Stress Corrosion Cracking of ferrito-pearlitic steel in aqueous environment containing dissolved CO2

Abstract : A confined aqueous environment is defined by a very low water-volume to exposed steel-area ratio. In such media containing dissolved CO2, siderite is formed and acts as a protective film. An addition of applied stress and/or environmental fluctuation can disturb the balance between the steel and this protective film, causing the fracture of the latter and leading to Stress Corrosion Cracking (SCC). The material studied is a cold drawn and rolled high strength steel composed of ferrite and spheroidized pearlite and has a strong microstructural anisotropy due to the specific cold work process. To investigate its susceptibility to SCC, Slow Strain Rate Tests (SSRT) were carried out on smooth and notched specimens allowing to separate crack initiation and crack propagation. The environment is an aqueous chloride solution saturated in CO2 at pH around 6. Tests were performed under open circuit potential (OCP) and at cathodic potential. Under OCP, localized anodic dissolution in shear bands is responsible for crack initiation. Under cathodic potential, crack initiation is delayed due to the absence of critical defect on the surface. Both fractographic analyses and results obtained on notched specimens showed that hydrogen plays an important role in crack propagation.