Vinyl Carbon Research Articles

A highly sensitive colorimetric and fluorometric sensor for the detection of cyanide

Certain anions play a vital role in metabolic reactions in our body, but the cyanide ion is highly toxic and poisonous to humans, animals, and the environment even at extremely low concentrations. To detect CN−ions, a highly sensitive electron donor-π-acceptor molecular system (IC) was designed based on the carbazole electron donor and 1,3-indanedione electron acceptor. The IC molecular probe was synthesized through simple organic transformations with good yields and characterized by all spectroscopic methods, including single-crystal X-ray diffraction. The IC probe molecule absorbs in the UV–visible region (200–550 nm), and fluorescence emission encompasses 400–750 nm with LE and CT emission bands. A systematic study suggests that the addition of cyanide ions caused both longer wavelength absorption bands and CT fluorescence emission intensity to decrease. Cyanide ion detection can be visualized with the naked eye, where the yellow color of the IC probe solution changes to colorless upon adding cyanide ion solution. Both UV–visible and fluorescence emission spectral changes with the addition of cyanide ions are attributed to the nucleophilic addition of cyanide at the vinylic carbon, which is confirmed by the 1H NMR, HRMS and fluorescence lifetime studies. The cyanide ion detection limit of the IC has been estimated to be 0.43 µM, far below the WHO permissible limit for cyanide ion concentration in potable water, thus broadening the utility of the IC probe in detecting cyanide in Tapioca. A prototype TLC testing strip has been fabricated for the qualitative determination of cyanide in THF/water medium.

Practical SPAN||Li cells enabled by insitu polymerized electrolyte

Sulfurized polyacrylonitrile (SPAN) is a promising cathode material for long-life lithium-sulfur batteries (LSBs) due to its enhanced electronic conductivity as well as the eliminated shuttle effect. However, the uncontrollable lithium dendrite issue as well as slow kinetics of thick electrodes still hinders its practical application. Herein, an in situ polymerized electrolyte (PGE) based on vinyl carbonate and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is designed and prepared to enhance the plating/stripping stability of the metal lithium while ensuring the cathode/electrolyte interfacial ion transportation. The optimized electrolyte, PGE-D, shows a satisfying ionic conductivity of 0.46 mS/cm at 25 °C; in the meantime, the polymer matrix hinders the diffusion of TFSI− anion and results in a high Li+ transference number (tLi+) of 0.73. Benefiting from the high affinity of the flowable PGE precursor to the thick SPAN cathode as well as enhanced lithium compatibility, the Li||SPAN battery with high areal loading of 14.1 mg/cm2 exhibits a high reversible specific capacity of 556.5 mAh/g and retains 76.3% of its capacity after 90 cycles.

Study on a New Reactive Dividing-Wall Column for the Reaction of Vinyl Carbonate and Methanol Ester Exchange to Produce Dimethyl Carbonate

Study on a New Reactive Dividing-Wall Column for the Reaction of Vinyl Carbonate and Methanol Ester Exchange to Produce Dimethyl Carbonate

Enhanced low-temperature resistance of lithium-ion batteries based on methyl propionate-fluorinated ethylene carbonate electrolyte

Low temperature has been a major challenge for lithium-ion batteries to maintain satisfied electrochemical performance, as it leads to poor rechargeability and low capacity retention. Traditional carbonate solvents, vinyl carbonate and dimethyl carbonate are indispensable components of commercial electrolytes. However, the higher melting point of these carbonate solvents causes their electrical conductivity to be easily reduced when temperatures drop below zero, limiting their ability to facilitate lithium ion transport. In this work, we demonstrate that the use of methyl propionate (MP) carboxylate and fluorocarbonate vinyl (FEC) electrolytes can overcome the limitations of low temperature cycling. Compared with carbonate electrolyte, MP has the characteristics of low melting point, low viscosity and low binding energy with Li+, which is crucial to improve the low temperature performance of the battery, while FEC is an effective component to inhibit the side reaction between MP and lithium metal. The carefully formulated MP-based electrolyte can generate a solid electrolyte interface with low resistance and rich in inorganic substances, which is conducive to the smooth diffusion of Li+, allowing the battery to successfully cycle at a high rate of 0.5 C at −20 °C, and giving it a reversible capacity retention rate of 65.3% at −40 oC. This work designs a promising advanced electrolyte and holds the potential to overcome limitations of lithium-ion batteries in harsh conditions.

