**5.2 Microstructural analysis of high temperature coatings**

Fig. 7a, presents the general cross section microstructure of a NiCr coatings with embedded alumina particles produced by the hybrid spray process. The coating is very dense and exhibits the characteristics of an HVOF coating rather than of an arc sprayed coating. The hybrid spray process is unique in the sense that while it yields comparable density to that of the HVOF process, the deposition rate is closer to that of an arc spray process. The observed density is advantageous for high temperature corrosion and erosion performance of the coatings. Details on the corrosion and erosion performance of the coatings are discussed in the forthcoming sections. The dispersion of the alumina particles (dark phase) in the NiCr matrix is shown in Fig. 7b.

Fig. 7. (a) SEM picture of NiCr coating with dispersed Al2O3 particulates and (b) higher magnification SEM picture of NiCr coating with dispersed Al2O3 particulates.

Corrosion of Metal – Oxide Systems 285

undergo changes upon reheating. If CrO2, which has higher oxygen content compared to Cr2O3, forms during the spray process; it can undergo stoichiometric changes to a stable oxide (Cr2O3) upon reheating and this could lead to an initial weight loss in the coatings. However, part of the initial weight loss could also be attributed to the evaporation of moisture absorbed by porosity in the coatings. According to Lars Mikkelsen (2003), the specimens may also lose weight due to vaporization of chromium containing species from the chromia scale. Whereas the oxidation of pure Cr to Cr2O3 and also the transformation of CrO and Cr3O4 to

The weight gain for arc sprayed NiCr coating was the highest compared to all other coatings and this could be due to the inherent porosity in the arc spray coatings. The pores in the coatings enhance the oxidation rate. The weight gain in the hybrid NiCr coatings (without any particulate) was much lower than the arc sprayed coating because of their dense splat structure. NiCr + SiO2 showed the lowest weight gain. The weight gain by NiCr + Al2O3 was comparable to that of the plain hybrid NiCr coatings. A large weight gain by the NiCr + Cr2O3 could be due to the changes associated in the chromium oxide composition. It is to be noted that there is no need to add Cr2O3 particles into NiCr coating using a precursor. The role of chromium nitrate precursor here is to stabilize the -alumina phase. However, excess addition could lead to undesirable consequence as observed in the case of the NiCr + Cr2O3 sample. Determining the appropriate level of chromium nitrate is beyond the scope of this study. From these studies we conclude that addition of SiO2 has the most remarkable effect on the oxidation behavior of NiCr coatings. It has been demonstrated that the presence of SiO2 enhances the high temperature resistance of the chromia scale, which helps

to improve the oxidation and corrosion resistance of the coatings (Carter et al., 1995).

The setup utilized for evaluating the hot erosion behavior was shown in Fig. 6a. The weight of cylinders was measured before and after the hot erosion test. Also, the amount of grit used for each test was measured. The measured weight loss of each sample was based on

Cr2O3 will lead to weight gain because of increasing oxygen content in the coatings.

Fig. 9. Weight gain measured using TGA for oxidation studies.

**5.3.2 Hot erosion test** 

The atomization of the liquid precursor (for oxide particles) prior to the injection into the combustion jet plays an important role on the size as well as on the distribution of the particles in the final coating. The requirements for the atomization system include: controlled and uniform flow, ability to operate against a back pressure of 30 psi pressure that exists in HVOF flame at the point of injection and the ability to generate monodispersed micron sized droplets. Details on the atomization and optimization of parameters could be found elsewhere (Mohanty et al., 2010). From Fig. 7(b), it is apparent that the distribution of the particles was uniform across the cross section. Similar observations were made in the case of NiCr + Cr2O3 and NiCr + SiO2 systems also. It is to be noted that composites made from premixed powders commonly exhibit large clusters of nanoparticles. Fig. 8 presents the TEM picture of a NiCr coating with embedded silica particles. Many fine particles are observed in the matrix, as well as along the grain boundaries. For enhanced creep resistance resulting from grain boundary pinning, the particles must be small and coherent with the matrix. Especially alloys with very high chrome content can substantially benefit from such ultrafine particle embedment as observed in Fig. 8.

### **5.3 Characterization of high temperature coatings**

#### **5.3.1 Oxidation studies**

The oxidation characteristics of all the coatings (a), (b), (c) and (d) (refer to page 9) including the arc sprayed NiCr coating, were investigated by TGA studies in air after removing them from the substrate. The TGA curves shown in Fig. 9 indicate an overall weight gain for all the coatings while heating, although there was an initial weight loss for most samples.

