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

coatings

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

Alfredo Morales, Oscar Piamba and Jhon Olaya * Grupo de Investigación en Corrosión, tribología y Energía, Department of Mechanical and Mechatronics Engineering; Universidad Nacional de Colombia, Bogotá 111321, Colombia * Correspondence: [email protected]; Tel.: +57-1-3165000 (COL)

Received: 17 July 2019; Accepted: 8 August 2019; Published: 10 August 2019

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.

Keywords: ﬂame thermal spray technique; aluminum bronze alloy; corrosion resistance; coatings

1. Introduction In industry, there is a need to improve the processes of protection and repair of naval, automotive, and aeronautical components. These structural elements are in a marine environment in the transportation system, where the high degree of deterioration, wear, and marine corrosion causes loss of material. In order to reduce the corrosion rate of these components, the application of protective coatings using thermal spraying is a strategy that has undergone rapid development in recent years. These techniques seek to extend the useful life of the components in the face of the difficulties of fabrication and the rise in price of the components in the replacement process. The practice of installing a new piece or discarding it once it has been worn out and replacing it with a spare part has been reduced, and now there is a tendency to install parts with protective coatings produced by thermal spraying. Some of the parts that fail most in the naval sector are made with bronze alloys, for example propellers, pumps, shafts, pipes, and bushings. As mentioned above, an economical and efficient alternative for protecting these components is the use of flame spraying. For this purpose, an aluminum–bronze alloy can be used, which is a good alternative for replacing parts subjected to corrosive environments [1] and which has a chemical composition very similar to that exhibited by the components that are used in the transportation industry. Previously, several researchers have produced Cu–Al coatings according to the application recommendations of the suppliers, but they have not obtained improvements in the quality of the

Coatings 2019, 9, 507; doi:10.3390/coatings9080507 www.mdpi.com/journal/coatings 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 (C2H2) and oxygen (O2), 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 order to predict the thermal effect of a particle in flight during the process of thermal projection by an oxyacetylene flame, numerical simulations have been made using Jets & Poudres software, 2019 [6]. This software is based on the GENMIX computer code, which was developed by Spalding and Patankar [7], and was improved by the use of thermodynamic and transport properties related to the local temperatures and the composition of the thermal sources that it simulates. Some of the most important conclusions of the two earlier studies were that the thermodynamic and kinetic parameters of the deposition were controlled by a combination of deposition conditions, such as the pressure, working distance, temperature of flame or substrate, and surface preparation, which in turn affects the properties of the coatings produced. Thermal spraying by flame is an effective system for modifying the microstructure of the coating, as the configuration of the deposition parameters can control the impact velocity of the particle from the flame towards the substrate, and this can be used to modify the properties of the coating. These studies have focused on the microstructural, corrosion, and mechanical properties of the coatings. However, to the authors’ knowledge, no studies have reported the effect of the collision speed of the particles on the electrochemical behavior of bronze–aluminum coatings deposited via thermal spraying. Furthermore, there are no studies related to investigating heat treatment and its effects on the corrosion resistance of these coatings. Therefore, the main objective of this research was to study the effect of the pressure of the feed gases on the corrosion resistance of the aluminum–bronze coatings that were deposited via thermal spraying by flame. To understand the effect of the gas pressure on the chemical and structural properties of the coatings deposited, previous simulations with the Jets & Poudres software were done, which allowed us to establish the main thermodynamic and kinetic parameters during the growth of the coating. Furthermore, in the present paper we have concentrated on improving the quality of the coatings produced; in particular, heat treatment was carried out in order to improve the density and adherence of the system produced.

2. Materials and Methods Before the application of the coatings, numerical simulations were carried out with the Jets & Poudres software (SPCTS, UMR-CNRS 6638 University of Limoges, 123, avenue Albert Thomas 87060 Limoges Cedex, France), in order to predict the behavior of the particles during the transport from the combustion zone to the substrate surface in the process of thermal spraying under different Coatings 2019, 9, 507 3 of 17 gas pressures. The Jets & Poudres software allows one to control the gas and the material feed conditions in order to simulate the thermodynamic variables during the spraying process of the material. This software is used to estimate the effect of the geometry of the torch nozzle and the volumetric flow of the gases involved in the combustion. Proxon 21071 powders (Castolin Eutectic, Messer Castolin Eutectic, Frankfurt, Germany) were used to produce the sample coatings by means of the flame thermal spray process. This raw material is a bronze–aluminum alloy (10% Al; 0.6% Si; Fe 2%; 87.4% Cu) used to recover soft sliding surfaces in the naval industry and is distinguished by its machinability. The aluminum–bronze coatings were obtained by the technique of thermal projection by flame, using TeroDyn® System 2000 equipment (Castolin Eutectic, Messer Castolin Eutectic, Frankfurt, Germany). The coatings were produced by varying the feed pressures of the combustion gases, oxygen and acetylene. For the application of the aluminum–bronze coatings, experimental variation of the combustion gases was carried out. Table1 presents the factors and their levels for the design of the experiment, which allowed us to see the modification of the feed pressures. During the fabrication of the coating, the projection distance was maintained at 160 mm, the preheating temperature of the substrate was maintained at approximately 367 K, the compressed air pressure was 275.8 kPa, and a flat position was used. The coatings were deposited on a navy brass substrate UNS No. C46400 (Cu 60%, Zn 39.25% and Sn 0.75% and lead-free) of 25 mm in diameter.

Table 1. Samples and parameters depositionof the experiment.

Oxygen Oxygen Acetylene Acetylene Testing Pressure Volumetric Volumetric Flow Pressure [kPa] [kPa] Flow [L/min] [L/min] B-1 331 33 69 27 B-2 345 35 69 27 B-3 359 37 69 27 B-4 331 33 83 28 B-5 345 35 83 28 B-6 359 37 83 28 B-7 331 33 97 29 B-8 345 35 97 29 B-9 359 37 97 29

The surface of the substrates was prepared by sanding with abrasive paper from #80–100, and then the surface was shot blasted with Al2O3 (aluminum oxide) with a particle size of 100 µm “No. 24” at a pressure of 689.5 kPa for 30 s. Then the cleaning of the surface was continued using dry air under pressure to remove the waste from the shot blasting. The average rugosity of the substrate was approximately 30 µm. In order to improve the quality of the coating, an annealing treatment was carried out on sample B-6. This process was carried out in a Lindberg/Blue (Thermo Fischer Scientific, Waltham, MA, USA), furnace at a temperature of 773 K, with a heating increase of 5 K/min and a controlled atmosphere of N2. The crystallographic structure was analyzed with X’PertPro(Panalytical, Almelo, The Netherlands) equipment operating in the Bragg–Brentano configuration with CuKα (1.5406 Å) radiation at 40 kV and 30 mA. The sweep range was from 10◦ to 60◦, with a step size of 0.05◦ and 2θ. The changes in the morphology and the chemical composition of coatings were determined through scanning electron microscopy (SEM) with Jeol JSM-7400F (Jeol, 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan) in a high vacuum with a voltage of 30 kV. Complementary studies to characterize the structure were done by means of transmission electron microscopy with FEI TECNAI 20 TWIN (Thermo Fischer Scientific, Waltham, MA, USA) equipment operating in diffraction mode, with a potential difference of 300 kV and with a constant chamber of 730 mm. The chemical composition was determined through X-ray dispersive energy spectroscopy (EDS) using Hitachi SU1510 (Hitachi High-Technologies, Coatings 2019, 9, 507 4 of 17

24-14 Nishi-Shimbashi 1-chome, Minato-ku, Tokyo 105-8717, Japan) equipment with an EDAX APOLLO probe (Edax Inc., Devon-Berwyn, PA, USA). The percentage of porosity in the coatings was determined from SEM images, from cross-sectional specimens. The SEM images were characterized at 300 and 250 through Image J (National Institutes × × of Health, Bethesda, MD, USA) image analysis software. Through grayscale contrast analysis, ImageJ software quantifies the total area and the gray areas of the cross-section micrography and automatically quantifies the percentage of porosity. The roughness and the morphology of the surfaces were determined using an OLYMPUS reference SX500 optical-digital microscope (Olympus Corporation, Shinjuku, Tokyo, Japan). Thickness measurements were taken in the cross section of the coating. For these measurements, Quartz PCI image analysis software was used (Quartz Image Corporation, Vancouver, BC, Canada). Ten measurements were made per record in four different continuous sections of the coating, and thicknesses between 127 µm and 299 µm were obtained. The corrosion and electrochemical behavior of the uncoated and coated specimens was studied using a three-electrode system and a Gamry Reference 600 potentiostat at room temperature (Gamry Instruments, Inc., Philadelphia, PA, USA). The samples were mounted in a special cell that limited the exposed area of the coated steel substrate face to 0.196 cm2. In the corrosion resistance experiments, the substrates were used as the working electrode, a platinum rod was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. The electrolytes that made up the solution had 3.5% NaCl. Prior to each corrosion experiment, the corrosion potential was allowed to stabilize for 0.75 h immersed in the test solution, followed by carrying out potentiodynamic polarization or electrochemical impedance tests. For the polarization, experiments were performed in a polarization range of 0.4 V to 0.5 V with respect to the open circuit potential, and the scanning rate − was 1 mV/s. The corrosion current density (Icorr) was estimated by a linear fit and Tafel extrapolation to the cathodic part of the polarization curve [8]. Electrochemical impedance spectroscopy (EIS) measurements were carried out using the same custom cell described above at a stable open circuit potential. The perturbing signal had an AC amplitude of 10 mV and a frequency range of 0.01 Hz to 100 kHz. Measurements were taken after 1 h of immersion. The EIS analysis was conducted using the equivalent circuit-fitting model with Gamry Echem Analyst software (Gamry Instruments, Inc., Philadelphia, PA, USA).

