Bobby Pejcic

An In Situ Synchrotron Radiation Grazing Incidence X-Ray Diffraction Study of Carbon Dioxide Corrosion

An in situ surface study of the carbon dioxide corrosion of mild steel has been undertaken using the tandem technique of mixed potential/synchrotron radiation-grazing incidence X-ray diffraction (SR-GIXRD). Long-term monitoring of the mixed potential showed there was an initial shift toward cathodic potentials attributable to preferential suppression of the cathodic half-reaction (viz., the reduction of carbonic acid), probably due to the formation of an adlayer of corrosion product physically blocking the cathodic reaction sites during the uniform corrosion of mild steel. Subsequently, the mixed potential displayed a gradual shift to anodic potentials, symbolizing the preferential suppression of the anodic half-reaction as the primary corrosion products [viz., Fe2(OH)2CO3, Fe2O2(CO3), Fe6(OH)12(CO3), and Fe6(OH)12(CO3)·2H2O] acquired a sufficient thickness to physically block the interface against further corrosion. Nevertheless, the corrosion product continues to grow over time, indicating that the corrosion product is discontinuous and porous, allowing the ingression of electrolyte to enable the further corrosion of the mild steel. This research has important ramifications for the field of corrosion research; it should enable researchers to develop improved methods of chemical inhibition by using compounds that can actually bind to the newly postulated corrosion products on mild steel.

The Influence of Diffusion Fluxes on the Detection Limit of the Jalpaite Copper Ion-Selective Electrode

Abstract : It has been suggested that electrode dissolution and the concomitant saturation of the electrodes diffusion layer restricts the detection limit of the jalpaite Cu ion-selective electrode (ISE) to samples with total Cu levels above 10(exp-5) mol dm(exp -3) [1, 2]. This article will use rotating disk electrode (RDE) data for San Diego Bay seawater and Ficks law of diffusion to demonstrate that the static commercial Orion Cu ISE (employing a jalpaite membrane) produces a background level of contamination of (2.0 plus or minus 0.5) x 10(exp -8) mol dm(exp -3) total Cu, and the reduced thickness of the Orion Cu ISEs diffusion layer in the presence of hydrodynamic flow [e.g., at an RDE, or in continuous flow analysis (CFA)] lowers the background contamination of Cu to < 10(exp-9) mol dm(exp -3). Furthermore, the RDE Cu ISE employing an electrode fabricated using jalpaite precipitated in 80% excess Na2S, so as to minimize the presence of occluded and leachable Cu salts and extraneous phases such as silver sulfide [3 - 5], reveals an improvement in the lower limit of detection compared to the commercial Orion Cu ISE.

Predicting the Adsorption Properties of Carbon Dioxide Corrosion Inhibitors Using a Structure-Activity Relationship

This paper presents a fundamental study of the influence of various chemical inhibitors on the corrosion rate of mild steel in brine electrolyte under carbon dioxide conditions. The corrosion inhibitors were fitted to a Temkin adsorption isotherm, and various fundamental constants of adsorption (i.e., adsorption equilibrium constants and molecular interaction constants) have been obtained. The inhibitor adsorption mechanism has been discussed in terms of thermodynamics (i.e., AH, AG, and ΔS), and this revealed that some compounds chemisorb onto the steel electrode. In addition, molecular modeling was undertaken using PCS-SPARTAN Plus and HyperChem Professional, and the various molecular parameters have been correlated with the thermodynamic adsorption properties of the inhibitors. Three-parameter and four-parameter fits for both negative and positive models are discussed. Multiple linear regression was performed on various combinations of molecular descriptors to describe the enthalpies and entropies of adsorption. Similarly, principal component analysis (PCA) was employed to corroborate the scientifically based selection of molecular descriptors used in the multivariate regression models.

Continuous flow analysis of iron (III) in seawater using a chalcogenide glass ion‐selective electrode

Continuous flow analysis (CFA) is suitable for rapid and reliable determination of free FeIII in organic-free, UV-oxidized seawater when used in conjunction with an extrapolation method that predicts the steady-state potentials of an FeIII chalcogenide glass ion-selective electrode (ISE) by using initial data associated with a step-wise change in aFe3+. It is shown that the CFA technique yields a Nernstian response of a 30 mV per decade change in aFe3+ in saline citrate buffers in the range of 10−22 to 10−1 M Fe3+. Validation of the CFA technique in organic-free, UV-oxidized seawater has demonstrated that the response of the FeIII ISE is internally consistent with the well-known inorganic speciation equilibria occurring in natural seawater.

Continuous flow analysis of mercury using a chalcogenide glass ion-selective electrode

A commercial Hg chalcogenide glass ion-selective electrode (ISE) has been characterized using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and the compositional data have been used in the fabrication of our own Hg ISE. It is shown that continuous flow analysis (CFA) is suitable for rapid determinations of ultratrace levels of mercury in saline media when used in conjunction with an extrapolation method that predicts the steady-state potential of the Hg ISE by using the initial data following a stepwise change in aHg2+. The CFA technique yields a near-Nernstian response of 28 mV per decade change in aHg2+ in Hg buffers in the range 10−18 to 10−12 M Hg2+. It is shown that the repeatability of the CFA technique in seawater is ±0.03 pHg units, and a seawater matrix effect causes a significant offset and apparent error in the response of the Hg ISE relative to Hg buffer standards. Standard addition CFA may be used to compensate for the seawater matrix interference.

A small angle neutron scattering and electrochemical impedance spectroscopy study of the nanostructure of the iron chalcogenide glass ion-selective electrode.

This paper presents a preliminary structural and interfacial study of the iron chalcogenide glass [i.e., Fe(x)(Ge(28)Sb(12)Se(60))(100-x)] ion-selective electrode (ISE) using small angle neutron scattering (SANS) and electrochemical impedance spectroscopy (EIS). SANS detected variations in the neutron scattering as a function of iron content in the chalcogenide glass. Furthermore, a change in the chalcogenide glass structure was observed at elevated iron dopant levels. Conversely, EIS was used to show that the iron chalcogenide membrane comprises various time constants, and the interfacial charge transfer reaction depends on the membrane iron content. Equivalent circuit modeling revealed that the charge transfer resistance decreases at elevated iron levels, and this may be related to the presence of iron defects in the glass. It is proposed that the iron chalcogenide membrane comprises an iron nanostructural network embedded in the amorphous matrix, and this directly influences the electrical conductivity and concomitant electrochemical reactivity of the glass.

Continuous flow analysis of iron in zinc electrowinning electrolyte using an iron chalcogenide glass ion-selective electrode: Part I. Synthetic media

It is shown that the iron(III) chalcogenide glass membrane ion-selective electrode (ISE) can be calibrated in continuous flow analysis (CFA) using acidified iron(III) nitrate standards, yielding a 60+/-3 mV per decade change in activity of Fe(3+) response in the range 10(-7)-10(-2) M total iron(III). Extended ageing of the iron(III) ISE in 2 M zinc(II) sulphate did not alter the potentiometric response characteristics of the electrode. Furthermore, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy in the presence and absence of zinc(II) sulphate failed to detect a zinc(II) interference on the iron(III) ISE. CFA/ISE determined activities of Fe(3+) in synthetic zinc electrolyte containing 2x10(-3)-2x10(-1) M total iron(III) yielded results falling within +/-0.2logaFe(3+) unit of the corresponding iron speciation data calculated using the minteqa2 program.