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Stuart Lyon - One of the best experts on this subject based on the ideXlab platform.
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The use of electrochemical impedance spectroscopy with segmented electrodes to study inhibition at the cut-edge of coil-coated systems
Progress in Organic Coatings, 2017Co-Authors: Lee A. Farren, Dave Francis, Stuart LyonAbstract:Abstract The use of multiple metal segmented electrodes has been applied to study the inhibition mechanisms for several anti-corrosion pigments that are commonly used in organic coatings. The relatively novel experimental methodology involves, firstly, polarisation of the coupled metal system at the common coupled potential but with the currents on the separated electrodes recorded individually and, secondly, the application of electrochemical impedance spectroscopy at the same coupled potential. For example, at the cut-edge of a coil-coated galvanised steel sheet, electrochemical data may be gathered simultaneously at an anodic potential for the zinc and at a cathodic potential for the steel substrate. Using this methodology the inhibitive efficiency for Strontium Chromate, calcium ion-exchanged silica and zinc molyphosphate inhibitors were determined under the electrochemical condition expected in service.
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Inhibition of mild steel by Strontium Chromate in artificial acid rain solution
Corrosion Engineering Science and Technology, 2005Co-Authors: I. M. Baghni, Stuart LyonAbstract:AbstractThe inhibition of mild steel in an artificial acid rain solution (pH 4·5) in low concentrations of Strontium Chromate pigment has been evaluated using a range of methods: potentiodynamic polarisation, electrochemical impedance, X-ray photoelectron spectroscopy and solution analysis. The individual effects of Strontium and Chromate have been evaluated and have been used to estimate inhibitor efficiency and film composition, and to determine the possible inhibition mechanism. The results show that, at low concentration, Strontium Chromate affects the cathodic reaction, with reduction of Cr6+ to Cr3+, and the surface film was composed of magnetite and hydrated chromium hydroxide. The presence of Strontium was not found significantly to affect the inhibitory performance.
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The effect of Strontium and Chromate ions on the inhibition of zinc
Surface & Coatings Technology, 2004Co-Authors: I. M. Baghni, Stuart Lyon, Baofeng DingAbstract:Abstract The corrosion and inhibition of zinc in artificial acid rain solution (pH 4.5) at a low Strontium Chromate concentration has been evaluated using electrochemistry, solution analysis and XPS. These techniques have been used to estimate inhibitor efficiency and to determine a possible inhibition mechanism. The results show that Strontium Chromate at low concentration inhibited significantly more of the cathodic corrosion process, by reduction of Cr6+ to a solid species containing Cr3+. The surface film contained a mixture of oxidised zinc and chromium, probably in the form of basic zinc carbonate (hydrozincite) and hydrated chromia. Importantly Strontium ions were found to play no significant role in the inhibition process.
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Corrosion control of galvanized steel using a phosphate/calcium ion inhibitor mixture
Corrosion Science, 2003Co-Authors: I. M. Zin, Stuart Lyon, V.i. PokhmurskiiAbstract:The corrosion inhibition of galvanized steel was studied in artificial acid rain solution using extracts of pigments normally used in organic coatings for corrosion control. It was established that a combination of zinc phosphate/molybdate and calcium ion exchange silica has a significant synergetic anticorrosion effect in the acid rain solution compared to the pigments used alone. Further, the charge transfer resistance of galvanized steel in acid rain solution saturated by the above pigment blend approaches that of Strontium Chromate in artificial acid rain solution. Use of the pigment blend was found to lead to development of a protective film, which is thought to be a complex mixture of calcium phosphates and zinc phosphate.
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Electrochemical cell study of cut edge corrosion inhibition on coil-coated steel sheet.
Transactions of the IMF, 2002Co-Authors: Stuart Lyon, Robert Howard, I. M. ZinAbstract:Corrosion and corrosion inhibition of coil-coated galvanised steel has been studied using an electrochemical sandwich cell which models the geometric situation at sheet steel cut edges. The galvanic currents flowing during immersion in a solution simulating typical acid rainwater were measured. Also, potentio-dynamic polarisation was carried out on the galvanic cell where the individual currents on the steel and zinc, as well as the total current, were determined. Where no inhibitor is present in solution, the corrosion rate is controlled by oxygen diffusion to the steel cathode. In the presence of Strontium Chromate the cathodic reduction of oxygen on the steel cathode is substantially reduced in addition to the expected anodic passivation of zinc. This is presumably due to reduction of Chromate at the cathode forming a protective chromium oxide film. Some alternatives to Chromate were also tested. Molybdate/phosphate pigments and calcium ion-exchange pigments alone gave poor inhibition. However they were found to act synergistically in combination giving an inhibitive efficiency closely approaching that of Chromate.
