Recent research progress in stress corrosion crack initiation of Alloy 690 in pressurized water reactor primary water

Alloy 690 exhibits much higher resistance to stress corrosion cracking (SCC) in pressurized water reactor (PWR) primary water than other engineering austenitic alloys as it is highly resistant to penetrative intergranular oxidation. Dynamic straining method can readily generate SCC initiation on Alloy 690 and increasing results indicate that there are some unique steps during the SCC initiation process of this alloy: the formation of protective oxide film above grain boundary and the subsequent fracture of such oxide film under dynamic straining. It was further revealed that local normal strain is the main driving force breaching the oxide film. Intergranular carbides further enhance the resistance to SCC initiation through: 1, decreasing the normal strain near grain boundary by impeding the movement of dislocations; 2, supplying additional Cr source for forming protective oxide when exposed to environment. Plastic deformation prior to or during SCC test can greatly weaken the mitigation effect of carbide and even reverse it. A certain amount of cold work and high tensile stress were shown to produce creep-induced cracking in the PWR operating temperature range due to the elevated density and enhanced diffusivity of vacancy. Moreover, although carbides diminish the local strain near grain boundary during dynamic straining, the dislocations accumulated near the matrix/carbide interface can expedite the ingress of oxygen once the surface oxide film is breached. There are still many critical research gaps that need to be filled in order to ensure the performance reliability of Alloy 690 during the lifespan of PWR.