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.

Introduction

In an earlier study1, a final tube product of super duplex stainless steel (UNS S32750) has been tested in NACE(1) standard solution A (50 g/L NaCl, 5 g/L NaCH3COO), under 0.5 bar (7.25 psi) H2S at temperature 23 °C, 50 °C, 80 °C and pH 2.0, 2.8, 3.5, 4.5, and 6. The super duplex tube product showed SCC susceptible and underwent selective dissolution in austenitic phase (γ) at 80°C when pH ≤ 3.5, see T-pH domain and stress-strain curves in Figure 1.

The investigation on the specimens in the SCC susceptible conditions is included in Part 1, AMPP conference paper C2022-179982. In this Part 2, a case study is presented on the local corrosion on specimen surface or shoulder area although high ductility ratios in two testing conditions.

Optical microscope observations of SSRT specimens

All the SSRT specimens have been observed using an optical stereo microscope. The specimens with local corrosion shown in Figure 2, are included in case study using SEM-EDS and EBSD in this case study. Although the high ductility ratios, corrosion has been observed on specimen surface tested at 50 °C, pH 2.0; whereas, localized corrosion has been observed on the specimen tested at 80 °C, pH 4.5, in one of the specimen shoulders, including two parts of circumferential corrosion lines, no corrosion observed on the fracture surface and gauge section.

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