Numerical Simulation for Crevice Corrosion of 304 Stainless Steel in Sodium Chloride Solution

The authors presented a mechanistic model describing the chemical reactions within a corroded thin, nar-row crevice. In the mathematical model, a two-dimensional steady-state was used to predict the crevice pH profile by taking into account dissolved oxygen and hydrogen ions within the crevice. It consists of six parallel electrochemical reactions: multi anodic reactions(Fe, Cr, Ni dissolution reactions) and three cathodic reactions(the oxygen reduction, the hydrogen reaction and water dissociation). Current density distribution and oxygen concentration distribution were determined to be corresponding to the evolution of potential distribution within the crevice. The contribution of each metal reaction to the overall corrosion process was in proportion to the mole fraction, and the simulation pro-vided a good agreement with published experimental results for the crevice corrosion of stainless steel in sodium chloride solution.     

A Theoretical Study of the Interaction of Hydrogen and Oxygen with Palladium or Gold Adsorbed on Pyridine‐Like Nitrogen‐Doped Graphene

The interaction of H₂ and O₂ molecules in the presence of nitrogen‐doped graphene decorated with either a palladium or gold atom was investigated by using density functional theory. It was found that two hydrogen molecules were adsorbed on the palladium atom. The interaction of these adsorbed hydrogen molecules with two oxygen molecules generates two hydrogen peroxide molecules first through a Eley–Rideal mechanism and then through a Langmuir–Hinshelwood mechanism. The barrier energies for this reaction were small; therefore, we expect that this process may occur spontaneously at room temperature. In the case of gold, a single hydrogen molecule is adsorbed and dissociated on the metal atom. The interaction of the dissociated hydrogen molecule on the surface with one oxygen molecule generates a water molecule. The competitive adsorption between oxygen and hydrogen molecules slightly favors oxygen adsorption.     

Mechanism for hydrogen evolution reaction on pipeline steel in near-neutral pH solution

Cyclic voltammetry, hydrogen permeation tests and electrochemical impedance spectroscopy measurements were combined to study the mechanism for hydrogen evolution reaction on X-70 pipe steel in near-neutral pH solution. It is found that hydrogen evolution reaction is dominated by the reduction of water molecules, followed by either an electrochemical hydrogen recombination reaction or a hydrogen absorption reaction. The near-neutral pH environment is capable of generating catalytic surface effect on hydrogen evolution on the pipe steel. The increasing dissolution of the cathodically pre-polarized steel could be due to the enhanced activation of the steel, rather than the increasing amount of hydrogen atoms in the steel. These results provide mechanistic information to understand the near-neutral pH stress corrosion cracking of pipelines.     

Single-point Attack of Two H2O Molecules towards a Lewis Acid Site on the GaAl12 Clusters for Hydrogen Evolution

The hydrogen evolution reaction (HER) of water with metallic aluminum-based materials provides an important way to address the global energy challenge; however, fundamental mechanism and reaction dynamics governing the chemical and electronic properties remain a debated research topic. Here we further study the HER mechanisms for water splitting on typical 13-atoms clusters, Al₁₂Ga and Al₁₃, by first-principles DFT calculations. We noted that the doping of a Ga atom into the Al₁₃ cluster could reduce the transition state barrier for H₂O dissociation on the metal cluster. Furthermore, it is interesting that the second water molecule prefers to adsorb on the same metal site giving rise to both thermodynamically and kinetically favorable reaction pathways. Based on the well-established complementary active sites (CAS) mechanism for metal cluster reactivity, we provide insights into the reaction dynamics of such metal clusters with two water molecules, which also sheds light on the Eley-Rideal and Langmuir-Hinshelwood mechanisms in surface science. Natural bond orbitals (NBO) analysis was conducted to evaluate the donor-acceptor charge-transfer interactions between the cluster and the nucleophilic reagent. These results gain a better understanding of the mechanism for water reacting with aluminum-based materials.     

