Role of hydrogen in stress corrosion cracking of austenitic stainless steels

Effects of hydrogen charging on the stress corrosion cracking (SCC) behavior of 304 and 310 stainless steels under sustained load were investigated in boiling 42% MgCl₂ solution. The cracking was accelerated by the incorporation of hydrogen into the steel without altering the crack growth mechanism. The fact that the active dissolution is almost unaffected by the hydrogen charging and tensile stress indicates that the phenomenon of hydrogen-promoted SCC is unlikely a result of hydrogen-facilitated active dissolution. In contrast, hydrogen significantly promotes anodic dissolution in the potential range where the active-to-passive transition occurs. The electrochemical noise detected in the SCC process implies that the crack propagation process is discontinuous and hydrogen charging can raise the frequency of film breakdown at the crack tip. These observations suggest that the hydrogen-promoted SCC may result from the hydrogen-induced passivity degradation.     

Localized dissolution electrochemistry at surface irregularities of pipeline steel

The localized electrochemical dissolution behavior at surface irregularities, including scratch, mechanically induced hole and corrosion pit, on pipeline steel was investigated in both near-neutral pH and high pH solutions by scanning vibrating micro-electrode and localized electrochemical impedance spectroscopy measurements. In near-neutral pH solution, the localized dissolution behavior at surface irregularities is dependent of their geometrical depth, which is critical to development of a local electrochemical condition to support the further localized dissolution. Therefore, surface irregularities exceeding a certain depth provide potential sites to initiate stress corrosion cracks in near-neutral pH solution. The strong passivating capability of high pH solution would result in the formation of oxide film over the whole electrode surface to “equalize” the electrochemical activity at irregularities to the intact area. Therefore, the irregularities would not result in localized dissolution electrochemistry. Consequently, localized corrosion and crack initiation are not anticipated to initiate from the geometrical irregularities in high pH solution. However, corrosion pits generating due to passive film breakdown could support the high local dissolution kinetics in high pH solution, providing potential sites for crack initiation. The effects of hydrogen-charging on anodic dissolution at regularities depend on the defect geometry and the solution pH.     

Detection of stress corrosion cracking of high-strength steel used in prestressed concrete structures by acoustic emission technique

The stress corrosion cracking (SCC) of high-strength steel used in prestressed concrete structures was studied by acoustic emission technique (AE). A simulated concrete pore (SCP) solution at high-alkaline (pH ≈ 12) contaminated by sulphate, chloride, and thiocyanate ions was used. The evolution of the acoustic activity recorded during the tests shows the presence of several stages related respectively to cracks initiation due to the local corrosion imposed by corrosives species, cracks propagation and steel failure. Microscopic examinations pointed out that the wires exhibited a brittle fracture mode. The cracking was found to propagate in the transgranular mode. The role of corrosives species and hydrogen in the rupture mechanism of high-strength steel was also investigated. This study shows promising results for an potential use in situ of AE for real-time health monitoring of eutectoid steel cables used in prestressed concrete structures.     

Niobian iron oxides as heterogeneous Fenton catalysts for environmental remediation

Heterogeneous Fenton or Fenton-like reagents consist of a mixture of an iron-containing solid matrix and a liquid medium with H₂O₂. The Fenton system is based on the reaction between Fe^2?+? and H₂O₂ to produce highly reactive intermediate hydroxyl radicals (^???OH), which are able to oxidize organic contaminants, whereas the Fenton-like reaction is based on the reaction between Fe^3?+? and H₂O₂. These heterogeneous systems offer several advantages over their homogeneous counterparts, such as no sludge formation, operation at near-neutral pH and the possibility of recycling the iron promoter. Some doping transition cations in the iron oxide structure are believed to enhance the catalytic efficiency for the oxidation of organic substrates in water. In this work, goethites synthesized in presence of niobium served as precursors for the preparation of magnetites (niobian magnetites) via chemical reduction with hydrogen at 400°C. These materials were used as Fenton-like catalysts. Both groups of (Nb, Fe)-oxide samples were characterized by ⁵⁷Fe Mössbauer spectroscopy at 298 K. The results show that increasing niobium contents raise the catalytic potential for decomposition of methylene blue, which was, in this work, used as a model molecule for organic substrates in water.     

ZrO2-reinforced Ni-P plate: An effective catalytic surface for hydrogen evolution

Nickel and its alloys have recently been emerged as potential catalytic electrode materials for hydrogen evolution reaction in alkaline media. The present work contemplates reinforcement of electroless Ni-P plate with ZrO₂. The plate showed very high stability and excellent electrocatalytic activity. In situ incorporation of ZrO₂ resulted in increase in the rate of deposition of Ni on steel substrate. There was high activation during the initial stage of the plating also. The electrocatalytic activity of the ZrO₂-reinforced Ni electroless plate was found to be highly reproducible and long lasting when used for hydrogen evolution reaction.     

