Surface oxide formation on IN625 and plasma sprayed NiCrAlY after high density and low density supercritical water testing

Abstract

In this study, IN625 and a NiCrAlY coated IN625 were tested in high density and low density supercritical water for 500 h. The NiCrAlY coating was applied using plasma spray method. The surfaces were finely polished before testing to assist microstructure observation after testing. The NiCrAlY coated samples were additionally heat treated in air furnace to encourage alumina formation. SEM and XRD analyses were carried out to determine surface microstructure changes, particularly oxide formation. The results showed that when tested under high density supercritical water, the bare IN625 sample suffered from intergranular attacking while low density supercritical water did not have the same effect. Both as‐sprayed NiCrAlY and heat treated NiCrAlY did not show any signs of intergranular attack or pitting after being tested in high density and low density supercritical water.

Comparative study of materials' behaviour in supercritical water and superheated steam

“…The dark depression (Figure 44), however, shows signs of possible grain boundary cracking, which may be a concern fo r long term use. Intergranular cracking in IN625 after SCW exposure was also observed by Selvig, Huang, et al [37]. Was, Ampornrat et al [23] determined that in supercritical w ater the Cr in IN625 migrates preferentially to the surface, leading to depletion and cracking [23].…”

Corrosion Characteristics of Typical Ni–Cr Alloys and Ni–Cr–Mo Alloys in Supercritical Water: A Review

“…54 The surface of Incoloy 625 can be covered with a NiCrAlY coating and then heated by a plasma spray method. Selvig et al 55 reported that the NiCrAlY coating could effectively inhibit the intergranular corrosion and pitting corrosion of the sample in SCW. Daigo et al 56 used a reactor made of Ni-44Cr-1Mo to study the corrosion behavior of the Ni-based alloy SCW, and they found that its corrosion rate was lower than that in the reactor made of titanium alloy under the same conditions.…”

Effect of Water or Steam Pressure on the Oxidation Behaviour of Alloy 625 and A286 at 625 °c

“…Weight gain for Alloy 625 at a subcritical temperature of 360°C (higher water density) was higher than the weight gain at above supercritical temperature (lower water density). The oxide layer on Alloy 625 after being tested under subcritical condition was found to be very thin and the presence of pitting was also noticed on the alloy surface [17]. Kritzer at al.…”