3. Results and Discussion

3.1. Numerical Simulation Figure1 shows the results obtained from the numerical simulation of the particle velocity at the time of the collision and the enthalpy of the different gas pressure conditions. In general, it can be seen that the velocities increase with the pressures of oxygen and acetylene, and that the highest collision velocity of the particles corresponds to an oxygen pressure of 359 kPa and 97 kPa of acetylene. The behavior observed for the velocities is consistent with the expected behavior for subsonic gas flows, in which the increase in velocity is a consequence of the increase in the flow rates of the gases through a constant section and the addition of heat generated during the combustion reaction. On the other hand, it can be seen that the enthalpy of the particles increases with the increase of oxygen and acetylene feed pressures. The lowest enthalpy values are exhibited for the sample produced at pressures of 331 kPa O2 and 69 kPa C2H6. In general, it is well known that a more energetic flame will transfer greater enthalpy to the powders, generating an over-melting in the particles and favoring the production of splashes when they reach the substrate [9].

Institutes of Health, Bethesda, MD, USA) image analysis software. Through grayscale contrast analysis, ImageJ software quantifies the total area and the gray areas of the cross-section micrography and automatically quantifies the percentage of porosity. The roughness and the morphology of the surfaces were determined using an OLYMPUS reference SX500 optical-digital microscope (Olympus Corporation, Shinjuku, Tokyo, Japan). Thickness measurements were taken in the cross section of the coating. For these measurements, Quartz PCI image analysis software was used (Quartz Image Corporation, Vancouver, BC, Canada). Ten measurements were made per record in four different continuous sections of the coating, and thicknesses between 127 μm and 299 μm were obtained. The corrosion and electrochemical behavior of the uncoated and coated specimens was studied using a three-electrode system and a Gamry Reference 600 potentiostat at room temperature (Gamry Instruments, Inc., Philadelphia, PA, USA). The samples were mounted in a special cell that limited the exposed area of the coated steel substrate face to 0.196 cm2. In the corrosion resistance experiments, the substrates were used as the working electrode, a platinum rod was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. The electrolytes that made up the solution had 3.5% NaCl. Prior to each corrosion experiment, the corrosion potential was allowed to stabilize for 0.75 h immersed in the test solution, followed by carrying out potentiodynamic polarization or electrochemical impedance tests. For the polarization, experiments were performed in a polarization range of −0.4 V to 0.5 V with respect to the open circuit potential, and the scanning rate was 1 mV/s. The corrosion current density (Icorr) was estimated by a linear fit and Tafel extrapolation to the cathodic part of the polarization curve [8]. Electrochemical impedance spectroscopy (EIS) measurements were carried out using the same custom cell described above at a stable open circuit potential. The perturbing signal had an AC amplitude of 10 mV and a frequency range of 0.01 Hz to 100 kHz. Measurements were taken after 1 h of immersion. The EIS analysis was conducted using the equivalent circuit-fitting model with Gamry Echem Analyst software (Gamry Instruments, Inc., Philadelphia, PA, USA).

3. Results and Discussion

3.1. Numerical Simulation Figure 1 shows the results obtained from the numerical simulation of the particle velocity at the time of the collision and the enthalpy of the different gas pressure conditions. In general, it can be seen that the velocities increase with the pressures of oxygen and acetylene, and that the highest collision velocity of the particles corresponds to an oxygen pressure of 359 kPa and 97 kPa of acetylene. The behavior observed for the velocities is consistent with the expected behavior for Coatings 2019, 9, 507 5 of 17 subsonic gas flows, in which the increase in velocity is a consequence of the increase in the flow rates of the gases through a constant section and the addition of heat generated during the combustion reaction.matrix rich On inthe copper other hand, in the itα canphase be seen and that the the presence enthalpy of theof theβ ‘(Cuparticles3Al) andincreasesκ1 (AlFe with3) the phases increase [10]. ofThe oxygen amount and of copperacetylene is infeed the pressures. range of 80 The wt.% lowest to 90 wt.%enthalpy according values to are the exhibited initial composition for the sample of the produceddeposition at material. pressures The of aluminum331 kPa O2 content and 69had kPa values C2H6. rangingIn general, between it is well 5% andknown 10 wt.%, that a close more to energeticthe concentration flame will value transfer of the grea materialter enthalpy in the powder to the butpowders, probably gene aﬀratingected inan itsover-melting measurement in bythe a particlespossible and segregation favoring of the aluminum production in theof sp mostlashes energetic when they processes. reach the substrate [9].

Coatings 2019, 9, x FOR PEER REVIEW 5 of 17

Figure 1. Variation in the velocity of particles colliding with the substrate vs. the variation of oxygen pressure in kPa.

3.2. Microstructural Characterization of Al–Cu–Fe Coatings The composition in the zone corresponding to the splats was analyzed. According to these results, the coating is an alloy of bronze and aluminum, whose matrix is rich in copper alloyed with Al and a small amount of Fe, which agrees with results published by other researches in which they identified a matrix rich in copper in the α phase and the presence of the β ‘(Cu3Al) and κ1 (AlFe 3) phases [10]. The amount (ofa) copper is in the range of 80 wt.% to 90 wt.%(b) according to the initial composition of the deposition material. The aluminum content had values ranging between 5% and Figure 1. Variation in the velocity of particles colliding with the substrate vs. the variation of oxygen 10 wt.%,pressure close in to kPa. the concentration value of the material in the powder but probably affected in its measurement by a possible segregation of aluminum in the most energetic processes. InIn Figure Figure 2,2, it it can can be be seen seen that that with with low low acet acetyleneylene and and oxygen oxygen pressures, pressures, there there is is a a decrease decrease in in thethe concentration concentration of of aluminum. aluminum. This This could could be be as associatedsociated with with high high values values of of the the maximum maximum flame ﬂame temperature,temperature, associated associated with lowerlower valuesvalues of of speed speed in in the the particles, particles, which which results results in longer in longer travel travel times timesof the of particles, the particles, which which favors favors the processes the processes of vaporization of vaporization of materials of materials with a lower with meltinga lowerpoint melting [10 ]. pointIt can [10]. also It be can seen also that be the seen coatings that the have coatings oxygen have concentrations oxygen concentrations greater than 2%greater as a consequencethan 2% as a of consequencethe oxidation of process the oxidation during process the deposition during the and deposition cooling. The and highestcooling. concentrations The highest concentrations of oxygen are ofpresent oxygen in are the present treatments in the in whichtreatments the simulations in which the indicated simulations higher indicated enthalpies higher of the enthalpies particles of during the particlesthe collision, during a factorthe collision, that favors a factor the reactionthat favors of thethe depositedreaction of metals the deposited with the metals oxygen with of the the ﬂame oxygen and ofthe the environment. flame and the environment.

Figure 2. Chemical composition of coatings. Weight percentage determined by EDS Analysis.

FigureFigure3 shows 2. Chemical the SEM composition image where of coatings. the chemical Weight compositionpercentage determined mapping by was EDS carried Analysis. out via EDS analysis. The red micrograph corresponds to the oxygen, the green to aluminum, the violet to copper, andFigure the yellow 3 shows to iron. the SEM The image EDS map where indicates the chem thatical the composition distribution mapping of most elementswas carried is uniform.out via EDSDue analysis. to the movement The red micrograph of the molten corresponds particles from to thethe combustionoxygen, the zonegreen to to the aluminum, surface of the the violet substrate, to copper,the elements and the react yellow and form to iron. some The oxides EDS of map copper indicate and aluminum,s that the whichdistribution have high of most negative elements values is of uniform.formation Due energies. to the movement In addition, of some the molten intermetallic particle compoundss from the combustion can be generated zone to under the surface the deposition of the substrate, the elements react and form some oxides of copper and aluminum, which have high negative values of formation energies. In addition, some intermetallic compounds can be generated under the deposition conditions. For example, Wen-sheng [10], in his studies of bronze and aluminum, found the formation of a matrix rich in Cu and accompanied by the β ‘phase (AlCu3) and the κ1 phase (AlFe3).

