Corrosion process in different environments

Corrosion processes are divided into the following three groups in a more comprehensive classification:

• Corrosion in aqueous solutions or environments
• Corrosion in molten salts and liquid metals
• Corrosion in gases

Corrosion in aqueous solutions or environments

In addition to atmospheric air, which is the most common environment, aqueous solutions including neutral water, atmospheric moisture, rain, and man-made solutions are considered as environments related to the phenomenon of aqueous corrosion.

Due to the presence of ionic conductivity in these types of environments, corrosion occurs as a result of electrochemical reactions. This type of corrosion is strongly influenced by factors such as electrode potential and solution acidity.

Corrosion in molten salts and liquid metals

This type of corrosion is more important in the nuclear industry, but this phenomenon can also be observed in many non-nuclear applications. In the case of corrosion in molten salts, the destruction mechanism is more varied than in aqueous corrosion. In corrosion caused by molten salts, preferential dissolution plays a more important role than in aqueous corrosion. Corrosion testing in these environments presents its own challenges and requires more expensive equipment than aqueous corrosion. Most commercially available corrosion monitoring methods and equipment study and monitor corrosion in aqueous or gaseous environments. Although basic information is available on corrosion in molten salt, relatively little thermodynamic and kinetic data is available. Corrosion in liquid metal is not only a concern in fast fission nuclear reactors but also in fusion reactors, but it is also important from an industrial perspective in other areas such as metal recovery, heat exchanger tubes, and the design of specific coolers. Corrosion in liquid metal is fundamentally different from aqueous corrosion and corrosion in molten salt because the environment (except for impurities) is non-ionized. Corrosion is greatly affected by the presence of non-metallic impurities in alloys and liquid metals.

Corrosion in gases

In this type of corrosion, the environment is non-conductive and ionic processes are limited to the metal surface and corrosion product layers. Because the reaction rate of industrial metals with common gases at room temperature is slow, gaseous corrosion is generally known as oxidation. Gaseous corrosion is only an industrial problem at high temperatures, where diffusion processes predominate. The most obvious result of oxidation at high temperatures is the formation of oxide scales. The degree of protection of these scales is determined by their properties and the development of stresses. In some cases of oxidation of metals, the reactions occur within the metal structure and are referred to as internal oxidation. Selective or preferential oxidation, similar to the corrosion process in liquids, is often observed in alloys containing elements and compounds with different thermodynamic stabilities.