Dissolution kinetics of CsHSO4 and CsHSO4/SiO2 composites in aqueous solutions

Using a differential dissolution method, we have studied the dissolution kinetics of CsHSO₄ and CsHSO₄/SiO₂ composites (40, 60, and 70 mol % SiO₂) in aqueous solutions. The results demonstrate that the composites and bulk CsHSO₄ differ markedly in dissolution rate. In addition, the dissolution rate depends on the composition of the composite and decreases significantly as the SiO₂ content is raised from 0 to 70 mol %, even though the particle size of the salt is notably smaller in the composites. The composites of the same composition may contain cesium hydrogen sulfate in several states, differing in dissolution rate.     

On the kinetics and mechanism of dissolution of MgO crystals

The kinetics of dissolution of {1 0 0}, {1 1 0} and{1 1 1} surfaces of MgO crystals have been investigated in H₂SO₄ and H₃PO₄. It is observed that in concentrated H₂SO₄ and in concentrated and dilute H₃PO₄, the etch rates change in the sequence {1 1 1}>{1 1 0}> {1 0 0}; and in dilute H₂SO₄ in the sequence {1 0 0}>{1 1 1}>{1 1 0}. From the etch rates determined for 4N H₂SO₄, 36N H₂SO₄ and 4N H₂SO₄-glycerine solutions at different temperature, the values of activation parameters are calculated. It is found that the value of the activation energy in a particular acid is practically constant for different planes, but that of the pre-exponential factor changes. Increase in the viscosity of 4N H₂SO₄ due to the addition of glycerine lowers the etch rate but the value of activation energy remains unchanged. However, in highly viscous 4N H₂SO₄ etchant, the value of the activation energy of selective etching decreases somewhat above 60° C, and the etch pits formed at dislocations are deeper and larger than those at screw dislocations. The results are discussed and the important conclusions are enumerated.     

Growth of gallium orthophosphate single crystals in acidic hydrothermal solutions

The hydrothermal growth of GaPO₄ single crystals has been studied in several solutions. Among many solutions, H₃PO₄, HCl and H₂SO₄ solutions were found to be effective solvents for the growth of GaPO₄ single crystals. Single crystals have been hydrothermally grown at temperatures over the range 210 to 290 °C in these solutions with seed crystals. HCl solution was found to be the most effective solvent in which to grow large single crystals. Morphologies of crystals grown at temperatures below 200 °C tended to be bounded by small major rhombohedral (1 0 ˉ1 1) faces. In the temperature range from 210 to 430°C, the crystals have morphologies bounded by prism (1 0 ˉ1 0), small major rhombohedral (1 0 ˉ1 1) and minor rhombohedral (0 1 ˉ1 1) faces, and grew with well developed basal (0 0 0 1) faces with increase in the growth temperature. Single crystals of GaPO₄ with lower dislocation density have been hydrothermally grown at 210 to 290°C in 3m H₃PO₄ solution.     

Etching of MgO crystals in acids: kinetics and mechanism of dissolution

Kinetics of etching of MgO crystals have been studied in H₂SO₄, HNO₃ and HCl. The effects of etching time, acid concentration and temperature on the growth of hillocks, on the selective etch rate and on the rate of overall dissolution are demonstrated. It is observed that etch rates are independent of time, but are determined by the temperature and concentration of the acid. The etch rate-concentration curves show maxima which are characteristic of an acid. The values of activation energy for the processes of dissolution, selective etching and hillock growth and the corresponding frequency factors are computed. It is established that the process of dissolution in concentrated H₂SO₄ is diffusion controlled, while in H₂SO₄ with concentrations below 18 N and in HNO₃ and HCl it is reaction rate controlled. The pre-exponential factor is found to be a function of acid concentration. The results are discussed from the standpoint of chemistry. A comment on the data published on MgO by previous workers is made.     

Surface micromorphology and dissolution kinetics of potassium dihydrogen phosphate (KDP) crystals in undersaturated aqueous solutions

The results of dissolution kinetics and surface micromorphology of (010) and (011) faces of potassium dihydrogen phosphate (KDP) crystals etched in undersaturated aqueous solutions are described. They are then analysed from the standpoint of the theories of dissolution and growth. It was found that Cabrera's mechanism of dislocation etch-pit formation and birthand-spread model of growth may be applied in the case of dissolution of KDP crystals.     

Dissolution of iron garnet LPE layers in acidic solutions containing reducing agents

The dissolution rates of liquid phase epitaxial iron garnet layers in acidic solvents are increased by up to two orders of magnitude in the presence of suitable reducing agents. Experiments with differently orientated layers and with single crystal spheres reveal a change in the anisotropy of dissolution rates.     

