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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]

The oxidation of Ni nanoparticles can reduce the oxygen partial pressure at the coating/substrate interface, which promotes the formation of protective Al 2 O 3 layer and enhance the oxidation resistance of TiAl alloy. [Display omitted] • Ni nanoparticles incorporated SiOC coating was fabricated to improve the oxidation resistance of TiAl alloy. • The reaction between Ni nanoparticles and SiOC coating promotes the generation of Ni 2 Si and affects the structure of the coating. • The oxidation of Ni nanoparticles reduces the oxygen partial pressure and promotes the selective oxidation of Al. In this work, SiOC coatings incorporated with Ni nanoparticles were fabricated on γ-TiAl alloy by dip-coating. The microstructure and high temperature oxidation behavior of the SiOC-Ni coatings with different Ni contents were investigated. Results showed that the incorporation of Ni nanoparticles improved the oxidation resistance of the SiOC coating at 850 ℃. During thermal exposure to air, the incorporated Ni particles would react with both oxygen and the SiOC coating, forming NiO and Ni 2 Si, respectively, therefore changing the structure of the SiOC coating. The generation of NiO and Ni 2 Si is beneficial in reducing the oxygen partial pressure at the coating/substrate interface, thereby facilitating the formation of a protective aluminum oxide layer. [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]

Tracking the variation of Cl− timely within the crevice is of great significance for comprehending the dynamic mechanism of crevice corrosion. The reported chloride ion selective electrodes are difficult to realize the long-time Cl− detection inside the confined crevice, due to their millimeter size or a relative limited lifespan. For this purpose, an Ag/AgCl ultra-micro sensor (UMS) with a radius of 12.5 μm was fabricated and optimized using laser drawing and electrodeposition techniques. Results show the AgCl film's structure is significantly impacted by the deposited current density, and further affects the linear response, life span and stability of Ag/AgCl UMS. The UMS prepared at current density of 0.1 mA/cm2 for 2 h shows a rapid response (several seconds), excellent stability and reproducibility, strong acid/alkali tolerance, sufficient linearity (R2 > 0.99), and long lifespan (86 days). Moreover, combined with the potentiometric mode of scanning electrochemical microscope (SECM), the Ag/AgCl UMS was successfully applied to monitor the in-situ radial Cl− concentration in micro-regions inside a 100 μm gap of stainless steel. The findings demonstrated that there was obvious radial difference in Cl− concentration inside the crevice, where the fastest rise in Cl− concentration was at the opening. The proposed method which combines the UMS with SECM has attractive practical applications for microzone Cl− monitoring in real time inside crevice. It may further promote the study of other localized corrosion mechanism and the development of microzone ions detection method. An Ag/AgCl UMS with a radius of 12.5 μm was developed for in-situ rapid, highly sensitive detction of radial Cl− concentration in micro-regions inside a 100 μm gap. [Display omitted] • A simple method for preparing Ag/AgCl UMS with rapid response to Cl− concentration change. • The UMS has excellent stability and reproducibility, strong acid/alkali tolerance, sufficient linearity, and long lifespan. • The deposition current density and time significantly affect AgCl film formation and microstructure. • The pores and microchannels on AgCl film reduce the stability of Ag/AgCl UMS. • In-situ radial monitoring Cl− concentration differences at different locations inside crevice. [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 duplex composite coating consisting of electrodeposited SiO 2 interlayer and glass-ceramic coating was fabricated on TiAl alloy. The oxidation behavior of the composite coating was studied in air at 1000 °C. The composition, morphology, and microstructure of the oxide scale were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). Results showed that the composite coating exhibited outstanding resistance against high temperature oxidation. After 100 h oxidation, no scale spallation was observed and the weight gain was only 1.16 mg cm−2. The inward diffusion of oxygen and outward diffusion of Ti were efficiently prevented. A continuous and adherent Al 2 O 3 layer was formed at the coating/substrate interface. The electrodeposited SiO 2 interlayer could improve the adhesion between the substrate and glass-ceramic coating. • Glass-ceramic coating and SiO 2 interlayer were fabricated on TiAl alloy. • SiO 2 interlayer can dissolve into the glass-ceramic coating and reduce its mobility. • The adhesion property of the glass-ceramic coating was enhanced. • The formation of nitride at the coating/substrate interface was suppressed. [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]