Mechanistic insights into C(sp2)-H activation in 1-Phenyl-4-vinyl-1H-1,2,3-triazole derivatives: a theoretical study with palladium acetate catalyst.

The activation of C-H bonds is a fundamental process in synthetic organic chemistry, which enables their replacement by highly reactive functional groups. Coordination compounds serve as effective catalysts for this purpose, as they facilitate chemical transformations by interacting with C-H bonds. A comprehensive understanding of the mechanism of activation of this type of bond lays the foundation for the development of efficient protocols for cross-coupling reactions. We explored the activation of C(sp2)-H bonds in 1-Phenyl-4-vinyl-1H-1,2,3-triazole derivatives with CH3, OCH3, and NO2 substituents in the para position of the phenyl ring, using palladium acetate as catalyst. The studied reaction is the first step for subsequent conjugation of the triazoles with naphthoquinones in a Heck-type reaction to create a C-C bond. The basic nitrogen atoms of the 1,2,3-triazole coordinate preferentially with the cationic palladium center to form an activated species. A concerted proton transfer from the terminal vinyl carbon to one of the acetate ligands with low activation energy is the main step for the C(sp2)-H activation. This study offers significant mechanistic insights for enhancing the effectiveness of C(sp2)-H activation protocols in organic synthesis. All calculations were performed using the Gaussian 09 software package and density functional theory (DFT). The structures of all reaction path components were fully optimized using the CAM-B3LYP functional with the Def2-SVP basis set. The optimized geometries were analyzed by computing the second-order Hessian matrix to confirm that the corresponding minimum or transition state was located. To account for solvent effects, the Polarizable Continuum Model of the Integral Equation Formalism (IEFPCM) with water as the solvent was used.

Tuning the cross-linked structure of basic poly(ionic liquid) to develop an efficient catalyst for the conversion of vinyl carbonate to dimethyl carbonate

Dimethyl carbonate (DMC) is a crucial chemical raw material widely used in organic synthesis, lithium-ion battery electrolytes, and various other fields. The current primary industrial process employs a conventional sodium methoxide basic catalyst to produce DMC through the transesterification reaction between vinyl carbonate and methanol. However, the utilization of this catalyst presents several challenges during the process, including equipment corrosion, generation of solid waste, susceptibility to deactivation, and complexities in separation and recovery. To address these limitations, a series of alkaline poly(ionic liquid)s, i.e., [DVBPIL][PHO], [DVCPIL][PHO], [TBVPIL][PHO], with different cross-linking degrees and structures were synthesized through the construction of cross-linked polymeric monomers and functionalization. These poly(ionic liquid)s exhibit cross-linked structures and controllable cationic and anionic characteristics. Research was conducted to investigate the effect of the cross-linking degree and structure on the catalytic performance of transesterification in synthesizing DMC. It was discovered that the appropriate cross-linking degree and structure of the [DVCPIL][PHO] catalyst resulted in a DMC yield of up to 80.6%. Furthermore, this catalyst material exhibited good stability, maintaining its catalytic activity after repeated use five times without significant changes. The results of this study demonstrate the potential for using alkaline poly(ionic liquid)s as a highly efficient and sustainable alternative to traditional catalysts for the transesterification synthesis of DMC.

LiF‐Rich Solid Electrolyte Interfaces Guarantee the Cycle Stability of Metal‐Organic Frameworks for Lithium Storage

Metal‐organic framework materials (MOFs) with hierarchical porosity, structural tunability, and rich redox‐active centers have attracted intensive attention as potential substitutes for carbonaceous anode materials. However, the practical commercialization of MOFs still remains a challenge, due to uncontrollable performance degradation. Inspired by the prolonged lifespans of traditional anode materials with LiF‐rich solid electrolyte interface (SEI), the roles of LiF are examined in enhancing the cycle stability of MOFs, with copper benzenedicarboxylate (Cu‐BDC) as an example. By adjusting the electrolyte composition, the cycle performance of Cu‐BDC material is optimized, due to the generation of LiF‐rich SEI. In the optimal 3 M LiPF6 electrolyte, the formed SEI membrane is the thinnest, and the LiF content is the highest, so the Cu‐BDC shows a specific charge/discharge capacity of 378/387.2 mAh g−1 after 100 cycles at 100 mA g−1, comparing to less than 200 mAh g−1 in the electrolyte with 1% vinyl carbonate.