The weight gain can be attributed to the oxidation of Cr in the NiCr matrix, as well as the changing oxidation state of the existing oxides. The later phenomenon can also lead to a weight loss in the initial stages because of the changing stoichiometry. Literature (Eschnauer et al., 2008; Hermansson et al., 1986; Richard et al., 1995; Schutz et al., 1991; Vippola et al., 2002) suggests that the oxidation of chromium during thermal spray processes could lead to nonstoichiometric compounds or metastable oxides (CrO2, CrO and Cr3O4) which can

The atomization of the liquid precursor (for oxide particles) prior to the injection into the combustion jet plays an important role on the size as well as on the distribution of the particles in the final coating. The requirements for the atomization system include: controlled and uniform flow, ability to operate against a back pressure of 30 psi pressure that exists in HVOF flame at the point of injection and the ability to generate monodispersed micron sized droplets. Details on the atomization and optimization of parameters could be found elsewhere (Mohanty et al., 2010). From Fig. 7(b), it is apparent that the distribution of the particles was uniform across the cross section. Similar observations were made in the case of NiCr + Cr2O3 and NiCr + SiO2 systems also. It is to be noted that composites made from premixed powders commonly exhibit large clusters of nanoparticles. Fig. 8 presents the TEM picture of a NiCr coating with embedded silica particles. Many fine particles are observed in the matrix, as well as along the grain boundaries. For enhanced creep resistance resulting from grain boundary pinning, the particles must be small and coherent with the matrix. Especially alloys with very high chrome content can substantially

benefit from such ultrafine particle embedment as observed in Fig. 8.

Fig. 8. TEM picture of NiCr coating with dispersed SiO2 particulates.

The oxidation characteristics of all the coatings (a), (b), (c) and (d) (refer to page 9) including the arc sprayed NiCr coating, were investigated by TGA studies in air after removing them from the substrate. The TGA curves shown in Fig. 9 indicate an overall weight gain for all the coatings while heating, although there was an initial weight loss for most samples.

The weight gain can be attributed to the oxidation of Cr in the NiCr matrix, as well as the changing oxidation state of the existing oxides. The later phenomenon can also lead to a weight loss in the initial stages because of the changing stoichiometry. Literature (Eschnauer et al., 2008; Hermansson et al., 1986; Richard et al., 1995; Schutz et al., 1991; Vippola et al., 2002) suggests that the oxidation of chromium during thermal spray processes could lead to nonstoichiometric compounds or metastable oxides (CrO2, CrO and Cr3O4) which can

**5.3 Characterization of high temperature coatings** 

**5.3.1 Oxidation studies** 

undergo changes upon reheating. If CrO2, which has higher oxygen content compared to Cr2O3, forms during the spray process; it can undergo stoichiometric changes to a stable oxide (Cr2O3) upon reheating and this could lead to an initial weight loss in the coatings. However, part of the initial weight loss could also be attributed to the evaporation of moisture absorbed by porosity in the coatings. According to Lars Mikkelsen (2003), the specimens may also lose weight due to vaporization of chromium containing species from the chromia scale. Whereas the oxidation of pure Cr to Cr2O3 and also the transformation of CrO and Cr3O4 to Cr2O3 will lead to weight gain because of increasing oxygen content in the coatings.

Fig. 9. Weight gain measured using TGA for oxidation studies.

The weight gain for arc sprayed NiCr coating was the highest compared to all other coatings and this could be due to the inherent porosity in the arc spray coatings. The pores in the coatings enhance the oxidation rate. The weight gain in the hybrid NiCr coatings (without any particulate) was much lower than the arc sprayed coating because of their dense splat structure. NiCr + SiO2 showed the lowest weight gain. The weight gain by NiCr + Al2O3 was comparable to that of the plain hybrid NiCr coatings. A large weight gain by the NiCr + Cr2O3 could be due to the changes associated in the chromium oxide composition. It is to be noted that there is no need to add Cr2O3 particles into NiCr coating using a precursor. The role of chromium nitrate precursor here is to stabilize the -alumina phase. However, excess addition could lead to undesirable consequence as observed in the case of the NiCr + Cr2O3 sample. Determining the appropriate level of chromium nitrate is beyond the scope of this study. From these studies we conclude that addition of SiO2 has the most remarkable effect on the oxidation behavior of NiCr coatings. It has been demonstrated that the presence of SiO2 enhances the high temperature resistance of the chromia scale, which helps to improve the oxidation and corrosion resistance of the coatings (Carter et al., 1995).

