Abstract
Boride coating applied on steam turbine parts of power plants has provided good particle erosion resistance under temperature of 550 o C, but it isn't able to protect the parts effectively any more in ultra super critical (USC) steam turbine which is being operated up to temperature of 650 o C. To ensure stable durability for USC steam turbine parts, an alternative coating replacing boride coating should be developed. In this study, multi-component boride coatings containing elements such as chromium (Cr) and vanadium (V) were formed on base metal (B50A365B) using thermochemical treatment method called by pack cementation. The thermochemical treatments involve consecutive diffusion of boron(B) and Cr or/and V using pack powders containing diffusion element sources, activators and diluents. The top layer of Cr-boride coating is primarily consisted of Cr2B3 and Cr5B3, while that of V-boride coating is mostly consisted of VB2 and V2B3. The (Cr,V)boride coating is consisted of Cr2B3, Cr5B3 and V2B3 mostly. The top surfaces of 3 multi-component boride coatings show hardness of 3200-3400 Hv, which is much higher than that of boride, about 1600-2000 Hv. In 5 wt.% NaCl solution immersion tests, the multi-component boride coatings show much better corrosion resistance than boride coating.
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