Stainless steels are structural materials that form the backbone of modern technological civilization, with SS304 steel, containing 18% chromium and 8% nickel, being the most widely used type. Today’s global industry relies heavily on this steel type, which can be found in subcutaneous needles, heat exchangers, and even spacecraft. Chromium is the alloying element responsible for the stainless property of this steel, as it forms a homogeneous, uniform Cr₂O₃ layer 2–3 nm thick on its surface, protecting it from environmental attacks. Monitoring and testing the barrier characteristics of this layer is essential for maintaining its intended function, especially when welding is required. In this master's thesis, a series of SS304 samples were subjected to electrochemical impedance spectroscopy to examine the protective properties of their passive films and to identify differences in impedance parameters when measurements were performed near the weld zone versus the edge of the plate, where the influence of welding is negligible. The samples were then passivated using two passivation methods, after which impedance spectra were recorded to evaluate the effectiveness of the passivation. Instead of the standard three-electrode system in a liquid electrolyte, an electrolyte paste and a polymer counter electrode with an integrated pseudo-reference electrode were used for the measurements. The described system, combined with the portable potentiostat, is highly mobile and opens up the possibility for broader applications outside of laboratory conditions, one of which is the monitoring of the stainless steel objects in the field.