SEM images of the coating layer produced by thermal spray are depicted in Figures 2 and 3. Metallic coatings with compositions, including Co-Ni and Cr-Ni, generally exhibited distinct splats or lamellae as a result of the impact during the thermal spray process. There are some microcracks but the addition of alloying elements has significantly improved the features of the coatings. For instance, fewer cracks are present in the coating layers containing cobalt, and the overall coating microstructure is homogeneous with few closed porosities along lamella boundaries. This type of structure resulted from the deposition of successive layers. The lamellar structure is anisotropic, and a dependency of properties with respect at the directions parallel to the substrate and that transverse to the coating thickness is expected (Ref 16). The choice of the deposition process depends strongly on the expected coating properties for the application and the coating cost. Coating properties are determined by the materials, its form and the parameter settings of deposition process (Ref 17).
EDX results indicate the phase composition of substrate/coating interface (Figures 2 and 3) for all conditions. Line analysis shows the element content along the substrate and the coating. Some dispersions in the element content are noted in the coating. Moreover, the structure of sprayed coatings is heterogeneous due to the scattering of individual particle impact caused by local cooling and flow conditions during the deposition (Ref 10). The total defect level of the coatings was quantified using image analysis technique. Although the mean value obtained for Condition 1 (3.87%) is greater than Condition 2 (3.28%), the relative difference between the two values (0.59%) is not very significant. In addition, the dispersion of results is very similar, since the standard deviation differs by only 0.01% when comparing Conditions 1 and 2. This result is in agreement with other authors (Ref 14, 15) who found a range of 0.2% to 10.0% of porosity in electric process deposition. The chemical composition of wires does not affect the defect level. Generally, the porosity of thermal spray coatings is typically less than 5% of volume. However, this porosity affects the heat transfer of certain mechanical component (Ref 18) that is a very important aspect for this application (Ref 19).
Figures 4 and 5 show the X-ray diffraction with Rietveld calculations of the coatings, Condition 1 and 2, respectively. The blue line corresponds to the experimental data, the red one corresponds to the calculated spectrum and the gray line is the difference between them. The quantitative measurements have revealed the presence of alpha and gamma Fe-Cr alloy phases with the formation of a third one, chromite, FeCr2O4. The specific ICSD (Inorganic Crystals Structure Database) files used for the Rietveld calculations are ICSD102751 for the gamma Fe-Cr alloy, ICSD 102748 for the alpha Fe-Cr alloy and ICSD 171121 for chromite. The goodness of fitting for both calculations are 1.257 for Condition 1 and 1.678 for Condition 2, which indicate a good adjustment for the experimental data. The weight concentrations for the three phases are the same for Condition 1 and 2.
Figure 6 exhibits the aspect of alloy coating surfaces after the pull-off test. The adhesive strength of coatings appears slightly dependent on the chemical composition. The adhesive strength of FeCr and CoCr alloys ranged from 24.9 to 29.7 MPa for both types of coatings, with an overall average adhesion tensile strength of 27.2 MPa. This strength is considerably higher than the typical mean values reported by Antunes et al. (Ref 10). The prevalent mode of fracture of samples was adhesion failure, which is the fracture between the adhesive and the coating. Moreover, a high oxide content and microcracks can be observed in Figure 2 (Condition 1). These microcracks are coating defects and can generate problems as low adhesion and even low resistance to corrosion (Ref 12).
Low porosity produces compact coatings and good substrate-coating bonds. Indeed, a close examination of the coating/substrate interface of deposit layers shows neither gaps nor cracks, which are characteristic features of its good adhesion. The adhesion test metallic coating system presents good adherence of both interfaces (intermediate bond-deposit and intermediate bond-substrate).
The causes of adhesion failure are relevant to define the most sensitive areas and further improvements (Ref 20). The deposit features change highly due to the affecting factors that bring heterogeneity, as substrate nature and even the workmanship. Moreover, during service, the corrosion provoked by the environment and the wear can reduce the life of structures. Thus, an evaluation of corrosion is necessary to obtain a corrosion resistance in representative media where the parts can operate.
Samples were tested in salt spray chamber for 36 hours at 35 oC to evaluate their resistance to corrosion in presence of chloride medium. Without sealant, the plate region with cobalt deposit shows the best results, when compared with the nickel and chromium-based alloys. In relation of the machined tube samples, the results were similar to those found for the plates, i.e., better results with cobalt alloy. Figure 7 shows the surface of coatings without/with sealant after salt spray test. The samples with epoxy sealant show negligible corrosion. Hence, the lack of corrosion of epoxy-coated samples for the studied condition indicates that its presence assures a high resistance against corrosion on carbon steel. Even considering the negligible corrosion intensity, and below the quantification limit of the used method, the microscopic evaluation reveals that Condition 1 (with cobalt) exhibited the lowest corroded area. The exposure of samples in aggressive condition of salt spray chamber can be used as a screening test for brine environment corrosion performance for unsealed/sealed samples. Thus, some samples were additionally tested in salt spray chamber for 36 hours at 35 oC. After this exposition, all samples were gently cleaned with water and dried with hot air. The sealants infiltrated into the coating and the correspondent corrosion resistances are higher than those found in the unsealed coatings.