• W/WOx-pH micro-sensor is in-situ fabricated on UME using constant current anodizing. • Micro-sensor shows good linearity, sensitivity and anti-interference in solutions. • Both anodic and cathodic sites appear on Fe surface under galvanic effect with Cu. Localized and interface pH values are important parameters for studying the corrosion progress and complex interface reactions. Although commercial glass pH sensors have been widely used to measure solution pH value due to their advantages of long stability and high sensitivity, they were unable to determine the interface and microregion pH value for their difficulty in miniaturization. In this work, an all-solid-state pH micro-sensor with strong adhesion strength was in-situ fabricated on diameter 15 μm tungsten ultramicroelectrode (UME) using constant current anodizing. The pH micro-sensor showed good linear response (R2 = 0.999), excellent sensitivity (with a slope of −53.8 mV/pH), quick response (<0.1 s), long lifetime (>44 days) and great anti-interference ability to most of the ions in marine (selectivity coefficients pK ij > 7), such as SO 4 2−, Cl−, NO 3 −, K+, Mg2+ and Fe2+. Then, the pH micro-sensor, combined with the potentiometric mode of SECM, was further employed to explore the localized pH value distribution during the galvanic corrosion of Fe-Cu couple in 0.1 M NaCl solution. To further verify the anti-interference ability of pH micro-sensor, potentiometric SECM experiment was also conducted in artificial seawater. Results showed that both acidic sites and alkalizing sites appeared on the Fe surface, while homogeneous alkalization took place overall Cu surface. This was consistent with the SVET results that anodic and cathodic areas existed simultaneously on Fe surface. Thus, it could be concluded that local corrosion occurred on Fe surface and only part of the Fe surface acted as the anodic even under galvanic effect. However, Cu acted as cathode with the homogeneously distributing cathodic current on the whole surface during the galvanic corrosion. [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 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 low density, high specific strength, and good creep properties make TiAl alloys being promising high-temperature structural materials in aerospace and automotive industries. However, the insufficient oxidation resistance limits their extensive practical applications. It is the intention of this review to illustrate the mechanism and development of sol–gel-based coatings, a surface modification approach for oxidation protection of TiAl alloys, but not to present a general review on the oxidation protection of TiAl alloys. The oxidation behavior and mechanism of TiAl alloys were addressed briefly. The discussion focused mainly on the sol–gel coatings preparation method and generation mechanism. Finally, the development trend of sol–gel-based coatings for high-temperature oxidation of TiAl alloys was prospected. [ABSTRACT FROM AUTHOR]

PDA-GO-CeO 2 is employed as the multifunctional filler to prepare PDA-GO-CeO 2 -EP composite coating. PDA-GO-CeO 2 shows excellent dispersion and compatibility in epoxy resin, which further improves the compactness of the coating. Meanwhile, thanks to the physicochemical characteristics of PDA-GO-CeO 2 and its interaction with epoxy resin, PDA-GO-CeO 2 -EP composite coating exhibits high adhesion force, physical barrier, and reliable active protection, anti-UV aging abilities. [Display omitted] • Low defect functionalized PDA-GO-CeO 2 are prepared via a facile stepwise method. • PDA-GO-CeO 2 strengthens the interface structure coating/metal system. • PDA-GO-CeO 2 -EP coating exhibits excellent anti-corrosion and anti-UV abilities. In the marine environment, besides external damage and internal defects, ultraviolet (UV) radiation is also a pivotal factor affecting the stable properties of coatings. Improving the anti-UV aging and active protection properties is beneficial to prolong the service life of coatings. Herein, the PDA-GO-CeO 2 composite material is obtained through a facile stepwise growth of polydopamine (PDA) and CeO 2 on GO nanosheets and further preparing a multifunctional anti-corrosion PDA-GO-CeO 2 -EP coating. As expected, composite coating displays strong adhesion force (6.9 MPa) and tensile strength (21.1 MPa), profiting from the enhanced crosslinking density and interface structure caused by PDA-GO-CeO 2 and resin matrix interaction. Since low defect functionalized PDA-GO-CeO 2 constructs the reinforced physical barrier, the anti-corrosion ability of PDA-GO-CeO 2 -EP coating (109.7 Ω·cm2) is more dependable than those of EP (106.7 Ω·cm2), GO-EP coating (108.8 Ω·cm2) and PDA-GO-EP (109.1 Ω·cm2) coatings after immersing in 3.5 wt.% NaCl solution for 100 d. Thanks to the synergistic effect of PDA/CeO 2 , the active protection performance of PDA-GO-CeO 2 -EP coating is endowed, which is further proved through the in situ scanning vibrating electrode technique (SVET). Following the UV aging treatment, PDA-GO-CeO 2 -EP coating shows minor gloss loss (20.4 %) and better adhesion force (4.3 MPa). Furthermore, the resistance value of PDA-GO-CeO 2 -EP coating is 7.03 × 106 Ω cm2 after 30 d immersion in saline solution, which is 242 times greater than that of EP coating (2.90 × 104 Ω cm2). This dependable performance of PDA-GO-CeO 2 -EP coating comes from the reflection, refraction, and absorption of UV rays by nanoscale CeO 2 particles. Consequently, this work offers a simple and promising strategy for fabricating multifunctional long-term protection coatings, thus hopefully extending the service life of marine engineering equipment. [ABSTRACT FROM AUTHOR]

