Corrosion Behavior of Alloy 625 in Subcritical- and Supercritical Water Containing 0.1 M HCl

摘要

Corrosion tests of alloy 625 in subcritical- and supercritical water containing 0.1 M HCl were performed in the range of temperature of 350 deg C - 450 deg C at 25 MPa. The corrosion rate of alloy 625 was faster in subcritical water than in supercritical water. 0.1 M of hydrogen peroxide seemed critical concentration of oxidizer that accelerated the corrosion rate of alloy 625. Below 0.1 M of hydrogen peroxide, corrosion rate was not influenced, however, over 0.1 M, corrosion rate was increased rapidly with increasing the hydrogen peroxide. Alloy 625 had two corroded layers. Intermediate layer was the porous structure layer in which Ni and/or Fe were dissolved out selectively. The outer layer was a powder-like porous oxide layer mainly composed with chromium oxide. It was found that the main corrosion mechanisms of alloy 625 were selective dissolution of nickel and iron under the subcritical water and selective dissolution of nickel under the highly oxidized supercritical water condition.