Studies on the preparation of active oxygen-deficient copper ferrite and its application for hydrogen production through thermal chemical water splitting

Hydrogen generation through thermal chemical water splitting technology has recently received in- creasingly international interest in the nuclear hydrogen production field. Besides the main known sulfur-iodine (S-I) cycle developed by the General Atomics Company and the UT3 cycle (iron, calcium, and bromine) developed at the University of Tokyo, the thermal cycle based on metal oxide two-step water splitting methods is also receiving research and development attention worldwide. In this work, copper ferrite was prepared by the co-precipitation method and oxygen-deficient copper ferrite was synthesized through first and second calcination steps for the application of hydrogen production by a two-step water splitting process. The crystal structure, properties, chemical composition and δ were investigated in detail by utilizing X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA), atomic absorption spectrometer (AAS), ultraviolet spectrophotometry (UV), gas chro- matography (GC), and so on. The experimental two-step thermal chemical cycle reactor for hydrogen generation was designed and developed in this lab. The hydrogen generation process of water splitting through CuFe2O4-δ and the cycle performance of copper ferrite regeneration were firstly studied and discussed.     

Hydrogen Generation Through Indirect Biophotolysis in Batch Cultures of the Nonheterocystous Nitrogen-Fixing Cyanobacterium <Emphasis Type="Italic">Plectonema boryanum</Emphasis>

The nitrogen-fixing nonheterocystous cyanobacterium Plectonema boryanum was used as a model organism to study hydrogen generation by indirect biophotolysis in nitrogen-limited batch cultures that were continuously illuminated and sparged with argon/CO₂ to maintain anaerobiosis. The highest hydrogen-production rate (i.e., 0.18 mL/mg day or 7.3 μmol/mg day) was observed in cultures with an initial medium nitrate concentration of 1 mM at a light intensity of 100 μmol/m² s. The addition of photosystem II (PSII) inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not reduce hydrogen-production rates relative to unchallenged controls for 50 to 150 h, and intracellular glycogen concentrations decreased significantly during the hydrogen generation period. The insensitivity of the hydrogen-production process to DCMU is indicative of the fact that hydrogen was not derived from water splitting at PSII (i.e., direct biophotolysis) but rather from electrons provided by intracellular glycogen reserves (i.e., indirect biophotolysis). It was shown that hydrogen generation could be sustained for long time periods by subjecting the cultures to alternating cycles of aerobic, nitrogen-limited growth and anaerobic hydrogen production.     

Surface Coating of Amorphous TiOx Enables Efficient Vapor-fed Photocatalytic Overall Water Splitting with Substantial Apparent Quantum Yield

Photocatalytic overall water splitting represents a promising strategy for sustainable hydrogen production. However, photocorrosion and dissolution of photocatalysts and cocatalysts are common concern for the solid-liquid phase reaction. Recently, the above issues could be addressed by Domen and coworkers, when the photocatalytic water splitting was conducted in the presence of water vapor, which dramatically restrain the undesired corrosion of the photocatalysts and cocatalysts. Besides, surface decoration of hygroscopic TiO_x layer promotes adsorption of water molecules and prevents the corrosion process. In addition, vapor-fed photocatalytic water splitting remains considerable apparent quantum yield compared with the liquid water photocatalytic overall water splitting at pressurized condition, which endows great potential in practical application.     

Dissociation, solvation, and dynamics of HBr in small water clusters

The dissociation of hydrogen bromide in a small water cluster (H₂O)_n (n=3–5) has been studied with quantum chemical methods. The dynamics of dissociation was followed by classical molecular dynamics, and stationary points were studied in order to compute the free energy change associated with the ionization process. The nudged elastic band method was used to map out the energy profile of the reaction paths. The results show that HBr can dissociate in the presence of just four water molecules if they are in the correct configuration.The relation of our results to recent experiments is discussed.     

