Abstract
A failure analysis has been performed and discussed in Part 1 on the tube material of UNS S32750 and the specimens which showed SCC susceptible after the SSRT in a sour environment in pH 2.8, pH 3.5 at 80 °C.
In this Part 2, a case study has been performed on local corrosion on surface corrosion attack observed on two specimens. Scanning electron microscope (SEM) equipped by energy-dispersive X-ray spectrometer (EDS) analysis and Electron backscatter diffraction (EBSD) measurements have been used. On a specimen tested in pH 2.0 at 50 °C, selective dissolution (SD) has been identified in austenite and thin uniform corrosion on the surface in ferrite, no crack has been observed under the corrode surface. On a specimen tested in pH 4.5 at 80 °C, both SD and cracking have been observed under the line corrosion in shoulder area. EBSD revealed that micro cracks initiated in ferrite with enhanced strain/deformation in frontend of selectively dissolved austenite. Dry-wet condition is supposed, which would make the local condition of a thin liquid layer more severe compared to the bulk liquid testing condition. The surface analysis methods showed a good way for failure analysis, which might be helpful in other complicated cases.
