Papers by Keyword: Chromate

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Authors: Peng Ge, Li Juan Wan, Ya Jing Xu
Abstract: Among the investigated clays and minerals (kaolinite, natural zeolite, manual zeolite, bentonite, sepiolite, sepiolite amianthus, tremolite amianthus, vermiculite and baritite), the baritite clay was selected as the optimal adsorbent for aqueous Cr (VI). The Cr (VI) adsorption capacity on baritite clay reached as high as 39.01 mg∙g−1 at 20°C. Then the adsorption kinetics of Cr (VI) by the baritite clay were investigated in details. Results showed that the pseudo-second-order model was a suitable description for the adsorption kinetics and fitted well with the experimental data.
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Authors: Jian Wei Lin, Yan Hui Zhan, Yun Qing Xing, Yu Liu
Abstract: Surfactant-modified zeolites (SMZ) with different coverage types were prepared by loading the cetylpyridinium bromide (CPB) onto the surface of the natural zeolite collected from Jinyun County, Zhejiang Province, China. The adsorption behavior of Cr(VI) on SMZ was investigated. Natural zeolite and SMZ with CPB monolayer coverage were inefficient for the removal of Cr(VI) from aqueous solution. However, SMZ with CPB patchy bilayer or bilayer coverage was efficient in the removal of Cr(VI) and the Cr(VI) adsorption capacity of SMZ increased with its CPB loading. The Cr(VI) adsorption capacity of SMZ showed a maximum at pH 3-6 and decreased with increasing pH from 6 to 11. The presence of competing anions such as chloride, sulfate and bicarbonate ions reduced the Cr(VI) adsorption capacity of SMZ. The adsorption process was strongly dependent on ionic strength, indicating that anionic exchange and electrostatic interaction were the main mechanisms that govern the adsorption of Cr(VI) on SMZ.
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Authors: Margarita del Arco, Daniel Carriazo, Cristina Martín, Amalia M. Pérez Grueso, Vicente Rives
Abstract: PXRD (powder x-ray diffraction), FT-IR (Fourier Transform infrared), N2 adsorption at - 196 °C and TG/DTA (thermogravimetric and differential thermal analyses) techniques have been used for characterisation of MgAl- and ZnAl-CrO4 LDHs, which had been prepared by the ion exchange method from the corresponding chloride LDH (layered double hydroxides) precursors. The results indicates that the oxometalate intercalation in both systems produces interlayer microporosity and a basal spacing of 8.7 Å; This gallery height decreases when the samples are calcined in the temperature range 100-300 °C, due to a grafting process. A larger thermal stability is detected for MgAl-CrO4 sample than for the zinc-containing one.
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Authors: Petter Andersson, Mats Norell, Régis Gautheron
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Authors: Jenifer S. Warner, Sang Shik Kim, Richard P. Gangloff
Abstract: The objective of this study is to quantify and understand the effectiveness of a hexavalent chrome replacement ion to inhibit environmentally assisted fatigue crack propagation (EFCP) in high strength aluminum alloys. Addition of molybdate (MoO4 2-) to bulk-low chloride solution effectively inhibits EFCP in peak aged 7075, comparable to that of CrO4 2-. The effectiveness of inhibition depends strongly on loading variables: .K, R, and frequency as explained qualitatively by mechanical instability of a crack tip passive film that otherwise hinders production and uptake of embrittling hydrogen. The critical loading frequency (and crack tip strain rate), below which film stability and inhibition occur, increases with increasing inhibitor concentration, but only for low stress ratio loading, perhaps due to occluded crack transport and reaction considerations. Molybdate could be a beneficial replacement for chromate and a candidate for inhibitor release from a tailored coating.
