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51318-11105-Effect of tungsten on the precipitation kinetics and localized corrosion resistance of super duplex

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The effect of W on the precipitation kinetics of three 25Cr SDSS grades was quantified in Time-Temperature-Transformation (TTT) diagrams. Optical microscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS), and Electron backscatter diffraction (EBSD) were used.

Product Number: 51318-11105-SG
Author: Mia Bernås / Ida Westermann / Anders Jernberg, Atle H. Qvale / Roy Johnsen / Cristian Torres / Mariano Iannuzzi
Publication Date: 2018
Industry: Science of Corrosion
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Although extensively used in oil and gas production due to their excellent combination of mechanical properties and corrosion resistance, 25Cr super duplex stainless steels (SDSSs) are susceptible to the precipitation of deleterious phases during heat treatment and welding. Deleterious phases, in turn, affect both localized corrosion resistance and mechanical properties. Much debate still exists as to whether alloying elements such as tungsten accelerate or retard the formation of detrimental precipitates.

In this work, the effect of W on the precipitation kinetics of three 25Cr SDSS grades, namely, UNS S32750 (W-free), S32760 (Low-W), and S39274 (High-W) was quantified in Time-Temperature-Transformation (TTT) diagrams. Optical microscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS), and Electron backscatter diffraction (EBSD) were used to characterize the microstructure evolution and construct the TTT diagrams. The effect of intermetallic compounds (IMCs) and tertiary phases on localized corrosion resistance was investigated as a function of volume fraction and type of precipitate, with a focus on σ– and 𝜒–phase formation. The localized corrosion resistance of the various metallurgical stages was determined using open circuit potential measurements as a function of temperature during immersion in 6 wt% FeCl3 pH = 1.0. The σ–phase precipitation rate was slower in the High-W SDSS compared to the other alloys, possibly due to 𝜒–phase precipitation at grain boundaries. At 846°C, the isothermal heating time required to observe a drop in Critical Pitting Temperature (CPT) doubled for the High-W SDSS. The implications of these findings in materials selection for oil and gas production equipment are discussed.

Key words: duplex stainless steels, seawater, critical pitting temperature, sigma phase, chi phase, chromium nitrides, time-temperature-transformation diagram.

Although extensively used in oil and gas production due to their excellent combination of mechanical properties and corrosion resistance, 25Cr super duplex stainless steels (SDSSs) are susceptible to the precipitation of deleterious phases during heat treatment and welding. Deleterious phases, in turn, affect both localized corrosion resistance and mechanical properties. Much debate still exists as to whether alloying elements such as tungsten accelerate or retard the formation of detrimental precipitates.

In this work, the effect of W on the precipitation kinetics of three 25Cr SDSS grades, namely, UNS S32750 (W-free), S32760 (Low-W), and S39274 (High-W) was quantified in Time-Temperature-Transformation (TTT) diagrams. Optical microscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS), and Electron backscatter diffraction (EBSD) were used to characterize the microstructure evolution and construct the TTT diagrams. The effect of intermetallic compounds (IMCs) and tertiary phases on localized corrosion resistance was investigated as a function of volume fraction and type of precipitate, with a focus on σ– and 𝜒–phase formation. The localized corrosion resistance of the various metallurgical stages was determined using open circuit potential measurements as a function of temperature during immersion in 6 wt% FeCl3 pH = 1.0. The σ–phase precipitation rate was slower in the High-W SDSS compared to the other alloys, possibly due to 𝜒–phase precipitation at grain boundaries. At 846°C, the isothermal heating time required to observe a drop in Critical Pitting Temperature (CPT) doubled for the High-W SDSS. The implications of these findings in materials selection for oil and gas production equipment are discussed.

Key words: duplex stainless steels, seawater, critical pitting temperature, sigma phase, chi phase, chromium nitrides, time-temperature-transformation diagram.

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Picture for Influence of tungsten on passivity breakdown and repassivation of 25Cr super duplex stainless steels
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51318-11102-Influence of tungsten on passivity breakdown and repassivation of 25Cr super duplex stainless steels

Product Number: 51318-11102-SG
Author: Cristian Torres / Mariano Kappes / Roy Johnsen / Mariano Iannuzzi
Publication Date: 2018
$20.00

Two super duplex stainless steels were investigated: a W-free (UNS S32750) and a 2.1 wt% W-containing (UNS S39274) grade. Goals: To study localized corrosion resistance of super duplex stainless steels and how W affects the same.
Picture for Effect of Oxygen in Back Shielding Gas on Corrosion Resistance of Super Duplex Stainless Steel Pipe
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51318-11107-Effect of Oxygen in Back Shielding Gas on Corrosion Resistance of Super Duplex Stainless Steel Pipe

Product Number: 51318-11107-SG
Author: Takayuki Tokumine / Takanori Mimoto / Gen Ogita / Yoshiyuki Shimizu / Masanori Gokou / Fu-Gao Wei / Toshiaki Ashitani / Masayuki Tanaka
Publication Date: 2018
$20.00

The effect of back shielding gas types and contained oxygen level on the corrosion resistance of welds was investigated for gas tungsten arc welding (GTAW) of UNS S32750 super duplex stainless steel (SDSS) pipes.
Picture for The Application Limitation Boundary of Duplex Stainless Steel under Sour Condition in Oil Fields
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51318-11084-The Application Limitation Boundary of Duplex Stainless Steel under Sour Condition in Oil Fields

Product Number: 51318-11084-SG
Author: Ziru Zhang / Zhu Wang / Junpeng Xue / Lei Zhang / Xian Tang / Minxu Lu
Publication Date: 2018
$20.00

Pitting failure boundary of duplex stainless steel under sour condition was investigated by electrochemical and immersion tests. Polarization measurements investigate the effect of temperature and H2S on the corrosion behavior.
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