Roman Cabrera-Sierra

Electrochemical characterization of the different surface states formed in the corrosion of carbon steel in alkaline sour medium

Abstract In this study the different surface states that manifest in the corrosion process of 1018 carbon steel in alkaline sour environment, solution prepared specifically to mimic the sour waters occurring in the catalytic oil refinery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH 4 ) 2 S and 10 ppm NaCN at pH 9.2) were prepared and characterized. The surface states of the carbon steel were formed by treating the surface with cyclic voltammetry at different switching potentials ( E λ + ), commencing at the corrosion potential ( E corr =−0.890 V vs sulfate saturated electrode, SSE). The surface states thus obtained were characterized using electrochemical impedance spectroscopy and scanning electron microscopy techniques. It was found that for E λ + =−0.7 and −0.6 V vs SSE a first product of corrosion formed, characterized by a high passivity. Moreover, it was very compact (with a thickness of 0.047 μm). However, at more anodic potentials ( E λ + >−0.5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 μm and very active) was observed. The diffusion of atomic hydrogen (H 0 ) was identified as the slowest step in the carbon steel corrosion process in the alkaline sour media. The H 0 diffusion coefficients in the first and second products that formed at the carbon steel–sour medium interface were of the order of 10 −15 and 10 −12 cm 2 /s respectively.

The Role of Hydroxide in the Electrochemical Impedance Response of Passive Films in Corrosion Environments

A simple model has been developed (within the framework of the point defect model, PDM) assuming that the impedance response of passivated metals is dominated by the barrier layer, which consists of a partly hydrated and highly defective oxide layer. Simulations were carried out for cation vacancy conducting barrier layers (with p-type electronic character) and O 2- vacancy conducting barrier layers (with n-type electronic character) including in both cases the possible contribution by transport of OH - vacancies. A very good agreement was obtained between calculated impedance diagrams and typical impedance spectra for passive films on nickel and titanium. For both types of films the impedance response is quite sensitive to variations in the formal rate constant k -3b involved in the generation of hydroxyl vacancies at the metal/film interface. Furthermore, in the case of films with p-type electronic character the shape of the Nyquist diagram is sensitive to variations in the ratio of the kinetics of hydroxyl vacancies generation to the kinetics of cation vacancies generation at the metal/film interface.

Experimental and Modeling Study of Nickel Electrodeposition Including H + and Water Reduction and Homogeneous Reactions

A comprehensive physicochemical model is presented for nickel electrodeposition onto nickel and copper rotating disk electrodes in sulfate media buffered with boric acid at various concentrations. The model accounts for three contributions comprising the total current: nickel deposition, reduction, and water reduction. Also considered are homogeneous reactions and transport of dissolved species to and from the electrode by diffusion and convection. The model is statistically fit to experimental linear sweep voltammetry data to obtain the various kinetic parameters. It is not possible to satisfactorily fit the curves obtained on a nickel substrate for the different concentrations with a single set of parameters. Analysis of the experimental data and fitted model shows that the cathodic reactions reach completion in the following order as the potential becomes more negative during the scans: reduction, nickel deposition, and water reduction. The extent of water reduction during scans does not vary with the metal ion concentration in solutions containing or more, but increases when the concentration is reduced to . The model confirms that hydrolysis mitigates the increase of the surface due to and water reduction.

Materials for aqueous sodium-ion batteries: cation mobility in a zinc hexacyanoferrate electrode

The coordination polymer Zn3Na2[FeII(CN)6]2 has an open porous framework that is stable in acidic and neutral aqueous solutions and appears to be an attractive solid for investigation as a material for sodium ion-based batteries. The easy ion exchange of Na+ by bigger cations (such as K+, Rb+ and Cs+) and the existence of the oxidized species Zn3[FeIII(CN)6]2 (as a stable phase) support the present study of the redox reaction and stability in aqueous media under electrochemical conditions for all the series, including the effect of the accompanying cation on the mobility of sodium in the framework. When a study was carried out in a solution of sodium ions, in the material structure K+ and Rb+ were progressively displaced by the sodium ions, which was ascribed to their higher mobility. However, when an experiment was conducted using a solution containing Cs+ or in an aqueous solution of NaNO3, from Zn3Cs2[FeII(CN)6]2, the mixed composition Zn3NaCs[FeII(CN)6]2 was formed. The results discussed here are supported by the electrochemical and spectroscopic measurements. Galvanostatic experiments on the Zn3Cs2[FeII(CN)6]2 system in a solution of sodium ions revealed a capacity retention of 75% during 40 charge/discharge cycles.