## 2 Atmospheric Corrosion and Impact on Natural Frequencies  
Open-pit mining in the Oltenia Carboniferous Basin, Romania, involves the excavation and storage of lignite. Essentially, after excavation and transport to the stockpiles of quarries or power plants, lignite is exposed to air, interacting with oxygen in the air and leading to the formation of carbon dioxide, sulphur dioxide, and water vapour, which affect the surfaces of steel parts in the resistance elements of mining machinery [20; 21; 22; 23; 24]. In order to evaluate the degree of damage to the structures by corrosion, as well as the decrease in the thickness of the material of the resistance elements and the influence of this degradation on the load-bearing elements, it is necessary to analyse this phenomenon by means of a method that takes into account the geometric elements of the load-bearing structure of the machine as well as the decrease in mass. The corrosion of the metal structures of mining equipment in surface pits for the exploitation of lignite is a complex process influenced by several environmental and chemical factors. The main causes of corrosion in this context are: high humidity (water from precipitation or ambient humidity can condense on metal surfaces, forming an environment conducive to corrosion); the presence of oxygen (oxygen in the air contributes to the oxidation of metals, a basic chemical process that contributes to corrosion); sulphides and sulphur compounds (lignite can contain sulphur, and mining can release these compounds into the environment); sulphur and its compounds (for example, sulphur dioxide (SO\({}_{2}\))) can accelerate the corrosion of metals; and abrasive particles (lignite dust and other abrasive particles (quartz from waste material) can mechanically damage the surface of metals, exposing more reactive areas to corrosion). In the presence of moisture and oxygen, iron oxidises to form rust (iron hydroxide). This reaction is one of the main corrosion processes for metallic materials containing iron. Sulphur and its compounds, which are present in lignite, can react with iron and oxygen in water to form iron sulphate, which accelerates the corrosion process and can contribute to the faster degradation of metal structures. Corrosion, being the chemical process that affects the surface or inner part of metal in resistance materials, can manifest itself in different forms, such as Fe\({}_{3}\)O\({}_{4}\) (black magnetite), \(\gamma\)-Fe\({}_{3}\)O\({}_{4}\) (brown rust) and \(\gamma\)(FeOOH) (yellow rust). All of these forms of corrosion are observed in the steel structure of the machinery, depending on the nature of the machinery's interaction with the environment and the general mechanism of corrosion [20; 21]. Thus, in general, three forms of corrosion can appear on mining equipment:  
* General corrosion (Figure 2) is the most common form of corrosion because it is characterised by a corrosive attack that extends almost uniformly over the entire exposed surface or over a large area [20; 22];
* Pitting corrosion (Figure 3) is a localised form of corrosion in which cavities or \"holes\" are produced in the material of the load-bearing structure when a liquid stagnates, flows slowly, or drips constantly and continuously on the metal surface. The central core of the cavity is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect at depth [11; 21; 23; 24; 25]. Pitting corrosion is characterised by localised and deep changes in natural frequencies due to the formation of cavities. These changes are detectable through modifications in the energy distribution in the frequency spectrum;
* Exfoliation corrosion (Figure 4) is an advanced form of intergranular corrosion and is manifested by the lifting of the surface grains of a metal due to the expansion force produced by the corrosion occurring at the grain boundaries just below the surface [20; 22; 26].