Stress cracking of nylon polymers in aqueous salt solutions

Craze-growth kinetics were measured for nylon 6 and nylon 6,6 immersed in aqueous salt solutions. Under all conditions of stress level, temperature, and salt type examined, the craze growth data exhibited a steady state growth rate which was linear with time. Comparison of these results with available mechanistic models for craze growth suggests that salt-induced crazing of nylon polymers is an example of relaxation-controlled craze growth. Activation energies for crazing are 2.1 × 105 J mol^–1 for nylon 6 and 3.9 x 10^–5 J mol^–1 for nylon 6,6. These values are similar to the activation energies for the alpha relaxation (glass transition) of the nylons. Although the activation energies depend to some extent on the initial stress intensity factor level and salt type, the results indicate that the polymer type is the primary factor determining the temperature dependence of craze growth. The effect of salt type on cracking was shown to correlate with the charge density ratio of the salt cation, namelyQ/R, whereQ is the oxidation number andR is the ionic radius. These results indicate that the stress cracking susceptibility of nylon polymers can be predicted based upon the polymer chemical structure, glass transition temperature, degree of crystallinity, and theQ/R ratio of the specific salt in question.     

Stress cracking of nylon polymers in aqueous salt solutions

The susceptibility of nylon polymers to environmental stress cracking in aqueous salt solution was investigated. The time to fail (complete rupture) was recorded for razor-cut precracked samples which were totally immersed in water, sodium chloride, or calcium chloride at various initial stress intensity levels and temperatures. Ductile failures were observed in both water and aqueous sodium chloride at all temperatures. However, in saturated aqueous calcium chloride the nylon failure mode changed from ductile tearing, at room temperature, to crazing and crachng at elevated temperatures (50 to 100° C). Lithium and magnesium chlorides were also observed to be potent cracking agents at the elevated temperatures. The temperature interval over which cracking occurred corresponded to the onset of the Oass transition temperature,T _g of the nylon. Thus, nylon 6, having a lowerT _g, was more susceptible to cracking than nylon 6,6 et any given temperature. [)n the other hand' nylon 11, having a lower ratio of amide tornathy!onegroups, wasnotuuuoaptib!eto salt-induced cracking, For the nylon 6 and nylon 6,6 the failure kinetics in salt solutions were similar, regardless of whether dry samples or previously moisturized samples were examined. Thus e simple tempmra1ure-moisture content equivalence does not apply forsalt-induced stress cracking in aqueous solutions.     

Growth mechanism of NaBrO3 crystals from aqueous solutions

To study the growth mechanism of {111} faces of NaBrO₃, crystals were grown at different supersaturations ranging from 2% to 8%. The growth mechanisms were investigated based on the growth rate versus supersaturation relation and from the surface features observed on {111} faces. The growth mechanism of these crystals appear to be due to 2D nucleation. The growth rate curve has been further investigated using Ohara and Reid equations. Polynucleation model in two-dimensional nucleation growth theory is suggested as the most possible growth mechanism for these crystals in the present supersaturation range.     

Cast iron deterioration with time in various aqueous salt solutions

The changes with time in the corrosion rate and corrosion current density on a cast iron electrode in various aqueous salt solutions have been carried out using total immersion test and potentiostatic polarization curves. The concentration of salts taken is expected to be present in potable water. The relative behaviour of these salts towards corrosion has also been studied, which is found to be different from previous studies. The total immersion test parameters viz. weight loss, corrosion rate as well as potentiostatic parameters, open circuit potential,I _corr, Tafel slopes, corrosion rate, have been calculated by standard methods. Besides these the relative increase in corrosion rate with time as well as the percentage to which corrosion rate should be decreased so as to provide protection towards corrosion have also been calculated. It was found that KCl and NaCl are major contributors than MnSO₄, Pb(NO₃)₂, KI and KBr. The relative increase in corrosion is high in KBr, KI, NaNO₃, CaCl₂, and less in Pb(NO₃)₂, NaHCO₃ and CaCO₃ test solutions. For the reliability of results the data has been statistically analysed.     

Degradation of AlPO4-5 by aqueous salt solutions

Calcined template-free AlPO₄-5 is attacked by liquid water and aqueous salt solutions at 25°C, whereas the “as-made” material is resistant to degradation. The extent to which the framework is attacked increases with the salt concentration and depends on the nature of the anion (OH⁻ > I⁻ > S₂O₃²⁻ > Br⁻ > Cl⁻). Phosphate is released to the solution phase and the pH decreases from ∼ 5.5 to ∼ 3.0. AlPO₄-11 and AlPO₄-25 are also attacked by salt solutions, but the effects are smaller than for AlPO₄-5.     

Flow of freezing water and aqueous salt solutions through tubes

The problem of internal freezing of pipes is formulated and results of numerical studies presented.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 5, 779–786, November, 1990.     

Corrosion of alumina ceramics in acidic aqueous solutions at high temperatures and pressures

Alumina is resistant against corrosive aqueous solutions and could be used as a reactor material in the Supercritical Water Oxidation (SCWO) process. For this reason, the corrosion resistance of alumina and zirconia toughened alumina (ZTA) ceramics was investigated in aqueous solutions containing 0.1 mol/kg H₂SO₄, H₃PO₄ or HCl at T = 240°C–500°C at p = 27 MPa. In sulfuric acid, the solubility of alumina and its corrosion products was high at temperatures of 240°C–290°C. The corrosion rate was still high at higher temperatures (340°C–500°C), but the corrosion products were less soluble and formed a non-protecting scale on the samples. Phosphoric acid was less corrosive due to the formation of berlinite (AlPO₄) on the surface of the specimens. In hydrochloric acid, the dissolution of the alumina grains was the predominant corrosion phenomenon at temperatures of 240°C–290°C. At higher temperatures, intergranular corrosion was observed, but a dissolution of the grains did not occur.     

