The effect of precipitated phases on the pitting corrosion of a Z3CN20.09M cast duplex stainless steel (CDSS) which has been widely used in primary coolant pipes of nuclear power plants was investigated by using isothermal aging treatment and potentiodynamic anodic polarization methods. It was found that M 23 C 6 carbide and · phase precipitated at the ferrite/austenite boundaries and in ferrite of the aged steel. The pitting potentials of the specimens aged at 700°C decreased with increasing of the precipitates content. The experimental results indicated that even a few percent of precipitates (about 1.0 vol%) in this steel could also worsen its pitting resistance. The pitting of the aged Z3CN20.09M specimens developed at the ferrite/austenite phase boundaries where precipitates formed often. These effects could be directly attributed to the presence of secondary austenite at the ferrite/austenite phase boundaries, which was found to be poor in Cr by energy dispersive X-ray (EDX).

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 nonuniform 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 21 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.