In order to modify the mechanical and magnetic characteristics of high-entropy alloys (HEAs), phase transformation has been extensively studied. However, the impact of phase transformation on corrosion resistance has received less attention. In this paper, the intensive investigation of BCC/B2 to FCC phase transformation of equiatomic AlCoCrFeNi reveals the correlation between corrosion resistance and microstructure transformation under different heat treatment temperatures. Combined with differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD), the phase transformation of equiatomic AlCoCrFeNi heat treated with increasing temperature is BCC + B2 (as-cast) → BCC + B2 (600 °C) → BCC + B2+ FCC +σ (850 °C) → BCC + B2+ FCC (1100 °C). Combined with the results of SKPFM, immersion tests, and electrochemical measurements, the alloy treated at 1100 °C has the most excellent corrosion resistance due to its unique phase composition with the highest FCC phase and no intermetallic. • The correlation between heat treatment temperature and corrosion resistance of equiatomic AlCoCrFeNi was studied. • The microstructure of AlCoCrFeNi is changed from BCC to σ + FCC + BCC (850 °C), and then to FCC + BCC (1100 °C). • With increasing temperature, the corrosion resistance of AlCoCrFeNi is gradually improved except for 600 °C. • AlCoCrFeNi treated at 1100 °C exhibits most excellent corrosion resistance due to maximum FCC phase without intermetallic. [ABSTRACT FROM AUTHOR]

[Display omitted] • The precursor containing NiSO 4 and TEOS was applied to prepare Ni-SiO 2 coating. • The stress in the sintered Ni-SiO 2 coatings is reduced due to the incorporated Ni. • The Ni particles can withstand stress change and strengthen the coating structure. • The oxidation product of nickel, NiO, can impede the occurrence of cracking. In this study, a novel SiO 2 coating with the incorporation of nickel was prepared on Ti48Al2Nb2Cr alloy by co-electrodeposition. The effect of Ni in SiO 2 coating on the oxidation resistance of Ti48Al2Nb2Cr alloy at 900 °C was investigated. Due to the incorporation of nickel, the high temperature performance of the SiO 2 coating is promoted. For the Ni-modified SiO 2 coating, diffusion of oxygen to the substrate is effectively inhibited by a compact oxide scale composed of SiO 2 outer layer, Al 2 O 3 middle layer, and Ti 5 Si 3 inner layer. The incorporated Ni reinforces the structure and withstands the stress change of the SiO 2 coating. Moreover, the occurrence of cracking is impeded by NiO which is formed during oxidation. [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]

[Display omitted] • A multifunctional coating was prepared via composite GO material (GO-CN/CON-U) and EP. • GO-CN/CON-U enhanced the mechanical and hydrophobic performance of epoxy coating. • The GO-CN/CON-U-EP coating exhibited excellent passive/active anti-corrosion ability. Effectively reducing microdefects and endowing coatings with active protection are critical factors in prolonging the service life of coatings. Herein, a novel ceria nanoparticle decorated cerium organic network (GO-CN/CON-U) is in-situ prepared on graphene oxide (GO) via a simple one-step method. Then, a multifunctional coating is obtained by introducing GO-CN/CON-U into the epoxy resin matrix. The coating surprisingly exhibits exceptional mechanical properties and hydrophobicity attributing to the optimized crosslinking degree, compactness, and toughness due to the interaction between composite materials and epoxy resin, while its sustained anti-corrosion performance (around 109 Ω·cm2) also displays three orders of magnitude higher than that of pure epoxy coating (less than 106 Ω·cm2) even after 60 d of immersion in the aggressive media. An active protective mechanism was further proposed according to the UV–Vis and microelectrochemical test results, revealing that a shielding film can be promptly formed on the surface of Q235 steel when damage occurs against the attack of aggressive ions due to the release of Ce3+/Ce4+ and organic inhibitors from GO-CN/CON-U under acidic responsiveness. In brief, the present work provides a simple but attractive strategy for the preparation of long-lasting multifunctional protective coatings. [ABSTRACT FROM AUTHOR]