Electrostatic interaction forces in aqueous salt solutions of variable concentration and valency

We use atomic force microscopy (AFM) to determine electrostatic interactions between Si tips and Si wafers in aqueous electrolytes of variable composition. We demonstrate that dynamic force spectroscopy (DFS) in the frequency modulation (FM) mode with stiff cantilevers and sharp tips allows for a continuous detection of the tip-sample interactions without mechanical jump-to-contact instability and with substantially higher lateral resolution than standard colloidal probe measurements. For four different species of salt (NaCl, KCl, MgCl(2), CaCl(2)) we find repulsive electrostatic forces at the lowest salt concentrations (1 mM) that become progressively screened until they are dominated by attractive van der Waals forces at the highest concentration (100 mM). For the divalent cations the crossover from repulsive to attractive forces occurs at lower concentrations than for monovalent cations. Surface charges extracted from fits to standard Poisson-Boltzmann double layer theory indicate a rather weak dependence of the surface charge on the ion concentration. The high lateral resolution of our approach is illustrated by a 2D force field measurement over a patterned surface of a supported lipid bilayer on a mica substrate.     

Attack of YBa2Cu3O6.8 by acidic aqueous solutions

The dissolution of YBa₂Cu₃O_6.8 in acidic aqueous solutions was studied at 298 Kas a function of pH, nature and concentration of present anions and time, t. In the range 2.0 pH 4.0, dissolution is congruent and proceeds to completion; dissolution profiles are deceleratory and comply with a contracting volume rate law up to t _0.5. The rate of dissolution, which is limited by slow surface reactions, decreases with increasing pH; the kinetic order on proton concentration is a fractional and variable number. At higher pH values, dissolution is arrested at intermediate conversion values. In these cases, congruent dissolution is followed by the precipitation of less soluble solid phases which form a passive layer. The nature of this passive layer depends on pH and on the nature and concentration of present anions, which also define the extent of conversion attained at passivation. The dissolution behaviour of YBa₂Cu₃O_6.8 is described in terms of two consecutive surface reactions: fast equilibrated protonation of surface metal ions is followed by the slow release of cations. Barium surface ions are identified as the more reactive sites. The effect of the fast initial leaching of barium ions on the reactivity of yttrium and copper sites is discussed. The influence of the formal Cu(III) oxidation state is also stressed.     

Mechanism of chemiluminescent reaction of U4+ with oxygen in weakly acidic aqueous solutions

The mechanism of the chemiluminescent reaction of U(IV) oxidation with atmospheric oxygen is substantiated using published data. The suggested mechanism includes the initiating step of successive oxidation of U(IV) and U(V) with the formation of the excited ion [UO ₂ ²⁺ ·aq]*, which forms with an unexcited ion UO ₂ ²⁺ ·aq an excimer [UO ₂ ²⁺ ·aq?UO ₂ ²⁺ ·aq]*. The decay of the excimer yields H₂O₂ and two U(V) ions, causing the development of the chain process of the U(IV) oxidation.     

Wear damage of MgO single crystals

Mechanisms of surface and sub-surface wear damage in MgO single crystals were investigated by scratching with two sintered alumina sliders, having tip radii of 60 and 120m, using a simple scratching apparatus in a controlled atmosphere. The degree of surface and sub-surface cracking is dependent on the shape of the slider and the normal contact load, which are related to the penetration into the crystal. The chevron crack on the (001) plane in the [100] sliding direction consists of cracks intersecting at an angle of 90°, and with a spread angle of about 120°, and the normal crack. The nature of the sub-surface damage is investigated; a parallel crack develops in front of the slider and an oblique crack propagates towards the front of the slider. Then an internal normal crack is formed between the oblique crack and the parallel crack. In the [110] direction, the oblique crack initiates from the top of the normal crack under the surface, and the parallel crack continues from the oblique crack. This wear damage is explained by the dislocation interactions occurring due to the distribution of resolved shear stresses during sliding. The wear caused by the chevron crack is a factor of 10 higher than that with plastic flow. Internal cracks do not have a direct influence on the increase of wear.     

Growth mechanism of NaClO3 and NaBrO3 crystals from aqueous solutions

A study of growth rates of NaClO₃ and NaBrO₃ has been carried out using a small growth cell by in situ observation. Normal growth rates of (100) faces of NaClO₃ and (111) faces of NaBrO₃ along 〈110〉 direction are measured under relatively high supersaturation ranging from 3–8%. In the initial stages of growth, (100), (110) and (111) faces develop in NaClO₃ and gradually all the faces are replaced by the (100) faces only. In the case of NaBrO₃, mostly (111) faces develop with occasional occurrence of small (100) faces at the intersection of (111) faces. The growth mechanisms are investigated from growth rate vs supersaturation plots and from the observations of surface features. In the present supersaturation range, the growth mechanism appears to be due to two-dimensional growth mechanism.