Electrochemistry is a science

Electrochemistry is a science, as well as an industrial domain, based on physicochemical phenomena taking place whenever electrical and chemical energy exchanges intervene in a reactive scheme. In this sense, most of the time electrochemistry investigates systems in which electrical currents flow, as in the case of electrochemical generators or batteries present in numerous everyday applications such as cars, mobile telephones, and so on. From this point of view, the archetypical corrosion process, that is a metallic piece freely exposed to a given aggressive environment is, to some extent, a bit marginal in the sense that the overall current flow observed from the outside of the system is zero; the system spontaneously evolves and is said to be at rest or at open-circuit. This is clearly the actual practical corrosion scenario, unless, an external source is applied as in the case of corrosion characterization or measurements and in some corrosion protection techniques such as cathodic or anodic protection. In spite of this feature, the concepts and tools involved in general electrochemistry remain valid and constitute the cornerstone of corrosion description and understanding.

It is possible to find redox reactions in a single phase, as in a bulk solution. Nevertheless, when talking about electrochemistry and more particularly corrosion, only heterogeneous systems are concerned, which means that the reactive system is composed of two or more distinct phases. As an example, let us consider a classical electrochemical system implying electrical energy exchanges such as a battery connected either to an electrical load (the battery hence discharges) or to an external electrical energy source (the battery hence charges). This is usually achieved by a multiphase ‘sandwich’ of at least three different materials with two different metals at the battery terminals. Since they involve electrical energy exchange, the electrochemical systems only employ conductive materials. Electrical conductivity exists as soon as the materials have mobile charges. Without going through an exhaustive survey, these key materials can be classified into three main categories according to the nature of the charge carriers: electronic conductors like metals or semiconductors, ionic conductors (or electrolytes), and finally mixed conductors (such as some conducting polymers) when the two types of electrical conduction coexist. To illustrate corrosion phenomena in the following, the text will concentrate only on metals, which are electronic conductors undergoing corrosion, and on electrolytes, which can be aqueous or organic solutions, molten salts, ionic solids, polymers, or even gases.