Aqueous Saline Environment Research Articles

Alkaline saline lakes: A chemical evolution experiment evaluating the stability of formaldehyde in an aqueous saline environment

Formaldehyde condensation in the presence of a mineral catalyst and under alkaline conditions is considered to be a "messy" chemical system due to its dependence on the complex chemical equilibrium between the reaction intermediates, which has a significant impact on the final products.This chemical system is extremely important in prebiotic chemistry and has been proposed as a potential pathway for carbohydrate formation in the early Earth. Saline and soda lakes are alkaline systems that could concentrate and accumulate a wide variety of ions (such as phosphate) and clay minerals, which can catalyze prebiotic chemical reactions. These geological environments have recently been suggested as ideal environments in which prebiotic chemical reactions could have occurred.This study uses Lake Alchichica in Mexico as a physicochemical analog of an early Archean saline lake to examine the stability of formaldehyde in these aqueous saline environments. Formaldehyde decomposes into sugar-like and CHO molecules in alkaline, high-salinity environments depending on the minerals phases present. As phosphate ion (HPO42−) is available in the aqueous medium, the results of our experiments also imply that phosphorylation processes may have occurred in these natural settings.

Fatigue crack propagation of aeronautic AA7050-T7451 and AA2050-T84 aluminum alloys in air and saline environments

AA7050-T7451 aluminum alloy is one of the most used material in aeronautical applications due to its high strength/density ratio and good fatigue resistance. More recently the AA2050-T84 alloy was introduced due to its reduced density and comparable static mechanical properties. However, fatigue crack propagation properties in corrosive environments are yet to be documented. In this paper fatigue crack propagation was evaluated in saline (3.5 wt% NaCl solution) and air environments. The AA2050-T84 alloy presented a better fatigue performance both in air and aqueous saline environments. Its superior performance was ascribed to non-homogeneous deformation and higher corrosion resistance. These results show the AA2050-T84 alloy as a potential candidate to replace AA7050-T7451 alloy in aeronautical components subjected to fatigue in corrosive environments.

New Developments in Scanning Microelectrochemical Techniques: A Highly Sensitive Route to Evaluate Degradation Reactions and Protection Methods with Chemical Selectivity

The scanning electrochemical microscope (SECM) offers a highly sensitive route to evaluate degradation reactions and protection methods with chemical selectivity by using ion-selective microelectrodes as tips, thus operating SECM potentiometrically. Spatially resolved imaging of electrochemical reactivity related to each component of the investigated material can thus be effectively monitored selectively both in situ and in real time. The applicability of this method has been illustrated using a practical example of a metal-coating system, consisting in the exposure of cut edges of coil-coated galvanized steel to aqueous saline environment. In this contribution, localized pH and zinc(II) ion distributions originated around cut edges of coil coated steel immersed in 1 mM NaCl solution are shown.

Study on Host-Guest Inclusion Complexation of a Drug in Cucurbit [6]uril

Abstract Assembly of pyridine-2-aldoxime drug with cucurbit [6]uril (CB[6]) has been investigated by 1H-NMR and 2D-ROESY NMR, UV-Vis spectroscopy, FT-IR spectroscopy, surface tension and conductivity measurements in aqueous saline environment. The distinct cationic receptor feature and the cavity dimension of the CB[6] emphasize that the macro-cyclic host molecule remain as complex with the nerve stimulus drug molecule. The results obtained from surface tension and specific conductivity measurements suggest 1:1 inclusion complex formation between drug and CB[6]. The stability constant evaluated by UV-Vis spectroscopic approach is 2.21×105 M−1 at 298.15 K, which indicates that the complex is sufficiently stable at physiological temperature.

Effect of cyclic frequency on fracture mode transitions during corrosion fatigue cracking of an Al-Zn-Mg-Cu alloy