Although the mechanical properties of CoCrFeNi system high entropy alloys (HEAs) could be enhanced with the increase of Al content, it is inevitable to produce a gradual transformation of multiphase microstructure with chemical segregations. It will lead to the change of work function and the appearance of localized corrosion. The present work investigates the correlation between the corrosion behavior and the microstructure of Al x (CoCrFeNi) 100−x (x = 0–20) HEAs in 0.5 M H 2 SO 4 solution by immersion test, electrochemical measurement, SKPFM and SEM. The microstructure of CoCrFeNi system HEAs with increasing the content of Al has been transformed from single FCC phase to mixture of FCC and BCC duplex phases, and then to BCC phase. The detected value of work function gradually decreases with Al addition. The addition of a little amount of Al which retains the single FCC phase structure is beneficial to improve the corrosion resistance of Al-containing CoCrFeNi system HEAs, and the addition of excessive Al elements which promotes the formation and growth of BCC phase weakens the corrosion resistance of Al-containing CoCrFeNi system HEAs. • The relationship between the microstructure and corrosion resistance of Al x (CoCrFeNi) 100−x was systematically studied. • The microstructure of Al x (CoCrFeNi) 100−x changes from single FCC phase to FCC + BCC, and then to BCC phase. • Little Al addition without phase change improves the corrosion resistance, while excessive Al addition weakens. [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]

[Display omitted] • Rapid growth of oriented zeolite film on diverse substrates at lower temperature was studied. • Thickness of as-prepared zeolite films under three conditions is all around 1 μm. • Lower temperature leads to longer synthesis time and worse hydrophobicity of films. • Films prepared at 373 K for 4 h show terrific hydrophobicity and corrosion resistance. A method to quickly prepare highly oriented silicalite-1 zeolite films on different substrates at lower temperature was provided in this work. Compared with the traditional hydrothermal synthesis, rubbing a layer of seed pastes on the pretreated substrate surface in advance and using (NH 4) 2 SiF 6 as silicon source for subsequent secondary growth process can effectively reduce the synthesis temperature and shorten the contact time between the substrate and the strong alkaline synthesis solution. The thickness of the zeolite films prepared under three optimal conditions is all around 1 μm and the best contact angle value of 142.5° was achieved for the film prepared at 373 K for 4 h, while this values of the film prepared at 363 K for 6 h and 353 K for 10 h were 97.8° and 41.7°, respectively. The results of SECM showed that the detected localized current value of the film prepared at 353 K for 10 h was similar to that on bare steel after immersion in 0.62 M NaCl solution for 24 h (around 0.4 nA). At the same time, the current value measured under the other two preparation conditions was much higher, suggesting that the corrosion resistance of the film prepared under 353 K was unsatisfactory. In addition, EIS results demonstrated that the corrosion resistance of the films prepared at 363 K for 6 h also decreased significantly after 72 h immersion, while the films prepared at 373 K for 4 h still maintained excellent corrosion resistance and no obvious corrosion products were detected even immersion time up to 720 h. This work provides a new strategy for the application of zeolite in the field of material protection. [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]

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]

• 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]

A novel Al–Si composite coating was fabricated on γ-TiAl by the combination of magnetron sputtered Al coating and subsequently electrodeposited SiO 2 coating. The oxidation performance of the specimens was evaluated in static air at 900 °C. Results shown that the Al–Si composite coating can act as an efficient oxygen migration barrier and provide good oxidation resistance. During the oxidation process, an Al 2 O 3 enriched layer formed beneath the composite coating. The interfacial reaction between the composite coating and alloy resulted in the generation of Al 2 O 3 and Ti 5 Si 3. Image 1 • A novel Al–Si composite coating was fabricated on γ-TiAl by a two-step method. • This Al–Si composite coating can prevent the formation of Al-depletion layer. • The interfacial reaction between the composite coating and alloy resulted in the generation of Al 2 O 3 and Ti 5 Si 3. • This Al–Si composite coating can reduce the coating/alloy interface hardness and improve the oxidation resistance. [ABSTRACT FROM AUTHOR]