Increase in corrosion resistance of Zn–22Al–2Cu alloy by depositing an Y2O3 film studied by Auger electron spectroscopy

Thin films of Y₂O₃ were deposited on the surface of a zinalco alloy (Zn–22Al–2Cu) in order to modify the surface and increase the corrosion resistance. By means of ion sputtering and surface analysis using Auger electron spectroscopy, in-depth relative elemental intensity profiles were obtained. The growth mechanism of the surface oxides layer is modified by the deposited yttrium oxide film. On samples without film, corrosion progresses mainly at the surface as indicated by the zinc excess, while on samples with film, the growth of the oxides layer occurs at inner points of the film where migrating anions and cations are allowed to find each other. The growth of the corrosion products layer is about nine times smaller in samples with a film of 1600 Å of Y₂O₃ with respect to samples without a film. Migration of aluminum particles is higher than that of zinc particles, producing a surface highly enriched in aluminum.     

Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques

Potentiostatically anodized oxide films on the surface of commercial pure titanium (cp-Ti) formed in sulfuric (0.5 M H₂SO₄) and in phosphoric (1.4 M H₃PO₄) acid solutions under variables anodizing voltages were investigated and compared with the native oxide film. Potentiodynamic polarization and electrochemical impedance spectroscopy, EIS, were used to predicate the different in corrosion behavior of the oxide film samples. Scanning electron microscope, SEM, and electron diffraction X-ray analysis, EDX, were used to investigate the difference in the morphology between different types of oxide films. The electrochemical characteristics were examined in phosphate saline buffer solution, PSB (pH 7.4) at 25 °C. Results have been shown that the nature of the native oxide film is thin and amorphous, while the process of anodization of Ti in both acid solutions plays an important role in changing the properties of passive oxide films. Significant increase in the corrosion resistance of the anodized surface film was recorded after 3 h of electrode immersion in PSB. On the other side, the coverage (θ) of film formed on cp-Ti was differed by changing the anodized acid solution. Impedance results showed that both the native film and anodized film formed on cp-Ti consist of two layers. The resistance of the anodized film has reached to the highest value by anodization of cp-Ti in H₃PO₄ and the inner layer in the anodized film formed in both acid solutions is also porous.     

Nano TiO2 film electrode for electrocatalytic reduction of furfural in ionic liquids

Nanoporous TiO₂ having enhanced surface area was synthesized by sol–gel method. An “environmental friendly” method for production of furfuryl alcohol was presented by electrocatalytic reduction of furfural to furfuryl alcohol in ionic liquid medium at the surface of nanoporous TiO₂ film electrode. The heterogeneous catalytic redox behaviour of a nanoporous TiO₂ film electrode surface was investigated by cyclic voltammetry (CV). It was found that the catalytic reduction of furfural by Ti(IV)/Ti(III) redox system on the nanoporous TiO₂ film surface. The electrode reaction mechanism is called catalytic (EC′) mechanism, current density can reach 38 mA/cm² and yielding an overall conversion efficiency of 61.7%.     

Cohesive zone modelling of anodic dissolution stress corrosion cracking induced by corrosion product films

Damage mechanics based on the cohesive zone model were applied to study the anodic dissolution stress corrosion cracking (SCC) in flat and U-shaped edge-notched specimens. The simulation results show that corrosion product films (CPFs) facilitate crack initiation in SCC due to the CPF-induced stress and CPF rupture. In the flat specimen, SCC susceptibility increases with the CPF thickness and CPF Young’s modulus, while it decreases with CPF fracture strength. For the U-shaped edge-notched specimen, the normalised threshold stress intensity factor K_ISCC/K_IC decreases with the CPF thickness and notch depth.     

Film forming kinetics and reaction mechanism of γ-glycidoxypropyltrimethoxysilane on low carbon steel surfaces

The film forming kinetics and reaction mechanism of γ-GPS on low carbon steel surfaces was investigated by FTIR-ATR, AFM, NSS and theoretical calculation method. The results from experimental section indicated that the reaction of γ-GPS on low carbon steel surfaces followed the conventional reaction mechanism, which can be described as reaction (I) (Me (Metal)-OH + HO-Si → Me-O-Si + H₂O) and reaction (II) (Si-OH + Si-OH → Si-O-Si + H₂O). During film forming process, the formation of Si-O-Fe bond (reaction (I)) exhibited oscillatory phenomenon, the condensation degree of silanol monomers (reaction (II)) increased continuously. The metal hydroxyl density had significant influence on the growth mechanisms and corrosion resisting property of γ-GPS films. The results from theoretical calculation section indicated that the patterns of reaction (I) and reaction (II) were similar, involving a nucleophilic attack on the silicon center. The formation of Si-O-Fe bond (reaction (I)) was kinetically and thermodynamically preferred, which had catalytic effect on its condensation with neighboring silanol monomers (reaction (II)). Our DFT calculations were good consistent with the experimental measurements.     

