Steam Oxidation Resistance of Ni-Cr Thermal Spray Coatings on 9Cr-1Mo Steel. Part 1: 80Ni-20Cr.

Abstract

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...

Steam Oxidation Studies on 50Ni-50Cr HVOF Coatings on 9Cr-1Mo Steel: Change in Structure and Morphology across the Coating/Substrate Interface

“…Apart from the chromium carbide formation, the coating did not undergo any other drastic changes such as diffusion of the coating elements to the substrate. In our earlier studies, 4) the 80Ni-20Cr coating showed significant diffusion of Ni from the coating to the steel substrate. In the present system the absence of such diffusion may be attributed to the formation of chromium carbide at the interface, which might have retarded the diffusion.…”

“…Steam oxidation tests were carried out at different temperatures viz., 873, 923, 973 and 1023 K. The procedure followed for the steam oxidation process was described elsewhere. 4) The sample pieces were taken out after different durations namely 36, 360 and 3600 kilo seconds (ks) of steam oxidation test. The surfaces of oxidized specimens were analyzed by XRD to identify the new phases formed during steam oxidation in comparison with the as-coated conditions.…”

“…Our group has undertaken a project to explore the suitability of thermal spray processes for this application and the results have been documented elsewhere. [4][5][6][7][8] HVOF coating of 50Ni-50Cr showed the excellent steam oxidation resistance in the tested duration of 1000 hours.…”

Behaviour of Mn and Si in the Spray Powders during Steam Oxidation of Ni-Cr Thermal Spray Coatings

“…[6][7][8] One such attempt was made in our earlier studies by thermal spray coating of Ni-Cr alloys. 9,10) The 50Ni-50Cr coatings showed an excellent protection against the steam oxidation till the tested duration of 1 000 h. The excellent protection of the coating against steam oxidation was attributed to the formation of chromium oxide film at the coating surface.…”

“…12) Thermal spray powders of Ni-Cr used in our studies 9,10) contain minor amounts of Mn and Si. It was reported that Mn diffusion is relatively rapid across the chromium oxide layer in the bulk Fe-Cr system.…”