In general, corrosion is understood to refer to material degradation through reaction with its environment. This has led to a common tendency to assess it in terms of the corrosion products which are formed, i. e., concentrating on the phenomenon rather than its cause. Recent developments in observing and measuring corrosion are increasingly changing this picture. As a result, it is necessary to give up commonly held assumptions in order to understand the nature of corrosion. Among other things, the order of standard potentials of the elements has been overemphasized for some time in terms of its relevance. It is hardly possible to describe the corrosion behavior of technical equipment and structural components by means of formulae, tables or guidelines. The reason for this is that their corrosion resistance, and thus corrosion itself, is not just a property of the material, but rather of the system as a whole.
The actual corrosion behavior is dependent in equal measure on the metal (as a technical material, taking into account all its properties), the environment (i. e., the concentration, temperature, flow rate, etc. of the corrosive medium), and the equipment design. In this context, design has to be understood in a broader sense to encompass everything from microscopically small surface roughness, methods of joining parts together, combinations of materials (including crevices resulting from the design) right through to the equipment construction as a whole. As a result, a large number of influencing factors are involved and the possible variations become difficult to comprehend.
Corrosion can be divided into two main types:
- Electrochemical corrosion (the atmospheric corrosion of steels, often equated with rusting, is an important example here)
- Chemical corrosion (high-temperature corrosion, leading to scale formation on steels, is a key area here, but the corrosion of glass, ceramics, and concrete is also primarily chemical in nature).