The steam oxidation resistance of 80%Ni-20%Cr metallic coatings has been evaluated at four different steam temperatures in the range of 600-750°C. Substrate used for the study was 9Cr-1Mo type steel. The thermal spray coatings were carried out using two different processes, viz., atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF) spray. Thickness of the coatings was about 40 and 60 mm respectively. The results show that the thick and dense HVOF coating showed a better steam oxidation resistance than the thin porous APS coatings. At prolonged aging (1 000 h) the HVOF coating showed the best protection till the temperature range of 650°C. Beyond this temperature the presence of Fe 2 O 3 was noticed at the coating surface. The reason for the protectiveness and failure at higher temperatures (above 650°C) are discussed in detail.KEY WORDS: steam oxidation; Cr-Mo steel; Ni-Cr coating; high velocity oxy fuel spray and atmospheric plasma spray.The effectiveness of a protective oxide scale depends on several factors. Ideally, it must be free of stoichiometric imperfection, porosity, crack, stress and be resistant to exfoliation and stable in service. Chromium oxide is one of such oxides that can exhibit such properties. To form a complete chromium oxide outer layer in Fe-Cr system, it requires around 12 % Cr in the air and oxygen environments. 1)However, in the steam environment, the amount of Cr required is around 25 %, to form a complete protective chromium oxide layer.7) Adding large amount Cr into the material will deteriorate the mechanical properties of the material. Thus, a way to incorporate a large amount of alloying elements to form a protective oxide is applying them as a coating either by diffusion or by deposition retaining the bulk mechanical properties of the component. Chromizing is widely used for steels to improve their steam oxidation resistance. 8,9) However, there are limitations to apply this technique for the large component and onsite modification. Thermal spray is an alternative technique in which deposition can be made at thicker level and flexibility to deal with onsite modifications.A few earlier attempts were made on the thermal spray coating for fossil power plants though the thermal spray process is extensively used for gas turbine applications. 10,11) A work from Aguero et al.,12) who attempted thermal spray coating FeAl, FeCrAl and NiAl powders on 9Cr-1Mo type steel to improve the steam oxidation resistance for USC boiler applications. In that FeCrAl and NiAl showed the promising results. Also, the thermal spray was attempted for the super heater/re-heater components where the material severely suffers on fireside corrosion. The powder used for the study is Fe-Si intermetallics with the undercoat of NiCr alloy. 13) In the present study, Ni-Cr based alloy powders have been coated on modified 9Cr-1Mo steel and the effect of coatings on steam oxidation were evaluated by X-ray diffraction (XRD), scanning electron microscope (SEM), energy despersive spectroscopy (EDS) and electr...

The present work is focused on the steam oxidation resistance of 50%Ni-50%Cr metallic coatings produced using atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF) spray processes on 9Cr-1Mo type steel substrate. Thickness of the coatings obtained in the HVOF and APS processes were about 60 and 40 mm respectively. The steam oxidation resistance of the coatings was evaluated at four different temperatures viz., 600, 650, 700 and 750°C. The results showed that the thick and denser HVOF coating yielded a better steam oxidation resistance than the thin and porous APS coatings. At prolonged aging (1 000 h), the HVOF coating showed the best protection in all tested steam temperatures. APS coating performed satisfactorily well till the 100 h of test duration. But, it started the scale initiation at the coating interface and incorporation of scales occurred at 1 000 h of steam oxidation test. The reason for the protectiveness by HVOF coating and failure of APS coatings at prolonged aging are discussed in detail.

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Steam oxidation resistance of 80Ni-20Cr and 50Ni-50Cr coatings has been evaluated in our previous study. The coatings were HVOF sprayed onto the modified 9Cr-1Mo steel substrate and the Cr content in the coating played an important role on the steam oxidation resistance. In the present study, effects of Mn and Si present in the powder and hence incorporated into the coatings were studied. In the 80Ni-20Cr coating, Mn segregated to the coating surface during steam oxidation duration of 1 000 h. Si also enriched at the surface of the coating. On the other hand, the 50Ni-50Cr coatings showed the absence of either Mn segregation or Si enrichment in the post-steam oxidized specimen. The results are discussed in conjunction with the diffusion of Mn and Si onto the chromium oxide layer at high temperatures and the influence of Cr content on the diffusion characteristics of these minor elements.KEY WORDS: steam oxidation; Cr-Mo steel; Ni-Cr coating; high velocity oxy fuel (HVOF) spray; manganese; silicon; diffusion. Experimental MethodModified 9Cr-1Mo type ferritic steel specified as ASME T91 was used as a substrate specimen with the following dimensions: 10 mm (length)ϫ10 mm (width)ϫ4 mm (thick) specimens coated with two Ni-Cr powders namely, 80Ni-20Cr and 50Ni-50Cr (Manufacturer: Praxair). The compositions of the alloy substrate and the powders are represented in Table 1. High velocity oxy-fuel (HVOF) spray process was adopted in the present study to coat these Ni-Cr powders on the sandblasted steel substrate. Totally 6 passes were adopted including the 2 pre-heating passes for pre-heating the substrate. Thickness of the coating obtained for both types were around 60 mm. The parameters used for HVOF are given elsewhere.9) The coated specimens were introduced to XRD and SEM investigations before placing them into the steam oxidation chambers. Steam oxidation tests were carried out in different temperatures viz., 600, 650, 700 and 750°C. The details of the test procedure are described elsewhere.6) Though the expected steam operating temperature in the power plants is at 650°C, importance is given to the results obtained from the specimens steam oxidized at 750°C in this paper. This is because, the 750°C is considered as the most accelerated condition among the four tested temperatures. This will help to simulate the material's behaviour in the longer duration with shorter testing time. The surfaces of oxidized specimens were analyzed by XRD (Cu K a ) to identify the phases formed during steam oxidation as compared with as-coated condition. EPMA elemental mapping was carried out on the cross sections of the all the steam oxidized conditions. Figure 1 shows the XRD pattern for 80Ni-20Cr coating on 9Cr-1Mo steel and their steam oxidized conditions at 750°C for different durations. The spectrum for as-coated specimen exhibited three major peaks responsible for fcc (Ni-Cr) (111), (200) and (220) indicating that the coating exists in single fcc phase. The 10 h steam oxidized specimen showed a few extra peaks compared to as-co...

The effect of alloying elements on the steam oxidation behavior of carbon free Fe 12Ni 9Co base martensitic alloys has been investigated in steam at temperatures between 873 and 1023 K for up to 3.6 Ms. The alloys tested are twelve new alloys developed for ultra super critical power plants and three commercial 9 12Cr ferritic steels. Mo and W affected steam oxidation resistance of Carbon Free Martensitic Alloys, and Fe 12Ni 9Co 5Mo alloy showed the best steam oxidation resistance among three alloys (Fe 12Ni 9Co 5W, Fe 12Ni 9Co 10W and Fe 12Ni 9Co 5Mo). The steam oxidation resistance is improved by an increase of Ni content at temperatures higher than 973 K, while remarkable effect is not observed by varying Si, Cr and Pd content. The mass gain due to steam oxidation of alloy with 18Ni at 973 K is approximately the same as that of three commercial 9 12Cr ferritic steels at 873 K. An austenitic layer with lower Fe and higher Ni than the base metal forms at the interface between Fe 3 O 4 oxide scale and the base metal in the carbon free martensitic alloys, which is responsible for the improvement of steam oxidation resistance in these alloys.

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