Coatings 2019, 9, 507 6 of 17 conditions. For example, Wen-sheng [10], in his studies of bronze and aluminum, found the formation of a matrix rich in Cu and accompanied by the β ‘phase (AlCu3) and the κ1 phase (AlFe3). Coatings 2019, 9, x FOR PEER REVIEW 6 of 17

(a) (b)

(e) (f)

FigureFigure 3.3. Chemical mapping of the surfacesurface coating at applied pressurespressures ofof 359359 kPakPa oxygenoxygen andand 9797 kPakPa acetylene:acetylene: ((aa)) EDSEDS mapmap ofof oxygen oxygen in in red; red; ( b(b)) EDS EDS map map of of aluminum aluminum in in green; green; (c )(c EDS) EDS map map of copperof copper in violet;in violet; (d) ( EDSd) EDS map map of ironof iron in yellow;in yellow; (e) ( SEMe) SEM micrograph micrograph of theof the surface; surface; (f) EDS(f) EDS spectrum. spectrum.

FigureFigure4 4 shows shows the the XRD XRD patterns patterns of of the the nine nine experiments experiments performed performed during during the the fabrication fabrication of of aluminum–bronzealuminum–bronze coatings. coatings. It It can can be be seen seen that that all theall applicationsthe applications exhibited exhibited the same the structuralsame structural phases andphases that and between that between them there them was there a small was a shift small to shif thet right. to theIn right. general, In general, the presence the presence of the phaseof the βphase’-AlCu3 β’-AlCu3 can be seen,can be and seen, its mostand its important most important planes are planes in position are in 2positionθ of 40.28, 2θ42.73, of 40.28, 44.81, 42.73, 46.85 44.81, and 73.0046.85 [JCPDSand 73.00 00-028-0005]. [JCPDS 00-028-0005]. In the study In bythe Li study Wen-she by Li and Wen-she colleagues and colleagues [10], they stated [10], they that stated the AlCu3 that stoichiometrythe AlCu3 stoichiometry corresponds corresponds to an intermetallic to an intermet compoundallic compound with a wide with range a wide of solubility, range of hassolubility, a FCC crystalhas a FCC structure crystal with structure a lattice with spacing a lattice of spacing 0.353 nm, of 0.353 and belongsnm, and tobelongs space groupto space 221. group In this221. spatial In this 3+ group,spatial thegroup, Al theions Al are3+ ions located are inlocated the vertices in the ofvertices the cell of and the the cell Cu and− ions the areCu− located ions are in located the centers in the of thecenters faces of of the the faces crystalline of the crystalline structure. Thestructure.α phase The of α Cu phase FCC of was Cu also FCC detected, was also which detected, can which be seen can in thebe seen peaks in locatedthe peaks at 42.73,located 50.5 at and42.73, 88.9 50.5 (JCPDS, and 88.9 96-901-3018). (JCPDS, 96-901-3018).

Coatings 2019,, 9,, 507x FOR PEER REVIEW 77 of of 17

Figure 4. X-rayX-ray diffraction diﬀraction (XRD) (XRD) of of the the nine nine variat variationsions of of oxygen and acetylene pressures for aluminum–bronze coatings.

Figure5 5 shows shows thethe SEMSEM imagesimages ofof thethe surfacesurface andand aa cross-sectioncross-section ofof thethe coatingscoatings producedproduced withwith an oxygenoxygen pressurepressure of of 331 331 kPa kPa and and an acetylenean acetylene pressure pressure of 69 kPa.of 69 On kPa. the surface,On the unmeltedsurface, unmelted spherical particles,spherical moltenparticles, particles molten thatparticles produce that splats,produce and splats, pores and can pores be seen, can and be seen, in some and cases in some dendritic cases structuresdendritic structures can be seen, can duebe seen, to the due solidification to the solidifi processescation processes of the molten of the drops.molten Indrops. the cross-section In the cross- ofsection thecoatings, of the coatings, a stacking a stacking of splats of splats can be can seen, be seen, as well as aswell the as presence the presence of defects of defects typical typical of the of technique,the technique, such such as pores,as pores, cavities, cavities, oxides oxides and and limits limits between between splats. splats. The The images images showshow thethe interface that separates the substrate from thethe coating.coating. In general, the coatings exhibited laminar structures made up up of of the the stacking stacking of of particles particles in ina molten a molten or semi-molten or semi-molten state state and andpores pores generated generated by the by short the shorttime of time the ofmaterial the material in the flame in the and flame the andrate theof cooling rate of of cooling the molten of the material molten during material the during formation the formationof the coating of the [11]. coating In [general,11]. In general, most of most the of coatings the coatings exhibited exhibited good good compaction, compaction, providing providing an acceptable homogeneity. This may be due to the great ease with which the oxyacetylene flameflame melts aluminum–bronze (melting point of aluminum–bronze: 1153–1193 K). Figure6 shows the cross-sectional TEM micrograph of the Al–bronze coating produced at pressures of 331 kPa oxygen and 69 kPa acetylene. It can be seen that the microstructure of the coating is made up of deformed particles, which are represented in the form of bands generated by shearing stress due to the high degree of deformation of the coating. In Figure6b, the structures in the form of sharp bands are identified more clearly, due to the high level of deformation, and in addition to this, areas of a high density of defects in the structure and fine grains due to dynamic recrystallization can be seen. These results agree with the investigations by Koivuluoto and W. Hincapie [12–14]. In Figure7, the variations of the porosity percentages vs. the variation of collision velocities on the substrate and the enthalpy can be seen. Figure7c,d show the SEM micrographics before and after the image treatment developed to quantify the percentage of porosity. In general, there is a tendency to reduce the porosity percentages(a) as the collision speeds and enthalpy(b) increase. The coatings have porosity percentages between 3.2% and 8.2%. The coating deposited with pressures of 331 kPa O2 and 69 kPa C2H6 exhibited the highest percentage of porosity, while the coating produced with higher pressures, acetylene at 97 kPa and oxygen at 345 kPa, had the lowest percentage of porosity. With an increase in particle velocity or pressure, the enthalpies and temperatures during the synthesis of the coating increased, which favors a greater atomization of the particles, generating smaller molten droplets, which are accelerated at a higher speed. The impact of these particles allows them to deform and form on the topography of the substrate before the solidification of a denser coating, with greater union between splats and the growth of a less porous structure. Similar results were reported by Y.

Kawaguchi [15], who indicated that high porosity is due to a low velocity of the drops, resulting from (c) (d) their poor acceleration by the compressed air during the thermal spraying. Figure 5. Application projected with a pressure of 331 kPa oxygen and 69 kPa acetylene. (a) and (b) SEM micrograph in cross-section; (c) and (d) SEM micrograph of the surface.

Coatings 2019, 9, x FOR PEER REVIEW 7 of 17

Figure 4. X-ray diffraction (XRD) of the nine variations of oxygen and acetylene pressures for aluminum–bronze coatings.

Figure 5 shows the SEM images of the surface and a cross-section of the coatings produced with an oxygen pressure of 331 kPa and an acetylene pressure of 69 kPa. On the surface, unmelted spherical particles, molten particles that produce splats, and pores can be seen, and in some cases dendritic structures can be seen, due to the solidification processes of the molten drops. In the cross- section of the coatings, a stacking of splats can be seen, as well as the presence of defects typical of the technique, such as pores, cavities, oxides and limits between splats. The images show the interface that separates the substrate from the coating. In general, the coatings exhibited laminar structures made up of the stacking of particles in a molten or semi-molten state and pores generated by the short time of the material in the flame and the rate of cooling of the molten material during the formation of the coating [11]. In general, most of the coatings exhibited good compaction, providing an Coatingsacceptable2019 ,homogeneity.9, 507 This may be due to the great ease with which the oxyacetylene flame melts8 of 17 aluminum–bronze (melting point of aluminum–bronze: 1153–1193 K).

(a) (b)

Coatings 2019, 9, x FOR PEER REVIEW 8 of 17

Figure 6 shows the cross-sectional TEM micrograph of the Al–bronze coating produced at pressures of 331 kPa oxygen and 69 kPa acetylene. It can be seen that the microstructure of the coating is made up of deformed particles, which are represented in the form of bands generated by shearing stress due to the high degree (ofc) deformation of the coating. In Figure (6b,d) the structures in the form of sharp bands are identified more clearly, due to the high level of deformation, and in addition to this, areasFigure of a high 5. Application density of projecteddefects in with the a structure pressure ofand 331 fine kPa grains oxygen due and to 69 dynamic kPa acetylene. recrystallization ( (aa)) and and ( (b)) can be seen.SEM These micrograph results in agree cross-section; with the (cc )investigat) andand ((d)) SEMSEMions micrographmicrograph by Koivuluoto ofof thethe surface.andsurface. W. Hincapie [12–14].

(a) (b)

Figure 6. ((aa)) TEM TEM images images with with microstructural microstructural details details in in the the transverse transverse direction direction of of the 331 kPa oxygen–69 kPa acetylene coating. The The cutting cutting bands are shown. ( (bb)) TEM TEM image image with with details of the band microstructure, a density dislocation in the structure, andand ﬁne-grainedfine-grained areas.areas.

