Effect of ferrite on pitting corrosion of Fe20Cr9Ni cast austenite stainless steel for nuclear power plant pipe

The ferrite content and grain size in a Fe20Cr9Ni (wt-%) cast austenite stainless steel which has been widely used in primary coolant pipes of nuclear power plants were found to vary due to non-uniform solidification of the large pipe with a thickness of 95 mm. The pitting corrosion of Fe20Cr9Ni cast austenite stainless steel with five ferrite contents, i.e. 13, 11, 10, 9·5 and 8 vol.-% and different ferrite grain sizes was investigated in a 0·5 mol L⁻¹ NaCl solution by means of potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). Both pitting potential and charge transfer resistance value of specimens increased with the decreasing ferrite content and grain size, indicating that few and small ferrite grains were beneficial for improving pitting resistance. The passive film properties studied by EIS correlated well with the polarisation results. Electron probe microanalysis results showed that the difference in the Cr concentration between ferrite and austenite decreased with a decreasing amount of ferrite phase. Accordingly, the composition microsegregation was reduced, and passive film homogeneity of the specimens was improved.