Z. Qin

Rate controlling reactions for copper corrosion in anaerobic aqueous sulphide solutions

Abstract The long term corrosion behaviour of copper in anoxic aqueous sulphide solutions has been studied using corrosion potential and electrochemical impedance spectroscopy measurements and scanning electron microscopy on corroded surfaces and cross-sections of surfaces prepared using a focused ion beam. Experiments were conducted in solutions containing either 5×10−4 or 5×10−5 mol L−1 sulphide for 1691 and 4000 h respectively. In the more concentrated solution, a coherent, compact and crystalline chalcocite (Cu2S) film accumulated on the corroding copper surface. A parabolic growth law was obtained, and the kinetics were controlled by Cu(I) ion transport either through the Cu2S matrix or along crystalline grain boundaries in the film. In the more dilute solution, the growth of a less crystalline, porous chalcocite layer followed approximately the linear growth kinetics controlled by sulphide ion transport through the pores. If the sulphide was allowed to deplete in the dilute solution, rate control switched to sulphide diffusion in the bulk solution. The implications for waste container corrosion in a nuclear waste repository are discussed.

Nominally Anaerobic Corrosion of Carbon Steel in Near-Neutral pH Saline Environments

Abstract Gas transmission pipeline corrosion commences when coatings disbond, exposing the steel to groundwater. When this occurs, a number of anaerobic and aerobic corrosion scenarios can be envisaged. The initial nominally anaerobic corrosion period has been investigated by applying a combination of electrochemical methods (i.e., corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy [EIS] measurements) and surface analytical techniques (scanning electron microscopy, energy-dispersive x-ray spectroscopy, and Raman spectroscopy). An evolution in film properties was observed and attributed to the entry of adventitious oxygen into faults within the preformed film. This leads to an increase in overall corrosion and a change in properties of the film as detected by EIS and Raman analysis. This article describes the mechanism involved in this transition, and provides a basis for a more extensive study of the corrosion process encountered on switching between anaerobic ...

Collective correlation effects in solid-state diffusion

Abstract Collective correlation factors are the correlated part of the phenomenological coefficients of irreversible thermodynamics. It is shown that collective correlation factors can be expressed in a manner similar to tracer correlation factors, that is they are associated with the collective cosines of the angle between successive jumps. These collective cosines can be determined by following a step-by-step walk of a vacancy. Illustrative calculations are performed on a binary random alloy with a vanishingly small concentration of vacancies in a simple-cubic lattice to an approximation that the vacancy will become random after six jumps. The results are found to fall between the data predicted by Manning and by Sato and coworkers over a large concentration region and for various exchange frequency ratios. When CA → 1, the present study gives the exact value for collective correlation factor for a pure system and, when CA → 0 and ωA = ωB, the present result is coincident with the known value for f0 for...

RELATIONS BETWEEN TRACER DIFFUSION COEFFICIENTS AND PHENOMENOLOGICAL COEFFICIENTS IN A BINARY SUBSTITUTIONAL ALLOY

Abstract In the course of a re-examination of the relations due to Darken and Manning we have derived two new relations each of which links the component tracer diffusion coeffients (D∗) and the phenomenological transport coefficients L for a binary substitutional alloy. From the first of these alone we can derive the relations proposed earlier without derivation by Dayananda. We have explored the accuracy of both the relations using the Monte Carlo computer simulation results of a previous study. The model chosen was of an order-disorder alloy A c B1-c in which atomic migration occurs via the movement of vacancies. At temperatures above and below the critical temperature for ordering (of AB) and at all compositions both relations do very well indeed; there is agreement almost within the statistical uncertainty of the Monte Carlo data.

Approximate expressions for the phenomenological coefficients of a binary alloy with short range order

Approximate expressions are derived for the phenomenological coefficients for vacancy-mechanism matter transport in a nearest-neighbour interaction lattice gas model of a concentrated binary alloy using the Kikuchi-Sato jump frequency model. These expressions are constructed from the leading coefficients (moments) in the Taylor series expansions in powers of time of the time correlation functions working within the Mori continued fraction representation. Numerical results are compared with earlier Monte Carlo simulations and with the predictions of the path probability method. Comparison with new Monte Carlo results shows that the approximate time correlation functions are good at short times but are too small at relatively long times.

