Corrosion Behavior of Alloy625 in Sub / Supercritical Water-Phosphate Environment

Abstract

Corrosion issues limit the development of sub / supercritical water oxidation technology.In particular， phosphates will form lowmelting-point mixed molten salts in a supercritical water environment，creating a more severe corrosion environment，and how to solve the problems of corrosion and salt deposition has become the focus of current research.In this work， the corrosion behavior of Alloy625 in subcritical/supercritical water-phosphate environment was studied by weight loss method， X-ray diffraction (XRD)， energy dispersive spectrum (EDS)and scanning electron microscopy (SEM)， and the evolution characteristics of surface morphology and corrosion products was characterized in detail.Results showed that in addition to metal phosphates， the corrosion products were mainly Cr2O3， NiO， FeO and NiCr2O4 under subcritical conditions.In contrast， the inner layer of the corrosion product film under supercritical conditions was composed of dense flaky Ni2 O3，Cr2O3 and NiCr2O4 spinel， and the outer layer was composed of loose needle-shaped Fe2O3 and FePO4.Moreover， phosphates could cause severe peeling of the oxide film， thereby exacerbating corrosion.The corrosion weight loss law under subcritical conditions at 350 ℃followed a parabolic curve.In the 450 ℃supercritical condition， the corrosion weight loss was influenced by the evolution process of the corrosion product film (generation-detachment-regeneration)， and the change in corrosion quality was much higher than that under subcritical conditions.

Keywords

• sub / supercritical water； molten salts of phosphate； corrosion behavior； alloy625