The effect of dissolved oxygen on stress corrosion cracking of 310SS in SCW

Austenitic stainless steels have been widely used as the major structural materials due to excellent combination of mechanical properties and corrosion resistance in high temperature. 310 SS has been regarded as a most promising material of fuel cladding for SCWR. It is well known that SCC of austenitic stainless steels is influenced by water chemistry, such as oxygen. In order to obtain a deep understanding of the SCC behavior of 310 SS in SCW, more research is urgently needed. The effects of dissolved oxygen content on the tensile properties and stress corrosion cracking (SCC) susceptibility of austenitic stainless steel 310 SS were studied by performing Slow Strain Rate Tensile (SSRT) tests. The SSRT tests were carried out in supercritical water at temperature of 620℃, a pressure of 25MPa, and a strain rate of 7.5×10- 7 s-1. The dissolved oxygen content was included 0, 500, 1000, 2000, and 8000 ppb. The SSRT tests were performed in a supercritical environment corrosion testing machine system, which can control the DO concentration, pH and conductivity. After the tests, strain–stress curves were analyzed to identify the mechanical properties of 310 SS. The morphologies of the side surface and fracture surface on the specimens were conducted by scanning electron microscope (SEM), and the chemical composition and structure of the oxide formed were examined by energy dispersive spectroscopy (EDS), in order to investigate fracture mode and to evaluate SCC susceptibility. The results show that the elongation decreased dramatically with the increasing of dissolved oxygen concentration. Cracks on 310 SS were widely distributed over the whole gauge section and a brittle fracture mode was observed on the fracture surface. The Cr content the oxide layer on the surface showed significant increase with the increasing of DO concentration. The corrosion products formed on 310 SS were Fe/Cr oxide layers, and they are classified into two layers, an outer Fe-rich oxide layer and an inner Cr-rich oxide layer.