The Kaplan turbine is a water turbine with adjustable blades, where the water inflow is axially. The gear box is filled with oil in order to prevent corrosion. In this thesis, a more environmentally friendly corrosion protection is investigated and the influence of the corrosion inhibitor on the corrosion fatigue is examined. The investigated materials are a X3CrNiMo13-4 and a 20Mn5 steel. Electrochemical studies have shown that both materials exhibit pitting attacks in the medium with addition of inhibitor. Furthermore, the fatigue behaviour is strongly influenced. The level of the fatigue strength X3CrNiMo13-4 in the medium without inhibitor is about 50 MPa lower than that of air. In the medium with inhibitor it is about 100 MPa lower. The 20Mn5 steel shows a fatigue limit in air, whereas in the medium all samples fail due to corrosion attack. Thus, the investigated inhibitor is not suitable for application at ambient temperature and in untreated water.