## 1 Introduction  
In the metal industry, corrosion is probably the most undesired phenomenon that affects all phases of material processing and service life, from the storage of materials up to the construction in the exploitation conditions [1, 2, 3, 4]. If the mentioned issue is presented as a financial cost, dealing with corrosion amounts to USD 2.5 trillion, most of which is spent to alleviate damages caused by corrosion [5, 6, 7]. In order to minimize such economic losses, it is necessary to choose the most appropriate surface protection for the material exposed to corrosion. Significant savings can be achieved by applying the appropriate technology, whether it concerns changing internal and external damage factors or creating a barrier between the protected material and an aggressive environment. Protection of metals by coating, especially by organic coating, is still the most common surface protection method. It is estimated that three fourths of all metal structures are protected in this way. The barrier created by coating prevents reactants from the environment from penetrating into the base material [8]. Coatings that create a non-conductive protective layer on base materials are commonly used in industries such as the automotive industry, shipbuilding, aviation and aerospace for the protection of tanks and pipelines, etc. [9; 10; 11; 12; 13; 14]. There are different application procedures and principles determined for certain protection system. This experiment investigates material protection by creating a barrier, which can be achieved by using different technological procedures. One of these is electrostatic powder coating, which provides a protective layer on a base material characterized by good resistance to corrosion and chemicals, as well as by good adhesion properties [15; 16]. Adhesion is indicated as a desirable property because no matter how good the individual components of the system are, if adhesion to the base material is poor, the protective potential of the coating will not be fully achieved [17; 18]. Besides those positive properties, powder coating is also environmentally friendly, since the coating agent is not diluted during application and does not contain volatile organic solvents. Melting the powder at temperatures of 180-250 \({}^{\circ}\)C enables homogeneous, stable and smooth layers to be applied in a thickness of 0.05-1.5 mm to the base material [19; 20]. In addition to the mentioned characteristics related to the final appearance of the coating, the success of the powder coating application depends on the process parameters related to voltage and electric current. The position of the spraying gun, i.e., the nozzle from which the powder comes out to the material surface (as shown in Figure 1), is also important. However, in this paper, the authors focused on investigating the influence of the previously mentioned electric current and voltage on the achievement of the target thickness of the protective layer on the base material. Proper powder coating requires a high voltage and a low current. In real production conditions, the process of applying a protective coating is often carried out on several types of materials with different conductivity and characteristics, so it is expected that the same process parameters (voltage and current) will not always give the same results regarding the thickness of the applied protective layer. If other parameters, such as the position and movement of the spraying gun, are automatically regulated as a constant, the voltage and electric current can be manipulated to obtain optimal thickness of the coating, which is demanded either by the technical documentation or by the product end user. The objective of this research is to explore the influence that different values of two input variables, i.e., voltage and electric current, have on different materials in order to determine the optimal values that provide the desirable protective layer thickness for each of the tested materials.