W. Depmeier - One of the best experts on this subject based on the ideXlab platform.
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The intermediate phase of Strontium Chromate aluminate sodalite
Zeitschrift für Kristallographie - Crystalline Materials, 2001Co-Authors: D. M. Többens, W. DepmeierAbstract:At high temperatures yellow Strontium Chromate aluminate sodalite Sr-8[Al12O24](CrO4)(2) has cubic symmetry with orientational disorder of the CrO42- ions within the sodalite cages. The existence of two phase transitions at 299 K and 289 K, which result in superstructures of tetragonal and orthorhombic symmetry, respectively, is well known. The structure of the orthorhombic low-temperature phase has been solved recently [1]. In this work we discuss the crystal structure of the intermediate phase and its determination from neutron and x-ray powder diffraction data. The structure of this phase shows only very small distortions of the framework of AlO4-tetrahedra. The CrO4-tetrahedra within the sodalite cages are partially ordered. The observed distribution of their orientations can be described as a combination of those of the low- and the high-temperature phase. Al-27-MAS-NMR spectra reveal the dynamic nature of the structure.
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Superstructure of Strontium Chromate aluminate sodalite at low temperatures
Zeitschrift für Kristallographie - Crystalline Materials, 2001Co-Authors: D. M. Többens, W. DepmeierAbstract:At high temperatures, yellow Strontium Chromate aluminate sodalite Sr-8[Al12O24](CrO4)(2) has cubic symmetry with orientational disorder of the CrO42--ions within the sodalite cages. The existence of two phase transitions. at 299 K and 289 K, which result in superstructures of tetragonal and orthorhombic symmetry respectively, is well known. In this work we describe the crystal structure of the orthorhombic low temperature phase which was determined from neutron and x-ray powder diffraction. Additional information from Al-27-MAS-NMR spectra and the Monte-Carlo-method was used to get a starting model for the structure refinement. In the orthorhombic phase the CrO4-tetrahedra are. fully ordered. Two thirds of the rings of four AlO4-tetrahedra forming the sodalite framework are nearly undistorted. The remaining, distorted, rings form, chains parallel to, the a-axis. The distortion of the rings is, of a type which occurs in other aluminate sodalite structures as well and is, known as the shearing mechanism. Despite of the different symmetry the structure of this phase is similar to the structure of Strontium tungstate aluminate sodalite. The characteristic differences can be explained by the smaller size of the CrO4-tetrahedra.
D. M. Többens - One of the best experts on this subject based on the ideXlab platform.
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The intermediate phase of Strontium Chromate aluminate sodalite
Zeitschrift für Kristallographie - Crystalline Materials, 2001Co-Authors: D. M. Többens, W. DepmeierAbstract:At high temperatures yellow Strontium Chromate aluminate sodalite Sr-8[Al12O24](CrO4)(2) has cubic symmetry with orientational disorder of the CrO42- ions within the sodalite cages. The existence of two phase transitions at 299 K and 289 K, which result in superstructures of tetragonal and orthorhombic symmetry, respectively, is well known. The structure of the orthorhombic low-temperature phase has been solved recently [1]. In this work we discuss the crystal structure of the intermediate phase and its determination from neutron and x-ray powder diffraction data. The structure of this phase shows only very small distortions of the framework of AlO4-tetrahedra. The CrO4-tetrahedra within the sodalite cages are partially ordered. The observed distribution of their orientations can be described as a combination of those of the low- and the high-temperature phase. Al-27-MAS-NMR spectra reveal the dynamic nature of the structure.
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Superstructure of Strontium Chromate aluminate sodalite at low temperatures
Zeitschrift für Kristallographie - Crystalline Materials, 2001Co-Authors: D. M. Többens, W. DepmeierAbstract:At high temperatures, yellow Strontium Chromate aluminate sodalite Sr-8[Al12O24](CrO4)(2) has cubic symmetry with orientational disorder of the CrO42--ions within the sodalite cages. The existence of two phase transitions. at 299 K and 289 K, which result in superstructures of tetragonal and orthorhombic symmetry respectively, is well known. In this work we describe the crystal structure of the orthorhombic low temperature phase which was determined from neutron and x-ray powder diffraction. Additional information from Al-27-MAS-NMR spectra and the Monte-Carlo-method was used to get a starting model for the structure refinement. In the orthorhombic phase the CrO4-tetrahedra are. fully ordered. Two thirds of the rings of four AlO4-tetrahedra forming the sodalite framework are nearly undistorted. The remaining, distorted, rings form, chains parallel to, the a-axis. The distortion of the rings is, of a type which occurs in other aluminate sodalite structures as well and is, known as the shearing mechanism. Despite of the different symmetry the structure of this phase is similar to the structure of Strontium tungstate aluminate sodalite. The characteristic differences can be explained by the smaller size of the CrO4-tetrahedra.
