The Corrosion Resistance of Aluminum–Bronze Coatings as a Function of Gas Pressure Used in the Thermal Spraying Process

Abstract

We report the results of the influence of acetylene and oxygen gas pressure on the corrosion resistance of bronze-aluminum coatings deposited on a naval brass substrate by means of the thermal (flame) deposition process. The coatings were characterized by means of scanning electronic microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), X-ray fluorescence (XRF), and transmission electron microscopy (TEM). The corrosion tests were carried out via Tafel and electrochemical impedance spectroscopy (EIS). In addition, some samples were selected in order to investigate heat treatment and its effects on corrosion resistance. The results indicate that changes in the pressure and flow of the gas affects the composition, morphology, and physical properties of the coatings, and these effects have consequences for the behavior of the coatings when they are immersed in corrosion environments. The collision speed of the particles was identified as the most significant factor that influences the properties and the performance of the coating. The gas pressure modified the oxides and the porosity level, which improved the corrosion resistance.Coatings 2019, 9, 507 2 of 17 coatings in terms of adherence, mechanical properties, and corrosion resistance [2]. In order to optimize the results, the coatings have been deposited while changing the work distance and substrate roughness, and the effect of the pretreatment of the surface using different physical and chemical methods has been studied [3]. However, a combination of parameters relating to the combustion and gas projection pressure for producing better strength and good adhesion has not been found. Aluminum-bronze coatings have good wear and corrosion resistance properties because a copper oxide layer is formed on the surface. They are recommended for the protection and dimensional restoration of parts that are subject to harsh operating conditions [1].In the process of thermal projection by combustion, the chemical energy of combustible and combustion gases is used for the generation of the flame responsible for melting the projected particles. Oxyacetylene torches use a mixture of acetylene (C 2 H 2 ) and oxygen (O 2 ), which in a stoichiometric ratio produces a flame that reaches a temperature of up to 3386 K at 1 atmosphere of pressure [4]. An oxidant or highly oxidizing flame is when an excess of oxidizer is applied, and as a result, a primary reaction zone (cone of the flame) is obtained that is much longer than in a reductive flame or a stoichiometric flame. The particles stay longer in the hottest area of the flame, and therefore they can achieve better heating.The axial and radial temperature distribution and the changes in the velocity of the flame gases significantly affect the temperature of the particles and their behavior in flight, which influences the quality of the coating. The hot gases generated by combustion undergo complex events that involve heat and mass transfer phenomena in the boundary layers of the particle [5]. In ord...

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