Formation Of Chromates Research Articles

Impact of molten salts composition on the corrosion behavior of NiMoCr and CoNiCrAl coatings on L-PBF 316L stainless steel for CSP plants

The high-temperature corrosion performance of NiMoCr and CoNiCrAl coatings produced by high velocity oxy-fuel (HVOF) on laser powder bed fusion (L-PBF) 316L stainless steel substate has been evaluated in presence of three different molten salts used as thermal energy storage (TES) materials for concentrated solar power (CSP) plants. The coatings have an excellent oxidation resistance at 700 °C, showing a percentage of affected thickness lower than 2 %, due to the formation of a protective layer of Cr2O3 and Al2O3 in both NiMoCr and CoNiCrAl coatings, respectively. The carbonates-based molten salt accelerates the corrosion of the NiMoCr coating, affecting around 43 % of the coating thickness, due to the formation of chromates and the high depletion of Mo in the coating. In contrast, the effective protection shown by the CoNiCrAl coating is explained because the grown LiAlO2 after the lithiation process has a high capacity to act as a strong diffusion barrier for metal ions and inhibited the formation of chromates. In the presence of two different ZnCl2 and MgCl2-rich molten salts, the CoNiCrAl coating behaved worse than NiMoCr coating due to the breakage of the (Al,Cr)-rich oxide layer as a consequence of an active oxidation mechanism and due to the formation of voids along the entire thickness, which were identified in 100 % of the coating thickness. The superior corrosion resistance of the NiMoCr coating in presence of the chloride rich molten salts was attributed to the presence of Mo, which fixed the Cr and inhibited the diffusion of oxygen.

Influence of PbCl2 and KCl salt mixture on high temperature corrosion of alloy 625

Aggressive corrosion can occur when firing waste or bio-based fuels, due to the presence of high concentrations of heavy metals, alkali metals, and chlorides. These deleterious compounds deposit on furnace walls and can form mixtures that can rapidly accelerate corrosion. The effect of salts containing lead had not been studied extensively at temperatures lower than 400 °C in nickel-based materials. This study investigates the effect of the individual salts PbCl2 and KCl and their mixture on the high temperature corrosion of alloy 625 at 340 °C and 380 °C. Samples of alloy 625 were covered with individual salts or a salt mixture and exposed to high temperatures in an atmosphere of synthetic air, 20-vol% H2O, and 100 ppm HCl. The results show that the presence of individual salts does not induce observable corrosion attack on alloy 625 after 168 h at any tested temperature. The salt mixture did cause a severe corrosion attack at 380 °C, observed after 24 h of exposure. It is suggested that the salt mixture induces the formation of lead chromates that may prevent or disrupt the formation of a protective chromia scale. It is believed that a key part of the mechanism is the formation of eutectic melts by the interaction of the scale with the salt mixture. Thermodynamic equilibria calculations show that the first melting temperature of PbCl2 and KCl salt mixture after reaction with oxygen can be as low as about 382 °C, and even lower (357 °C) if chromates are present.

Role of 5 mol% rare earth (La3+) on the structural, microstructural, and magnetic properties of cobalt chromite ceramics

