PSI - Issue 14

12

Author name / Structural Integrity Procedia 00 (2018) 000–000

S.M. Muthu et al. / Procedia Structural Integrity 14 (2019) 290–303

301

Fig10. X-ray mapping analysis of HVOF sprayed Ni-20Cr coated super alloy Inconel 825 after subjected hot corrosion in Na 2 SO 4 -60%V 2 O 5 in 900°C after 50 cycles.

4. Discussion Hot corrosion study was carried out on uncoated and Ni-20%Cr coated superalloy 825 in Na 2 SO 4 -60% V 2 O 5 molten salt environment at 900°C for 50 cycles. During the hot corrosion study, the bare and coated specimens got corroded and oxide layers formed on the specimen surface. Subsequently oxide spallation occurs which is clearly observed from the macro images. In bare specimen, it could be observed that aggressive hot corrosion took place and there is a steady weight gain up-to the 24 th cycle. In the following cycles there was drastic weight loss. This could be attributed to the severe spallation and sputtering of the oxide scale. During cooling phase of the 25 th cycle, it was observed that the oxide scale sputtered outside the ceramic boat holding the specimen; this is believed to be due to the excessive thermal stresses developed in the oxide scale. The difference in coefficient of thermal expansion of oxide scale and the substrate is expected to induce cracks in the oxide scales by stress generation Chatha et al. (2012). The sputtered oxides during the cooling cycle could not be retained in the boat during the weight measurements and hence the drastic drop in the weight in thermogravimetric chart between the 25 th and 26 th cycle. In the following cycles, it could be observed that there is weight gain suggesting that the corrosion reaction resumed. The thermogravimetric chart gives a good indication of sputtering away of the scale during the study. The HVOF sprayed Ni-20%Cr coated superalloy samples show a lower weight gain compared to the uncoated specimens; there was no spallation of the oxides nor sputtering. This shows that a strong adherent protective oxide layer was formed on the HVOF coated samples and explains the high resistance to hot corrosion of the coated sample in the Na 2 SO 4 -60% V 2 O 5 environment at 900 ◦C. The thermogravimetric analysis shows the corrosion kinetics of the bare and coated specimens. The overall weight gain at the end of 24 cycles of the uncoated sample was found to be 7.197 mg/cm 2 , whereas it was found to be 3.134 mg/cm 2 for the HVOF coated sample at the end of 50 th cycle. The corrosion rate of the uncoated and coated superalloy value is found to be 14.388 ×10 -9 g 2 cm -4 s -1 (24 cycles) and 2.727×10 -9 g 2 cm -4 s -1 (50 cycles). The various corrosion products formed on the uncoated and HVOF coated samples were analysed with the SEM/EDS in combination with the XRD results. In the XRD plots reaction products such as NiO, Cr 2 O 3, FeCr 2 O 4 , NiCr 2 O 4 and Fe 2 O 3 were identified on hot corroded uncoated superalloy. In contrast, in the HVOF coated samples NiO, Cr 2 O 3 and NiCr 2 O 4 were observed. From the cross sectional SEM/EDS point analysis of the uncoated sample in Fig. 7 it could be observed that the Fe content is more at the surface. The high weight % Fe present in the