Hamilton Neil Mcmurray - One of the best experts on this subject based on the ideXlab platform.
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Inhibition of corrosion driven delamination on iron by smart-release bentonite cation-exchange pigments studied using a scanning Kelvin probe technique
Progress in Organic Coatings, 2017Co-Authors: Geraint Williams, Hamilton Neil McmurrayAbstract:Abstract Low-cost, environmentally friendly, cation exchange pigments derived from naturally occurring bentonite clay are shown to significantly enhance resistance to corrosion-driven cathodic delamination in organic coatings adherent to iron surfaces. A scanning Kelvin probe (SKP) is used to study the delamination kinetics of pigmented and unpigmented poly-vinyl-butyral (PVB)-based coatings applied to polished iron substrates. The bentonite clay is used both in its native form and exhaustively exchanged with a range of divalent alkali earth and trivalent rare earth metal cations. For the best performing divalent cation-exchanged pigment, the dependence of coating delamination rate on pigment volume fraction is determined and compared with that of a conventional Strontium Chromate (SrCrO 4 ) inhibitor. An inhibition mechanism is proposed for the bentonite pigments whereby underfilm cation release and subsequent precipitation of sparingly soluble hydroxides reduces the conductivity of the underfilm electrolyte.
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Chromate Mitigation in Atmospheric Corrosion By the Use of Smart-Release Pigments
2013Co-Authors: Hamilton Neil Mcmurray, Geraint WilliamsAbstract:Despite concerted efforts over the last few decades, it is arguable that a credible, environmentally acceptable, equal for Chromate-based inhibition of atmospheric corrosion (as it affects organic coated metals) has yet to be identified. This paper outlines an effective and immediately applicable approach for massively reducing the quantity of Cr(VI) used in a protective coating whilst retaining the full corrosion inhibition effect. The trick for doing this is to avoid using sparingly soluble Chromate salts such as Strontium Chromate or zinc Chromate and instead to store Chromate ions in a waterinsoluble smart-release pigment. The advantages of such an approach are twofold. Firstly the inhibitor is only released “on-demand”. For smart release pigments based on anion-exchange minerals (such as those to be described here) release is triggered only when aggressive anions such as chloride or hydroxide are encountered. In contrast a pigment comprising a sparingly soluble Chromate salt (e.g Strontium Chromate) will continuously leach out Chromate ions when in contact with an aqueous medium. Secondly, the smart-release pigment comprises a significantly lower Chromate content per gram of solid. Whereas a Strontium Chromate (SrCrO4) pigment has a CrO4 2content of 4.9 mmol g, a Chromate exchanged hydrotalcite pigment (HT-CrO4 ) comprises 0.35 mmol g, representing a 15 fold decrease in Cr(VI) on a weight-for-weight basis. In this paper a brief description is first given of the preparation of a HT-CrO4 2pigment and its incorporation into a model coating comprising polyvinylbutryal lacquer containing various volume fractions of pigment dispersion. Two types of atmospheric corrosion experiment are then described, in both of which the scanning Kelvin probe (SKP) is used to follow the rate of corrosion-driven coating failure (disbondment) and provide mechanistic information on inhibitor action. In the first type of experiment the cathodic disbondment of a PVB coating is followed on galvanized steel exposed to sodium chloride at 96%RH. In the second type of experiment filiform corrosion (FFC) is followed on PVB coated AA2024-T3 exposed to HCl at 93%RH. Fig. 1 shows the effect of various in-coating volume fractions of HT-CrO4 2on the rate of corrosioninduced cathodic disbondment of a PVB coating on galvanised steel. For pigment volume fractions > 0.1 delamination rates are reduced to a minimal level. It is shown that a similar performance is obtained for SrCrO4 and HT-CrO4 2pigments incorporated at roughly comparable pigment volume fractions. It can be estimated that if a pigment volume fraction of 0.1 is used in each case, then the SrCrO4 and HT-CrO4 2containing coatings comprises 1.9 and 0.07 mmol cm respectively (i.e. the HT-CrO4 2pigmented coating contains 27 times less Chromate). Fig. 2 shows the effect of incorporating 20% w/w SrCrO4 and HT-CrO4 2pigments on the rate of FFCinduced coating disbondment on PVB coated AA2024-T3 It may be seen that to a similar extent, For both pigments the initial coating delamination rate is ca. 1.8 × 10 mm min, compared with 1.1×10 mm min for the unpigmented case. However, as argued previously, the HT-CrO4 2pigment contains 15 times less available Chromate than the same weight of SrCrO4. On the basis of the above we can conclude that, the use of smart-release pigments can allow a massive reduction of Chromate use without compromising protection against atmospheric corrosion.