Asymmetric synthesis of allylic sulfonamides with axially and central chirality via palladium-catalyzed of atroposelective N-allylic alkylation

Axially chiral anilide compounds are an emerging but scarcely investigated class of stereogenic molecules with potential applications as chiral catalysts, biologically active scaffolds and functional materials. Compared to the C–C axially chiral molecules, the synthesis of axially chiral anilide compounds is a challenge owing to the lower rotation barriers. Herein, the construction of chiral allylic amine bearing axial chirality and central chirality via Pd-catalyzed atroposelective N-allylic alkylation reaction of cyclic vinyl carbonates and sulfonamides is reported. A diverse range of chiral sulfonamides bearing axial chirality and central chirality were synthesized in high yields and excellent enantioselectivities (up to 92 ​% yield and 99 ​% ee). In addition, the practical application of this protocol was also demonstrated by gram-scale synthesis and derivatives of the compound.

Theoretical Investigation on the H Atom Abstraction Reaction from C1-C4 Alkanes and Alkenes by ṄH2 Radicals.

The H atom abstraction reactions from alkanes and alkenes by ṄH2 are decisive in predicting the combustion characteristics of NH3/CxHy binary fuels. Theoretical investigation is carried out on the energy barriers of H atom abstraction reactions from C1-C4 alkanes/alkenes by ṄH2 radicals at the QCISD(T)/CBS//M06-2X/6-311++G(d,p) level of theory. Single-point energies of each species are computed using QCISD(T)/cc-pVDZ, TZ level of theories with basis set corrections from MP2/cc-pVDZ, TZ, and QZ methods. One-dimensional hindered rotor potentials are obtained by the M06-2X/cc-pVTZ method with 10° increment. Rate constants of each channel across temperatures of 298.15-2000 K are calculated by solving the RRKM/Master Equation with conventional transition state theory. For alkanes, rate constants order follows ktertiary> ksecondary> kprimary, while for alkenes the order follows kallylic> kprimary> kvinylic. Among the vinylic carbon sites within the same alkene species, the hydrogen atom sharing the same carbon with the allylic carbon on the C-C double bond is the preferred site for the H atom abstraction reaction. The branching ratio results indicate that the abstraction from tertiary or secondary carbon sites on alkanes and allylic carbon sites on alkenes are dominating during the investigated temperature range but become less important as the temperature increases. The data provided in this work are in good agreement with the literature data, but for the ṄH2+alkenes system, the literature data are scarce and further investigation is needed.

Aging Behavior of Polyethylene and Ceramics-Coated Separators under the Simulated Lithium-Ion Battery Service Compression and Temperature Field

In this paper, a device was set up, which could simulate the separator environment in the battery to track the influence of compression, temperature, and the electrolyte on the structure and electrochemical performance of separators. A commercial polyethylene separator and alumina- or boehmite-coated separators were selected, and the high-temperature cyclic compression was carried out in a mixed solvent environment with a ratio of vinyl carbonate and diethyl carbonate of 1:1. Compared with that compressed for 50 cycles under room temperature, the compression at 60 °C resulted in pore structure deterioration in the polyethylene separator. The oxidative voltage limit was reduced to 3.6 V, and after 200 charge and discharge cycles, the capacity was reduced by more than 50%. For the coated separator, the presence of a coating layer exhibited some protective effects, and the microporous structure in the base membrane was preserved. The oxidative voltage limit was above 4.2 V. However, as a result of the compression, the coating particles were still inserted into the pore structure, leading to a decrease in porosity and a decrease in discharge capacity, especially at a rate of 4 C. Compared with that coated with alumina particles, the interface resistance for the separator coated with boehmite particles was minimally affected, and the electrochemical performance after cyclic compression under 60 °C was better, exhibiting higher application ability.