Spinel oxides, which have the structure AB2O4 and are distinguished by their exceptional magnetic, electric, and multiferroic properties and exhibit a wide range of functional responses in the category of oxides. The cobalt chromate (CoCr2O4) is well-known as a multiferroic-magnetoelctric (ME) material. It belongs to the magnetic spinal oxide family and is noted for its complicated spiral ferromagnetic(FM) orderings. To enhance the magnetic properties of the CoCr2O4, in this work we have chosen Lanthanum (La3+) 0.05 mol% doped cobalt chromites (CoCr2O4) nanoparticles. The solution combustion method was utilized to synthesize the nanomaterial, where urea and glucose were used as fuels. The Structural exploration of the Co0.95La0.05 Cr2O4 nanoparticles were clarified using powder X-ray diffraction. To understand the microstructure properties we have carried out high-resolution electron microscopy and it reveals highly porous nature. In FTIR spectra the two main absorption bands at 507 cm−1 and 616 cm−1 were observed which confirm the formation of spinal chromates. The TGA results revealed a slow thermal degradation, and the sample has a thermal stability that is greater than the temperature range tested. Researchers have examined the changes in temperature and magnetic field affected the magnetic properties of materials over time (ZFC & FC). In comparison to the bulk sample, the Curie temperature at which samples transition from a paramagnetic (PM) state to a collinear short-range ferromagnetic (FM) state is significantly lower. This temperature is denoted as TC-98 K deteriorated with time in terms of quality. At low temperatures, there is no magnetic transition that can be identified between the short-range FM order and the long-range spiral spin structure. This is in contrast to the magnetic transitions that can be discovered in bulk. What gives rise to all of the intriguing qualities that have been described above is the presence of weak magnetic frustration, which is very much connected to super-exchange interactions and is found along different paths such as A-O-A, B–O–B, and A-O-B. This presence is what gives rise to all of the intriguing characteristics that have been described above.

High temperature corrosion behavior of Ni and Co base HVOF coatings exposed to NaCl-KCl salt mixture

The high-temperature corrosion behavior of NiMoCrW and CoNiCrAlY base coatings produced by high velocity oxy-fuel technique (HVOF) has been analyzed in dry air atmosphere and in presence of NaCl-KCl salt mixture at 650 °C for 360 h. Their behavior was compared with that of NiCr coating used as reference. Their corrosion resistance in presence of NaCl-KCl was measured by dimensional metrology analysis and the corroded samples were characterized by XRD and SEM techniques. The coatings exhibited excellent oxidation resistance in dry air atmosphere due to the formation of Cr2O3 and Cr-rich spinels, as well as the formation of Al2O3 in CoNiCrAlY coating. Their corrosion resistance in presence of NaCl-KCl was significantly lower due to chlorine-induced active oxidation and due to the formation of chromates, identified by XRD as K2CrO4. However, both coatings presented higher corrosion resistance in presence of the alkali salts compared to that of NiCr coating. The presence of Mo in the NiMoCrW coating improved the high-temperature corrosion resistance in presence of the alkali salts and the formation of a stable Al2O3 oxide scale on CoNiCrAlY coating reduced the penetration of chlorides and the degradation of the protective oxide scale by chromates formation.

Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

AbstractThe effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine‐containing atmosphere using air mixed with the salt‐vapor mixture of NaCl–KCl–CaCl2 at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo‐free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr2O3 scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr2O3 scale, resulting in internal chromium chloride precipitates and a Cr‐depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high‐temperature chlorination of Ni–Cr alloys in salt‐vapor‐containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.

Green one-step roasting method for efficient extraction of vanadium and chromium from vanadium-chromium slag

An effective and environment-friendly method based on one-step sodium hydroxide roasting was proposed to extract vanadium and chromium from vanadium-chromium slag. In optimal conditions (roasting temperature 800 °C and NaOH addition dosage 30%), vanadium spinel was converted into NaVO3 while chromium spinel was converted into Na2CrO4 and CaCrO4. Leaching efficiencies of vanadium and chromium reached 94.87% and 80.51%, respectively. During sodium roasting, Na also reacted with Si, Fe, Ti and Al to produce sodium salts of high melting points, which inhibits sintering of reaction mixture and facilitates the formation of vanadates and chromates. The leaching residues consist mainly of Fe2O3, which can be recycled to converter as flux of slagging materials to achieve resource recycling and zero-discharge of solid wastes. This work has provides a simple green way to extract vanadium and chromium from vanadium-chromium slag, which puts forward the green comprehensive utilization of vanadium-titanium magnetite ores with high chromium contents.