Hydrogen dissociation on oxide covered MgH2 by catalytically active vacancies

MgH₂ is an important ingredient in modern reactive hydride composites to be used as hydrogen storage materials. The surface composition and chemical state of ball-milled MgH₂ is studied during hydrogen desorption by means of X-ray photoelectron spectroscopy. Simultaneously, the desorption rate of hydrogen is monitored, which is compared to dissociative properties of the surface investigated by hydrogen-deuterium exchange experiments. It is found that MgH₂ is also oxide covered during desorption demonstrating that MgO is able to recombine atomic hydrogen. The corresponding catalytic sites are associated with low coordinated surface vacancies on the oxide. The maximum surface concentration of these vacancies is very small, which is countered by a very high turnover frequency due to a small activation energy for dissociation of hydrogen of 0.1 eV on the single vacancy. The study provides insight into the catalytic role played by the oxide additives in MgH₂, which are superior catalysts for hydrogen sorption even when compared to 3d-metals.     

The corrosion of weathering steel by SO2 polluted atmospheres at its very early stages

CEMS was used in conjunction with AES to study the protective film formed on a weathering steel by exposure to a highly SO₂-polluted atmosphere. Ferrous species (sulphite) and ferric oxyhydroxides (ferrihydrite and α-FeOOH) were identified as corrosion products. From the correlation of CEMS and AES results the evolution with time of the different compounds is obtained, and a possible reaction sequence is outlined.     

Influence of annealing temperature on the nanostructure and corrosivity of Ti/stainless steel substrates

Titanium films of 80 nm thickness were deposited on stainless steel type 304, and they were post-annealed under flow of oxygen at different temperatures. The prepared samples were corrosion tested in 1.0 M H₂SO₄ solution using potentiodynamic and galvanometric polarization technique. The variation of corrosion resistance of these samples showed that the optimum annealing temperature is 473 K. The reduction of corrosion resistance of the sample with increasing the temperature above 473 K is attributed to the phenomena which are confirmed by AFM results: (a) increase of surface roughness, and (b) formation of larger grains with large grooves between them on the film surface. Hence larger effective surfaces for chemical reactions are provided. The films’ crystallographic and morphological structures were analysed using XRD and AFM, respectively before corrosion test and SEM after corrosion test. It is observed that the crystallographic structure of the film goes through a sudden change at 943 K annealing temperature and three phases of titanium oxide (i.e., rutile, anatase and brookite) are formed.     

Structure and catalytic properties of MoSe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films containing Mo nanoparticles in electrochemical production of hydrogen in solution

The introduction of molybdenum nanoparticles in MoSe _x thin films formed by pulsed laser deposition led to changes in the film structure. The base planes of the layered atomic packing of the MoSe _х matrix around Mo nanoparticles rotated; as a consequence, the edge sites that formed during the “breaking” of the Se–Mo–Se layered atomic packing came out to the film surface. At high nanoparticle concentrations, this effect led to high density of edge sites possessing increased catalytic activity (compared with that of the base planes) for initiating the electrochemical evolution of hydrogen in a 0.5 M H₂SO₄ solution. Voltammetric measurements at room temperature showed that when the carbon cathode was coated with MoSe _x thin films under optimum conditions, the hydrogen overvoltage considerably decreased, and the cathodic current increased. The results indicate that developments in the field of preparation of nanostructured electrodes based on layered transition metal dichalcogenides show promise as an alternative to expensive electrodes based on platinum group metals for electrocatalysts of hydrogen evolution.     

Transformation Process of the Magnetron-Sputtered Ag2O Film in Hydrogen Annealing

The current paper addresses the effect of the hydrogen partial pressure on the microstructure and transformation of the Ag₂O film. The transformation process and mechanism were also analyzed in detail. Increasing the hydrogen partial pressure can accelerate the transformation of Ag₂O to Ag and lower the critical transformation temperature of the film due to the enhanced hydrogen reduction and to both of the lowered activation energy of the reaction of Ag₂O with hydrogen and enhanced lattice strain of the Ag₂O film. Hydrogen-involved reaction in the whole hydrogen annealing process is mainly hydrogen reduction reaction with Ag₂O. Diffusion of hydrogen and gaseous H₂O molecules accompanies the whole hydrogen annealing process.     

