Abstract

For safely using duplex stainless steel and super duplex stainless steel, temperature-pH domains of stress corrosion cracking (SCC) resistance were provided by using slow strain rate testing (SSRT) screening in sour conditions in an earlier study.

To follow-up the study, a failure analysis has been performed on the tube material of UNS S32750 and the specimens after the SSRT in a sour environment in pH 2.8, pH 3.5 at 80 °C. Scanning electron microscope (SEM) equipped by energy-dispersive X-ray spectrometer (EDS) analysis and Electron backscatter diffraction (EBSD) measurements have been used for the investigation on specimen surface, fracture surface and cross-sections of the SSRT specimens. Initiation of selective dissolution (SD) and cracking has been identified. EBSD reveals that cracking initiated in ferrite with enhanced strain/deformation in frontend of selectively dissolved austenite, where possible SCC mechanism seems to be hydrogen embrittlement. It observed that SD stopped in the brittle-ductile transition area in the fracture surface, which probably attributed to repassivation of the material in the condition of pH 3.5. Combined SSRT and analysis methods by SEM and EBSD showed a good way for the material evaluation before decision making for the materials selections and supplementary long-term exposures.

Introduction

Standards NACE(1) MR0175 / ISO(2) 15156-31 have temperature-H2S-Cl-pH limits for safely using duplex stainless steels based on their Pitting Resistance Equivalent Number (PREN) of subcategory such as duplex stainless steel (DSS, PREN 30-40, Mo>1.5%) and super duplex stainless steel (SDSS, PREN 40-45), temperature-pH domain of stress corrosion cracking (SCC) resistance can be used for material evaluation and provided by using slow strain rate testing (SSRT) screening in sour testing conditions2.

In earlier studies2, 3, the plate- and tube products were tested in NACE 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 2.8, 3.5, 4.5, and 6. The super duplex tube product showed higher SCC resistance compared to the duplex plate product while the hyper duplex stainless steel plate product did not show SCC or selective dissolution (SD) in the same testing conditions. It has been found that a UNS S32750 tube product was 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.

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