The influence of NaCl and CaCl2 induced high temperature corrosion on the aqueous corrosion resistance of stainless steels

The corrosion resistance of the steel grades 1.4509 and 1.4301 against sodium and calcium chloride, at 650 and 750 °C is investigated by means of cyclic exposure tests (CETs) in dry air. The formation of chromates as well as volatile metal chlorides and oxychlorides leads to non‐protective oxide scales on both steel grades. At 650 and 750 °C and in presence of a salt layer the alloy 1.4301 shows a higher corrosion resistance than the material 1.4509. At 750 °C and in presence of salt, the surface of the steel grade 1.4301 is transformed from a face centered cubic (fcc) to a body centered cubic (bcc) lattice, due to corrosion induced de‐alloying. The surfaces of both tested materials deplete in chromium, especially at 750 °C. Consequently, the corrosion resistance of the materials exposed to CET decreases sharply in aqueous corrodants.

Thermodynamic Properties of M2EO4, M2Mo x O3x + 1 and Double Chromates (M = Li, Na, K, Rb, Cs; E = Cr, Mo, W)

The phase transition temperatures of chromates and molybdates of certain alkali metals, and the melting temperature and enthalpy of polymorphic transformations for tungstates, are determined by means of thermal analysis. Enthalpies of dissolution of rubidium and cesium chromates in water and enthalpies of dissolution of alkali metal tungstates in a melt at 923 K are measured via calorimetry. Standard enthalpies of formation of sought chromates are calculated. The linear correlations between the enthalpies of formation of sulfates, selenates, chromates, tungstates, and molybdates are established, and a linear correlation within − (ΔGoox)-1-(ΔMV)ox)−1 coordinates is found for isopolymolybdates.

Spinel-Structured ZnCr2O4 with Excess Zn Is the Active ZnO/Cr2O3 Catalyst for High-Temperature Methanol Synthesis

A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260–300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn–Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn–Cr spinel with an optimum amount of oxygen vacancies resul...

Formation of Chromium-Containing Molten Salt Phase during Roasting of Chromite Ore with Sodium and Potassium Hydroxides

AbstractChromium has a wide range of applications including metals and alloys manufacturing, pigments, corrosion resistance coatings and leather tanning. The production of chromium chemicals is based on the oxidative alkali roasting of chromite ores, which leads to the formation of water-soluble alkali chromates. Previous investigations reported that when chromite is roasted with soda-ash, a molten salt containing chromium, which is mainly composed of sodium carbonate and sodium chromate (Na2CO3-Na2CrO4 binary mixture), forms under typical roasting conditions. The physical properties of the liquid phase, which are dependent on the temperature, charge and gangue compositions, play an important role on the oxidation reaction and may limit the chromate recovery by hindering the oxygen transport to the reaction interface. This investigation focuses on the alkali roasting of chromite ore at 1,000 oC using NaOH and KOH, followed by water leaching. The influence of the alkali ratio on the chromium extraction yield is analysed, and the results obtained with both hydroxides are compared. Sample characterisation and thermodynamic analysis, including phase diagrams, equilibrium calculations and computation of liquidus curves, are combined with the purpose of studying the formation of the molten salt phase under different roasting conditions and its effect on the final chromium recovery.

Kinetic studies on acid- and base-catalyzed aquation of bis-alaninatochromium(III) and on oxidation of tris- and bis-Aa–chromium(III) complexes (Aa = Gly, Ala, Asn) by H2O2

Two aqua derivatives of [Cr(Ala)3] were characterized in solution. Acid-catalyzed aquation of cis-[Cr(Ala)2(H2O)2]+ leads to inert [Cr(Ala)(H2O)4]2+, whereas base hydrolysis of cis-Cr(Ala)2(OH)2]− causes dissociation of both the Ala ligands and formation of chromates(III). Kinetics of these processes have been studied spectrophotometrically in both 0.1–1.0 M HClO4 and 0.2–0.9 M NaOH under first-order conditions. A linear dependence of the kobs,H on [H+] and a small dependence of the (kobs)OH on [OH−] were established. In the proposed mechanism, the rate determining step is Cr–N bond breaking in the reactive form of the substrate, i.e., in the protonated aqua- or dihydroxo complex. The effect of pH on the complex reactivity is discussed. The kinetic results are compared with those determined previously for analogous glycine and asparagine complexes. Additionally, oxidation of tris- and bis-Aa–chromium(III) complexes, where Aa = Gly, Ala or Asn, by hydrogen peroxide in alkaline medium was studied. Two reaction products were detected: thermodynamically stable CrO42− and [Cr(O2)4]3− that under a large excess of hydrogen peroxide is metastable. The rate-limiting stage of this process is an inner sphere two-electron transfer within the peroxido intermediate.