3.3. Electrochemical Measurements In Figure 7, the variations of the porosity percentages vs. the variation of collision velocities on the substrateFigure8 showsand the the enthalpy potentiodynamic can be seen. polarization Figure 7c,d show curves the for SEM the micrographics coatings deposited before at and various after workingthe image pressures. treatment It developed can be seen to that quantify the anodic the percenta and cathodicge of porosity. branches In have general, a regular there pattern is a tendency and are toalmost reduce the the same porosity as the percentages coatings, with as the small collisio changesn speeds in the and value enthalpy of the increase. corrosion The potential coatings and have the porositydensity of percentages the corrosion between current 3.2% (Icorr). and Comparatively,8.2%. The coating among deposited them itwith is shown pressures that theof 331 curve kPa that O2 andcorresponds 69 kPa C2 toH the6 exhibited application the highest with 97 percentage kPa acetylene of porosity, and oxygen while is the the coating one with produced the lowest with values higher of pressures,current in the acetylene ﬁgure, whichat 97 kPa indicates and oxygen that it isat the 345 sample kPa, had with the the lowest best resistance percentage to corrosion.of porosity. In With general, an increasethe good in corrosion particle resistance velocity or of pressure, aluminum–bronze the enthalpies and waterborne and temperatures coatings during is associated the synthesis with a passive of the coatingoxide ﬁlm increased, consisting which of Cu 2favorsO and a Al greater2O3, which atomization self-repairs of automaticallythe particles, [16generating]. However, smaller the corrosion molten processdroplets, of which aluminum–bronze are accelerated coatings at a higher surely speed. occurs The impact through of thethese dissolution particles allows of copper them to to form deform the andcomplex form CuClon the2, topography which can be of hydrolyzedthe substrate when before in the saline solidification environments of a denser [17–19 coating,]. This compound with greater is union between splats and the growth of a less porous structure. Similar results were reported by Y. Kawaguchi [15], who indicated that high porosity is due to a low velocity of the drops, resulting from their poor acceleration by the compressed air during the thermal spraying.

(a) (b)

Coatings 2019, 9, x FOR PEER REVIEW 8 of 17

Figure 6 shows the cross-sectional TEM micrograph of the Al–bronze coating produced at pressures of 331 kPa oxygen and 69 kPa acetylene. It can be seen that the microstructure of the coating is made up of deformed particles, which are represented in the form of bands generated by shearing stress due to the high degree of deformation of the coating. In Figure 6b, the structures in the form of sharp bands are identified more clearly, due to the high level of deformation, and in addition to this, areas of a high density of defects in the structure and fine grains due to dynamic recrystallization can be seen. These results agree with the investigations by Koivuluoto and W. Hincapie [12–14].

(a) (b)

Figure 6. (a) TEM images with microstructural details in the transverse direction of the 331 kPa oxygen–69 kPa acetylene coating. The cutting bands are shown. (b) TEM image with details of the band microstructure, a density dislocation in the structure, and fine-grained areas.

In Figure 7, the variations of the porosity percentages vs. the variation of collision velocities on the substrate and the enthalpy can be seen. Figure 7c,d show the SEM micrographics before and after the image treatment developed to quantify the percentage of porosity. In general, there is a tendency to reduce the porosity percentages as the collision speeds and enthalpy increase. The coatings have porosity percentages between 3.2% and 8.2%. The coating deposited with pressures of 331 kPa O2 and 69 kPa C2H6 exhibited the highest percentage of porosity, while the coating produced with higher pressures, acetylene at 97 kPa and oxygen at 345 kPa, had the lowest percentage of porosity. With an increase in particle velocity or pressure, the enthalpies and temperatures during the synthesis of the coating increased, which favors a greater atomization of the particles, generating smaller molten Coatingsdroplets, 2019 which, 9, x 507 FOR are PEER accelerated REVIEW at a higher speed. The impact of these particles allows them to deform99 of of 17 and form on the topography of the substrate before the solidification of a denser coating, with greater union between splats and the growth of a less porous structure. Similar results were reported by Y. formed by the reaction of chlorine ions that diﬀuse through the pores of the coating and the dissolved Kawaguchi [15], who indicated that high porosity is due to a low velocity of the drops, resulting from copper in the coating and the substrate. their poor acceleration by the compressed air during the thermal spraying.

CoatingsFigure 2019 , 7.9, x( aFOR) Porosity PEER REVIEW percentage vs. collision velocity of the particles; (b) porosity percentage vs.9 of 17 (a) (b) enthalpy; (c) and (d) SEM micrography processing by gray contrast

3.3. Electrochemical Measurements Figure 8 shows the potentiodynamic polarization curves for the coatings deposited at various working pressures. It can be seen that the anodic and cathodic branches have a regular pattern and are almost the same as the coatings, with small changes in the value of the corrosion potential and the density of the corrosion current (Icorr). Comparatively, among them it is shown that the curve that corresponds to the application with 97 kPa acetylene and oxygen is the one with the lowest values of current in the figure, which indicates that it is the sample with the best resistance to corrosion. In general, the good corrosion resistance of aluminum–bronze and waterborne coatings is associated with a passive oxide film consisting of Cu2O and Al2O3, which self-repairs automatically [16]. However, the corrosion process of aluminum–bronze coatings surely occurs through the dissolution of copper to form the complex CuCl2, which can be hydrolyzed when in saline Figure 8. Potentio-dynamic polarization curves of coatings deposited under different pressure environmentsFigure 8. Potentio-dynamic[17–19]. This compound polarization is formed curves by of th coe atingsreaction deposited of chlorine under ions different that diffuse pressure through conditions of oxygen and acetylene. the poresconditions of the of coating oxygen and acetylene.the dissolved copper in the coating and the substrate. Figure9 shows the corrosion current density (Jcorr) of the coatings as a function of the porosity. In general,Figure there9 shows is a the tendency corrosion for ancurrent increase density in the (Jcorr) Jcorr valuesof the coatings with an increaseas a function in the of porosity the porosity. of the In general, there is a tendency for an increase in the Jcorr values with an increase in the porosity of coating. For example, the coating deposited with a pressure of 331 kPa O2 and 69 kPa C2H6 exhibits the coating. highest For degree example, of porosity. the coating The poresdeposited in the with coatings a pressure act as of “paths” 331 kPa orO2trajectories and 69 kPa C by2H which6 exhibits the thecorrosive highest electrolyte degree of can porosity. flow to The the splatpores of in the the coating coatings and act even as “paths” reach the or substrate trajectories itself, by promotingwhich the corrosiveactive or electrolyte anodic zones. can flow Similarly, to the Walshsplat of et the al. co reportedating and that even the reach increase the substrate in thickness itself, reduces promoting the activedegree or of anodic average zones. porosity Similarly, in Ni deposits, Walsh et consequently al. reported optimizing that the increase the corrosion in thickness resistance reduces in saline the degreeenvironments of average of 3% porosity NaCl [in20 ].Ni Likewise, deposits, Gavrilaconsequently et al., inoptimizing their research the corrosion on the eff ectresistance of the degree in saline of environments of 3% NaCl [20]. Likewise, Gavrila et al., in their research on the effect of the degree of

Figure 8. Potentio-dynamic polarization curves of coatings deposited under different pressure conditions of oxygen and acetylene.

Figure 9 shows the corrosion current density (Jcorr) of the coatings as a function of the porosity. In general, there is a tendency for an increase in the Jcorr values with an increase in the porosity of the coating. For example, the coating deposited with a pressure of 331 kPa O2 and 69 kPa C2H6 exhibits the highest degree of porosity. The pores in the coatings act as “paths” or trajectories by which the corrosive electrolyte can flow to the splat of the coating and even reach the substrate itself, promoting active or anodic zones. Similarly, Walsh et al. reported that the increase in thickness reduces the degree of average porosity in Ni deposits, consequently optimizing the corrosion resistance in saline environments of 3% NaCl [20]. Likewise, Gavrila et al., in their research on the effect of the degree of

Coatings 2019, 9, 507 10 of 17 Coatings 2019, 9, x FOR PEER REVIEW 10 of 17 Coatings 2019, 9, x FOR PEER REVIEW 10 of 17 porosityporosity on on the the corrosion corrosion resistance resistance of of electrodep electrodepositedelectrodepositedosited Zn–Ni Zn–Ni deposition deposition on on steels, steels, showed showed thatthat an an increaseincrease in in the the porosityporosity of of the the depositsdeposits by by aa factorfactor ofof 1010 10 entailsentails entails anan an equivalentequivalent equivalent increaseincrease increase inin thethe the corrosioncorrosion corrosion raterate [21].[21]. [21].