Effect of silver and copper ions on the decomposition of ozone in water

Features of the kinetics of ozone decomposition in water at pH 2 are studied depending on the concentration of silver and copper ions that are present. The existence of a critical concentration of metal ions (??3?6 × 10^?6 M) is established, below which ions slow the rate of ozone decomposition and above which the accelerate the process. It is concluded that the first region is due to the capture of hydroxyl and other radicals by metal ions, inhibiting the chain of ozone decomposition in water. A further increase in the concentration of ions leads to dominance of their direct interaction with molecules of ozone. A mechanism for the process is proposed and the rate constants of reaction of ozone with silver ions and copper are calculated (0.033 and 0.06 M^?1 s^?1, respectively).     

Analysis of Aluminum-Brass Corrosion Products in Sea Water and in Chloride Solutions

Abstract

A separation and chemical analysis method has been developed of corrosion products forming on aluminum brass surfaces immersed in sodium sulfate solutions at pH= 7.25 (simulating river water) and in sea water at pH= 8.2. This method is based upon a selective solubilization of various oxidation compounds with the aid of proper solvents (i.e. dissolving the metal matrix only to a negligible extent).

These analyses can be performed on oxidation films of more than 100 Å thickness. The following solvents were used:

1) Methanol for solubilization of Na⁺, Cu⁺⁺ and Zn⁺⁺ chlorides and sulfates;

2) Glycine to solubilize bivalent metal compounds:Cu and Zn oxydes oxysulfates, oxycarbonates, oxychlorides of Cu and Zn;

3) NH₄ OH to solubilize Cu⁺⁺ and aluminum compounds.

A fair agreement was observed between the formulae derived by stoichiometric calculation applied to chemical analyses and the values obtained by X-ray analysis.

Agreement was satisfactory for all compounds, except aluminum, determined by chemical analysis and identified by X-rays only in very thick oxidation films (≥ 10³ Å). The role of chemical analysis is therefore determinant to finalize the formation mechanism of aluminum brass passivation films.     

Evidence for the occurrence of electrochemical reactions and their interaction with chemical reactions during the corrosion of pure Fe with solid NaCl deposit in water vapor at 600 °C

The results of this work prove that electrochemical reactions are involved in the whole corrosion process of pure Fe with a solid NaCl deposit in water vapor at 600 °C. The interaction of chemical and electrochemical reactions accelerates the corrosion rate of pure Fe significantly. The electrochemical reaction is coupled with a preceding chemical reaction process (ce), in which Fe first reacts chemically with NaCl and water vapor to generate HCl_(g). And then, the electrochemical reaction proceeds via a one-electron electrochemical reduction to form H₂.     

Intermolecular interaction of myoglobin with water molecules along the pH denaturation curve

A method of diffusion coefficient (D) measurement for proteins based on the pulsed laser-induced transient grating method using a photosensitive cross-linker was applied to the characterization of the pH denaturation process of holo- and apo-myoglobin (Mb) from the viewpoint of protein-water interaction. It was found that the pH denaturation curve monitored by D agrees quite well with that determined by the circular dichroism intensity for holo-Mb. This fact indicates that the changes in intermolecular interaction and the alpha-helix content occur simultaneously during the unfolding process. However, the pH dependence of D for apo-Mb was different from that of alpha-helix content. This different behavior can be explained in terms of the different denaturation steps for the secondary structure and the hydrogen bonding network of the intermediate species around pH 4; i.e., this intermediate is partially unfolded, but the hydrogen bonding network is dominantly an intramolecular one. Taking previously reported properties of this species into account, we conclude that water molecules are trapped in the hydrophobic core of the apo-Mb pH 4 intermediate. This fact suggests that the kinetic intermediate state of the protein folding process is a swollen state without water molecular exchange with the bulk phase.     

