Corrosion Fatigue of Copper in Aqueous Media

The failure of the cooling systems of engines and propellers of the ship poses the serious hazard to passengers and crew. Damage to these units is often caused by corrosion, with localized corrosion being the most dangerous. In the case under consideration, when the metal is simultaneously affected by a corrosive environment and alternating mechanical loads, conditions arise for the behaviour (occurrence) of corrosion fatigue. The prospects of cathodic electrochemical protection of these units are considered in the paper. Since the cooling systems and propellers are made of copper alloys, the effect was studied concerning the potential and composition of the solution on the corrosion of the main component of the structural material — copper. Copper samples with a smooth mechanical pressure concentrator cantilevered in a fluoroplastic electrochemical cell were tested. Alternating bending loads were applied to them, and the number of load cycles before the destruction of the sample (durability) was determined. The experiments were carried out in deaerated solutions of the surfactantly inactive electrolyte NaF, as well as fluoride solutions with additives of surfactants and organic surfactants - benzotriazole. At the same time, an interval of potentials was set, in which the only Faraday process is possible — the reduction of traces of dissolved oxygen. It was found that at potentials less than the zero charge potential, the durability of copper increases significantly. And the introduction of chloride ions and benzotriazole into the solution, on the contrary, leads to a strong decrease in its durability. The potential of zero charge of copper was determined from the measurement data of the differential capacitance of the metal in sodium fluoride solutions with different concentrations of fluoride. The increase in the durability of the metal is explained by the occurrence of hydrogen bonds between the water molecules adsorbed by the hydrogen atoms. This manifests the so-called negative Rebinder effect. For electrochemical protection, the potential range is recommended below the potential of zero charge of the metal, but above the potential of water decomposition. For cooling systems, it is recommended to use deionized water as a coolant.