The adhesion of the coating to the substrate relies predominantly on the mechanical bonding, thus careful cleaning and pretreatment of the surface to be coated is extremely important. Sealing sprayed coatings serve primarily to ﬁll the pores and microcracks of the coating, which provides additional protection against corrosive media that would otherwise penetrate into the base material by cracks of the coating, reducing the corrosion resistance. Thus, sealing is a protective layer that closes pores near the surface but does not form an effective coating film (Ref 21) and the efficiency of each sealant could be considered by its barrier capability (i.e., its capability of blocking a corrosive liquid to penetrate towards the interface coating/substrate) (Ref 18, 21).
Electrochemical measurements are an efficient method to analyze corrosion resistance of coatings (Ref 22, 23), and also allow to evaluate the coating porosity (Ref 24). Figure 8 shows the evolution of the open circuit potential versus time. All samples exhibited a potential dropping at the beginning of immersion until reaching a stable plateau related to corrosion potential Ecorr. This behavior indicated that a steady-state of corrosion had been reached with the 2-hour exposition. The depletion of dissolved oxygen at metallic interface can reduce the potential. The sealed coatings always exhibit nobler corrosion potential than unsealed coatings, revealing lower corrosion. This feature was associated with the formation of barrier layer over the pores with metallic surface on the coating surface (Ref 25) that increases the corrosion resistance.
The polarization curves for unsealed and sealed coatings in 3.5% wt. sodium chloride electrolyte are shown in Figure 9. The current density-potential shows an active process, with higher corrosion potential for sealed specimens and lower current density. The uncoated samples The corrosion parameters calculated from Tafel extrapolation (i.e. corrosion potential, Ecorr; corrosion current density, Jcorr; cathodic, βc, and anodic, βa, Tafel slopes) are present in Table 3. The corrosion current density for the non-sealed samples are similar, regardless the difference in the composition. Tafel slopes is related to the electrochemical mechanism of cathodic and anodic processes, but as the alloys are complex, besides the morphology of surfaces, the clear interpretation of the values is a tricky task.
The porosity P of the coating can be estimated by the electrochemical parameter according to Equation 1, take into account that the current comes chiefly from the active surface not sealed by the epoxy. P is the connected porosity of the coating where the electrolyte reaches the metal, the corrosion current density of the unsealed surface, and is the corrosion current density of the sealed surface in the same electrolyte (Ref 18). Once the porosity represents the connected porosity of the coating and the effectiveness of sealing treatments is related to the percentage of the open porosity, the Equation 2 can be used to estimate the sealing efficiency, , (Ref 26, 27,28,29,30). The porosity of Conditions 1 and 2 are, respectively, 21 and 29%. These values are higher than those obtained by optical microscopy because the corrosion current can penetrate through tortuous paths not sensitive to detected by image analysis.
The calculated sealing efficiencies of coated samples are present in the Table 3. For the Condition 1, the sealant was more effective (78.6±4.7%) than from the Condition 2 (70.5±2.4%). As the corrosion current densities of unsealed coating are well similar, the efficiency relies on the lower current of epoxy sealed surfaces.
The EIS spectra measured at corrosion potential are displayed as Bode plots in Figure 10. Grosso modo, the sealant increases tenfold the impedance modulus for both conditions in the major part of frequency range (Figure 10a). The maximum angle is also higher in the sealed sample than unsealed ones. Although the precise relationship between the corrosion current density and electrochemical impedance is complex, generally higher impedance modulus is associated to higher corrosion resistance of the surface. EIS data are dependent on the surface area, then the unsealed samples have a higher active area that produces lower impedance modulus. In this case, a positive effect of epoxy is the reduction of active surface. Moreover, the real TS surface is difficult to be evaluated, thus we used a large sample area to reduce the possible influence on measurement and obtain a representative response from the surface. However, the phase reveals the presence of more than one electrochemical process. It is worth noting that unsealed samples exhibit at least two relaxation times, clearly shown in angle phase (Figure 10b), and likely also related to open porous. The low frequency loop around 20 mHz just one is observed for unsealed samples, and likely it is related to surface imperfections. Probably the sealant covers the coating regions whose electrochemical response occurs close to 20 mHz. The low frequency can be ascribed to processes at confined area such as pores or even a slow corrosive process, where the diffusion of species can play a role in the electrochemical process. This behavior is not related to surface area, but to other physical phenomena, such as diffusion and the localized corrosion processes. The important effect of epoxy is the blocking of the sites where these processes take place, increasing the overall corrosion resistance. The observed microcracks and porosities can be sites responsible for the low frequency loop. Due to the high capillarity the used resin, it enters in the confined region and blocks the access to the electrolyte, avoiding the corrosive attack.
Moreover, as cobalt is costly, its effect deserves to be further understanding in the microstructure and corrosion results. The cobalt increases the corrosion potential, for an unsealed and sealed sample, but the corrosion current density is not significantly modified, but Tafel slope. Similar results were observed in impedance spectroscopic data. Hence, it is not possible to ascribe to the cobalt a specific improvement of the corrosion resistance. In the obtained data, the main effect was caused by the epoxy sealant that clearly improve the corrosion resistance for the studied conditions of this work.