Insight of the evolution of structure and energy storage mechanism of (FeCoNiCrMn)3O4 spinel high entropy oxide in life-cycle span as lithium-ion battery anode

Recently, spinel high-entropy oxide (HEO) anode materials have garnered extensive attention for high-energy lithium-ion batteries due to their high specific capacity. Many studies have explored its energy storage mechanism, but few have paid attention to its characteristics in long cycle life. In this work, the whole life cycling performance of (FeCoNiCrMn)-HEO is presented and investigated for the first time. Distinct capacity trends are studied to divide the cycling life into three stages: activation, upgradation, and degradation. Ex situ morphological characterizations are conducted to reveal that the driving force of stage change is continuous particle fragmentation. As a result, refined particles promote conversion reaction reversibility and induce extra interfacial capacity, achieving 2831 mAh g−1 at 825 cycles. Density functional theory (DFT) calculations provide a deeper insight into the high entropy effect on the ultra-high extra interfacial capacity. However, continuous refinement brings about huge electrolyte interface-derived layers, which are also responsible for pre-decay and post-collapse. The view of the evolution of the lithium storage mechanism is explicitly presented and experimentally verified by high-energy ball milling and fluorinated vinyl carbonate (FEC). These above results can give practical solutions to design high specific capacity and long-life cycle stability HEO anode in the future.

Preparation of Spirocyclic Vinylic Carbonates from Allylic Alcohols.

We present the synthesis of exo- and endo-spirovinylethylene carbonates, starting from various cyclic allylic alcohols. This one-pot cascade reaction to the spirocyclic scaffold was optimized using a design of experiments approach. The introduction of spirovinylethylene carbonates broadens the scope of using these in catalytic applications and provides an easy synthetic entry into spirocyclic scaffolds of various ring sizes.

New insights into the mechanism and kinetics of the addition reaction of unsaturated Criegee intermediates to CF3COOH and tropospheric implications.

In this work, we have investigated the mechanism, thermochemistry and kinetics of the reaction of syn-cis-CH2RzCRyCO+O- (where Rz, Ry = H, CH3-) unsaturated Criegee intermediates (CIs) with CF3COOH using quantum chemical methods. The rate coefficients for the barrierless reactions were calculated using variable reaction coordinate variational transition state theory (VRC-VTST). For the syn-cis-CH2RzCRyCO+O- conformation in which conjugated CC and CO double bonds are aligned with each other, we propose a new pathway for the unidirectional addition of an OC-OH molecule (CF3COOH) to the CC double bond of syn-cis-CH2RzCRyCO+O-. The rate coefficient for the 1,4-CC addition reaction at 298 K is ∼10-10 to 10-11 cm3 s-1, resulting in the formation of CF3C(O)OCH2CRzRyCOOH trifluoroacetate alkyl allyl hydroperoxide (TFAAAH) as a new transitory adduct. It can act as a precursor for the formation of secondary organic aerosols (SOAs). This novel TFAAAH hydroperoxide was identified through a detailed quantum chemical study of the 1,4-addition mechanism and will provide new insights into the significance of the 1,4-addition reaction of unsaturated Cls with trace tropospheric gases on -CRzCH2 vinyl carbon atoms.

Exposure to ambient air toxicants and the risk of amyotrophic lateral sclerosis (ALS): A matched case control study

BackgroundAmyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder with few risk factors identified and no known cure. Gene-environment interaction is hypothesized especially for sporadic ALS cases (90–95%) which are of unknown etiology. We aimed to investigate risk factors for ALS including exposure to ambient air toxics. MethodsThis population-based case-control study included 267 ALS cases (from the United States [U.S.] Centers for Disease Control and Prevention/Agency for Toxic Substances and Disease Registry National ALS Registry and Biorepository) and 267 age, sex, and county-matched controls identified via a commercial database. Exposure assessment for 34 ambient air toxicants was performed by assigning census tract-level U.S. Environmental Protection Agency (EPA) 2011 National Air Toxics Assessment (NATA) data to participants’ residential ZIP codes. Conditional logistic regression was used to compute adjusted odds ratios (aORs) and 95% confidence intervals (CIs) for individual compounds, chemical classes, and overall exposure. Sensitivity analyses using both conditional logistic regression and Bayesian grouped weighted quartile sum (GWQS) models were performed to assess the integrity of findings. ResultsUsing the 2011 NATA, the highest exposure quartile (Q4) compared to the lowest (Q1) of vinyl chloride (aOR = 6.00, 95% CI: 1.87–19.25), 2,4-dinitrotoluene (aOR = 5.45, 95% CI: 1.53–19.36), cyanide (aOR = 4.34, 95% CI: 1.52–12.43), cadmium (aOR = 3.30, 95% CI: 1.11–9.77), and carbon disulfide (aOR = 2.98, 95% CI: 1.00–8.91) was associated with increased odds of ALS. Residential air selenium showed an inverse association with ALS (second quartile [Q2] vs. Q1: aOR = 0.38, 95% CI: 0.18–0.79). Additionally, residential exposure to organic/chlorinated solvents (Q4 vs Q1: aOR = 2.62, 95% CI: 1.003–6.85) was associated with ALS. ConclusionsOur findings using the 2011 NATA linked by census tract to residential area provide evidence of increased ALS risk in cases compared to controls for 2,4-dinitrotoluene, vinyl chloride, cyanide, and the organic/chlorinated solvents class. This underscores the importance of ongoing surveillance of potential exposures for at-risk populations.

