Corrosion negatively affects industrial and other equipment and products in a way that shortens their life span and resulting in high costs and possible damage and accidents. This paper seeks to show how the problem of corrosion needs to be approached by an adequate understanding of the corrosion process and the influence of various factors on the speed and flow of corrosion in lowalloy steels. The present thesis consists of two parts: theoretical review of the topic of corrosion with emphasis on the influence of medium on corrosion and corrosion in low-alloy steels and experimental work. The theoretical part presents types of corrosion according to the corrosion mechanisms and forms of destruction with special emphasis on the corrosion factors and influence of the medium on corrosion and variaty of the corrosion protection modalities. The corrosion and corrosion protection mechanisms for low-alloy steels are also discussed, since this material is used in the experimental part for electrochemical testing of corrosion reactions. In the experimental part of the thesis, the electrochemical testing of the corrosion reaction rate in low-alloy steels was performed to analyze the influence of media (NaCl solution) of different concentrations and temperatures on the corrosion behavior of specific steel 42CrMo4. According to the obtained results, it is possible to conclude that the concentration, and especially the temperature of the medium, affects the speed and flow of the corrosion reaction in the tested samples of low - alloy steel.