Heat exchangers can be coated with functional layers to reduce the negative effects of established fouling films. In the process, long-term layer stability is a requirement for the fouling-inhibiting properties of the surface coatings. Various atomic layer deposition (ALD), with layer thicknesses between 10 and 50 nm, and Parylene (polymeric) coatings, with layer thicknesses < 50 µm, are applied to stainless steel (1.4571) and structural steel (1.0038) substrates. The stability against critical thermal process conditions and cleaning media (acids, alkalis, solvents) is examined. The surface free energy of the substrates is determined before and after durability tests using a contact angle measuring device. In this study the targeted water contact angle was between 80° and 110° and the related surface free energy < 45 mN/m. The change in the surface free energy is a measure of the coating durability. The tested Parylene-coated substrates show no significant changes and no layer defects. In the case of the ALD-coated substrates, only the coating with TiO₂ deposition using a chlorine-free precursor shows promising results. Non-systematical layer defects and corrosion can sometimes be optically observed on the TiO₂ and Al₂O₃ ALD-coated substrates. A strong increase in the polar part of the surface free energy is observed. This may be due to the physisorption and chemisorption of water with the coating, increasing polar interactions. In further research, fouling experiments must be performed to investigate the influence of the coatings on the induction time.