Thermodynamic parameters for the binding process of the OH<Superscript>−</Superscript> ion with the dissipative structures. Calorimetric and conductometric titrations

Conductometric and calorimetric titrations of Extremely Diluted Solutions (EDS) were performed by adding HCl or NaOH solutions. The aim of this study is to obtain further confirmation of the hypothesized presence, in the EDS, of molecular aggregates of water molecules. The measurements on the EDS evidenced some relevant differences compared to those on solutions with just water as solvent. The conductivity and the pH caused by adding the titrant, namely NaOH or HCl, were markedly different to those of the control solutions. We suppose that the preparation procedure of the EDS could produce non-equilibrium changes in the supramolecular structure of water. The experimental results were interpreted by considering the interactions that can take place between the OH⁻ or H₃O⁺ and the hypothesized molecular aggregates of water molecules i.e. dissipative structures. A comparison was made about the nature of the driving force that leads to the formation of the complexes between the two ions deriving from probes and the molecular aggregates of water molecules (dissipative structures). In this study, we have determined the thermodynamic parameters of association between molecular aggregates of water molecules (dissipative structures) in the EDS and OH⁻ or H₃O⁺ probe ions. The experimental results were interpreted by considering a favorable interaction between the H₃O⁺ and OH⁻ ions and the dissipative structures, due, probably, to steric hindrance and chemical affinity with the aggregates.     

On the mechanisms of oxidation of organic sulfides by H2O2 in aqueous solutions

The mechanism of oxidation of organic sulfides in aqueous solutions by hydrogen peroxide was investigated via ab initio calculations. Specifically, two reactions, hydrogen transfer of hydrogen peroxide to form water oxide and the oxidation of dimethyl sulfide (DMS) by hydrogen peroxide to form dimethyl sulfoxide, were studied as models of these processes in general. Solvent effects are included both via including explicitly water molecules and via the polarizable continuum model. The former was found to have a much more significant effect than the latter. When explicit water molecules are included, a mechanism different from those proposed in the literature was found. Specific interactions including hydrogen bonding with 2-3 water molecules can provide enough stabilization for the charge separation of the activation complex. The energy barrier of the oxidation of DMS by hydrogen peroxide was estimated to be 12.7 kcal/mol, within the experimental range of the oxidation of analogous compounds (10-20 kcal/mol). The major reaction coordinates of the reaction are the breaking of the O-O bond of H2O2 and the formation of the S-O bond, the transfer of hydrogen to the distal oxygen of hydrogen peroxide occurring after the system has passed the transition state. Reaction barriers of the hydrogen transfer of H2O2 are an average of 10 kcal/mol or higher than the reaction barriers of the oxidation of DMS. Therefore, a two-step oxidation mechanism in which, first, the transfer of a hydrogen atom occurs to form water oxide and, second, the transfer of oxygen to the substrate occurs is unlikely to be correct. Our proposed oxidation mechanism does not suggest a pH dependence of oxidation rate within a moderate range around neutral pH (i.e., under conditions in which hydronium and hydroxide ions do not participate directly in the reaction), and it agrees with experimental observations over moderate pH values. Also, without including a protonated solvent molecule, it has activation energies that correspond to measured activation energies.     

Metallurgical factors in stress corrosion cracking (SCC) and hydrogen-induced cracking (HIC)

Nonmetallic inclusions can affect resistance of steels to both general and localized corrosion, including pitting corrosion, stress corrosion cracking (SCC), and hydrogen-induced cracking (HIC). Because stress corrosion cracks frequently initiate at pits, and pits nucleate at sulfides, the presence of sulfides is likely to affect the SCC process. Nonmetallic inclusions increase susceptibility of steel to HIC, which occurs by the formation of internal hydrogen blisters or blister-like cracks at internal delaminations or at nonmetallic inclusions in low strength materials. HIC occurs when H atoms diffusing through a linepipe steel become trapped and form H₂ molecules at inhomogeneities in the steel. A planar, gas-filled defect is created, which grows parallel to the pipe surface as it continues to trap more diffusing H atoms. If the defect grows sufficiently large, it may develop into a blister. HIC failure occurs if a mechanism exists for linkage of defects or blisters with the internal and external surfaces. The H atom source is normally the cathodic reaction of an acid corrosion mechanism occurring at the internal linepipe surface, i.e., the reduction of hydrogen ions, H⁺: Dedicated to the 85th birthday of John OM. Bockris.     