Chromium Speciation in Coal and Biomass Co-Combustion Products

Chromium speciation is vital for the toxicity of products resulting from co-combustion of coal and biomass. Therefore, understanding of formation processes has been studied using a combination of X-ray absorption fine structure (XAFS) spectroscopy and thermodynamic equilibrium calculations. The influence of cofiring on Cr speciation is very dependent on the type of fuel. Cr(VI) contents in the investigated fly ash samples from coal and cofiring average around 7% of the total chromium. An exception is cofiring 7-28% wood for which ashes exhibited Cr(VI) concentrations of 12-16% of the total chromium. Measurements are in line with thermodynamic predictions: RE factors of Cr around 1 are in line with volatile Cr only above 1400 °C; lower Cr(VI) concentrations with lower oxygen content and Cr(III) dissolved in aluminosilicate glass. Stability of Cr(VI) below 700 °C does not correlate with Cr(VI) concentrations found in the combustion products. It is indicated that Cr(VI) formation is a high-temperature process dependent on Cr evaporation (mode of occurrence in fuel, promoted by organic association), oxidation (local oxygen content), and formation of solid chromates (promoted by presence of free lime (CaO) in the ash). CaCrO(4)(s) is a probable chemical form but, given different leachable fractions (varying from 25 to 100%), different forms of Cr(VI) must be present. Clay-bound Cr is likely to dissolve in the aluminosilicate glass phase during melting of the clay.

Equilibrium, FTIR, scanning electron microscopy and small wide angle X-ray scattering studies of chromates adsorption on modified bentonite

The study presents a discussion about the adsorption mechanism of chromate anions on bentonite modified by hexadecyltrimethylammonium bromide (HDTMA-Br). The formation of alkylammonium chromates: HDTMAHCrO 4, (HDTMA) 2Cr 2O 7 and to the lesser extent (HDTMA) 2CrO 4 at the water–bentonite interface is examined based on the Scanning Electron Microscopy and surface tension measurements. The histograms of HDTMA/Cr(VI) molar ratio on the bentonite surface, found from Scanning Electron Microscopy (SEM) measurements, show that for the majority of points of bentonite surface the value of this ratio is in 1–2 range. FTIR spectra of modified bentonite samples show the change from gauche to trans conformation in the surfactant arrangement in the clay interlayer accompanying its concentration increase. In turn Small Wide Angle X-Ray Scattering (SWAXS) patterns evidently suggest incorporation of chromate anions into the interlamellar space of bentonite structure.

Synthesis of rare-earth chromates in the presence of certain oxides

The effect of Al2O3, SiO2, MgO, 3Al2O3·2SiO2 on the synthesis of LnCrO3 (where Ln=Nd, Sm, Yb) at high temperatures is considered. Chromium oxide in all the compositions Ln2O3−Cr2O3−Al2O3 (SiO2, MgO, 3Al2O3·2SiO2) reacts mostly with Ln2O3, i.e., silicon oxide alumina, magnesium oxide, and mullite do not interfere with formation of rare-earth chromates. This makes it possible to predict high durability for LnCrO3 electric heaters operating in contact with corundum, dinas, aluminosilicate, and periclase refractories in laboratory and industrial furnaces.

Low-temperature preparation and characterization of phase-pure lanthanide chromates (V) by the citrate gel process

Phase-pure, crystalline lanthanide chromates LnCrO 4 (V), where Ln = La, Pr, Nd, Sm, Gd, Dy, Ho, Yb, Lu and Y, have been prepared by the controlled combustion of the corresponding lanthanide biscitrato chromium (III) complexes at comparatively low temperatures. Formation of chromates (V) was confirmed by X-ray diffraction, infrared and electronic spectroscopy. Phase purity of the materials has also been confirmed by X-ray photoelectron spectroscopy.

