Microelectrochemical imaging of corrosion reactions

Abstract

Corrosion reactions – anodic, cathodic half-cell and side reactions – are occurring simultaneously on the surfaces exposed to corrosive environment. Localized information helps to get a deeper insight into the degradation of materials and facilitates the development of new anti-corrosive treatments. Simultaneously collected, localized information is a great step further. In order to obtain spatially resolved chemical information of multiple species in the same time, new type of electrodes has to be developed. Monitoring of pH changes along with metal-ion concentration is particularly attractive. Metal-ion concentration in the adjacent solution can be readily measured using ion selective microelectrodes. However, poor selectivity can be a limiting factor in these measurements. The problem of selectivity was subdivided into two areas in this research: the role of the time-dependent selectivity coefficient was investigated, as well as the „conventional” selectivity coefficient, the one everybody knows from the Nikolsky equation. As a trumpet player, I am interested in the corrosion of brass, but the lack of copper(II) ion selective microelectrode made the developement of the new ISME necessary. In potentiometric SECM, the electrice field contribution in the case of polarized samples or galvanic corrosion is an old problem, I also faced in my earlier studies. This is very emphatic in the case of magnesium corrosion, hence from the zinc and copper based materials I turned my eyes toward magnesium. Dealing with magnesium, I’ve seen peculiar behavior of magnesium anodes that is often referred as the „negative difference effect” in the literature. For this reason, as I finished with the electric field related experiments, I wanted to dig deeper into this problem. In this work, it seemed obvious to detect the hydrogen evolved on the magnesium surface that can be done amperometrically. Although my results are sorted into 4 apparently independent chapters in the Results and discussion, one ensue from the other: as I started to work on something, my experiences inspired to next direction of the research.