The evolution with cycling of the three-dimensional (3D) microstructure of a silicon/carbon/carboxymethylcellulose (Si/C/CMC) electrode for Li-ion batteries is investigated by combined focused ion ...beam (FIB) / scanning electron microscopy (SEM) tomography. Using appropriate image processing methods, a volume of 20 × 8 × 11 μm3 is reconstructed in which the Si and pore phases are clearly identified. Their respective morphological characteristics (volume fraction, spatial distribution, size, connectivity, and tortuosity) are determined before and after 1, 10 and 100 cycles. The Si particles (37 vol.%, median diameter = 0.37 μm) and pores (57 vol.%, median diameter = 0.40 μm) are homogeneously distributed and fully connected in the pristine electrode. Major changes in the electrode morphology occur upon cycling due to electrode cracking and the growth of the solid electrolyte interphase (SEI) layer. It also appears that the size and shape of the Si particles change upon cycling. After 100 cycles, they display a non-spherical morphology (axial ratio of 4.6) with a median size of 0.14 μm.
The increase of the rebars corrosion rate due to the concrete carbonation is the major cause of reinforced concrete degradation. The aim of this study was to investigate the transition from passive ...to active corrosion of mild steel rebars in carbonated concrete. For this purpose, electrochemical techniques (polarization curves, free corrosion potential measurements) and surface analyses (EDS, XRD, XPS) were used. Five different electrolytes, with pH ranging from 13 to 8.3, were chosen to simulate the interstitial concrete pore water at various degrees of carbonation. The results indicate that the transition pH is between 10 and 9.4. XPS results indicate a passivation of mild steel for pH values ranging from 13 to 10 due to the formation of a thin iron III oxide layer. Immersion tests highlight the importance of the buffering effect of the carbonate content. At the free corrosion potential in an aerated solution, a decrease of the carbonate content increases the corrosion rate. On the opposite, at low electrode potential, the kinetics of oxidation increases with the carbonate content.
We outline the ability of zinc phosphate coatings, obtained by chemical conversion, to protect mild steel rebars against localized corrosion, generated by chloride ions in alkaline media. The ...corrosion resistance of coated steel, in comparison with uncoated rebars and coated and uncoated steel rebars embedded in mortar, were evaluated by open-circuit potential, potentiodynamic polarization, cronoamperometry and electrochemical impedance spectroscopy. The coated surfaces were characterized by x-ray diffraction and scanning electron microscopy. First, coated mild steel rebars were studied in an alkaline solution with and without chloride simulating a concrete pore solution. The results showed that the slow dissolution of the coating generates hydroxyapatite Ca
10
(PO
4
)
6
(OH)
2
. After a long immersion, the coating became dense and provided an effective corrosion resistance compared with the mild steel rebar. Secondly, the coated and uncoated steel rebars embedded in mortar and immersed in chloride solution showed no corrosion or deterioration of the coated steel. Corrosion rate is considerably lowered by this phosphate coating.
The propagation of localized corrosion on API 5L X65 carbon steel in hydrogenocarbonated solution containing chloride ions was monitored during long immersion times by electrochemical noise (EN) ...measurements in ZRA mode. Monitoring corrosion propagation from the EN amplitude is possible but interpreting the potential and current transients after long immersion times is made difficult by the different corrosion processes (metastable or stable pits, crevices, corrosion products detachment…) occurring simultaneously. The noise impedance at 0.1 Hz, directly derived from the EN measurements, is shown to decrease and stabilize in the same time scale as the coupled potential of the electrodes.
•Study of localized corrosion of API 5L X65 carbon in NaHCO3 solution with Cl- ions.•Corrosion propagation studied with EN during long immersion times (100 h) in ZRA mode.•Corrosion mechanism identification by noise transients only at short immersion times.•Corrosion propagation may be monitored from the increase in noise amplitude.•Noise impedance follows same time evolution as the coupled potential of the WEs.
Electrochemical dilatometry experiments were performed on silicon/carbon/carboxymethylcellulose (Si/C/CMC) composite electrodes prepared with pH7 and buffered pH3 slurries. It was shown that the pH3 ...electrode better accommodates the severe volume change of the micrometric Si particles, inducing a much better capacity retention with cycling (70% after 10 cycles compared to only 6% for the pH7 electrode). During the first discharge (lithiation), a maximum electrode thickness expansion of ∼170% was observed for the pH3 electrode compared to ∼330% for the pH7 electrode. A lower irreversible expansion was also observed at the end of the 1st cycle (∼50% compared to ∼180% for the pH7 electrode). It was explained by the fact that the pH3 of the slurry, which is known to favor the formation covalent bonds between the Si particles and the CMC chains, greatly improves the cohesive strength of the electrode as supported by the higher hardness and elastic modulus of the pH3 electrode. When the discharge capacity was limited to 1200 mAh g−1, a progressive and irreversible swelling of the pH3 electrode was observed upon prolonged cycling, which was attributed to the accumulation of solid electrolyte interface (SEI) products.
•The Na3PO4 addition in mortar decreases the chloride effect on localized corrosion.•FePO4 and Fe3(PO)2 formations on anodic sites of steel protects against corrosion.•The acoustic emission technique ...is a powerful tool to monitor the corrosion process.
