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Engelhardt G.R.,OLI Systems Inc | MacDonald D.D.,Pennsylvania State University | MacDonald D.D.,King Fahd University of Petroleum and Minerals
Electrochimica Acta | Year: 2012

The case when the potential distribution inside a corrosion cavity obeys Ohm's law is considered. Mathematically, the potential drop in the crevice is described by a Poisson-type equation with a non-linear source term. A simple method for finding all possible solutions in a one-dimensional approximation and for investigating their stability has been developed. We derive a simple relation for estimating the critical depth of the crevice, L c (which is defined as the depth at which the active-passive transition just occurs within the crevice) as a function of the width of the crevice, w, electrolyte conductivity, κ, metal potential, E met, and a polarization curve. It is shown that L c is proportional to √(wκ) and is a linear function of E met. Calculation of the corrosion damage (maximum depth of the penetration into the metal, w max) as a function of time and position inside the crevice has been performed. It is shown that during the initial stages of crevice corrosion, when the one-dimensional approximation is valid, w max is determined mainly by the polarization curve for the anodic dissolution of the metal. It is shown that, in the general case, it is impossible to neglect the potential drop in the external environment when quantitatively describing crevice corrosion. © 2012 Elsevier Ltd. All rights reserved. Source

Zhang Y.,Pennsylvania State University | Urquidi-MacDonald M.,Pennsylvania State University | Engelhardt G.R.,OLI Systems Inc | MacDonald D.D.,Pennsylvania State University | MacDonald D.D.,King Fahd University of Petroleum and Minerals
Electrochimica Acta | Year: 2012

Passivity on steel surfaces plays a crucial role in the development of pitting corrosion damage. In this work, the passivity on Type 403 stainless steel (SS), low pressure steam turbine (LPST) blade alloy, and A470/471 low alloy steel (LPST) disk/rotor steel has been studied in borate buffer solution. The highly defective barrier layers on Type 403 SS and A470/471 steel, formed at potentials in the passive region, exhibit n-type semiconductor behavior, due to the predominance of oxygen vacancies and/or cation interstitials as crystallographic point defects. The defective Cr 2O 3 barrier layer on the Type 403 SS surface has a greater donor concentration and a smaller thickness than does the barrier layer (defective Fe 3O 4) on the A470/471 steel surface. Increasing oxygen and chloride concentration in the electrolyte increase the donor concentration in the passive film on Type 403 SS. In the passive regions of Type 403 SS and A470/471 steel, the steady-state current density is nearly independent of, and the steady-state barrier layer thickness increases linearly with, increasing film formation potential. These experimental observations are interpreted in terms of the point defect model (PDM) for the passive state. Mechanistic analysis of electrochemical impedance spectroscopy (EIS) data by the impedance model developed from the PDM, is also performed, and the validity of the impedance model is demonstrated. © 2012 Elsevier Ltd. Source

Choudhary V.,University of Delaware | Mushrif S.H.,Nanyang Technological University | Ho C.,University of Delaware | Anderko A.,OLI Systems Inc | And 5 more authors.
Journal of the American Chemical Society | Year: 2013

5-(Hydroxymethyl)furfural (HMF) and levulinic acid production from glucose in a cascade of reactions using a Lewis acid (CrCl3) catalyst together with a Brønsted acid (HCl) catalyst in aqueous media is investigated. It is shown that CrCl3 is an active Lewis acid catalyst in glucose isomerization to fructose, and the combined Lewis and Brønsted acid catalysts perform the isomerization and dehydration/rehydration reactions. A CrCl3 speciation model in conjunction with kinetics results indicates that the hydrolyzed Cr(III) complex [Cr(H2O)5OH]2+ is the most active Cr species in glucose isomerization and probably acts as a Lewis acid-Brønsted base bifunctional site. Extended X-ray absorption fine structure spectroscopy and Car-Parrinello molecular dynamics simulations indicate a strong interaction between the Cr cation and the glucose molecule whereby some water molecules are displaced from the first coordination sphere of Cr by the glucose to enable ring-opening and isomerization of glucose. Additionally, complex interactions between the two catalysts are revealed: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of [Cr(H2O)5OH]2+. In contrast, Lewis acidity increases the overall rate of consumption of fructose and HMF compared to Brønsted acid catalysis by promoting side reactions. Even in the absence of HCl, hydrolysis of Cr(III) decreases the solution pH, and this intrinsic Brønsted acidity drives the dehydration and rehydration reactions. Yields of 46% levulinic acid in a single phase and 59% HMF in a biphasic system have been achieved at moderate temperatures by combining CrCl3 and HCl. © 2013 American Chemical Society. Source

Macdonald D.D.,Pennsylvania State University | Engelhardt G.R.,OLI Systems Inc
ECS Transactions | Year: 2010

The Point Defect Model (PDM) has been shown to accurately describe the properties of passive films that form on metal surfaces in contact with aggressive environments under both open circuit and anodic polarization conditions. However, the commonly-employed PDM, known henceforth as Generation II or PDM-II assumes that passivity arises from the properties of the barrier layer and that the outer layer, if present, contributes negligibly to the interfacial impedance. In this paper, we describe a third generation of the PDM, PDM-III, in which a resistive outer layer exists on the surface and contributes substantially to the impedance of the interface and hence to the corrosion resistance. The outer layer is shown to have a profound impact on the properties of the barrier layer and under certain circumstances the barrier layer is predicted to disappear. This new form of depassivation is observed experimentally in the corrosion of carbon steel in CO2-acidified oil-field brines, for example. The use of electrochemical impedance spectroscopy to characterize passive films having resistive outer layers is describe and illustrated with reference to the passive state on zirconium in simulated PWR (Pressurized Water Reactor) primary coolant. ©The Electrochemical Society. Source

Engelhardt G.R.,OLI Systems Inc | Macdonald D.D.,Pennsylvania State University
Corrosion Science | Year: 2010

The application of standard mathematical techniques for the solution of mass transport equations, in the case of advection that is caused by the pulsating movement of crack walls in the case of corrosion fatigue, can be very time consuming. This problem arises, due to the requirement that the time step that must be employed, when solving the non-stationary equations numerically, must be significantly smaller than the period of oscillation. For overcoming these time-consuming limitations, a simple algorithm, which is based on eliminating the convective term from the equations of mass transfer in the pulsating slab by a suitable change of variables, was developed. The estimation of the advection effect on the rate of corrosion fatigue has been performed for the cases of diffusion and mixed kinetic control at high frequencies of applied stress. It is shown that, in many cases, it is possible to use codes that were developed for describing stress corrosion cracking, i.e. for the case of mass transfer without advection at zero loading frequency, to predict corrosion fatigue crack propagation rate, by simply substituting an effective crack length. Numerical calculations that have been performed in this work also show that the method developed here yields results that are applicable not only to the elevated frequencies, but to the any arbitrary frequency from 0 to ∞ for estimating corrosion fatigue crack propagation rate. © 2009 Elsevier Ltd. All rights reserved. Source

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