#### **5.3.2 Hot erosion test**

The setup utilized for evaluating the hot erosion behavior was shown in Fig. 6a. The weight of cylinders was measured before and after the hot erosion test. Also, the amount of grit used for each test was measured. The measured weight loss of each sample was based on

Corrosion of Metal – Oxide Systems 287

the difference was not that significant. After 24 hours of immersion, the current values significantly differed between the coatings. The arc spray coating measured two times greater current, Io, after 24 hours. Io is a measure of the corrosion resistance of a material and higher current values indicate lower corrosion resistance. These results confirm that the hybrid coating being denser than the arc spray coating restricts the migration of the corrosive solution/ions to the substrate interface and, therefore, provides more protection to the substrate. Although aqueous corrosion is not an issue for these high temperature coatings, this test has some significance in terms of molten deposit (sulfates) migration

The hot corrosion test results are shown in Fig. 12. This chart compares the weight loss data obtained on weld overlay coating (with and without salt), 304 stainless steel (304 SS) sample and the coatings – NiCr by arc spray, NiCr and NiCr + SiO2 by hybrid gun . The chromia stabilized alumina embedded coatings were not included in the test due to their unfavorable oxidation results presented in Fig. 9. NiCr + SiO2 coatings showed the lowest weight loss compared to all the other samples. Plain NiCr coating by hybrid spray also exhibited lesser weight loss compared to the arc spray coating and this could be attributed to the improved density of the hybrid spray coatings. The superior corrosion resistance of the NiCr + SiO2 coating is possibly due to the enhanced stability of the chromia scale and the improved oxidation resistance caused by SiO2. Weld overlay coating showed least weight loss in the absence of the salt; however, when salt was present, it showed poor corrosion resistance

The case study demonstrates that even the base NiCr hybrid spray coatings outperformed the alloy 625 coatings in the presence of corrosive salts. The presence of second phase particles, especially SiO2, showed improved oxidation and corrosion characteristics. Incorporation of ultrafine and nano sized oxide particles is expected to improve the creep properties by pinning the splat boundaries and reduce the oxidation rate. Chromia addition by itself did not help improve the properties significantly. However, it could act as a

through the coating in a coal fired boiler environment.

**5.3.4 Hot corrosion test** 

compared to the hybrid spray coatings.

Fig. 12. Weight loss measured in hot corrosion test.

200 gm of grit being used. Samples tested included arc sprayed coatings, plain hybrid coatings, and hybrid coatings with alumina, chromia and silica, respectively. The results of the tests, shown in Fig. 10, indicate that the hybrid coatings are up to 30% more resistant to erosion than the arc sprayed coatings at 7500C and this is thought to be due to the higher density of the hybrid coatings. However, the weight loss was slightly higher in the case of oxide particulate embedded coatings. This is contrary to the observation of Jiang Xu et al. (Xu et al., 2008), who have reported improved erosion resistance with the addition of nanoparticles in Ni based alloys. Especially, in the case of chromia embedment, the difference was evident. This may be linked to the large bubble shaped features with internal voids that were observed in chromia particles (which are not shown here).

Fig. 10. Weight loss measured in hot erosion test.

#### **5.3.3 Wet corrosion test**

The corrosion currents measured from the electrochemical tests are shown in Fig. 11. At zero hours, although the hybrid coating showed less current compared to the arc sprayed coating;

Fig. 11. Wet corrosion of thermally sprayed NiCr coatings in NaCl solution at room temperature.

200 gm of grit being used. Samples tested included arc sprayed coatings, plain hybrid coatings, and hybrid coatings with alumina, chromia and silica, respectively. The results of the tests, shown in Fig. 10, indicate that the hybrid coatings are up to 30% more resistant to erosion than the arc sprayed coatings at 7500C and this is thought to be due to the higher density of the hybrid coatings. However, the weight loss was slightly higher in the case of oxide particulate embedded coatings. This is contrary to the observation of Jiang Xu et al. (Xu et al., 2008), who have reported improved erosion resistance with the addition of nanoparticles in Ni based alloys. Especially, in the case of chromia embedment, the difference was evident. This may be linked to the large bubble shaped features with internal

The corrosion currents measured from the electrochemical tests are shown in Fig. 11. At zero hours, although the hybrid coating showed less current compared to the arc sprayed coating;

Fig. 11. Wet corrosion of thermally sprayed NiCr coatings in NaCl solution at room temperature.

voids that were observed in chromia particles (which are not shown here).

Fig. 10. Weight loss measured in hot erosion test.

**5.3.3 Wet corrosion test** 

the difference was not that significant. After 24 hours of immersion, the current values significantly differed between the coatings. The arc spray coating measured two times greater current, Io, after 24 hours. Io is a measure of the corrosion resistance of a material and higher current values indicate lower corrosion resistance. These results confirm that the hybrid coating being denser than the arc spray coating restricts the migration of the corrosive solution/ions to the substrate interface and, therefore, provides more protection to the substrate. Although aqueous corrosion is not an issue for these high temperature coatings, this test has some significance in terms of molten deposit (sulfates) migration through the coating in a coal fired boiler environment.