Abstract There are complex issues remaining to be resolved before environment-assisted cracking models can be included in structural mechanics programs that are currently used to analyze mechanical fatigue crack growth. Considered here is the effect of cyclic frequency on fracture mode transitions that occur during corrosion fatigue of high-strength Al-Zn-Mg-Cu alloys. These alloys are used in civilian and military aircraft applications where they are exposed to detrimental aqueous saline environments. A previously developed “critical hydrogen at a critical distance” crack growth model is used to rationalize the observed transitions in crack-path fracture-modes, from intergranular (IG), to brittle transgranular (BTG), to ductile transgranular (DTG), as the alternating stress intensity factor and cyclic frequency increase beyond critical values. Corrosion fatigue crack growth rate data obtained on Al-Zn-Mg-Cu alloy 7017-T651 base metal and heat-affected “white zone” metal tested in aqueous NaCl solutions over a frequency range of 0.01–70 Hz are analyzed. For the white zone metal, dependence of the critical crack velocity on the critical frequency, at which the IG-BTG transition occurs, undergoes an abrupt reversal as the critical frequency increases above about 0.1 Hz. Mechanisms potentially responsible for this change in frequency dependency are discussed in the context of the critical hydrogen model. The transition from low to intermediate frequency behavior is speculated to be due to a change in the critical distance from microstructural control, for frequencies at or below 0.1 Hz, to control by the critical hydrogen criterion at higher frequencies. The low frequency behavior is discussed relative to the transition from static load stress corrosion cracking to low frequency corrosion fatigue, which occurs as cyclic frequency increases above zero.

Evaluation of the Fatigue Life of High-Strength Low-Alloy Steel Girth Welds in Aqueous Saline Environments with Varying Carbon Dioxide Partial Pressures

High-strength low-alloy steel girth weld specimens were subjected to fatigue tests in saline environments saturated with different carbon dioxide partial pressures. As expected, results show that increases in gas concentration initially affect fatigue life adversely, but when higher partial pressures are associated with low stresses, a reduction in the negative impact of environmental conditions is seen. This may be related to a competition between corrosion rates and mechanisms of crack initiation and propagation. Data is presented with the aim of contributing toward the establishment of a database of results in literature which may lead to better understanding of the phenomena involved through association of these with ongoing research.

Toward an Understanding of the Aqueous Solubility of Amino Acids in the Presence of Salts: A Molecular Dynamics Simulation Study

Ion-specific effects on the aqueous solubilities of biomolecules are relevant in many areas of biochemistry and life sciences. However, a general and well-supported molecular picture of the phenomena has not yet been established. In order to contribute to the understanding of the molecular-level interactions governing the behavior of biocompounds in aqueous saline environments, classical molecular dynamics simulations were performed for aqueous solutions of four amino acids (alanine, valine, isoleucine, and 2-aminodecanoic acid), taken as model systems, in the presence of a series of inorganic salts. The MD results reported here provide support for a molecular picture of the salting-in/salting-out mechanism based on the presence/absence of interactions between the anions and the nonpolar moieties of the amino acids. These results are in good qualitative agreement with experimental solubilities and allow for a theoretical interpretation of the available data.

Book Review: Cold Aqueous Planetary Geochemistry with FREZCHEM – From Modeling to the Search for Life at the Limits

Through the last decades the importance of understanding the chemistry of cold aqueous environments, both on the Earth and other planetary bodies, has become increasingly clear. The FREZCHEM program, which constitutes the core of the book written by Giles Marion and Jeffrey Kargel (as clearly stated in the title), is a powerful theoretical tool to contribute to this task. Since 1994 – date of publication of the first version of FREZCHEM – almost ten versions and adaptations progressively have increased its power, versatility and capability of dealing with chemically complex scenarios. Now, the program is mature enough to justify a book. Indeed, this book is a guide for the introduction to, and the use of, FREZCHEM (in fact, the appendix A is literally a “program guide”), but it is more. For example, the explanation of the theoretical basis for the program building constitutes an excellent review of the principles of the thermodynamics of aqueous solutions. An additional merit is a clear specification of the program limits and conditions of applicability. Therefore, the book will be of great interest for researchers on the chemistry of cold aqueous solutions, and it is an excellent guide to the use of FREZCHEM. But the book has other prominent merits. For example, chapter 5 includes a synthesis of results obtained by previous works using FREZCHEM, which deal with a variety of topics. For Earth, they include analysis of the chemistry of aqueous saline environments (which can be standing water bodies, but also soils or sea ice), the ocean chemistry during snowball or hothouse episodes, or the evolution of freezing seawater. FREZCHEM has also been applied to study the composition and geochemical evolution of ancient waters on Mars, and to the composition of both the icy shell and internal ocean of Europa. As recognized by the authors, the results for Mars Orig Life Evol Biosph (2008) 38:371–372 DOI 10.1007/s11084-008-9142-5

