The decay that the metallic materials undergo in contact with various environments due to the passage of their constituent elements to the state of combination with environmental substances is called corrosion. This process is called anti-metallurgy because it tends to bring the metals to their natural state. The natural state is represented by the combination of the elements of the metal with other elements, in particular with oxygen, from which it derives the metallic material through the administration of energy. This energy is then transferred again to the environment through the corrosion process (Figure 1) .
Corrosion involves in particular the energy sector, transport, chemical, food, oil and mechanical. The damage caused by this is enormous because it takes into account the intrinsic value of the metal corroded, costs for their replacement, and costs to prevent the destructive process, in simple terms, they are called direct costs. Indirect costs are related to those resulting from the reduction of the useful life of the metal, from the loss of products, pollution, production stop, from sudden failure or bursting. The indirect costs are difficult to predict and often exceed the direct costs. Corrosion does not mean always just damage and loss. There is a constructive corrosion, such as corrosive attack which is carried out on metals to highlight its microstructure, to make wrinkled or glossy surface, to coat with protective layers, to produce matrix-type, to perform selective removal of material, to develop hydrogen or to create artistic decorations (Figure 2).
Figure 2 Processes of corrosion Art
The corrosion of metallic material can be of two types: wet or dry. It has wet corrosion when the metal material is in contact with an environment containing water; dry corrosion when the environment is instead constituted by gaseous atmospheres at high temperatures. There are other environments, such as salts or molten metals or non-aqueous solutions, whose aggressive action cannot fall distinctly into one of two classes: in these cases the corrosive phenomena can take on characteristic aspects of both forms of corrosion. The distinction of the types of corrosion concerns also the mechanism that governs the phenomenon. In the wet corrosion, the mechanism is of the electrochemical type, in which the corrosion process is the product of a process of anodic dissolution of the metal material (with release of electrons) and a cathodic process in which a chemical species present in the environment consumes electrons that provides the metal. Thus, the corrosion wet can be described by the laws of thermodynamics and kinetics of electrochemical. In the dry corrosion instead the mechanism is chemical type. This phenomenon is described by the laws of thermodynamics and the kinetics of heterogeneous reactions. Taking into the account the stage relative to the growth of oxide layers on the metal surface as a mechanism of the electrochemical type, the kinetics of the process is very complicated because it is linked by several factors:
- Adhesion and degree of compactness of the oxide film
- Type of management (ionic or electronic) and conductivity value.