Random walk analysis of the collective cosine formulation of correlation effects in atom transport in a concentrated alloy

Abstract The phenomenological coefficients Lij, which are defined in non-equilibrium thermodynamics and which characterize atom transport near thermodynamic equilibrium, can be expanded in terms of so-called collective cosines. A typical such quantity is (cos θji (n)) which is defined as the average of the cosine of the angle between the direction of an initial jump of an atom of species j and a final jump of an atom of species i when there are exactly n jumps of atoms of species i following the initial j atom jump and n = 1,2,3,…New exact relations between the four quantities (cos θji (n)), for i,j = A,B and arbitrary n are derived for a binary random alloy of two atomic species (A,B) with transport by a very small concentration of vacancies. A new simplified formula for the off-diagonal coefficient Lij,j ≠ i, is also derived in terms of these collective cosines. The approximate calculation of the collective cosines by enumeration of random walks is examined; results for n = 1,2 and 3 are in very good ag...

Study of Amyloid β-Peptide (Aβ12-28-Cys) Interactions with Congo Red and β-Sheet Breaker Peptides Using Electrochemical Impedance Spectroscopy

A surface-based approach is presented to study the interactions of Aβ12-28-Cys assembled on gold surfaces with Congo red (CR) and a β-sheet breaker (BSB) peptide. The various aspects of the peptide film have been examined using different electrochemical and surface analytical techniques. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) results using redox probes [Fe(CN)(6)](3-/4-) show that Aβ12-28-Cys on gold forms a stable and reproducible blocking film. EIS analysis shows that CR and BSB have different effects on the electrochemical properties of Aβ12-28-Cys films, presumably due to changes in the interactions between the film and CR and BSB. EIS results indicate that in the case of CR film resistance decreases significantly presumably due to better penetration of the solution-based redox probe into the film, whereas in the case of BSB, the film resistance increases. We interpret this difference to BSB being able to interact with the Aβ12-28-Cys on the surface and presumably forming a film that presents a higher resistance for electron transfer from the redox probe to the solution.

Relations between atomic transport coefficients in a binary system

Abstract A number of relations formally linking the phenomenological coefficients and other atomic transport coefficients such as the tracer diffusion coefficients, intrinsic diffusion coefficients and ionic conductivities are developed in the context of the Onsager linear formalism of non-equilibrium thermodynamics and two new compound relations recently derived by Lidiard et al. The relations are then tested by Monte Carlo computer simulation of the asymmetric form of the order-disorder model of a binary alloy. It is found that these relations perform considerably more accurately than the Manning relations over the entire region of compositions and at temperatures above and below the critical temperature. The relations would appear to be useful for accurately determining the phenomenological coefficients through a knowledge of measurable quantities.

The Electrochemistry of Copper in Aqueous Sulphide Solutions

Using a variety of electrochemical and surface analytical techniques, the mechanism and kinetics of Cu corrosion in anoxic, aqueous, sulphide-containing environments are being investigated. Under these conditions ([S]total = 10 −4 to 3 ×10 −3 mol/L), the anodicgrowth ofa film (XRD identifies Cu 2 S/Cu 1.8 S as major/minor phases, respectively) is supported by the cathodic reduction of water thereby destabilizing the copper surface. For more oxidizing conditions, the subsequent growth of a partially passivating film is observed. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry at rotating disc electrodes show film growth occurringwith negligible dissolution and under partial SH - transport control. Current-potentialrelationships as a function of [S]total give Tafel slopes of ∼ (40 mV) −1 suggesting reaction occurs via a 2step process: Aninitial rapid adsorption of SH - leading to an equilibrium surface concentration, followed by a rate determining electron transfer to form a sulphide film. It is proposed thatfilm growth propagates via transport of Cu I through the film to the solution interface. The primary goal of this research is the development of a mathematical model which can be used to assess the performance of copper nuclear waste canisters in granitic repositories.

Computer simulation of chemical diffusion in a binary alloy with an equilibrium concentration of vacancies

Abstract In the present work, chemical diffusion is explored by computer simulation in a (Ising) binary alloy wherein an equilibrium concentration of vacancies is established and maintained. The equilibrium was set up using a “two-phase equilibrium” approach. The thermodynamic factors and intrinsic diffusion coefficient were calculated on the basis of the simulated data. Instead of only one thermodynamic factor as in the binary system with a vanishingly small vacancy concentration, there are, in general, two different thermodynamic factors. Monte Carlo results show that, whenever the vacancy concentration is not negligible (greater than 1%), one can no longer treat the thermodynamic factors for chemical diffusion of the components A and B as a single factor.