A comparative investigation on molten salt hot corrosion of GTA and SMA-welded super-heater pipes of 70 MW CFBC boilers

Abstract The tubular SA 335 P91 joints are generally used as super-heater pipes in CFBC boilers. This research work presents hot corrosion studies on similar SA 335 P91 joints using ErNiCrMo-3 and E9018 B9 fillers via GTAW and SMAW. The HTC studies were conducted at 770 ± 1˚C employing Na 2 SO 4 +60% V 2 O 5 molten salt mixture on parent metal (PM) and GTA and SMA-weldments with 50 cycles. To predict the corrosion kinetics, TGA were generated. The layer of oxide formed on the surface of weldments and PM has been methodically described using SEM/EDS. The phases of oxide layers were examined through XRD analysis. The investigation confirms that the Ni-rich filler fusion zone coupon exhibits superior oxidation resistance due to the formation of spinel oxide (NiFe 2 O 4 ), which acts as a protective barrier against the penetration of corrosive species. The cross-sectional SEM image and XRD results confirmed that the thin scale formed on the oxidized weldments which were well adherent with the substrate. Blistering was experienced at the E9018 B9 weld zone and found to be more vulnerable to degradation. Consequently, ERNiCrMo-3 filler is recommended for joining similar joints of P91 metals employing the GTAW process used in CFBC boilers.