En el presente trabajo se ha investigado el efecto de los elementos aleantes Zn (0,95% en masa) y Ca (0,15% en masa) en el mecanismo de degradación del Mg. Las superficies del Mg puro y de la aleación Mg-Zn-Ca han sido caracterizadas durante su exposición a la solución fisiológica de Hanks (a 37 °C) hasta siete días, utilizando las técnicas SEM-EDS y XPS. La capa formada en la superficie de la aleación contiene Ca10(PO4)6(OH)2, que puede mejorar la compatibilidad ósea. Las partículas intermetálicas compuestas de la fase Mg2Ca, así como la presencia de Zn, han promovido la formación de una capa protectora más uniforme. Las pruebas de EIS y ruido electroquímico (EN) han indicado que la resistencia a la polarización (Rp) del Mg puro es un orden de magnitud menor y la resistencia al ruido de la corriente (Rn) ≈ 5 veces menor, que los de la aleación de Mg-Zn-Ca. Los valores del índice de picadura (PI) de cada material fueron inferiores a 0,6; lo que sugiere que el ataque de corrosión no está altamente localizado. Al final de las pruebas de inmersión, la concentración de iones de Mg liberados durante la degradación ha sido ≈ 4,5 veces mayor para Mg puro (1,63 ± 0,02 mg·cm−2) que para Mg-Zn-Ca (0,35 ± 0,03 mg·cm−2). En consecuencia, la densidad de corriente de corrosión (jcorr) calculada para el Mg puro fue dos veces mayor (1,33 μA·cm−2) que la de la aleación de Mg ZX10 (0,59 μA·cm−2).

Mapas de Espectroscopia de Impedancia Electroquímica Localizada (LEIS) y de Sonda Kelvin de Barrido (SKP) fueron adquiridos para caracterizar las etapas iniciales de corrosión localizada de AA6061-T6 en agua de mar sustituta diluida hasta 24 h, simulando un ambiente atmosférico poco contaminado en cloruros. Ambas mediciones fueron llevadas a cabo in situ en potencial de circuito abierto. Los mapas de LEIS mostraron la distribución de la magnitud de la impedancia |Z| en la superficie de la aleación, sugiriendo una heterogeneidad en el desarrollo de sitios activos frente a la corrosión (primeras picaduras). Los mapas de SKP evidenciaron la aparición de sitios anódicos (con potencial más negativo) y catódicos en la superficie de la aleación, lo que confirmó el carácter localizado de la corrosión. El análisis SEM-EDX reveló el inicio de la disolución de aluminio, localizado en la vecindad de partículas intermetálicas de Al-Fe-Si, ricas en hierro, y en áreas con carbono segregado, todas éstas actuando como cátodos y permaneciendo en la superficie con el avance del proceso.

This paper presents a theoretical and experimental evaluation of benzidine derivatives as electroactive molecules for organic redox flow batteries. These redox indicators are novel electroactive materials that can perform multielectron transfers in aqueous media. We performed the synthesis, electrochemical characterization, and theoretical study of the dimer of sodium 4-diphenylamine sulfonate, a benzidine derivative with high water solubility properties. The Pourbaix diagram of the dimer shows a bielectronic process at highly acidic pH values (≤ 0.9) and two single-electron transfers in a pH range from 0 to 9. The dimer was prepared in situ and tested on a neutral electrochemical flow cell as a stability diagnostic. To improve cell performance, we calculate and calibrate, with experimental data, the Pourbaix diagrams of benzidine derivatives using different substitution patterns and functional groups. A screening process allowed the selection of those derivatives with a bielectronic process in the entire pH window or at acidic/neutral pH values. Given the redox potential difference, they can be potential catholytes or anolytes in a flow cell. The couples formed with the final candidates can generate a theoretical cell voltage of 0.60 V at pH 0 and up to 0.68 V at pH 7. These candidate molecules could be viable as electroactive materials for a full-organic redox flow battery.

In this article, we present experimental and theoretical studies of pyridine derivatives (pyDs) inspired by natural systems to investigate the electron transfer processes occurring in aqueous media and elaborate a theoretical model that adequately predicts the behavior of new derivatives. Our results might be relevant to scientific and technological applications, including energy storage, redox-active scaffolds for organic synthesis, photoredox catalysis, and new materials. The synthesis of eight pyDs is reported. To improve water solubility, six new compounds are hexafluorophosphate alkylammonium salts. The pyDs exhibit irreversible redox processes, with electron-donating substituents decreasing the cathodic peak potential while electron-withdrawing groups increase it; when both substituents are present, the latter effect prevails. A computational study was performed to investigate the electrochemical behavior of the synthesized compounds and design new electroactive pyDs. DFT calculations provided the predominant species’ redox potentials and acidity constants to elaborate Pourbaix diagrams for each compound. The synthesized molecules exhibit a two-electron-one-proton dismutation process in the water pH window. Beyond this range, stabilized radical species undergo one-electron exchange processes. We correlated experimental and calculated parameters, screening 22 additional derivatives to evaluate their electrochemical behavior, identifying potential candidates capable of performing a one-electron transfer process in the pH window of water, revealing new applications for pyDs.

The electrochemical behaviour and surface changes on AA6061-T6 alloy exposed to Caribbean seawater from the Cozumel Channel for 30 days under laminar flow (0.1 ms−1) were studied, these contrasting then with stationary conditions. Open circuit potential monitoring and electrochemical current fluctuations, considered as electrochemical noise (EN), were employed as two nondestructive methods. The calculated corrosion current, based on Rn, was one order higher in laminar flow. The fluctuations of current were transformed in the frequency domain. Their power spectral density (PSD) plots were obtained in order to gain information concerning the dynamic of the spontaneous release of energy during the corrosion process. The value of the exponent β in PSD graphs suggested that the localised corrosion on AA6061-T6 surface occurs as a persistent stationary process, which dynamic is controlled by oxygen diffusion. The changes in the morphology and elemental composition of the formed layers revealed that the localised attacks occurred in the vicinity of intermetallic particles rich in Fe and Cu, which act as cathodes.