Study of aluminium anodization in sulphuric and chromic acid solutions—I. Kinetics of growth and composition of oxides

Abstract The kinetics of growth and the chemical composition of anodic alumina films formed in chromium acid electrolyte and its mixtures with sulphuric acid are studied. It is shown that the oxide growth in the electrolyte, containing equal molar parts of chromium and sulphuric acids, occurs in a different way depending on the anodic current density. At j a lower than J ao = 5 mA/cm 2 porous oxide is formed, whereas higher current promotes barrier oxide formation. At j a ⋍ j ao oxide starts to grow in the porous oxidation mode and then switches to barrier oxide growth. However, the increase of the electrolyte temperature is found to inhibit this effect. The chemical composition of alumina films formed in chromate electrolyte exhibited the presence of Cr(VI) and Cr(III) ionic species at the oxide surface. The observed effect of the combined porous-and/or-barrier oxide growth in the mixed electrolyte is interpreted in terms of enhanced formation of insoluble aluminium chromates at high electric fields. The other possible interpretation employs the theoretically predicted effect of the extreme-possessing dependence of pore growth rate on the electric field applied.

ChemInform Abstract: FORMATION AND ACTIVITY OF CERTAIN CHROMITE CATALYSTS

AbstractDie Kinetik der Reaktionen A in Pulvergemischen wird isotherm bei 400‐1000°C an der Luft atomabsorptionsspektrophotometrisch und durch eine konventionelle Lösungsmittelmethode untersucht.

Thermodynamics of Aqueous Hydrogen Chromate and Dichromate Ions. Heats of Formation of Chromates and Dichromates

Thermodynamics of Aqueous Hydrogen Chromate and Dichromate Ions. Heats of Formation of Chromates and Dichromates

Nature of the Passive Film on Iron in Concentrated Nitric Acid

The film responsible for passivity when iron is immersed in concentrated nitric acid is shown to be ferric acid or a related higher valence iron compound. This was proved (a) by demonstrating formation of chromates when a 2.84% Cr‐Fe alloy is passivated, (b) by showing complete suppression of the iron‐nitric acid reaction when iron is previously immersed in potassium ferrate solution, and (c) by immersing passive iron in solution and observing higher valence iron compounds displaced from the surface by adsorbed OH−. The maximum of that was found is the same order of magnitude as moles chromate adsorbed on iron from chromate solutions as reported by other investigators. This amount of ferric acid, when it decomposes, is calculated to form a film of ferric oxide 40–125 Aå thick, which thickness range agrees with that of residual oxide films on passive iron measured previously by others.The higher valence iron compound forms at anodic areas of the iron surface during the iron‐nitric acid reaction. When sufficient local concentration is achieved, the compound adsorbs (chemisorbs) on iron, satisfies metal surface affinities, and is itself stabilized against decomposition by adsorption. The metastable compound breaks down by cathodic reduction, mechanical shock, or presence of halide ions, the latter either displacing the adsorbed compound or hastening its decomposition. The roles of urea and hydrogen peroxide in nitric acid are discussed.

The chemistry of some complex zinc and cadmium chrome pigments

AbstractWith the help of pH curves, conditions have been found for the formation of basic double chromates of general formula R2CrO43MCrO4,M(OH)2,2H2O, where R is Na, K or NH4 and M is Zn or Cd. No NaCd compound was encountered, and the NH,Zn compound was of a different type. The decomposition of these compounds by alkali and by copious leaching with hot and cold water (KZn and KCd only) into basic zinc and cadmium compounds has been studied. Cadmium chromate and the monoxychromates of zinc and cadmium have been prepared. The identity of each compound has been characterized by equation of formation, composition, X‐ray diffraction pattern and solubility. Many of these compounds are of commercial importance as paint pigments. The NaZn, KZn, KCd and NH,Cd compounds have been prepared.