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Chrome free pigments for corrosion protection in coil coated galvanised steels
Corrosion Engineering Science and Technology, 2006Co-Authors: M. J. Loveridge, Hamilton Neil Mcmurray, David A. WorsleyAbstract:AbstractA range of bentonite clays such as Wyoming (W), hybond (H), cat litter (CL) and versions exchanged with Ce3+, Y3+, La3+ and Ca2+ cations used as the corrosion inhibitors in the 5 μm polyester primer layer applied to 0·7 mm gauge hot dip zinc (0·15%Al) galvanised steel materials, over coated with a 20 μm architectural polyester coating. Similar primer systems have been prepared containing Strontium Chromate (Cr), Shieldex (Shd) and titanium dioxide (TiO2) for the purposes of comparison. The scanning vibrating electrode technique (SVET) was used to probe the corrosion kinetics at sample cut edges immersed in 5%NaCl on a limited number of the coatings based on the Wyoming bentonite. The SVET determined zinc losses for the pigments tested over 24 h were as follows: Shd (920 μg), Cr (740 μg), W–Y (655 μg), W–Ce (560 μg), W (550 μg) and W–Ca (496 μg). The same samples together with all of the other pigmented coatings were also subjected to salt spray testing. The average maximum cut edge corrosion delam...
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Chromate Inhibition of Filiform Corrosion on Organic Coated AA2024-T3 Studied Using the Scanning Kelvin Probe
Journal of The Electrochemical Society, 2004Co-Authors: Hamilton Neil Mcmurray, Geraint Williams, S. O’driscollAbstract:The influence of in-coating Strontium Chromate (SrCrO 4 ) pigment dispersions on the kinetics and mechanism of HCl-induced filiform corrosion (FFC) affecting polyvinyl butyral coated AA2024 T3 aluminum alloy is studied using a scanning Kelvin probe. FFC initiation and propagation are detected by imaging time-dependent changes in local free corrosion potential (E corr ) distribution patterns. Area rates of coating delamination fall from 1.25 X 10 -8 m 2 min -1 to zero as the SrCrO 4 volume fraction (Φ SC ) is increased from zero to 0.06. Increasing Φ SC over the same range produces minimal change in filament-head E corr values but depresses filament tail and intact coating E corr values by up to 0.3 V. It is proposed that filament tail E corr depression results from increased polarization of cathodic O 2 reduction reactions at sites of high pH, where insoluble Cr(OH) 3 is the predominant CrO 2- 4 reduction product. It is further proposed that intact coating E corr depression results from reversible, nonfaradaic adsorption of CrO 2- 4 at the oxide-covered aluminum surface. Both effects are implicated in FFC inhibition, as is pH buffering by SrCrO 4 which acts in conjunction with CrO 2 4 adsorption to forestall anodic attack at the coating-substrate interface.
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Inhibitor pretreatment synergies demonstrated using a scanning Kelvin probe technique
Corrosion Engineering Science and Technology, 2003Co-Authors: Hamilton Neil Mcmurray, Geraint Williams, David A. Williams, David A. WorsleyAbstract:AbstractA scanning Kelvin probe technique has been used to study the influence of pretreatment and pigment interaction on the kinetics and mechanism of corrosion driven delamination processes affecting model polyvinyl butyral coatings adherent to the intact zinc surface of hot dip galvanised steel. The effects of incorporating dispersions of Strontium Chromate (SrCrO4), silica and bentonite based Cerium (III) cation exchange pigments within the polyvinyl butyral coating are investigated on galvanised steel specimens in the absence and presence of a mixed oxide/Chromate rinse surface pretreatment. Placing aqueous sodium chloride electrolyte onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O2 reduction at the delamination front is coupled to anodic zinc dissolution at the coating defect by a thin layer of electrolyte which ingresses beneath the delaminated polyvinyl butyral film. The SrCrO4 pigmented polyvinyl butyral films allow CrO42- diffusion directly ...
