and TiO 2 formed during anodization act as crystal nucleus for the generation of α-Al 2 O 3 -based scale. Al 2 O 3 and TiO 2 formed at the anodization process, together with the fluorine effect, promote the generation of protective α-Al 2 O 3 scale. [Display omitted] • A fluorine-enriched region can be found beneath the anodic film. • The anodized Ti48Al2Nb2Cr alloy exhibits excellent oxidation performance. • Al 2 O 3 and TiO 2 formed during anodization promote the generation of α-Al 2 O 3 -based scale. • The improved oxidation resistance can be ascribed to the fluorine effect. [ABSTRACT FROM AUTHOR]

A modified tip generation/substrate collection mode of SECM was used to separate the hydrogen evolution reaction (HER) current from the coupled multireaction current on pure titanium spontaneously passivated in air (P-Ti), activated by electrochemical method (EA-Ti) and fluoride ion (FA-Ti). Besides, the HER kinetics of Ti electrodes pretreated by different approaches were also simulated. The pretreatments influence the HER behavior and kinetics on Ti through changing the electronic structure of passive film. [Display omitted] • Effect of pretreatment on HER behavior and kinetics of Ti was studied. • Coupled multireaction current on Ti was quantitatively separated by modified TG/SC mode of SECM. • Kinetics parameters of HER on Ti with different pretreatment were simulated. • Pretreatment change the electronic structure of passive film thus affect HER on Ti. The behaviors and kinetics of hydrogen evolution reaction (HER) on pure titanium spontaneously passivated in air (P-Ti), activated by electrochemical method (EA-Ti) and fluoride ion (FA-Ti) were investigated by impedance and scanning electrochemical microscopy (SECM). During cathodic polarization, the passive film and proton were simultaneously reduced on P-Ti, only HER current was detected on EA-Ti, while both HER and active/passive transition behavior presented on FA-Ti. Corresponding transfer coefficients and standard rate constants of HER are 0.48/6.00 × 10−12 cm/s, 0.25/7.60 × 10−7 cm/s and 0.24/1.14 × 10−6 cm/s, respectively. These differences originate from the change in electronic structure of passive film by pretreatments. [ABSTRACT FROM AUTHOR]

• Features of naturally aged (NA) and artificially aged (AA) alloys stored for two years were studied. • Microstructure of NA alloys has changed, while that of AA alloys is almost the same as two years ago. • Numerous finer precipitates better mechanical response and lower corrosion resistance of NA alloys. • Coarse precipitates and high-density dislocations limit the elongation of AA alloys. • The better protective rust and weaker galvanic effect improve the corrosion resistance of AA alloys. This work presents a comparison of microstructure, mechanical properties and corrosion behavior of naturally aged (NA) and artificially aged (AA) alloys after two years of storage. The results showed that numerous finer precipitates distributed in NA alloys, resulting in better mechanical response but higher corrosion sensitivity, while the microstructure of AA alloys were almost the same as two years before, the tensile results showed the initial necking phenomenon but limited elongation caused by presence of coarse precipitates and high-density dislocations, whilst its corrosion resistance was better than NA alloys, reflecting from smaller pits and more protective components of corrosion products. [ABSTRACT FROM AUTHOR]

• Electrodeposited SiO 2 coating can efficiently improve the hot corrosion resistance of TiAl alloy. • Compact and adherent corrosion scale generates during the hot corrosion process. • The dissolution of SiO 2 into the molten sodium salt leads to the generation of Na 2 Si 4 O 9. • The compact Al 2 O 3 layer at the coating/alloy interface benefits to improve the hot corrosion resistance. To improve the hot corrosion resistance, thickness controllable SiO 2 coating was electrodeposited on TiAl alloy. The hot corrosion behavior of the specimens contaminated with 75 wt.% Na 2 SO 4 + 25 wt.% NaCl salt mixture was investigated at 700 °C. Results showed that the generation of compact amorphous SiO 2 embedded with Na 2 Si 4 O 9 and cristobalite was responsible for the enhanced hot corrosion resistance. Moreover, the SiO 2 coating does not only act as a diffusion barrier to sulfur and chlorine but also decreases the oxygen partial pressure at the coating/alloy interface, therefore promoting the selective oxidation of Al to form protective Al 2 O 3 layer. [ABSTRACT FROM AUTHOR]

• Ti45Al8.5Nb was anodized in NH 4 F containing solution. • Nb can reduce the critical Al content to form a continuous alumina layer. • The anodized Ti45Al8.5Nb exhibited excellent oxidation resistance at 1000 °C. • The improved oxidation resistance can be ascribed to halogen effect. Ti45Al8.5Nb was anodized in ethylene glycol and NH 4 F containing solution to improve the oxidation resistance. Isothermal oxidation test shows that the anodized specimens can provide excellent oxidation resistance and spallation resistance under 1000 °C. During the high temperature oxidation process, a compact and continuous alumina layer would generate on the anodized Ti45Al8.5Nb alloy. This protective alumina layer can efficiently decrease the inward diffusion of oxygen and outward diffusion of the substrate element. The improved oxidation resistance is assigned to the halogen effect caused by the fluorine component generated during the anodization process. [ABSTRACT FROM AUTHOR]