This work is based on the practical problem where corrosion of the inner walls of the water tanks (boilers) has occurred. For this reason, the boilers are protected by cathodic protection with an external current source. The aim of this work was to analyze the efficacy of the cathodic protection applied for the boiler protection. In the experimental part of this final work anodic and cathodic polarization measurements were conducted on carbon steel samples at four different temperatures (26 °C, 40 °C, 60 °C and 80 °C), simulating the actual conditions in the boiler. Also, depolarization measurements were performed on boilers for hot water. From the obtained polarization curves, it can be concluded that with increase of temperature decreases the corrosion resistance of the metal, and the corrosion potential shifts to more negative values. As the temperature rises, the required protection current increases. At the highest temperature the protection potential range is narrow, and unwanted hydrogen reduction reaction easily occurs, which is why the cathodic protection is more difficult to apply. Three groups of depolarization curves were obtained by depolarization measurement. The first group make those with expected behaviour where, uppon switching off cathodic protection, steel potentials approach asymptotically corrosion potential. The second and third group consist of curves that show unexpected behavior, sudden drop or sudden increase of potential. This behavior is associated with the presence of copper heaters that can form galvanic couple with carbon steel.