I. M. Zin - One of the best experts on this subject based on the ideXlab platform.
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Contradictory effect of Chromate inhibitor on corrosive wear of aluminium alloy
Corrosion Science, 2011Co-Authors: V.i. Pokhmurskii, I. M. Zin, V. A. Vynar, L.m. BilyAbstract:Abstract The corrosive wear of D16T aluminium alloy in artificial acid rain was studied. A special tribometer with the linear reciprocating ball-on-flat geometry was used. The setup allows to measure simultaneously an open circuit potential, to carry out potentiostatic and potentiodynamic polarization studies of the alloy corrosion and to record the friction coefficient. It was established that the addition of Strontium Chromate inhibitor to the working environment decreases an electrochemical corrosion of the aluminium alloy under wear conditions, but in general accelerates its destruction due to insufficient wear resistance of a formed surface film.
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Corrosion control of galvanized steel using a phosphate/calcium ion inhibitor mixture
Corrosion Science, 2003Co-Authors: I. M. Zin, Stuart Lyon, V.i. PokhmurskiiAbstract:The corrosion inhibition of galvanized steel was studied in artificial acid rain solution using extracts of pigments normally used in organic coatings for corrosion control. It was established that a combination of zinc phosphate/molybdate and calcium ion exchange silica has a significant synergetic anticorrosion effect in the acid rain solution compared to the pigments used alone. Further, the charge transfer resistance of galvanized steel in acid rain solution saturated by the above pigment blend approaches that of Strontium Chromate in artificial acid rain solution. Use of the pigment blend was found to lead to development of a protective film, which is thought to be a complex mixture of calcium phosphates and zinc phosphate.
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Electrochemical cell study of cut edge corrosion inhibition on coil-coated steel sheet.
Transactions of the IMF, 2002Co-Authors: Stuart Lyon, Robert Howard, I. M. ZinAbstract:Corrosion and corrosion inhibition of coil-coated galvanised steel has been studied using an electrochemical sandwich cell which models the geometric situation at sheet steel cut edges. The galvanic currents flowing during immersion in a solution simulating typical acid rainwater were measured. Also, potentio-dynamic polarisation was carried out on the galvanic cell where the individual currents on the steel and zinc, as well as the total current, were determined. Where no inhibitor is present in solution, the corrosion rate is controlled by oxygen diffusion to the steel cathode. In the presence of Strontium Chromate the cathodic reduction of oxygen on the steel cathode is substantially reduced in addition to the expected anodic passivation of zinc. This is presumably due to reduction of Chromate at the cathode forming a protective chromium oxide film. Some alternatives to Chromate were also tested. Molybdate/phosphate pigments and calcium ion-exchange pigments alone gave poor inhibition. However they were found to act synergistically in combination giving an inhibitive efficiency closely approaching that of Chromate.
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Model Electrochemical Cell Study of Cut-Edge Corrosion Inhibition on Coil-Coated Steel Sheet by Chromate-, Phosphate-, and Calcium-Containing Pigments
Journal of The Electrochemical Society, 2001Co-Authors: I. M. Zin, V.i. Pokhmurskii, J.d. Scantlebury, Stuart LyonAbstract:Corrosion at cut edges is the most important failure mechanism of org.-coated, profiled galvanized steel architectural claddings. Currently, edge corrosion is generally controlled by the addn. of Strontium Chromate in the paint primers; however, there is substantial interest in Chromate replacements due to environmental reasons. This work describes an exptl. study of inhibition with specific relevance to the cut-edge situation; essentially equiv. to a small galvanic cell between zinc and steel. Although Chromate initially acts as an anodic inhibitor for zinc corrosion at the cut edge, over a few hours of immersion, it was found to also strongly inhibit the steel cathode, hence reducing the cathodic protection current requirement on the zinc and thus acted as a mixed inhibitor in the cut-edge galvanic cell. Although individually, zinc phosphate and calcium ion-exchanged silica pigments had relatively poor inhibition, they showed a strong synergistic effect. Thus, a mixt. of the two compds. had comparable inhibitive efficiency to Chromate. This is due to a similar mixed inhibition mechanism as Chromate. Thus, anodic inhibition of zinc was evident as well as strong cathodic inhibition on the steel due to the formation of a compact, thin film contg. zinc, calcium, and phosphate species. [on SciFinder (R)]
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The effect of Chromate pigment on the protective properties of an epoxy prime coat
Materials Science, 1999Co-Authors: I. M. ZinAbstract:The protective function of Strontium Chromate pigment in the epoxy prime coat on the galvanized steel is manifested simultaneously with penetration of the corrosive medium to the metal surface and consists of an increase (by an order of magnitude) in polarization resistance and inhibition of corrosion of the galvanized steel at the places of through defects of the polymer coating.
