Javier Izquierdo

Electrochemical characteristics of bioresorbable binary MgCa alloys in Ringer's solution: Revealing the impact of local pH distributions during in-vitro dissolution.

Biodegradable magnesium-calcium (MgCa) alloy is a very attractive biomaterial. Two MgCa alloys below the solid solubility of Ca were considered, as to solely investigate the effect of Ca content on the behavior of magnesium and the pH changes associated to metal dissolution. X-ray diffraction analysis and optical microscopy showed that both Mg-0.63Ca and Mg-0.89Ca alloys were solely composed of α(Mg) phase. Degradation characteristics and electrochemical characterization of MgCa alloys were investigated during exposure to Ringers solution at 37 °C by electrochemical impedance spectroscopy and scanning electrochemical microscopy. The impedance behavior showed both capacitive and inductive features that are related to the alloy charge transfer reaction and the relaxation of the absorbed corrosion compounds, and can be described in terms of an equivalent circuit. Scanning electron microscopy (SEM) was employed to view the surface morphology of the MgCa samples after 1 week immersion in Ringers solution showing extensive precipitation of corrosion products, whereas the substrate shows evidence of a non-uniform corrosion process. Energy dispersive analysis showed that the precipitates contained oxygen, calcium, magnesium and chlorine, and the Mg:Ca ratios were smaller than in the alloys. Scanning electrochemical microscopy (SECM) was used to visualize local pH changes associated to these physicochemical processes with high spatial resolution. The occurrence of pH variations in excess of 3 units between anodic and cathodic half-cell reactions was monitored in situ.

Scanning electrochemical microscopy studies for the characterization of localized corrosion reactions at cut edges of coil-coated steel

The local distribution of anodic and cathodic sites in a corroding cut edge using the scanning electrochemical microscope is presented. Platinum and antimony microdiscs were used as tips for combined amperometric/potentiometric operation. Local changes in electrochemical activity, oxygen concentration and pH distribution associated to the onset of localized anodes and cathodes were imaged in situ, as well as their evolution with time. Local alkalization and oxygen consumption related to cathodic activity were detected above the zinc layer covered with the thinner polymeric coating, providing a direct evidence of a differential aeration process effectively operating in an asymmetrical cut edge, in addition to the galvanic coupling of aluzinc and steel. Anodic activity leading to local acidification occurred at the other aluzinc layer at all exposures.

Development of Solid Contact Micropipette Zn-Ion Selective Electrode for Corrosion Studies

Micron-size ion selective micropipettes can be used in scanning electrochemical microscopy (SECM). They can provide excellent spatial resolution. Unfortunately the resistance of these small sensors is high. Their application needs special shielding and slow scanning rates. Usually their lifetime hardly exceeds a few days. Zinc layer or dispersed zinc particles containing films are often used for providing cathodic protection against corrosion in case of metal surfaces. Therefore, in corrosion studies, measurements of local zinc ion concentration can give important information about the nature of the process. For corrosion studies we needed SECM measuring tips for imaging concentration profiles of Zn2+ions involved in surface processes. Based on our earlier experience, solid contact micropipettes for selective measurements of Zn2+ion concentration were prepared with a tip size of a few micrometers. The properties of the micropipettes were investigated. They were also used in SECM imaging. In this paper, details of Zn2+ion selective microelectrode preparation are described. Data about their properties, lifetime, resistance, and ion activity response are shown. Preliminary findings in SECM imaging of zinc ion concentration profiles are shown. The improvement of the scanning rate achieved by lowering tip resistance is a main advantage in potentiometric SECM.

Scanning electrochemical microscopy: an analytical perspective

Scanning electrochemical microscopy (SECM) has evolved from an electrochemical specialist tool to a broadly used electroanalytical surface technique, which has experienced exciting developments for nanoscale electrochemical studies in recent years. Several companies now offer commercial instruments, and SECM has been used in a broad range of applications. SECM research is frequently interdisciplinary, bridging areas ranging from electrochemistry, nanotechnology, and materials science to biomedical research. Although SECM is considered a modern electroanalytical technique, it appears that less attention is paid to so-called analytical figures of merit, which are essential also in electroanalytical chemistry. Besides instrumental developments, this review focuses on aspects such as reliability, repeatability, and reproducibility of SECM data. The review is intended to spark discussion within the community on this topic, but also to raise awareness of the challenges faced during the evaluation of quantitative SECM data.