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51316-7308-Effect of Tungsten on Pitting and Crevice Corrosion of Type 25Cr Super Duplex Stainless Steel

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To examine the effect of Tungsten (W) on localized corrosion of 2 superduplex stainless steels (SDSS)- high & low-W. Both plain samples (pitting corrosion) and samples with artificial crevices (crevice corrosion) were exposed. Tests in 3.50-wt% NaCl solution or natural seawater. T= 30 to 90°C.

Product Number: 51316-7308-SG
ISBN: 7308 2016 CP
Author: Roy Johnsen
Publication Date: 2016
Industry: Oil and Gas Production
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The oil and gas industry frequently uses type 25Cr super duplex stainless steels (SDSS) both for seawater applications and for pipes and components exposed to hydrocarbon environments topside and subsea. During the last years, the effect of tungsten on pitting and crevice corrosion resistance has been debated among corrosion experts. In particular, some authors claim that W has a strong synergistic effect with molybdenum when added above a certain threshold value. In the solution annealed condition, tungsten-containing SDSS have been claimed to offer corrosion resistance on a par with higher-grade steels such as type 6Mo (UNS S31254) super austenitic stainless steel grades.

The objective of this investigation was to examine the effect of W on localized corrosion of two SDSS: a high-W (UNS S39274) and a low-W (mod. UNS S32750) grade. Both plain samples (pitting corrosion) and samples with artificial crevices (crevice corrosion) were exposed. Tests were conducted in a 3.50-wt% NaCl solution or natural seawater with temperature ranging from 30°C to 90°C. Cyclic Potentiodynamic polarization scans according to ASTM G-61, critical pitting temperature tests according to ASTM G-150, and long-term seawater open circuit potential exposures of creviced samples were conducted.

The outcome of the testing showed that W addition has a positive effect on both pitting and crevice corrosion resistance. We propose a new parametric definition of Pitting Resistant Equivalent.

Keywords: downloadable, super duplex stainless steels, seawater, critical pitting temperature, critical crevice temperature, cyclic potentiodynamic testing

The oil and gas industry frequently uses type 25Cr super duplex stainless steels (SDSS) both for seawater applications and for pipes and components exposed to hydrocarbon environments topside and subsea. During the last years, the effect of tungsten on pitting and crevice corrosion resistance has been debated among corrosion experts. In particular, some authors claim that W has a strong synergistic effect with molybdenum when added above a certain threshold value. In the solution annealed condition, tungsten-containing SDSS have been claimed to offer corrosion resistance on a par with higher-grade steels such as type 6Mo (UNS S31254) super austenitic stainless steel grades.

The objective of this investigation was to examine the effect of W on localized corrosion of two SDSS: a high-W (UNS S39274) and a low-W (mod. UNS S32750) grade. Both plain samples (pitting corrosion) and samples with artificial crevices (crevice corrosion) were exposed. Tests were conducted in a 3.50-wt% NaCl solution or natural seawater with temperature ranging from 30°C to 90°C. Cyclic Potentiodynamic polarization scans according to ASTM G-61, critical pitting temperature tests according to ASTM G-150, and long-term seawater open circuit potential exposures of creviced samples were conducted.

The outcome of the testing showed that W addition has a positive effect on both pitting and crevice corrosion resistance. We propose a new parametric definition of Pitting Resistant Equivalent.

Keywords: downloadable, super duplex stainless steels, seawater, critical pitting temperature, critical crevice temperature, cyclic potentiodynamic testing

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