Severe Hot Corrosion of the Superalloy IN718 in Mixed Salts of Na2SO4 and V2O5 at 700 °C

This study presents hot corrosion behavior of the superalloy IN718 in 100 wt.% NaCl (salt S) and in salt mixtures of 60 wt.% Na2SO4 + 40 wt.% V2O5 (SM1) and 75 wt.% Na2SO4 + 15 wt.% NaCl + 10 wt.% V2O5 (SM2), deposited separately by spray gun technique, at elevated temperature of 700 °C. The weight gain per unit area at 700 °C was increased by 49% for salt S, 153% for the dual salt mixture SM1, and only 8% for the triple salt mixture SM2, in comparison with that observed earlier at 600 °C. The marked increase in the severity of corrosion at 700 °C is attributed to formation of the highly damaging compound NaVO3 that significantly enhances the oxygen activity, in the SM1 coated samples. The effect of surface roughness and dislocation density on corrosion behavior at 700 °C, however, is found to be similar to that at 600 °C. Ultrasonic shot peening, a novel technique of surface modification, is found to enhance the hot corrosion resistance in the salt mixture SM1 at 700 °C appreciably due to extensive grain refinement to nanoscale and formation of highly protective Cr2O3 layer in the surface region.