Interactions of water and methanol with a mixture of copper and zinc metals: a theoretical <Emphasis Type="Italic">ab initio</Emphasis> study

Ab initio cluster quantum chemical calculations at the Hartree–Fock and second-order Møller–Plesset perturbation theory levels were carried out to mimic the interactions of water and methanol with a mixture of Cu and Zn metals. It was shown that both molecular and dissociative adsorption of methanol on a mixture of Cu and Zn metal catalyst are preferred over the corresponding adsorptions of water. Estimated transition-state structures for dissociation of methanol into CH·₃ and OH· lie about 9.0 and 22.0 kcal/mol higher compared to the dissociated (forward reaction) and molecular adsorption (reverse reaction) complexes, respectively. Based on distinct radicals' bond energies with the active sites of the catalyst considered, it is suggested that hydrogen molecules could be formed through a chain of homogeneous reactions of methyl radicals released into the gas phase with the water and/or methanol molecules.     

Transition structure selectivity in enzyme catalysis: a QM/MM study of chorismate mutase

Two different transition structures (TSs) have been located and characterized for the chorismate conversion to prephenate in Bacillus subtilis chorismate mutase by means of hybrid quantum-mechanical/molecular-mechanical (QM/MM) calculations. GRACE software, combined with an AM1/CHARMM24/TIP3P potential, has been used involving full gradient relaxation of the position of ca. 3300 atoms. These TSs have been connected with their respective reactants and products by the intrinsic reaction coordinate (IRC) procedure carried out in the presence of the protein environment, thus obtaining for the first time a realistic enzymatic reaction path for this reaction. Similar QM/MM computational schemes have been applied to study the chemical reaction solvated by ca. 500 water molecules. Comparison of these results together with gas phase calculations has allowed understanding of the catalytic efficiency of the protein. The enzyme stabilizes one of the TSs (TS_OHout) by means of specific hydrogen bond interactions, while the other TS (TS_OHin) is the preferred one in vacuum and in water. The enzyme TS is effectively more polarized but less dissociative than the corresponding solvent and gas phase TSs. Electrostatic stabilization and an intramolecular charge-transfer process can explain this enzymatically induced change. Our theoretical results provide new information on an important enzymatic transformation and the key factors responsible for efficient selectivity are clarified. Received: 25 March 2000 / Accepted: 7 August 2000 / Published online: 23 November 2000     

Preparation and characterization of RuO2/polyaniline composite electrodes

RuO₂–polyaniline (PANI) composites have been prepared by a novel method resulting in a composite material at the electrode surface. The method is based on the utilization of the chemical oxidation of aniline by the RuO₂ attached to the gold substrate in acid media. Electrochemical quartz crystal nanobalance combined with cyclic voltammetric and chronoamperometric measurements was used to study the oxidative deposition process as well as the exchange of ions and solvent molecules during cycling. The chemical reaction between RuO₂ and aniline results in a surface mass increase at the open circuit, and it also manifests itself in the substantial decrease of the open-circuit potential after addition of aniline into the supporting electrolyte. The potential range, the nature of the electrolyte, and the pH have been varied. The results of the piezoelectric nanogravimetric studies obtained for the redox transformation of RuO₂ and the composites are elucidated by the transport of ions and water molecules. It has been shown that the behavior of RuO₂ as well as of the composites strongly depends on the spontaneous and potential-dependent solvent sorption, the nature of the electrolyte, and the potential range. It has been found that the value of the specific capacitance was substantially increased by the polyaniline present without any significant deterioration of stability of the capacitor.