FigureFigure 9. 9. CurrentCurrent density density vs. vs. percentage percentage of of porosity. porosity. Figure 9. Current density vs. percentage of porosity. In Figure 10, the SEM micrograph of the surface of the aluminum–bronze coating can be seen InIn FigureFigure 10,10, thethe SEMSEM micrographmicrograph ofof thethe surfacsurfacee ofof thethe aluminum–bronzealuminum–bronze coatingcoating cancan bebe seenseen after the corrosion test on the surface of the coating, 69 kPa C2H6–345 kPa O2. There is an increase after the corrosion test on the surface of the coating, 69 kPa C2H6–345 kPa O2. There is an increase in afterin the the roughness, corrosion andtest aon layer the surface is formed of the with coating, some corrosion69 kPa C2H products6–345 kPa adhered O2. There to theis an coating increase after in the roughness, and a layer is formed with some corrosion products adhered to the coating after being thebeing roughness, exposed and to the a layer NaCl is solutionformed with at 3.5 some wt.%. corro Itsion is also products possible adhered to observe to the the coating precipitation after being of exposed to the NaCl solution at 3.5 wt.%. It is also possible to observe the precipitation of electrolyte exposedelectrolyte to crystals,the NaCl especially solution at in 3.5 the wt.%. porous It is areas also possible of the surface to observe and the the formation precipitation of the of electrolyte protective crystals, especially in the porous areas of the surface and the formation of the protective patina [22], patinacrystals, [22 especially], which isin characterizedthe porous areas by itsof the green surf colorace and and the by formation an appearance of the similar protective to that patina of fungi [22], which is characterized by its green color and by an appearance similar to that of fungi produced by producedwhich is characterized by moisture. Theseby its resultsgreen color agree and with by the an research appearance of Pompo similar et al. to [23that] and of fungi M. M. produced Sadawy [24 by], moisture. These results agree with the research of Pompo et al. [23] and M. M. Sadawy [24], who whomoisture. reported These that results the corrosion agree with products the research formed of during Pompo the et processal. [23] stronglyand M. M. adhere Sadawy to the [24], surface. who reported that the corrosion products formed during the process strongly adhere to the surface. J.A. J.A.reported Wharton that etthe al. corrosion [25] established products that formed the corrosion during resistancethe process of aluminum–bronzestrongly adhere to canthe besurface. attributed J.A. Wharton et al. [25] established that the corrosion resistance of aluminum–bronze can be attributed to Whartonto the formation et al. [25] of established a protective that layer, the corrosion perhaps 900resistance to 1000 of nm aluminum–bronze thick, that contains can be aluminum attributed and to the formation of a protective layer, perhaps 900 to 1000 nm thick, that contains aluminum and copper thecopper formation oxides. of Figures a protective 11 and layer, 12 show perhaps the 900 SEM to image 1000 nm and thick, the EDS that mappingcontains aluminum of the elements and copper in the oxides. Figures 11 and 12 show the SEM image and the EDS mapping of the elements in the corrosion oxides.corrosion Figures ﬁlm formed 11 and on12 show the surface the SEM of theimage coating. and the In EDS general, mapping the presence of the elements of Cu, Al, in andthe corrosion Fe can be filmfilm formedformed onon thethe surfacesurface ofof thethe coating.coating. InIn general,general, thethe presencepresence ofof Cu,Cu, Al,Al, andand FeFe cancan bebe seen,seen, whichwhich areseen, the which most areimportant the most elements important of elementsthe coating, of the with coating, Zn from with the Zn substrate from the and substrate Na and and Cl Na from and the Cl arefrom the the most corrosive important electrolyte. elements The of presence the coating, of these with elements Zn from makes the substrate it possible and to Na conclude and Cl thatfrom after the corrosivecorrosive electrolyte.electrolyte. TheThe presencepresence ofof thesethese elemenelementsts makesmakes itit possiblepossible toto concludeconclude thatthat afterafter thethe electrochemicalthe electrochemical tests, tests, a coating a coating of thin of thin salt salt crystals crystals and and corrosion corrosion products products with with cuprous cuprous chloride chloride (CuCl)electrochemical was produced tests, [a26 coating]. of thin salt crystals and corrosion products with cuprous chloride (CuCl)(CuCl) waswas producedproduced [26].[26].

(b) ((aa)) (b)

Figure 10. Surface coating of aluminum–bronze after the impedance test for the coating, 69 kPa C2H6– Figure 10. SurfaceSurface coating coating of aluminum–bronze aluminum–bronze after the impedance test for the coating, 69 kPa C22H6–– 345 kPa O2. (a) Salt crystals, and (b) corrosion products. 345 kPa O2..( (aa)) Salt Salt crystals, crystals, and and ( (bb)) corrosion corrosion products. products.

Coatings 2019, 9, 507 11 of 17 Coatings 2019, 9, x FOR PEER REVIEW 11 of 17 Coatings 2019, 9, x FOR PEER REVIEW 11 of 17

(a) (b) (a) (b)

(c) (c) Figure 11. SEM morphology, energy dispersion spectroscopy, and elementary mapping of the FigureFigure 11.11. SEMSEM morphology,morphology, energyenergy dispersiondispersion spectroscopy,spectroscopy, andand elementaryelementary mappingmapping ofof thethe aluminum–bronze film after the Tafel tests for the coating, 69 kPa C2H6–359 kPa O2. aluminum–bronzealuminum–bronze ﬁlm film after after the the Tafel Tafel tests tests for for the the coating, coating, 69 69 kPa kPa C 2CH2H6–3596–359 kPa kPa O O2.2.

Figure 12. Elementary mapping of the EDS coating, 69 kPa C2H6–359 kPa O2. Figure 12. Elementary mapping of the EDS coating, 69 kPa C2H6–359 kPa O2. Figure 12. Elementary mapping of the EDS coating, 69 kPa C2H6–359 kPa O2. Figure 13 shows the Bode diagrams of Al–Cu coating applications at 1 h immersion time. On the Figure 13 shows the Bode diagrams of Al–Cu coating applications at 1 h immersion time. On the impedanceFigure vs. 13 frequencyshows the graph,Bode diagrams it can be of seen Al–Cu that coating at high frequenciesapplications the at 1 values h immersion of resistance time. On to the the impedance vs. frequency graph, it can be seen that at high frequencies the values of resistance to the solutionimpedance do not vs. varyfrequency signiﬁcantly, graph, whichit can be indicates seen that that at thehigh coating frequencies did not the experience values of a resistance high degree to the of solution do not vary significantly, which indicates that the coating did not experience a high degree degradation.solution do not In general,vary significantly, these results which show indicate two relaxations that thetimes, coating which did not are experience related to twoa high slopes degree in of degradation. In general, these results show two relaxation times, which are related to two slopes theof degradation. impedance graph In general, and two these loops results in the show Bode two diagram. relaxation To better times, understand which are related the electrochemical to two slopes in the impedance graph and two loops in the Bode diagram. To better understand the electrochemical behaviorin the impedance of these coatings, graph and the two EIS loops results in werethe Bode ﬁt to diagram. an equivalent To better circuit understand model proposed the electrochemical by Dermaj behavior of these coatings, the EIS results were fit to an equivalent circuit model proposed by Dermaj behavior of these coatings, the EIS results were fit to an equivalent circuit model proposed by Dermaj et al. [27] and Rahmouni et al. [28] for bronze subjected to corrosive processes in a 3% NaCl solution. et al. [27] and Rahmouni et al. [28] for bronze subjected to corrosive processes in a 3% NaCl solution.

Coatings 2019, 9, 507 12 of 17

Coatings 2019, 9, x FOR PEER REVIEW 12 of 17 et al. [27] and Rahmouni et al. [28] for bronze subjected to corrosive processes in a 3% NaCl solution. AA couple couple of of elements, elements, CPEc CPEc (c (c is is the the coating) coating) and and Rpo, Rpo, were were used used in in parallel parallel to to replace replace the the dielectric dielectric propertiesproperties of of the the coating. coating. TheThe otherother pairpair ofof elements,elements, CPEdcCPEdc andand RctRct inin parallel,parallel, waswas adaptedadapted toto describedescribe the the charge charge transfer transfer at at the the coating coating/substrat/substrate interfacee interface due due to theto the presence presence of micropores.of micropores. Rpo Rpo is theis the resistance resistance of theof the circuit circuit to current to current ﬂow flow through through the the pores, pores, while while Rsln Rsln represents represents the the resistance resistance of theof the electrolyte electrolyte between between the the working working electrode electrode and and the the reference reference electrode, electrode, and and Rct Rct is the is the resistance resistance to chargeto charge transfer. transfer. The timeThe time constant constant at higher at higher frequencies frequencies in this circuitin this representscircuit represents the dielectric the dielectric pattern (CPEcpattern and (CPEc M) of theand coating, M) of the and coating, the time constantand the attime lower constant frequencies at lower represents frequencies the coating represents/substrate the interfacecoating/substrate (CPEcd and interface N) properties (CPEcd [and26]. N) properties [26].

(a) (b)

FigureFigure 13. 13.Spectra Spectra for for Cu–Al Cu–Al coatings. coatings. ( a(a)) Impedance Impedance vs. vs. frequency;frequency; ( b(b)) phase phase angle angle vs. vs. frequency. frequency.

