Thermally sprayed cermet coatings are adequate solutions to improve cavitation and wear resistance of hydraulic turbines made of stainless steel (SS), especially in rivers with a high sediment load, such as the Madeira River in Brazil. However, some cermets are easily dissolved in river water, leading to premature failure of the coating and costly maintenance. Moreover, galvanic corrosion induced by coupling the cermet to a SS can accelerate the coating dissolution. Therefore, the corrosion resistance of six cermets (WC-12Co, WC-10Ni, WC-10Co-4Cr, Cr 3 C 2 -25NiCr, Cr 3 C 2 -10NiCr and Cr 3 C 2 -10Ni) and the galvanic corrosion resistance of these materials coupled to CA6NM SS were evaluated in a solution that simulated Madeira River water. WC-12Co and WC-10Ni cermets exhibited the highest corrosion rates, 0.077 and 0.068 mm/ year, respectively, whereas the Cr content in the WC-10Co-4Cr (0.017 mm/year) and Cr 3 C 2 -based coatings (0.005 to 0.007 mm/year) led them to corrode at slower rates. Moreover, the WC-10Co-4Cr and Cr 3 C 2 -based cermets exhibited negligible galvanic corrosion current when coupled to the CA6NM SS, making them good options to coat hydraulic turbines. In contrast, WC-12Co and WC-10Ni coatings underwent a more severe galvanic corrosion process, which would drastically reduce the lifespan of these materials as hydraulic turbine coatings.Keywords corrosion resistance Á Cr 3 C 2 -based coatings Á galvanic corrosion Á power plant Á turbines Á WC-based coatings This article is an invited paper selected from abstracts submitted for the 2020 International Thermal Spray Conference, ITSC2020, that was to be held from June 10-12, 2020, in Vienna, Austria. The conference was canceled due to the coronavirus (COVID-19) pandemic. The paper has been expanded from the planned presentation.
The present paper describes the study and the repair of a steel water pipe concerning the dimensional stability by finite elements methods, selection of materials and the repair procedure. The water steel pipe transports water from a dam to an electrical power. The structure presented some areas with corrosion and other parts with lack of material "pitting". There were some solutions for the repair, but due to atmospheric conditions, local access and the advanced corrosion conditions, it was decided that the right solution would be with composite materials. Composite materials turn out to be the most attractive solution to the repair because it can easily be done, possibility to transport the raw materials near repair local and good mechanical properties of the composite laminates. The finite element method predicted the number of carbon fibre layers necessary to recover the initial mechanical resistance of the water steel pipe. Dry fibres and a thermosetting matrix were selected according to the atmospheric conditions, its mechanical properties and the joining of dissimilar materials. A repair procedure manual was done according to the results obtained.
Hydroelectric turbines are strongly affected by cavitation and the damage it can cause to critical part surfaces and profiles. The study of thermal spray processes and materials is thus relevant to improving turbine performance. The main objective of this work is to evaluate the influence of fuel-oxygen ratio on tungsten- and chromium-carbide cermet coatings deposited by HVOF. Particle velocity and temperature were measured as were coating hardness, porosity, and cavitation resistance. Higher particle velocities were obtained at higher fuel ratios, producing harder, denser coatings with better cavitation resistance. Based on test results, the wear mechanism starts with the nucleation of the cavitation that occurs in the pores, resulting in the formation of craters and the eventual detachment of lamellae as indicated by the smoothness of the surface.
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