Corrosion Properties of Thermally Sprayed Bond Coatings Under Plasma-Sprayed Chromia Coating in Sulfuric Acid Solutions
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Corrosion Properties of Thermally Sprayed Bond Coatings Under Plasma-Sprayed Chromia Coating in Sulfuric Acid Solutions Giovanni Bolelli1 • Damiano Meschini1 • Tommi Varis2 • Veronica Testa1 Stefania Morelli1 • Luca Lusvarghi1,3 • Petri Vuoristo2
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Submitted: 29 June 2019 / in revised form: 9 August 2019 ASM International 2019
Abstract Plasma-sprayed chromia coatings are known to have excellent corrosion and wear properties in highly acidic conditions at ambient and elevated temperatures, but are not watertight due to their intrinsic porosity. Therefore, in applications involving aggressive environments, the whole component is usually made of a corrosion-resistant alloy, to which the Cr2O3 coating imparts the necessary wear resistance. However, in such aggressive environments, the survival of thermal spray metallic bond layers becomes an issue. The present study deals with the performance in sulfuric acid solutions of coated systems consisting of a Hastelloy C-276 substrate and a plasmasprayed Cr2O3 top coating with four different intermediate bond coatings. The bond coatings were HVOF-sprayed Ni20Cr, Hastelloy C-276 and Ultimet alloys and plasmasprayed tantalum. Open-circuit measurement, electrochemical polarization and electrochemical impedance spectroscopy tests were carried out at room temperature This article is an invited paper selected from presentations at the 2019 International Thermal Spray Conference, held May 26-29, 2019 in Yokohama, Japan and has been expanded from the original presentation. & Giovanni Bolelli [email protected] 1
Department of Engineering ‘‘Enzo Ferrari’’, University of Modena and Reggio Emilia, Via Pietro Vivarelli 10/1, 41125 Modena, MO, Italy
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Faculty of Engineering and Natural Sciences, Materials Science and Environmental Engineering, Tampere University, P.O. Box 589, 33101 Tampere, Finland
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Local Unit Universita` di Modena e Reggio Emilia, Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Via Pietro Vivarelli 10/1, 41125 Modena, MO, Italy
(RT) in solutions with various concentrations. Also, static immersion tests were performed at RT and 60 C. The results revealed that the HVOF-sprayed Ni-20Cr and Ultimet alloy coatings were significantly attacked by the sulfuric acid electrolyte, especially at 60 C, whereas the HVOF-sprayed Hastelloy C-276 and plasma-sprayed Ta coatings performed significantly better. Keywords bond coat chromium oxide (Cr2O3) corrosion electrochemical impedance spectroscopy (EIS) electrochemical polarization high-velocity oxygen fuel (HVOF) plasma spray
Introduction Thermally sprayed ceramic oxide coatings are used in many industrial sectors because of their very good wear resistance and chemical stability (Ref 1, 2). In most cases, it is also required that the whole coated system consisting of the substrate material, bond coating and ceramic top coating possesses good corrosion resistance. The operating environments can indeed be corrosive. However, it is well known that
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