The corrosion behavior of M5 (Zr–1Nb-0.12O) alloy in 360 °C water with dissolved oxygen

Abstract

The corrosion behavior of M5 alloy in 360 °C/20 MPa water with 1.5 ppm dissolved oxygen (DO) was studied. The test was conducted by using a circulating autoclave loop system. The results indicated that DO accelerated the corrosion of M5 alloy in high temperature water. The corrosion behavior of β-Nb phase in the barrier layer and the post-transition layer in oxygenated water is completely different from that in deoxygenated water. For materials exposed to oxygenated water, β-Nb phases are sequentially oxidized to crystalline NbO (Nb2+) and crystalline NbO2 (Nb4+), then part of it is completely oxidized to Nb2O5 (Nb5+) in the barrier layer. In the post-transition layer, these crystalline oxidation products of β-Nb phase change to amorphous state, wherein Nb is totally oxidized to Nb5+ state. In the outer layer, the dissolution of β-Nb oxide is not completed when exposed to oxygenated water for even 100 days. Oxygenated water can accelerate corrosion of β-Nb SPPs, which could be responsible for high corrosion rate of Nb-containing Zr alloy in DO water.