Abstract: The results of the investigation into the morphology and phase and chemical composition of protective films and corrosion products deposits that form spontaneously on the steel surface during operation of the heat-exchange equipment with the oxidizing water chemistry in the tertiary coolant circuit of a BN-800 reactor unit are reported. A comparison is made of the phase composition of protective oxide films on the working surfaces in the tertiary coolant circuit of steam generators at BN-600 and BN-800 reactor units operating with different water chemistries. It has been demonstrated that, unlike type PGV horizontal steam generators employed at power units with pressurized water power reactors, where oxide films and deposits of corrosion products in the secondary circuit are determined only by magnetite (Fe₃O₄), films and deposits in the N-272 steam generator contain three phases: magnetite (Fe₃O₄), hematite (α-Fe₂O₃), and lepidocrocite (γ-FeOOH). A protective oxide film of magnetite (100%) is formed directly on the metal. On the film surface are formed a layer of deposits tightly bound to it and consisting of 70% of magnetite, 25–30% of hematite, 1–5% of lepidocrocite, and loose surface corrosion products deposits of corrosion products. The phase compositions of the dense layer (hematite) and loose corrosion products' deposits (hematite + lepidocrocite) on the working surfaces of the heat-transfer tubes in the N-272 steam generator correspond to the weakly alkaline oxidizing water chemistry for the tertiary circuit of the BN-800 reactor unit. It has been established that, irrespective of the type of water chemistry (oxidizing for the BN-800 reactor or reducing for the BN-600 reactor), an oxide layer of magnetite with protective properties is formed spontaneously on the surface of pearlitic steel during operation.