Fire side erosion-corrosion protection of boiler tubes by nanostructured coatings

Abstract

This article presents a systematic evaluation of erosion-corrosion (E-C) behavior of nanostructured Ni-20Cr coatings on two boiler steels. A nanostructured Ni-20Cr powder was synthesized using ball milling technique by the investigators. The powder was deposited on SAE-213-T22 and SA 516-Grade 70 steels by high-velocity oxy-fuel (HVOF) spraying under set parameters. The uncoated and coated samples were exposed to super heating zone of a power plant boiler at 750°C under cyclic conditions for 15 cycles. Each cycle consisted of 100 h of heating followed by 1-h of cooling. Attempt was made to study the kinetics of E-C using weight change and thickness loss data of the samples. Several characterization techniques such as XRD, SEM/EDS, and X-ray mappings analyses were used to characterize the eroded-corroded samples. The coating was found to reduce the E-C rate of T22 steel and SA 516 steel by 85 and 84%, respectively, in terms of thickness loss. Moreover, the investigated nanostructured Ni-20Cr coating was found to be more effective to reduce the E-C rates of the steels in comparison with its conventional (Ni-20Cr) micron-size counterpart. E-C resistance shown by the investigated coating under actual boiler conditions may be attributed to the presence of protective NiO and Cr2O3 phases in its oxide scale and its superior as-sprayed microhardness.