TheThe CPEs CPEs are are a a mathematical mathematical supportsupport thatthat representsrepresents severalseveral elementselements ofof anan electricalelectrical circuit.circuit. TheThe impedance impedance value value of of a a CPE CPE is is shown shown by by the the following following equation: equation:

n n ZZCPECPE= =Z Zo(jo(jω)ω) (1)(1) (1) where Zo and n are frequency-independent fit parameters, j = (−1)1/2, and ω is the angular frequency. where Zo and n are frequency-independent fit parameters, j = ( 1)1/2, and ω is the angular frequency. The factor n, defined as a CPE power, always had a value between− 0.5 and 1. The Bode plot was The factor n, defined as a CPE power, always had a value between 0.5 and 1. The Bode plot was obtained from the slope of |Z|. When n = 0.5, CPE represents the Warburg impedance, W, with a obtained from the slope of |Z|. When n = 0.5, CPE represents the Warburg impedance, W, with a diffusion character, and CPE becomes reduced to an ideal capacitor for n = 1 and to a simple resistor diffusion character, and CPE becomes reduced to an ideal capacitor for n = 1 and to a simple resistor for n = 0. Table 2 shows the results obtained through fitting the proposed equivalent circuit to the for n = 0. Table2 shows the results obtained through fitting the proposed equivalent circuit to the experimental data obtained in the EIS results of the aluminum–bronze coatings. In general terms, it experimental data obtained in the EIS results of the aluminum–bronze coatings. In general terms, it can can be seen that most coatings exhibit a capacitive behavior (n = 0.8), which indicates that the surface be seen that most coatings exhibit a capacitive behavior (n = 0.8), which indicates that the surface of of the electrode is heterogeneous. Furthermore, as the value of n is lower than 1, it can be deduced the electrode is heterogeneous. Furthermore, as the value of n is lower than 1, it can be deduced that that the surface under study behaves like a capacitor with leakage of charge or mass. the surface under study behaves like a capacitor with leakage of charge or mass.

Table 2. Results of the equivalent circuit settings of the EIS impedance results obtained from the Table 2. Results of the equivalent circuit settings of the EIS impedance results obtained from the aluminum–bronze coatings. aluminum–bronze coatings. Testing Rsoln [Ω] Rct [Ω] Rpo [Ω] CPE1 n CPE2 m Rp [Ω] Testing Rsoln [Ω] Rct [Ω] Rpo [Ω] CPE1 n CPE2 m Rp [Ω] B-1 54.06 3157 819 1.39 × 10−4 0.672 2.93 × 10−4 0.556 3977 B-1 54.06 3157 819 1.39 10 4 0.672 2.93 10 4 0.556 3977 − −8 −−5 B-2 80.84 16,650 3878 2.66× × 108 0.998 1.89 × 10 5 0.786 20,528 B-2 80.84 16,650 3878 2.66 10− 0.998 1.89 10− 0.786 20,528 × 6 −6 × −6 B-3B-3 64.16 64.16 13,280 13,280 15,990 15,990 5.58 5.58 10× 10− 0.954 8.328.32 × 10− 0.8360.836 29,27029,270 × 6 × 6 B-4B-4 80.74 80.74 10,310 10,310 14,590 14,590 6.90 6.90 10× 10− −6 0.831 6.056.05 × 10−−6 0.8450.845 24,90024,900 × 6 × 5 B-5B-5 56.21 56.21 4595 4595 5413 5413 9.73 9.73 10× 10− −6 0.99 1.801.80 × 10−−5 0.8040.804 10,00810,008 × 6 × 6 B-6 84.26 9236 56,620 3.10 10− −6 0.99 2.87 10−−6 0.869 65,856 B-6 84.26 9236 56,620 3.10× × 106 0.99 2.87 × 10 6 0.869 65,856 B-7 57.66 18,960 9118 6.22 10− 0.740 6.60 10− 0.849 28,078 × −6 × −6 B-8B-7 59.4657.66 967618,960 7897 9118 5.73 6.22 10× 106 0.9700.740 6.606.77 × 10 6 0.8490.822 2817,573,078 × − × − B-9B-8 81.51 59.46 25,050 9676 7567 7897 5.12 5.73 10× 106 −6 0.6610.970 6.776.17 × 10−6 0.8220.848 17,57332,617 × − × − B-9 81.51 25,050 7567 5.12 × 10−6 0.661 6.17 × 10−6 0.848 32,617

In general, the values of Rct and Rpo are high, which shows that the coating has good anticorrosive properties, which can be attributed to the formation of copper and aluminum oxides during the corrosive test. The sum of Rpo and Rcor is used to represent the polarization resistance

Coatings 2019, 9, 507 13 of 17

In general, the values of Rct and Rpo are high, which shows that the coating has good anticorrosive Coatingsproperties, 2019, 9 which, x FOR PEER can beREVIEW attributed to the formation of copper and aluminum oxides during13 of the 17 corrosive test. The sum of Rpo and Rcor is used to represent the polarization resistance (Rp), which is (Rp), which is inversely proportional to the corrosion current density, that is, the higher the value of inversely proportional to the corrosion current density, that is, the higher the value of the sum, the sum, the slower the corrosion rate. Figure 14 shows the results of the polarization resistance as a the slower the corrosion rate. Figure 14 shows the results of the polarization resistance as a function of function of the porosity of the produced coatings. An increase in the values of Rp is shown in the the porosity of the produced coatings. An increase in the values of Rp is shown in the samples with low samples with low porosity percentage, that is, the reduction of the pores reduces the paths for the porosity percentage, that is, the reduction of the pores reduces the paths for the ions of the corrosive ions of the corrosive electrolyte to circulate through the thickness of the coating and the substrate to electrolyte to circulate through the thickness of the coating and the substrate to form pitting. According form pitting. According to the microstructural study, the less porous coatings were deposited with to the microstructural study, the less porous coatings were deposited with the highest values of collision the highest values of collision speed and temperature, which favored a reduction in the size of the speed and temperature, which favored a reduction in the size of the particles, their greater plasticity particles, their greater plasticity on the substrate, and better union between splats. This structure is on the substrate, and better union between splats. This structure is more compact and generates more compact and generates greater obstacles for the diffusion of ions such as the Cl of the corrosive greater obstacles for the diﬀusion of ions such as the Cl of the corrosive medium inside the coating, medium inside the coating, thus increasing the polarization resistance. On the other hand, some thus increasing the polarization resistance. On the other hand, some investigations [28–30] have found investigations [28–30] have found that during the time of the test, the pores or microcracks in the that during the time of the test, the pores or microcracks in the coating can improve the resistance coating can improve the resistance to corrosion by plugging these defects with the corrosion products to corrosion by plugging these defects with the corrosion products generated. In this way, the ionic generated. In this way, the ionic conduction and the density of the electrolyte diffusion zones towards conduction and the density of the electrolyte diﬀusion zones towards the substrate can be decreased. the substrate can be decreased. Polarization resistance can also be increased by partial passivation of Polarization resistance can also be increased by partial passivation of the coating by the formation the coating by the formation of protective oxide layers such as Al2O3 and Cu2O, which are known to of protective oxide layers such as Al O and Cu O, which are known to be highly protective against be highly protective against corrosion,2 3as they ar2 e easily formed and are also dense, homogeneous, corrosion, as they are easily formed and are also dense, homogeneous, and of high adherence [31]. and of high adherence [31].

Figure 14. Variation of the polarization resistance as a function of the percentage of porosity for Figurealuminum–bronze 14. Variation coatings. of the polarization resistance as a function of the percentage of porosity for aluminum–bronze coatings. Figure 15 shows images obtained via scanning electron microscopy of the coating applied with 331 kPaFigure oxygen 15 shows and 69images kPa acetylene. obtained via The scanning surface ofelectron the coating microscopy is seen beforeof the coating immersion applied and with after 331carrying kPa oxygen out the and corrosion 69 kPa acetylene. tests. This The comparative surface of studythe coating allows is observationseen before immersion of the deterioration and after carryingundergone out by the these corrosion coatings tests. due This to thecomparativ electrochemicale study tests,allows with observation a cracked of surfacethe deterioration and some undergonecorrosion products. by these This coatings failure due mode to canthe beelectrochemical explained by thetests, corrosive with a attack cracked of the surface electrolyte and some when corrosionit penetrates products. into the This coating failure through mode the can pores, be explained cracks, andby the defects, corrosive such attack as boundaries of the electrolyte between when splats itand penetrates the grain, into into the which coating O2 ions through can di theﬀuse pores, at a lower cracks, velocity and defects, than Cl −suchions, as generating boundaries a process between of splatscrack corrosionand the grain, [29]. Ininto this which process, O2 ions anodic can zones diffuse are at generated a lower velocity by the dissolution than Cl− ions, of Cu, generating Al and Zn a process(from the of substrate)crack corrosion that later [29]. react In this with process, oxygen anodic and chlorine zones toare form generated the corrosion by the products,dissolution plugging of Cu, Althe and pores Zn and (from the the surface substrate) of the coating.that later The react dissolution with oxygen of the and metals chlorine in the to substrate form the generates corrosion a products,loss of adherence plugging to the coating,pores and which the cansurface later of generate the coating. small The delaminations dissolution and of athe severe metals corrosive in the substrateattack on generates the coating–substrate a loss of adherence system to [32 the]. Incoat addition,ing, which it mustcan later be borne generate in mind small that delaminations during the andcorrosion a severe test, corrosive passivation attack layers on canthe becoating–substrate generated that increase system in[32]. thickness In addition, until localizedit must be corrosion borne in is mindgenerated that byduring grain the boundaries corrosion [ 32test,]. passivation layers can be generated that increase in thickness until localized corrosion is generated by grain boundaries [32].