Assessing the kinetics and mechanisms of corrosion of cast and HIPed Stellite 6 in aqueous saline environments

Cobalt–base (Stellite) alloys have seen extensive use in wear environments mainly due to their high strength, corrosion resistance and hardness. Co–base superalloys rely primarily on carbides, formed in the Co matrix and at grain boundaries, for their strength and the distribution, size and shape of carbides depends on processing conditions. Currently use of Stellite alloys has extended into various industrial sectors (e.g. pulp and paper processing, oil and gas processing, pharmaceuticals, chemical processing) and the need for improved information regarding corrosion (and often tribo-corrosion) of Stellite alloys has increased. It has been recognised that processing changes, which affect the microstructure of Stellite alloys, will most probably affect the corrosion performance. In this paper the corrosion behaviour of Stellite 6 in the as-cast and the HIP consolidated forms has been compared and contrasted using DC electrochemical techniques in static saline conditions. It has been shown that there is a significant difference in the corrosion performance of HIP consolidated Stellite 6 and it is possible to link the corrosion mechanisms to the microstructure. The benefits of using HIPing as a manufacturing process for the corrosion performance of Stellite 6 are discussed.

Use of Advanced Thermal Spray Processes for Corrosion Protection in Marine Environments

The microstructure and aqueous electrochemical corrosion characteristics of stainless steel 316L (UNS S31603) and nickel alloy 625 (UNS N06625) coatings deposited onto a carbon steel substrate using commercial high velocity oxyfuel (HVOF) spraying systems and a new high velocity wire flame spraying system were assessed. The electrochemical corrosion behaviour of the as prepared coatings was compared with their respective materials in wrought form. A HVOF sprayed coating of nickel alloy 625 was found to be more corrosion resistant in an aqueous saline environment than a coating of stainless steel 316L. However, the nickel alloy coating did not match the corrosion resistance of the same nickel alloy in wrought form, but may have the ability to offer corrosion resistance in saline aqueous environments approaching that of a wrought stainless steel. A porosity level of < 2.5% may not have a significant effect on the electrochemical activities of the HVOF sprayed coatings near the free corrosion (rest) potential. However, the level of oxidation depending upon the spraying system was shown to have an effect on the corrosion resistance of the sprayed coatings. The stainless steel and nickel alloy coatings produced using the high velocity wire flame spray system displayed much higher oxides compared with those produced using HVOF processes and. consequently, poorer corrosion resistance.

The fatigue crack propagation resistance of Ti–6Al–4V under aqueous saline environments

Fatigue crack propagation rates are characterised for the titanium alloy Ti 6/4 in atmospheric air and under complete immersion in 3.5% salt solution. The alloy is assessed in three microstructural forms: mill annealed, bimodal and fully transformed lamellar. Through the employment of a corner crack specimen geometry containing a part through crack with a maximum length of 2.5 mm, the lamellar variant demonstrates microstructurally sensitive growth for cracks shorter than 1 mm. The growth rate response is highly dependent on both environment and mean stress. Relative to this, both the mill annealed and bimodal microstructures are shown to be insensitive to salt solution conditions.

Feasibility of on-line supercritical fluid extraction of steroids from aqueous-based matrices with analysis via gas chromatography-mass spectrometry.

The solubility of testosterone, boldenone, androstenone, etiocholanolone, and epitestosterone are measured in pure supercritical CO2. Testosterone exhibited the highest solubility in supercritical CO2. The solubility of all steroids except epitestosterone increased by one order of magnitude with increasing pressure from 100 to 400 atm. Epitestosterone had the lowest solubility in supercritical CO2 and its solubility was not affected by pressure. The extraction efficiency of steroids from an aqueous saline environment exceeded 95%. Because of the partial solubility of water in supercritical CO2, the addition of a moisture trap after the aqueous vessel is necessary to prevent the plugging and deterioration of the gas chromatographic (GC) column. It is demonstrated that on-line supercritical fluid extraction-GC-mass spectrometry is feasible for the quantitative extraction and analysis of steroids from both saline and urine solutions. However, it is determined that the adsorbent vessel filled with Hydromatrix is not sufficient to trap all the moisture, and after 3 to 4 extractions, the GC column efficiency lowered.