The influence of a phosphate inhibitor on the corrosion behavior of steel reinforcement in mortar immersed in 3% NaCl solution was investigated. For this purpose, characterization methods (SEM, XRD), mechanical (flexural strength and compressive strength), electrochemical and acoustic emission measurements were carried out. It was demonstrated that the addition of phosphate inhibitor induces a slight decrease of the mortar compressive strength but no change of the flexural strength. The electrochemical measurements indicate that the addition of Na3PO4 in mortar leads to an improvement of the steel reinforcement corrosion resistance in chloride solution. It was also confirmed by the acoustic emission technique that the phosphate inhibitor decreases the corrosion rate. Furthermore the visual observation of the mortar corroborates these results. Thus it can be concluded that the addition of this phosphate inhibitor in concrete decreases the chloride ions effect on localized corrosion.
•In alkaline medium, Na3PO4 is an efficient inhibitor against the Cl− ions agressivity.•Na3PO4 increases the resistance to localized corrosion of reinforcement in concrete.•Na3PO4 increases the ...threshold (Cl−/OH−) of steel corrosion from 0.6 to 15.•The acoustic emission technique is a powerful tool to monitor the corrosion process.
In this paper, a phosphate-based inhibitor was used to increase the well-known critical ratio Cl−/OH− (R), inducing the localized corrosion of steel reinforcements in saturated Ca(OH)2 solution. For this purpose, electrochemical measurements and characterization methods (SEM, EDS, XRD, etc.) were first employed to identify the phosphate layer formation, and then to evaluate the inhibition effectiveness for the localized corrosion of steel reinforcements. The process of pitting corrosion is evaluated by cyclic polarization and is followed by acoustic emission.
The experimental results show that electrochemical techniques can evaluate the pitting corrosion and reveal that the introduction of phosphate-based inhibitor increases strongly the critical ratio Cl−/OH− from which the depassivation and pitting corrosion can be initiated. This clearly confirms its positive effect on the localized corrosion resistance of steel reinforcements in saturated Ca(OH)2 solution. A perfect correlation with the evolution of the acoustic activity is also demonstrated. Thus, it was significantly possible to highlight the acoustic signature of depassivation damage and to chloride concentration.
The influence of the Si particle size (85 nm versus 230 nm) on the mechanical stability of composite Si/C/carboxymethyl cellulose (CMC) electrodes is evaluated from in-operando dilatometry and ...acoustic emission measurements. A lower, more progressive and more reversible expansion/contraction of the electrode is observed with the Si 230 nm powder, with a maximum expansion of ∼140% and a residual irreversible expansion of ∼25% measured during the first cycle compared to ∼350% and ∼90% for the Si 85 nm based electrode. Moreover, during the 2nd cycle, an abrupt and very large expansion/contraction (up to ∼400%) is observed for the Si 85 nm based electrode, which results in the irreversible cracking and exfoliation of the electrode as confirmed by post-mortem scanning electron microscopy observations. This is also in accordance with the more intensive acoustic activity measured during the Si 85 nm electrode cycling. The lower mechanical strength of the Si 85 nm electrode is interpreted as the consequence of an insufficient amount of CMC binder relative to the larger specific surface area of the Si 85 nm powder. This tends to be confirmed by the significant improvement of its electrochemical cycling performance as its CMC content is increased.
•Mechanical degradation of Si anodes is studied by dilatometry and acoustic emission.•Decrease of the Si particle size results in a lower electrode mechanical stability.•The proper amount of binder depends on the specific surface area of the Si powder.
The binder used in the formulation of sulfur electrodes for Li/S batteries plays a crucial role in their electrochemical performance. In the present study, the impact of using a polyelectrolyte ...binder (poly(diallyldimethylammonium) bis(trifluromethane sulfonyl)imide) on the morphological degradation of sulfur electrodes is evaluated by in situ dilatometry, acoustic emission (AE) and synchrotron X-ray tomography (XRT), and compared to more conventional binders (poly(vinylidene difluoride) (PVdF) and carboxymethylcellulose (CMC)). The dilatometry study shows that during the initial sulfur dissolution process, the polyelectrolyte-based electrode displays a lower irreversible thickness contraction of ~16% compared to ~22% and ~31% for CMC and PVdF, respectively. This is confirmed by the XRT measurements showing a reduced thickness variation for the polyelectrolyte electrode compared to the CMC electrode. The same trend is found in the AE results, where a lower acoustic activity attributed to the rupture of the binder/carbon/sulfur network is detected during the 1st discharge plateau for the polyelectrolyte electrode. All these results confirm the major role of the binder for the Li/S system. Thanks to its multifunctionality, it impacts both the diffusion of the active material outside the electrode and the electrode integrity and therefore the conduction paths and accessible active surface for electrochemical processes.
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•The impact of the binder on the morphological change of sulfur electrodes is studied.•PVdF, CMC and cationic polyelectrolyte binders are compared.•In situ dilatometry, acoustic emission and X-ray tomography are used.•The better mechanical strength of the polyelectrolyte based electrode is highlighted.
The present work evaluated the ability of zinc phosphate coating, obtained by cathodic electrochemical treatment, to protect mild steel rebar against the localized attack generated by chloride ions ...in alkaline medium. The corrosion behaviour of coated steel was assessed by open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. The chemical composition and the morphology of the coated surfaces were evaluated by X-ray diffraction and scanning electron microscopy. Cathodically phosphated mild steel rebar have been studied in alkaline solution with and without chloride simulating the concrete pore solution. For these conditions, the results showed that the slow dissolution of the coating generates the formation of calcium hydroxyzincate (Ca(Zn(OH)
3)
2·2H
2O). After a long immersion time in alkaline solution with and without Cl
−, the coating is dense and provides an effective corrosion resistance compared to mild steel rebar.
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