Coatings 2019, 9, 507 14 of 17 Coatings 2019, 9, x FOR PEER REVIEW 14 of 17

(a) (b)

Figure 15. ((aa)) and and ( (bb)) SEM SEM micrograph micrograph of of the the surface surface of of the coating with 359 kPa oxygen–69 kPa acetylene appliedapplied beforebefore the the EIS EIS test. test. ( c()c and) and (d ()d Micrography) Micrography of of the the surface surface of theof the coating coating with with 359 kPa359 kPaoxygen–69 oxygen–69 kPa kPa acetylene acetylene applied applied after after the EISthe testEIS attest 350 at 350×and and 600 600×.. × × Figure 16 shows the potentiodynamic polarizatio polarizationn curves and the Bode diagrams for the sample B6, before and after thermal treatment.treatment. Thermal Thermal treatments treatments were were carried carried out out in in order to reduce the defects in the producedproduced coatings.coatings. InIn thisthis investigation,investigation, sample sample B6 B6 was was selected selected to to compare compare the the eff effectect of ofthe the treatment treatment on the on corrosion the corrosion resistance. resistance. Table3 shows Table the 3 mainshows results the main obtained results in the obtained electrochemical in the electrochemicaltest, and the porosity test, ofand the the coatings porosity with of andthe coatin withoutgs treatmentwith and arewithout presented. treatment The heat-treatedare presented. coating The heat-treated(B-6T) exhibited coating a decrease (B-6T) inexhibited its current a decrease density comparedin its current to the density coating compared without annealingto the coating (B-6). withoutThe effect annealing of the mechanisms (B-6). The ofeffect diff usionof the onmechanisms the microstructure of diffusion was on more the significantmicrostructure in the was response more significantto corrosion in of the the response coatings. to Thiscorrosion is surely of the due coatings. to the formation This is surely of a more due to compact the formation structure, of greatera more compactadherence structure, between greater the coating adherence and the between substrate, the co andating the and increase the substrate, in the oxide and content. the increase In Table in the3, oxideresults content. of the potentio-dynamic In Table 3, results tests of of the the potentio-dynamic coatings are shown. tests Finally, of the Table coatings3 also showsare shown. the results Finally, of Tablethe polarization 3 also shows resistance the results obtained of the via polarization electrochemical resistance impedance obtained spectroscopy. via electrochemical The higher impedance the value spectroscopy.of Rp, the slower The will higher be thethe corrosion value of Rp, rate. the These slower results will allowbe the us corrosion to confirm rate. that These thermally-treated results allow uscoatings to confirm improve that corrosionthermally-treated resistance. coatings The increase improv ine corrosion Rp in of the resistance. annealed The coatings increase is the in resultRp in of thean improvementannealed coatings in their is structurethe result due of an to poreimprov plugging,ement in increased their structure oxide concentration, due to pore plugging, increased increasedadhesion betweenoxide concentration, splats, etc., and increased thus reduction adhesion in between the density splats, of zones etc., ofand electrolyte thus reduction diffusion in intothe densitythe coating of zones and theof electr substrate.olyte diffusion into the coating and the substrate.

Table 3. Results of the potentiodynamic (current density) and EIS test (Rp) of the samples of coatings B-6 and B-6T without and with heat treatment.

Acetylene Oxygen Current Density Testing Porosity [%] Rp [Ω] Pressure [kPa] Pressure [kPa] [µA/cm2] B-6 69 345 4.35 4847 2.93 10 4 × − B-6T 69 345 2.65 24,850 1.89 10 5 × −

These results allow us to conclude that using this process, economical coatings can be achieved that can be used successfully for the protection and repair of naval, automotive, and aeronautical components or as coatings for increasing the durability of various industrial components subject to conditions of severe degradation.(a) These structural elements are in(b a) marine environment in the

Figure 16. Electrochemical test. (a) Potentio-dynamic polarization curves; (b) Bode spectra for Cu–Al coatings: Impedance vs. frequency and phase angle vs. frequency.

Coatings 2019, 9, x FOR PEER REVIEW 14 of 17

(a) (b)

Figure 15. (a) and (b) SEM micrograph of the surface of the coating with 359 kPa oxygen–69 kPa acetylene applied before the EIS test. (c) and (d) Micrography of the surface of the coating with 359 kPa oxygen–69 kPa acetylene applied after the EIS test at 350× and 600×.

Figure 16 shows the potentiodynamic polarization curves and the Bode diagrams for the sample B6, before and after thermal treatment. Thermal treatments were carried out in order to reduce the defects in the produced coatings. In this investigation, sample B6 was selected to compare the effect of the treatment on the corrosion resistance. Table 3 shows the main results obtained in the electrochemical test, and the porosity of the coatings with and without treatment are presented. The heat-treated coating (B-6T) exhibited a decrease in its current density compared to the coating without annealing (B-6). The effect of the mechanisms of diffusion on the microstructure was more significant in the response to corrosion of the coatings. This is surely due to the formation of a more compact structure, greater adherence between the coating and the substrate, and the increase in the oxide content. In Table 3, results of the potentio-dynamic tests of the coatings are shown. Finally, Table 3 also shows the results of the polarization resistance obtained via electrochemical impedance spectroscopy. The higher the value of Rp, the slower will be the corrosion rate. These results allow Coatingsus to confirm2019, 9, 507 that thermally-treated coatings improve corrosion resistance. The increase in Rp15 in of 17of the annealed coatings is the result of an improvement in their structure due to pore plugging, increased oxide concentration, increased adhesion between splats, etc., and thus reduction in the transportation system, where the high degree of deterioration and marine corrosion causes loss density of zones of electrolyte diffusion into the coating and the substrate. of material.

(a) (b)

FigureFigure 16.16. ElectrochemicalElectrochemical test.test. (aa)) Potentio-dynamicPotentio-dynamic polarizationpolarization curves;curves; ((bb)) BodeBode spectraspectra forfor Cu–AlCu–Al coatings:coatings: ImpedanceImpedance vs.vs. frequency andand phasephase angleangle vs.vs. frequency.frequency.

4. Conclusions Aluminum–bronze coatings on naval brass were produced by means of thermal projection by the flame technique, varying the pressure of the feed gases. In this study, the following results were achieved. In general, there is a tendency for a reduction in the porosity percentages with an increase of feed pressures and an increase in the Jcorr values, with an increase in the porosity of the coating. The coating deposited with oxygen at 359 kPa and acetylene at 97 kPa exhibited lower corrosion current, higher corrosion potential, and higher polarization resistance compared with the other depositions. By contrast, the results obtained show that the deposition carried out with pressures of 331 kPa oxygen and 69 kPa acetylene exhibited the lowest potential values, which automatically converts it into a less noble coating, with a low degree of protection against corrosion. The good resistance to corrosion of these coatings is associated with a thin passive oxide film; however, the anticorrosive properties are reduced with an increase in the porosity of the coatings. This could be explained by the pores in the coatings acting as “paths” or trajectories through which the corrosive electrolyte can flow to the splat of the coating and even reach the substrate itself, promoting active or anodic zones. In this way, the corrosion process begins with the dissolution of the elements of the coating and the substrate, and then there is a loss of adherence in the interface of the coating–substrate system and plugging of pores by some corrosion products. The thermal treatments of the B-6 coatings improved the resistance to corrosion. This was explained by the significant effect of the diffusion mechanisms on the microstructure; that is, a more compact structure, greater adherence between the coating and the substrate, better union between splats, and an increase in the oxide content were achieved. These coatings can be used for the protection and repair of naval, automotive, and aeronautical components and for coating structural components in order to improve their durability in applications subject to severe corrosion. For future investigations, the influence of the pressure on the mechanical and tribological properties could be studied, and these coatings could be applied under the conditions of the greatest pressure achieved in this research to some industrial components, in order to determine their performance for different industrial sectors.

