Novel images of the evolution of stress corrosion cracks from corrosion pits

SCC in turbine disc steels exposed to simulated steam-condensate tends to nucleate preferentially from corrosion pit precursors. The evolution of these cracks is not straightforward and not well understood. In this work, unique three-dimensional X-ray microtomographic images have confirmed that cracks develop predominantly at the shoulder of the pit, near the pit/surface interface, for specimens stressed to 50–90% σ0.2. In support of this observation, FEA of model pits indicate that strain is a maximum on the pit wall just below the pit mouth. Implications of these observations for the pit-to-crack transition and predictive-modelling of crack nucleation and growth are discussed.

Very early stages of crack evolution from corrosion pit. Internal view (i.e., looking at the surface skin of the pit and crack as viewed from inside the bulk of the specimen), via X-ray tomographic rendering, of the pit-to-crack transition within a steel specimen stressed at 90% σ0.2 and exposed to simulated condensate at 90 °C for 668 h (the surface of the sites rendered on the left are shown in the SEM image on the right for comparison). Two pits are imaged showing crack initiation on the pit wall near the pit mouth. Finite element analysis of U-shaped pit. Maximum principal stress and maximum principal strain of a 500 μm deep U-shaped pit in a 6.4 mm diameter cylindrical specimen at 90% σ0.2 viewed in cross section. The maximum stress, as expected, is located at the base of the pit while the maximum strain occurs along the pit wall close to the pit mouth.Figure optionsDownload as PowerPoint slideHighlights
► X-ray tomography is a powerful technique for pit-to-SCC transition characterisation.
► The preferred site for crack initiation is on the pit wall close to the pit mouth.
► FEA indicate that localised plastic strain provides explanation for SCC nucleation.
► Kondo’s criteria at the point of transition are not supported for SCC in this system.
► There is a need to reassess quantitative modelling of SCC growth from pits.