Experimental studies of ignition of ammonium nitrate in interaction with magnesium and sulfur powders

Выполнены экспериментальные исследования взрывоопасности взаимореагирующих веществ: нитрата аммония (аммиачной селитры), который является сильным окислителем, относится к пожароопасным соединениям, в смеси с магнием и серой. Исследования свидетельствуют о взрывоопасности металлов и серы при их взаимодействии с пожароопасным веществом - окислителем (нитратом аммония). Эти особенности аммиачной селитры должны учитываться в процессе ее обращения (производство, хранение, транспортирование, применение, утилизация), при предотвращении и тушении пожаров. Ammonium nitrate (ammonia nitrate) is widely used in industry and in agriculture. The use of ammonium nitrate as a fertilizer gives this strong oxidizer a reputation of harmless agricultural chemical. At the same time, many cases of fires and explosions were registered during the storage, transportation and processing of saltpeter. Saltpeter decomposes with the release of highly toxic nitrogen oxides, which actively maintain combustion, and in case of fire also with release of oxygen. The analysis of statistical data on fires and explosions during the circulation of ammonium nitrate indicates serious problems in ensuring fire and explosion safety of buildings and structures in particular facilities of its storage. Clean and dry saltpeter explodes at normal temperature only in the presence of large volumes of the substance under the influence of a strong initiator, for example, a detonator. However, in the presence of some substances of both organic and inorganic origin the decomposition temperature of saltpeter decreases and this process of chemical spontaneous combustion can lead to an explosion. There are conducted experimental investigations by the method of constant volume bomb to determine the excess explosion pressure (ΔР) at interaction of ammonium nitrate with magnesium and sulfur powders and to compare these data with the calculated data of the excess explosion pressure (ΔР). It is established that the explosion pressure increases continuously with an increase in the concentration of the oxidant for mutually reactive substances consisting of combustible chemical element (magnesium, sulfur) and ammonium nitrate. This feature of the explosion of metals with an oxidizer - ammonium nitrate is a consequence of metal oxidation as well as the direct influence of decomposition products of the oxidizer in explosion. The growth of the overpressure of the explosion is also determined in the experiments when changing the volume of the reaction vessel (bomb camera) At the same specific weight of the mixture of mutually reactive substances (magnesium - ammonium nitrate) the explosion pressure increases with the volume of the bomb (the camera). The performed studies show that the contact of ammonium nitrate (ammonia nitrate) with foreign inclusions (additives) and, in particular, with metals and sulfur can lead to an explosion.

Airflow Dynamics and Aluminum Coating Oxidation Behavior under Electric-Arc Spraying with Airflow Pulsations

The paper aims to investigate the airflow dynamics of electric-arc spraying (EAS) with airflow pulsation. The study is focused on the dynamic structure of the airflow with an obstacle in the form of crossed electrodes at the steady and the pulsating air supply (with a frequency up to 120 Hz). The work was fulfilled using a computer simulation, the airflow “shadow” photo visualization, and the microstructure characterization of the coatings formed. It was found that when air flows along the crossed electrodes with a gap of 2 mm, a depression zone appears in the flow with a pressure drop from 0.56 MPa to 0.01 MPa. The air pulsation resulted in a change in a flow’s dynamic structure towards an increase in the length of the depression zone, which covers most of the arc, affecting the liquid metal oxidation. It is established that the frequency of a droplet formation should match the frequency of the airflow pulsation to minimize the metal oxidation. With the air pulsating at about 65 Hz, the oxide volume fraction in the aluminum coating was reduced by 3.6 times compared to the steady airflow. EAS with airflow pulsation has the potential for technological cost reduction.

Phase field modeling for the morphological and microstructural evolution of metallic materials under environmental attack

The complex degradation of metallic materials in aggressive environments can result in morphological and microstructural changes. The phase-field (PF) method is an effective computational approach to understanding and predicting the morphology, phase change and/or transformation of materials. PF models are based on conserved and non-conserved field variables that represent each phase as a function of space and time coupled with time-dependent equations that describe the mechanisms. This report summarizes progress in the PF modeling of degradation of metallic materials in aqueous corrosion, hydrogen-assisted cracking, high-temperature metal oxidation in the gas phase and porous structure evolution with insights to future applications.

Metallurgy and Mechanism of Underwater Wet Cutting Using Oxidizing and Exothermic Flux-Cored Wires

This paper considers the metallurgical processes of dissociation, ionization, oxidation, deoxidation, and dissolution of oxides during underwater wet cutting. A multiphase mechanism of underwater wet cutting consisting of working and idle cycles of the electrical process in a pulsating vapor gas bubble is proposed. A model of arc penetration into metal due to metal oxidation and stabilization of the arc by the inner walls of a narrow kerf is proposed. For underwater cutting of 10 KhSND, 304L steel, CuAl5, and AlMg4.5Mn0.7 alloy, we provide a principle of modeling the phase composition of the gas mixture based on high oxygen concentration, improving ionization, enthalpy, heat capacity, and thermal conductivity of plasma through the use of a mixture of KNO3, FeCO3, and aluminum. The method of improving the thermophysical properties and ionization of plasma due to the exothermic effect when introducing Fe3O4, MoO2, WO2 oxides and Al, Mg, Ti deoxidizers is proposed. Although a negative effect of refractory slag was revealed, it could be removed by using the method of reducing surface tension through the ionic dissolution of refractory oxides in Na3AlF6 cryolite. In underwater cutting of 10 KhSND and 304L, the steel welding current was 344–402 A with a voltage of 36–39 V; in cutting of CuAl5 and AlMg4.5Mn0.7 alloy, the welding current was 360–406; 240 A, with a voltage of 35–37; 38 V, respectively, with the optimal composition of flux-cored wire: 50–60% FeCO3 and KNO3, 20–30% aluminum, 20% Na3AlF6. Application of flux-cored wires of the KNO3-FeCO3-Na3AlF6-Al system allowed stable cutting of 10KhSND, AISI 304L steels, and CuAl5 bronze with kerf width up to 2.5–4.7 mm.