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Authors: S. Karlsson, J. Pettersson, J.E. Svensson, L.G. Johansson
Abstract: The effect of SO2 on the oxidation of alloy 304L in O2+H2O and O2+H2O+KCl environment has been investigated at 600°C. Exposure time was 1-168 hours. The exposed samples were analyzed by SEM/EDX, XRD and IC. In dry O2, a protective and chromium-rich corundum-type oxide forms. In the presence of H2O(g), chromium is volatilized in the form of CrO2(OH)2(g). The corresponding chromium depletion of the protective oxide triggers a partial loss of protective properties resulting in the formation of oxide islands on the alloy grain centers. The oxide islands consist of an outward growing hematite layer and an inward growing FeCrNi spinel layer. By coating the samples with KCl the chromia depletion of the protective oxide dramatically increases due to the formation of K2CrO4. This leads to breakaway corrosion, a rapidly growing scale forming all over the surface. The resulting thick scale has a similar structure as the oxide islands formed in the absence of KCl. The addition of 300 ppm SO2 to the O2+H2O and O2+H2O+KCl environments results in a drastic reduction of corrosion rate. In O2+H2O environment the effect of SO2 is attributed to the formation of a thin sulphate film on the oxide surface that impedes chromium volatilization and decreases the rate of oxygen reduction on the oxide surface. In O2+H2O+KCl environment the corrosion mitigating effect of SO2 is mainly attributed to the rapid conversion of KCl to K2SO4. In contrast to KCl, K2SO4 does not deplete the protective oxide in chromium by forming K2CrO4.
224
Authors: Petter Andersson, Mats Norell
Abstract: The scale growth on two austenitic alloys, Alloy 310 and Sanicro 28, under KCl deposits was examined. This is relevant to the long term corrosion of superheater tubes in biofuel combustion. Coupons were encapsulated in tablets so that 1 mm of KCl with a relative density of 91% covered the metal. Samples were tested at 500°C in 5%O2-10%H2O-N2 for 24, 168 and 672 h. After exposure the salt was broken off and the scale was characterised by using SEM-EDX and AES. After 24 h a 50 nm thick oxide surrounded 500 nm thick chromates on the surface. No oxide layer was detected under the chromates and no Cl was found under either layer. The chromate growth requires lateral transportation of Cr along the surface. This reduces the protectiveness of the oxide and accelerates the formation of less protective Fe rich oxides. The formation of chromates also releases HCl inside the KCl tablet. The chromates did not grow significantly between 24 and 168 h, but the oxide grew equally thick beneath and between them and Cl was enriched around the metal oxide interface. After 672 h the oxides were about 5 μm thick and only few chromates were seen. Crystals of KCl formed in areas with thick porous and Fe rich oxides on both alloys.
333
Authors: Sarah E. Galyon Dorman
Abstract: Corrosion fatigue is an area of concern for the United States Air Force (USAF) and other Department of Defense (DoD) organizations. Often DoD corrosion prevention systems include chromate containing coatings, typically in the form of chromate conversion coatings and polymer primers. Chromate has been used successfully for many years within the DoD to prevent corrosion damage. However the environmental and personnel risks associated with chromate coatings have caused the USAF to pursue non-chromate containing corrosion prevention coatings [1]. To fully quantify chromate replacement coatings, an understanding of the effects that chromate has on corrosion fatigue crack growth rates must be fully characterized. Some researchers have shown that high levels of chromate added to 0.6 M NaCl full immersion corrosion fatigue tests on 7xxx series aluminum alloys slow the fatigue crack growth rate substantially [2,3]. The limitation of that research was that the amount of chromate present in the test solution environment was not connected to expected leach rates of chromate from polymeric coatings and a high solubility salt was used. The majority of DoD assets are protected from corrosion by polymer coatings loaded with corrosion inhibitors. For these coatings to slow fatigue crack propagation the corrosion inhibitors must become mobile as a consequence of hydration of the polymer coating matrix. Based on this mechanism of corrosion inhibitor release, the examination of atmospheric corrosion fatigue becomes important to help understand how inhibitors work in real world situations with hydrated salt layers rather than only fully immersed solutions.
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