Building a Flexible and Highly Ionic Conductive Solid Electrolyte Interphase on the Surface of Si@C Anodes by Binary Electrolyte Additives.

Si@C as a high specific capacity anode material for lithium batteries (LIBs) has attracted a lot of attention. However, the severe volume change during lithium de-embedding causes repeated rupture/reconstruction of the solid electrolyte interphase (SEI), resulting in poor cycling stability of the Si-based battery system and thus hindering its application in commercial batteries. Using electrolyte additives to form an excellent SEI is considered to be a cost-effective method to meet this challenge. Here, the classical film-forming additive vinyl carbonate (VC), and the newly emerging lithium salt additive lithium difluorophosphate (LiDFP), are chosen as synergistic additives to improve the electrode-electrolyte interface properties. Final results show that the VC additive generates flexible polycarbonate components at the electrode/electrolyte interface, preventing the fragmentation of Si particles. However, the organic components show high impedance, inhibiting the fast transport of Li+. This defect can be supplemented from the decomposition substances of the LiDFP additive. The derived inorganic products, such as LiF and Li3PO4, can strengthen the reaction kinetics of the electrode, reduce the interfacial impedance, and promote the Li+ transport. Thus, the synergistic effect of VC and LiDFP additives builds an effective SEI with good flexibility and high ionic conductivity and then significantly improves the cycling and rate stability of Si@C anodes. The experimental results show that the utilization of LiDFP and VC additives to modify the Si@C anode interface enhances the capacity retention of the Si@C/Li half-cell after 100 cycles from 68.2% to 85.1%. Besides, the possible mechanism of action between VC and LiDFP is proposed by using the spectral characterization technique and density functional theory (DFT) calculations. This research opens up a new possibility for improvement of SEI, and provides a simple way to achieve high-performance Si-based LIBs.

Heterocyclic polymer supported cathode/Li interface layers to lower the operational temperature of PEO-based Li-batteries

High-performance poly (ethylene oxide) (PEO)-based solid-state polymer Li-batteries (SSLBs) at low temperature (≤ 25 ℃) is hindered by high interfacial resistance and low ionic conductivity. Herein, we simultaneously construct cathode and anode interface layers via in-situ heat-induced heterocyclic polymerization reaction, where the cathode-electrolyte interface layer (PPL) consists of bromine-doped poly (3,4-ethylene-dioxy-thiophene) (PEDOT), PEO and bis-trifluoromethanesulfonimide (LiTFSI), featuring with mixed ionic/electronic conduction, and the anode-electrolyte interface layer (PVCL) was formed by poly (vinyl carbonate) (PVC), PEO and LiTFSI, having a robust fast ionic conduction. Benefiting from the PEDOT conductivity, high mechanical strength of PVC and good interface affinity aroused by heat-induced reaction, the solid-state PEO-based LiFePO4||Li cell shows low interfacial resistance of 11.02 Ω that decreased by 92% compared to the cell without interface modification, and delivering a discharge capacity of 164.4 mAh g−1 at 0.1 C (25 ℃), as high as 115.9 mAh g−1 at 10 ℃. X-ray photoelectron spectroscopy revealed the enrichment of LiF, Li3N and LiBr on the lithium metal surface after cycling, further enhancing stable cycling Li-batteries. The dual interface-constructed strategy provides a reliable way to resolve the bottle-neck interfacial issues of SSLBs.

