Steam and air oxidation behaviour of thermal spray chromium carbide-based composite coatings

Thermal spray chromium carbide-based composites have been widely used industrially because of their superior oxidation resistance at elevated temperatures compared to other metal matrix composite coatings. In this work, the serviceability of HVOF sprayed coatings from an atomized chromium carbide-based feedstock (Cr23C6- 40NiCr) was compared with those from a conventional Cr3C2-25NiCr agglomerated and sintered feedstock. Coatings were exposed to 540 degrees C and 610 degrees C for 168 h in both air and steam environments. These conditions are indicative of industrial power plant working conditions to assess their real-time performance. Quantitative Rietveld analysis of the Cr23C6-40NiCr coating XRD patterns indicated a comparable concentration of Cr23C6 in both the powder and coating, implying minimal carbide dissolution. However, the Cr3C2 content in the conventional Cr3C2-25NiCr coating was markedly reduced compared to the initial powder composition, indicating that carbide dissolution occurred to a greater extent. Air oxidation led to the formation of coarse surface oxide, due to the initial formation of Ni-based oxides before a continuous Cr2O3 layer could develop underneath. A notably finer and more uniform oxidized surface composed only of Cr2O3 was formed during steam treatment across both coating types. More importantly, the oxide scale was intact and crack-free in Cr23C6-40NiCr compared to Cr3C2-25NiCr. Moreover, despite Cr23C6 being the main carbide phase and the high binder content (40%NiCr), the microhardness of the Cr23C6-40NiCr coating was comparable to that of the conventional Cr3C2- 25NiCr coating, both in the as-sprayed and heat exposed states.

Automated summary

Thermal spray chromium carbide-based composites are extensively used in industry due to their superior oxidation resistance at high temperatures. This study compared the performance of HVOF sprayed coatings from an atomized chromium carbide-based feedstock with those from a conventional agglomerated and sintered feedstock. The coatings were exposed to high temperatures in air and steam environments to simulate industrial power plant working conditions. The results showed that the chromium carbide content remained stable in the atomized feedstock, while carbide dissolution was observed in the conventional feedstock. Air oxidation resulted in the formation of a coarse oxide surface, while steam treatment formed a finer and more uniform Cr2O3 surface. The Cr23C6-40NiCr coating exhibited an intact and crack-free oxide scale, and its microhardness was comparable to that of the Cr3C2-25NiCr coating.