Influence of aqueous salt environments on fatigue response of SiCp reinforced aluminium alloys

The low cycle fatigue response of aluminium alloy 2124 reinforced with silicon carbide particulates produced by a commercial powder metallurgy route has been evaluated. This assessment includes measurements of fatigue crack growth rates and ‘total life to failure’ experiments. The relative performance under aqueous saline environments, namely total immersion in a 3.5% salt solution and exposure to a salt fog of similar concentration, were then characterised. While the alloy essentially demonstrates an insensitivity to saline environments in terms of its crack growth characteristics, a process of surface corrosion and pitting is highlighted and deemed responsible for significant reductions in the total fatigue life. The implications for engineering designs attempting to make use of this class of metal matrix composite are discussed.

Synthesis and characterization of a poly(methyl methacrylate-acrylic acid) copolymer for bioimplant applications

This paper describes the development of a new crosslinked poly(methyl methacrylate-acrylic acid) copolymer for potential applications in bone implants. This copolymer, comprising hydrophobic and hydrophilic components, has been designed to provide small amounts of controllable swelling strains at saturation when exposed to an aqueous saline environment. The volume fraction of the hydrophobic methyl methacrylate monomer to the hydrophilic acrylic acid monomer strongly influenced the swelling behavior of the copolymer. Two different cross-linking agents, allyl methacrylate and diethylene glycol dimethacrylate, were evaluated for their effectiveness in cross-linking and limiting the saturated swelling levels. The influences of the amounts of cross-linking agents and other processing parameters on the swelling behavior of the copolymer were studied using differential scanning calorimetry measurements, solubility tests, and swelling measurements in saline solutions. These measurements provided a good understanding of the structure of the copolymer, the effectiveness of the cross-linkers, the swelling mechanisms in this system, and the factors that strongly affect the swelling weight gain levels in this copolymer. © 1997 John Wiley & Sons, Inc.

The effects of saline aqueous corrosion on fatigue crack growth rates in 316 grade stainless steels

A study of fatigue crack propagation rates of 316 grade stainless steels in air and in an aqueous saline environment was carried out in an attempt to assess the fatigue properties encountered when such materials are used as surgical implants. The effects of variables such as temperature, pH, oxygenation level, bulk electrode potential, mean stress, frequency and stress waveform on the Paris crack growth law parameters were determined. Corrosion fatigue effects were observed in the aqueous saline environment, and a mechanism to describe this effect is proposed.

Origin of Ca, Fe, and Mg Carbonates in Oil Shales of Eocene Green River Formation in Colorado, Wyoming, and Utah: ABSTRACT

Calcium and magnesium carbonates are perhaps the most widespread minerals in the oil-shale sequence of the Green River Formation. Crystallography and chemical composition of the principal carbonate minerals in the oil shale indicate a wide range in mineral composition that includes calcite, Mg-calcite, dolomite which is iron-free and has normal 1:1 Ca:Mg, dolomite with low iron and excess Ca, ankerites with excess Ca, siderite, magnesiosiderite with low Ca, and aragonite. Recognition of the ubiquitous presence of Mg-calcite in these rocks seems important in reconstructing a plausible depositional model for the sedimentary environment and origin of the kerogen-rich shales. Recent studies show the ability of algae and many invertebrates to precipitate high Mg-calcite in aqueous saline environments. Biogenic precipitation of calcite in the hypersaline waters of the Green River lake may account for the higher Mg-content of laminae adjacent to and within high-kerogen zones than in low-kerogen zones. Subsequent to the inferred biologic precipitation or accumulation of high Mg-calcite in the upper levels of the lake, the Mg-calcite sank to a lower zone of extreme salinity after death of the organism. Postdepositional processes in the lower zone of accumulation may have converted high Mg-calcite to low Mg-calcite and dolomite, as suggested by the presence of this mineral assemblage in many of the samples studied. Development of ankerite, siderite, and magnesiosiderite is believed to be authigenic or diagenetic. Metastable carbonate minerals such as Mg-calcite plus dolomite with excess Ca, or Mg-calcite plus ankerite with excess Ca have been identified in a single polymineralic grain. This finding makes untenable an interpretation of the oil-shale mineral assemblage based on conventi nal equilibrium conditions for the system CaCO3-MgCO3-MgCO3-FeCO3 at 25°C. We find no compelling mineralogic or chemical evidence that indicates precipitation of calcite and protodolomite at the basin margin followed by reworking and transport of the carbonate sediments to the basin center, as other workers have recently proposed for their playa-lake model. End_of_Article - Last_Page 913------------