A Dual Organic Solvent Zn-Ion Electrolyte Enables Highly Stable Zn Metal Batteries.

Aqueous zinc (Zn) batteries have been regarded as an alternative to lithium-ion batteries due to their high abundance, low cost, and higher intrinsic safety. However, the low Zn plating/stripping reversibility, Zn dendrite growth, and continuous water consumption have hindered the practical application of aqueous Zn anodes. Herein, a hydrous organic Zn-ion electrolyte based on a dual organic solvent, namely hydrated Zn(BF4)2 zinc salt dissolved in dimethyl carbonate (DMC) and vinyl carbonate (EC) solvents [denoted as Zn(BF4)2/DMC/EC], can address these problems, which not only inhibits the side reactions but also promotes uniform Zn plating/stripping through the formation of a stable solid state interface layer and Zn2+-EC/2DMC pairs. This electrolyte enables the Zn electrode to stably undergo >700 cycles at a rate of 1 mA cm-2 with a Coulombic efficiency of 99.71%. Moreover, the full cell paired with V2O5 also demonstrates excellent cycling stability without capacity decay at 1 A g-1 after 1600 cycles.

Palladium-Catalyzed Asymmetric Cycloaddition/Cope Rearrangement Relay: Synthesis of Chiral Endocyclic Allenes

AbstractThe synthesis of chiral endocyclic allenes, especially the medium-sized ones, remains a challenge in allene chemistry due to unfavorable tension and difficult stereocontrol. Herein, an efficient protocol for the construction of chiral nine-membered endocyclic allenes via palladium-catalyzed asymmetric cycloaddition/Cope rearrangement relay of vinyl carbonates with activated enynes is highlighted. This process provides rapid access to a variety of chiral nine-membered endocyclic allenes in good yields with excellent enantioselectivities. In particular, a chiral P,S-ligand shows good performance on stereoinduction, generating central and axial chirality in a single transformation, which is rationalized by DFT calculations and by a proposed transition state.1 Introduction2 Pd-Catalyzed Asymmetric Cycloaddition/Cope Rearrangement Relay3 Plausible Mechanism and Stereochemical Outcome4 Conclusion and Outlook

Stereoselective Three-Component Allylic Alkylation Enabled by Dual Photoredox/Ni Catalysis

A modular and practical approach has been developed for the stereoselective formation of elaborate allylic alcohols through a three-component reaction (3CR) between vinyl cyclic carbonates, olefins, and various radical precursors using dual photoredox/Ni catalysis. The developed process expands the potential of radical-based allylic alkylation to multicomponent approaches under high regio- and stereocontrol with ample variation of the individual reagents and offers access to compounds featuring sterically congested and functionalized quaternary carbon centers. The catalytic protocol can be expanded to other allylic precursors, and the stereoselectivity is reversed by simply changing the photocatalyst.

Vinyl acetate polymers modified by siloxane for improving reinforced concrete: Water proofness and anticorrosion

Concrete, which is commonly destroyed by erosion in adverse environments, cannot satisfy the demands of foundation construction engineering applications. The optimal wayto ensure concrete's durability is to cover its surface with a barrier coating that prevents potentially harmful particles from penetrating the inside. Vinyl acetate polymers were applied as coatings owing to excellent flexibility and strong adhesion to the substrate. However, their hydrophilicity and low hardness will inevitably limit the usage in various areas, in particular the construction materials field. In this study, a method of modifying vinyl acetate polymers (PVAc) by vinyl triethoxysilane (VTES) and vinyl neononanoate (VeoVa-9), vinyl neodecanoate (VeoVa-10) in tertiary vinyl carbonates was reported to enhance the anticorrosion of concrete, where the former VTES could act a coupling agent, while the latter tertiary vinyl carbonates provide shielding against invasion through steric effect. Following modification, the hydrophilic PVAc becomes a hydrophobic coating with a contact angle as high as 98°; the coating's water absorption rate in one week is merely 1.25%; higher hardness and modulus were validated by nanoindentation measurements, while sandpaper friction and lap-shear trial indicated an enhancement in wear resistance and adhesion; three-electrode electrochemical analysis revealed that the modified coating has a 98% anti-corrosion efficacy, and its excellent anticorrosion performance is further confirmed by 480-hour salt spray test on reinforced concrete models.