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ORR activity and selectivity on native oxide film-covered Ti were quantitatively studied by the reactive tip generation/substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) and further clarified by theoretical calculations. The ORR behavior on Ti is highly film characteristics-dependent. [Display omitted] • ORR activity and selectivity on spontaneous film-covered Ti were studied. • Effects of key factors were quantified by SECM and further clarified by theoretical calculations. • ORR is sensitive to film features, especially on the highly reduced Ti surface. • Hydrogen bonds and/or electrostatic effect improve 4e– selectivity while Cl– suppress reactivity. • This work provides theoretical support and possible guidance for ORR on oxide-covered metals. Unlike noble metals, the oxygen reduction reaction (ORR) behavior on Ti is more complicated due to its spontaneously formed oxide film. This film results in sluggish ORR kinetics and tends to be reduced within ORR potential region, causing the weak and multi-reaction coupled current. Though Ti is being used in chemical and biological fields, its ORR research is still underexplored. We innovatively employed the modified reactive tip generation-substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) with high efficiency of 97.2 % to quantitatively study the effects of film characteristics, solution environment (pH, anion, dissolved oxygen), and applied potential on the ORR activity and selectivity of Ti. Then, density functional theory (DFT) and molecular dynamics (MD) analyses were employed to elucidate its ORR behavior. On highly reduced Ti, film properties dominate ORR behavior with promoted 4e– selectivity. Rapid film regeneration in alkaline/O 2 -saturated conditions inhibits ORR activity. Besides, ORR is sensitive to anion species in neutral solutions while showing enhanced 4e– reduction in alkaline media. All the improved 4e– selectivities originate from the hydrogen bond/electrostatic stabilization effect, while the decayed ORR activity by Cl– arises from the suppressed O 2 adsorption. This work provides theoretical support and possible guidance for ORR research on oxide-covered metals. [ABSTRACT FROM AUTHOR]

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]

In the notch microenvironment caused by the stress corrosion of pipeline steel, the local pH value changes mediated by anodic-cathodic corrosion reactions played an important role in comprehending the local corrosion behavior. To address this issue, a radius 5 µm all-solid-state Pt/RuO x ultra-micro pH sensor was developed using cyclic voltammetry deposition method, which was combined with scanning electrochemical microscopy (SECM) to achieve in-situ monitoring the local pH distribution and evolution of the pre-notched X80 pipeline steel. The micro-sensor demonstrates a near Nernstian slope of −60.0 ± 3 mV/pH within the pH value range of 2.00–12.00, accompanied by an exceptionally rapid response time of 200 ms. Moreover, it exhibits high accuracy, strong long-term monitoring ability and remarkable resistance to the interference of common ions in seawater and ferrous ions generated by the corrosion of pipeline steel. Through monitoring the local pH distribution in simulated seawater solution at the notch of X80 pipeline steel, it was found that obvious pH alkalization occurs first on both the surface and inside of the notch, followed by gradual acidification, and this phenomenon is more obvious at the front of the notch. Combined with other characterization methods, a possible corrosion reaction mechanism at the notch of stress corrosion was proposed. [Display omitted] • A simple method for fabricating an all-solid-state Pt/RuO x micro-sensor with rapid response to pH value change. • The micro-sensor exhibits high accuracy, strong long-term monitoring ability and remarkable anti-interference performance. • In-situ monitoring the pH value change on the surface and inside of the pre-notch X80 pipeline steel. • Obvious pH alkalization occurs first on the notch, and then gradually acidification, especially at the notch front. [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]

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]

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]

Although the existence of short-lived univalent magnesium cation (Mg+) was supposed to play an important role on negative difference effect (NDE) during anodic dissolution of magnesium in previous work, no direct proof of the existence of Mg+ has ever been reported. Here, a modified substrate generation/tip collection mode of scanning electrochemical microscopy was applied to detect Mg+ via the reaction of generated Mg+ and potassium ferricyanide (K 3 [Fe(CN) 6 ]). Based on the new design method and experimental data, our studies provide a direct evidence of the presence of Mg+ in aqueous solution combining with open circuit and polarization condition, which is benefit to understand NDE. A modified substrate generation/tip collection (SG/TC) mode of SECM with carbon fiber ultramicroelectrode as tip was applied to detect the unstable univalent Mg cation (Mg+) via the reaction of Mg+ and [Fe(CN) 6 ]3− during the dissolution of Mg under OCP and anodic polarization. After 3 h of immersion, the tip current is much lower than that before immersion due to the surface film formed on Mg substrate. The tip current is basically unchanged and tends to 0 when the tip was placed far away from the Mg substrate Unlabelled Image • A novel electrochemical method was proposed for the detection of Mg+. • The reaction products of Mg+ and [Fe(CN) 6 ]3− was monitored using SECM in a modified SG/TC mode. • The corrosion rate of Mg decreases after 3 h immersion in the case of 2 μm gap. • The production of Mg+ was promoted under anodic polarization than that at OCP. [ABSTRACT FROM AUTHOR]