EFFECT OF Nb ON THE CORROSION RESISTANCE OF Zr-4 ALLOY IN SUPERHEATED STEAM AT 500 °C

The corrosion resistance of Zr-4+xNb alloys (x=0.1-0.3, mass fraction, %) was investigated in a superheated steam at 500 degrees C and 10.3 MPa by autoclave tests. The microstructure of the alloys and fracture surface morphology of the oxide film formed on the alloys were observed by TEM and SEM, respectively. Results show that no nodular corrosion appears on Zr-4+xNb alloys in 500 degrees C /10.3 MPa superheated steam even for 500 h, which is related to the higher Nb concentration dissolved in alpha-Zr matrix. The Nb dissolved in alpha-Zr matrix can restrain the nucleation of nodular corrosion, thus improve the nodular corrosion resistance. The uniform corrosion resistance of Zr-4+xNb alloys is lowered with the increase of Nb content, which is related to the decrease of the solid solution concentration of (Fe+Cr) in alpha-Zr matrix and the precipitation of the second phase particles of Zr(Fe, Cr, Nb)(2), and the two aspects will accelerate the microstructural evolution of the oxide film during corrosion process to promote the formation of pores and micro-cracks.