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09300 Stress Corrosion Cracking of Super Duplex Stainless Steels above and below Pitting Potentials

Product Number: 51300-09300-SG
ISBN: 09300 2009 CP
Author: Janardhan Rao Saithala, John Atkinson, Harvindher Singh Ubhi, Andy Houghton and Steve McCoy
Publication Date: 2009
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Pitting potentials (Ep) of super duplex stainless steel (SDSS) grade (UNS S32760) were determined using potentiodynamic polarization methods. The effect of chloride concentration on Ep was determined at 130oC in an autoclave at various chloride ppm levels (15, 100, 1000, 10000) for UNS S32760. This study also focussed on investigating the stress corrosion cracking (SCC) behaviour at + 200 mV from the measured pitting potential (EP), using slow strain rate testing (SSRT) methods in 15 & 1000 ppm chloride level for UNS S32760. Results show that below EP isolated small cracks initiate, around EP small cracks start to coalesce and above EP, the attack was very rapid and appeared to be galvanic/pitting in both phases. Slow strain rate tests indicate that applied potential (EAPP), and EP play a major role in the failure process. Electron Back Scattered Diffraction (EBSD) and Scanning Electron Microscopy (SEM) has shown that below Ep, SCC is associated with short cracks and selective phase attack and above Ep pits tend to link with cracks. It was also revealed that grain size distribution of both ferrite and austenite phases in SDSS played an important role in the final failure process. The phase morphology and the balance of the two phases was also found to be of significance with continuous single phase paths sometimes created along the rolling direction providing active corrosion pit paths.

Keywords: stress corrosion cracking, pitting potential, super duplex stainless steel, chloride solution
Pitting potentials (Ep) of super duplex stainless steel (SDSS) grade (UNS S32760) were determined using potentiodynamic polarization methods. The effect of chloride concentration on Ep was determined at 130oC in an autoclave at various chloride ppm levels (15, 100, 1000, 10000) for UNS S32760. This study also focussed on investigating the stress corrosion cracking (SCC) behaviour at + 200 mV from the measured pitting potential (EP), using slow strain rate testing (SSRT) methods in 15 & 1000 ppm chloride level for UNS S32760. Results show that below EP isolated small cracks initiate, around EP small cracks start to coalesce and above EP, the attack was very rapid and appeared to be galvanic/pitting in both phases. Slow strain rate tests indicate that applied potential (EAPP), and EP play a major role in the failure process. Electron Back Scattered Diffraction (EBSD) and Scanning Electron Microscopy (SEM) has shown that below Ep, SCC is associated with short cracks and selective phase attack and above Ep pits tend to link with cracks. It was also revealed that grain size distribution of both ferrite and austenite phases in SDSS played an important role in the final failure process. The phase morphology and the balance of the two phases was also found to be of significance with continuous single phase paths sometimes created along the rolling direction providing active corrosion pit paths.

Keywords: stress corrosion cracking, pitting potential, super duplex stainless steel, chloride solution
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