Roughness simulation for thin films prepared by atomic layer deposition

The kinetic lattice Monte Carlo method for film growth simulation without taking crystallization into account was applied to study the roughness of the HfO₂ film grown by atomic layer deposition at 100–500°C from HfCl₄ and H₂O. The calculations were performed using a simplified kinetic mechanism of the growth of HfO₂ films obtained by reducing the detailed kinetic mechanism developed earlier. Ab initio quantum-chemical calculations were performed to determine the kinetic parameters of diffusion processes on the surface of hafnium oxide that could influence film roughness. Because of the special features of atomic layer deposition, the rate of film growth and film roughness were finite even if surface relaxation was ignored. It was found that, irrespective of the temperature, the diffusion of hydrogen and adsorbed HfCl₄ complexes did not change the profile of the growing film and only insignificantly increased the mean rate of growth. The results obtained were also qualitatively applicable to zirconium dioxide at fairly low (≤100°C) temperatures in the absence of crystallization.     

Cobalt Based Nanoparticles Embedded Reduced Graphene Oxide Aerogel for Hydrogen Evolution Electrocatalyst

Efficient water electrolysis catalyst is highly demanded for the production of hydrogen as a sustainable energy fuel. It is reported that cobalt derived nanoparticle (CoS₂, CoP, CoS|P) decorated reduced graphene oxide (rGO) composite aerogel catalysts for highly active and reliable hydrogen evolution reaction electrocatalysts. 7 nm level cobalt derived nanoparticles are synthesized over graphene aerogel surfaces with excellent surface coverage and maximal expose of active sites. CoS|P/rGO hybrid aerogel composites show an excellent catalytic activity with overpotential of ≈169 mV at a current density of ≈10 mA cm^?2. Accordingly, efficient charge transfer is attained with Tafel slope of ≈52 mV dec^?1 and a charge transfer resistance (R_ct) of ≈12 Ω. This work suggests a viable route toward ultrasmall, uniform nanoparticles decorated graphene surfaces with well‐controlled chemical compositions, which can be generally useful for various applications commonly requiring large exposure of active surface area as well as robust interparticle charger transfer.     

n型有序多孔硅基氧化钨室温气敏性能研究

利用电化学腐蚀方法制备了n型有序多孔硅, 并以此为基底用直流磁控溅射法在其表面溅射不同厚度的氧化钨薄膜. 利用X射线和扫描电子显微镜表征了材料的成分和结构, 结果表明, 多孔硅的孔呈柱形有序分布, 溅射10 min的WO₃薄膜是多晶结构, 比较松散地覆盖在整个多孔硅的表面. 分别测试了多孔硅和多孔硅基氧化钨在室温条件下对二氧化氮的气敏性能, 结果表明, 相对于多孔硅, 多孔硅基氧化钨薄膜对二氧化氮的气敏性能显著提高. 对多孔硅基氧化钨复合结构的气敏机理分析认为, 多孔硅和氧化钨薄膜复合形成的异质结对良好的气敏性能起到主要作用, 氧化钨薄膜表面出现了反型层引起了气敏响应时电阻的异常变化. 关键词： 有序多孔硅 氧化钨薄膜 二氧化氮 室温气敏性能     

Influence of pH on microstructural and optical properties of electrosynthesized CdSe thin films

Cadmium selenide (CdSe) thin films were electrosynthesized onto well cleaned stainless steel and fluorine-doped tin oxide (FTO) coated glass (10-15 Ω/cm²) substrates at different pH of the electrolytic solution. X-ray diffraction study reveals a cubic structure with preferential orientation along the (1 1 1) direction. The structural parameters such as grain size (D), lattice constant (a), strain (ε), dislocation density (δ), average internal stress (S) and degree of preferred orientation (I₁₁₀/I₂₂₀) in the film are calculated and they are found to be dependent on the pH of the depositing bath. EDAX analysis confirms nearly stoichiometric composition of the film deposited at pH 2.70. AFM analysis shows uniform deposition of the film over the entire substrate surface. In optical studies, the transition of the deposited film is found to be a direct allowed transition. The optical energy gaps are found to be in the range from 1.87 to 2.04 eV depending on the pH of the depositing bath. Photoluminescence (PL) spectrum shows blue shift in PL peak position and reduction in luminescence intensity for the film deposited at pH other than 2.70.