Author Contributions: O.P. and J.O. conceived and designed the experiments; A.M. performed the experiments; and O.P., J.O. and A.M. wrote the paper. Funding: This research was funded by Departamento Administrativo de Ciencia, Tecnología e Innovación: Colciencias, convocatoria doctorados nacionales No. 727 de 2015. Coatings 2019, 9, 507 16 of 17

Acknowledgments: The authors wish to acknowledge the technological support provided by: Universidad Nacional de Colombia; Centro de Investigación de MaterialesAvanzados CIMA and Research Associate Kleberg Advanced Microscopy Center, University of Texas at San Antonio. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Castolin Eutectic. PROXON®21071: One Step Alloy Powder for the Cold Spray Process Rebuilding Worn Parts; Castolin Eutectic: Lausanne, Switzerland, 2011. 2. Morales, J.A.; Olaya, J.J. An Approach to Thermal Spray Technology; UNAL–COTECMAR: Cartagena, Colombia, 2009. 3. Dimaté, L.M. Resistencia a la Corrosión en Recubrimientos Comerciales METACERAM 25050 y Proxon 21071 Producidos con el Sistema de Proyección Térmica por Llama. Master’s Thesis, Universidad Nacional de Colombia, Bogotá, Colombia, 2011. Available online: http://www.bdigital.unal.edu.co/5222/ (accessed on 1 June 2019). 4. Fauchais, M.I.; Heberlein, P.L.; Boulos, J.V.R. Thermal Spray Fundamentals, from Powder to Part; Springer: New York, NY, USA, 2014; pp. 228–238. 5. Djebali, M.; Pateyron, R.; ElGanaoui, B. Scrutiny of plasma spraying complexities with case study on the optimized conditions toward coating process control. Case Stud. Therm. Eng. 2015, 6, 171–181. [CrossRef] 6. JETS and POUDRES. Available online: http://jets.poudres.free.fr/en/home.html (accessed on 15 July 2019). 7. Delluc, G.; Mariaux, G.; Vardelle, A.; Fauchais, P.; Pateyron, B. A Numerical Tool for Plasma Spraying Part I: Modelling of Plasma Jet and Particle Behaviour. In Proceedings of the 16th International Symposium on Plasma Chemistry, Taormina, Italy, 22–27 June 2003. 8. Ortiz, J.L.; Manzano, A.; Olvera, O.; Perez, M.A. Principios Básicos de Corrosión y Sus Prácticas de Laboratorio, 1st ed.; Limusa: Ciudad de Mexico, Mexico, 2009; pp. 21–40. 9. Cadavid, E.; Parra, C.; Vargas, F. Estudio de llamas oxiacetilénica usadas en la proyección térmica. Rev. Colomb. Mater. 2016, 9, 15–16. Available online: http://aprendeenlinea.udea.edu.co/revistas/index. php/materiales/article/view/326490/20783786 (accessed on 1 July 2019). 10. LI, W.S.; Yi, L.I.U.; Wang, Z.P.; Chao, M.; Wang, S.C. Effects of Ce in novel bronze and its plasma sprayed coating. Trans. Nonferrous Met. Soc. China 2012, 22, 2139–2145. [CrossRef] 11. Gonzales, A.G. Estudio de la Influencia de Las Propiedades Físicas y Mecánicas en el Comportamiento Tribológico de Recubrimientos Duros Para Herramientas de Corte y Procesamiento de Polietileno. Master’s Thesis, Universidad de Antioquia, Medellin, Columbia, 2008. Available online: http://tesis. udea.edu.co/handle/10495/48 (accessed on 1 June 2019). 12. Koivuluoto, H.; Honkanen, M.; Vuoristo, P. Cold-sprayed copper and tantalum coatings—Detailed FESEM and TEM analysis. Surf. Coat. Technol. 2010, 204, 2353–2361. [CrossRef] 13. Hincapie-campos, W.S.; Olaya, J.J.; Alfonso-Orjuela, J.E. Microstructural, mechanical and wear characterization of CuxAly coatings deposited via thermal projection. DYNA 2017, 84, 155–163. [CrossRef] 14. Richer, P.; Zúñiga, A.; Yandouzi, M.; Jodoin, B. CoNiCrAl Y microstructural changes induced during Cold Gas Dynamic Spraying. Surf. Coat. Technol. 2008, 203, 364–371. [CrossRef] 15. Kawaguchi, Y.; Miyazaki, F.; Yamasaki, M.; Yamagata, Y.; Kobayashi, N.; Muraoka, K. Coating Qualities Deposited Using Three Different Thermal Spray Technologies in Relation with Temperatures and Velocities of Spray Droplets. Coatings 2017, 7, 27. [CrossRef] 16. Richardson, I. Guide to Nickel Aluminium Bronze for Engineers. Copp. Dev. Assoc. 2016, 222, 8–9. Available online: https://www.copper.org/applications/marine/nickel_al_bronze/pub-222-nickel-al-bronze- guide-engineers.pdf (accessed on 1 June 2019). 17. Kear, G.; Barker, B.D.; Walsh, F.C. Electrochemical Corrosion of Unalloyed Copper in Chloride media—A Critical Review. Corros. Sci. 2004, 49, 109–135. [CrossRef] 18. Wharton, J.A.; Stokes, K.R. The Influence of Nickel-aluminium Bronze Microstructure and Crevice Solution on the Nitiation of Crevice Corrosion. Electrochim. Acta 2008, 53, 2463–2473. [CrossRef] 19. Kabasakalo˘glu,A.A.M.; Kiyak, T.; ¸Sendil,O. Electrochemical Behavior of Brass in 0.1 M NaCl. Appl. Surf. Sci. 2002, 193, 167–174. [CrossRef] Coatings 2019, 9, 507 17 of 17

20. Walsh, F.C.; De León, C.P.; Kerr, C.; Court, S.; Barker, B.D. Electrochemical characterisation of the porosity and corrosion resistance of electrochemically deposited metal coatings. Surf. Coat. Technol. 2008, 202, 5092–5102. [CrossRef] 21. Gavrila, M.; Millet, J.P.; Mazille, H.; Marchandise, D.; Cuntz, J.M. Corrosion behaviour of zinc-nickel coatings, electrodeposited on steel. Surf. Coat. Technol. 2000, 123, 164–172. [CrossRef] 22. Bendezú, R.P.; Gonçalves, R.P.; Neiva, A.C.; De Melo, H.G. EIS and microstructural characterization of artificial nitrate patina layers produced at room temperature on copper and bronze. J. Braz. Chem. Soc. 2007, 18, 54–64. [CrossRef] 23. Pombo Rodriguez, R.M.H.; Paredes, R.S.C.; Wido, S.H.; Calixto, A. Comparison of aluminum coatings deposited by flame spray and by electric arc spray. Surf. Coat. Technol. 2007, 202, 172–179. [CrossRef] 24. Sadawy, M.M.; Ghanem, M.; Zohdy, K.M. Corrosion and Electrochemical Behavior of Leaded-Bronze Alloys in 3.5 wt% NaCl Solution. Am. J. Chem. Appl. 2014, 1, 1–6. 25. Wharton, J.A.; Barik, R.C.; Kear, G.; Wood, R.J.K.; Stokes, K.R.; Walsh, F.C. The corrosion of nickel-aluminium bronze in seawater. Corros. Sci. 2005, 47, 3336–3367. [CrossRef] 26. Saud, S.N.; Hamzah, E.; Abubakar, T.; Bakhsheshi-Rad, H.R.; Farahany, S.; Abdolahi, A.; Taheri, M.M. Influence of Silver nanoparticles addition on the phase transformation, mechanical properties and corrosion behaviour of Cu–Al–Ni shape memory alloys. J. Alloys Compd. 2014, 612, 471–478. [CrossRef] 27. Dermaj, A.; Hajjaji, N.; Joiret, S.; Rahmouni, K.; Srhiri, A.; Takenouti, H.; Vivier, V. Electrochemical and spectroscopic evidences of corrosion inhibition of bronze by a triazole derivative. Electrochim. Acta 2007, 52, 4654–4662. [CrossRef] 28. Rahmouni, K.; Keddam, M.; Srhiri, A.; Takenouti, H. Corrosion of copper in 3% NaCl solution polluted by sulphide ions. Corros. Sci. 2005, 47, 3249–3266. [CrossRef] 29. Xu, C.; Du, L.; Yang, B.; Zhang, W. Study on salt spray corrosion of Ni-graphite abradable coating with 80Ni20Al and 96NiCr-4Al as bonding layers. Surf. Coat. Technol. 2011, 205, 4154–4161. [CrossRef] 30. Epelboin, I.; Keddam, M.; Takenouti, H. Use of impedance measurements for the determination of the instant rate of metal corrosion. J. Appl. Electrochem. 1972, 2, 71–79. [CrossRef] 31. Chen, Y.; Qi, D.M.; Wang, H.P.; Xu, Z.; Yi, C.X.; Zhang, Z. Corrosion behavior of aluminum bronze under thin electrolyte layers containing artificial seawater. Int. J. Electrochem. Sci. 2015, 10, 9056–9072. Available online: http://www.electrochemsci.org/papers/vol10/101109056.pdf (accessed on 1 June 2019). 32. Lister, T.E.; Wright, R.N.; Pinhero, P.J.; Swank, W.D. Corrosion of thermal spray hastelloy C-22 coatings in dilute HCl. J. Therm. Spray Technol. 2002, 11, 530–535. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).