A Novel Enrichment Culture Highlights Core Features of Microbial Networks Contributing to Autotrophic Fe(II) Oxidation Coupled to Nitrate Reduction

Fe(II) oxidation coupled to nitrate reduction (NRFO) has been described for many environments. Yet very few autotrophic microorganisms catalysing NRFO have been cultivated and their diversity, as well as their mechanisms for NRFO <i>in situ</i> remain unclear. A novel autotrophic NRFO enrichment culture, named culture BP, was obtained from freshwater sediment. After more than 20 transfers, culture BP oxidized 8.22 mM of Fe(II) and reduced 2.42 mM of nitrate within 6.5 days under autotrophic conditions. We applied metagenomic, metatranscriptomic, and metaproteomic analyses to culture BP to identify the microorganisms involved in autotrophic NRFO and to unravel their metabolism. Overall, twelve metagenome-assembled genomes (MAGs) were constructed, including a dominant <i>Gallionellaceae</i> sp. MAG (≥71% relative abundance). Genes and transcripts associated with potential Fe(II) oxidizers in culture BP, identified as a <i>Gallionellaceae</i> sp., <i>Noviherbaspirillum</i> sp., and <i>Thiobacillus</i> sp., were likely involved in metal oxidation (e.g., <i>cyc2</i>, <i>mtoA</i>), denitrification (e.g., <i>nirK</i>/<i>S</i>, <i>norBC</i>), carbon fixation (e.g., <i>rbcL</i>), and oxidative phosphorylation. The putative Fe(II)-oxidizing protein Cyc2 was detected for the <i>Gallionellaceae</i> sp. Overall, a complex network of microbial interactions among several Fe(II) oxidizers and denitrifiers was deciphered in culture BP that might resemble NRFO mechanisms <i>in situ</i>. Furthermore, 16S rRNA gene amplicon sequencing from environmental samples revealed 36 distinct <i>Gallionellaceae</i> taxa, including the key player of NRFO from culture BP (approx. 0.13% relative abundance <i>in situ</i>). Since several of these <i>in situ</i>-detected <i>Gallionellaceae</i> taxa were closely related to the key player in culture BP, this suggests that the diversity of organisms contributing to NRFO might be higher than currently known.

STUDY OF KINETICS OF COPPER OXIDATION BY ELECTROLYSIS UNDER NON-STATIONARY CONDITIONS

Data on the study of the kinetics and mechanism of copper electro-oxidation -reduction processes in alkaline and neutral solutions were obtained in this work. Patterns of the electrochemical oxidation of copper using alternating current were established. Based on the data of polarisation measurements kinetic parameters were calculated: the heterogeneous rate constant, the effective activation energy of copper electro-oxidation, which allow to establish the course of the metal oxidation process in alkaline and neutral environments and its characteristics. Potentiodynamic measurements in alkaline solutions at a copper anode indicate the formation of Cu2O, CuO и Cu(OH)2. Anodic dissolution of copper in neutral salt solutions includes two main stages: the formation of Cu(I) and Cu(II) ions. It was found that the product of oxidation at high potentials is the Cu(II) ion. The nature of the anion electrolyte affects the rate of copper anodic oxidation, while the process occurs more efficiently in solutions of sulfate and sodium chloride and is accompanied by significant polarisation. Based on the study of kinetics the possibility of obtaining powders of metal oxide was shown, which further allowed to determine the parameters of the electrolysis process in non-stationary conditions in order to obtain highly dispersed materials based on copper oxides with specific physico-chemical properties.

In Situ Atomic-Scale Observation of Silver Oxidation Triggered by Electron Beam Irradiation

Understanding the mechanism of metal oxidation processes is critical for maintaining the desired properties of metals and catalysts, as well as for designing advanced materials. In this work, we investigate the electron beam induced oxidation of silver using in situ transmission electron microscopy. The additions of Ag-O columns on {111} and {110} planes were captured with atomic resolution. Interestingly, oscillatory growth on {110} planes was observed, which resulted from the double effect of electron beam irradiation. It was found that not only thermodynamic factors but also kinetic factors played significant roles in morphology evolutions. These results can facilitate the fundamental understanding of the oxidation process of Ag and provide a promising approach for the fabrication of desired nanostructures.

Cave Decorating with Microbes: Geomicrobiology of Caves

Microorganisms are important for the formation and biogeochemistry of caves. Some caves are energy-rich systems with abundant organic or inorganic chemical energy inputs that support robust microbial ecosystems, but most are extremely oligotrophic settings with slow-growing microbial communities that rely on limited energy resources. Microorganisms are catalysts for element cycling in subterranean environments and act as agents of mineral precipitation and dissolution. Microbes can contribute to cave formation by producing acids and corroding limestone bedrock, and they can form secondary mineral deposits by catalyzing metal oxidation and inducing carbonate precipitation. We describe the energy sources for microbial life in caves, and we review three situations in which microorganisms may play a direct role in mineral deposition and bedrock corrosion.