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Determining the Dissolved Oxygen (DO) Concentration Limit for Safe Operation of CRAs in Saline Solution

Study to assess pitting corrosion resistance of 316L ASS (UNS S31603) and 25%Cr SDSS (UNS S32750) in salt solutions containing dissolved oxygen(DO). The DO levels examined were: 20, 50, and 100ppb, and the concentration of chloride ions were up to 152g/L Cl-, at 50 and 60°C. The results are reported herein.

Product Number: 51317--9062-SG
ISBN: 9062 2017 CP
Author: Qing Lu
Publication Date: 2017
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It is well established that the corrosion rate of carbon/low alloy steels in aqueous solutions (e.g. seawater) is affected by the concentration of dissolved oxygen (DO) along with other environmental factors including temperature chloride ion concentration pH etc. Several models have been developed and used to predict the corrosion rate of carbon steels at different DO levels. However carbon/low alloy steels cannot offer sufficient corrosion resistance in saline solutions at high temperatures (e.g. produced water systems in the oil field) leading to excessive metal loss due to the high corrosion rates. In such cases corrosion resistant alloys (CRAs) may offer a more cost effective/long-term solution. However there have been cases that CRAs were used with the premise that they were immune to corrosion whereas CRAs suffer from localised corrosion in oxygen-containing saline solutions.Depending on the grade of CRAs employed the tolerance level of the DO may vary e.g. a lower level for lower grades of stainless steels (e.g. 316/316L austenitic stainless steel (ASS) and a slightly higher level for higher grades (e.g. 25%Cr superduplex stainless steel (SDSS). However there is a lack of defined DO limits on CRA applications where the materials are considered to be ‘safe’ (free of corrosion) in the operation system.In order to establish the DO limits pitting and crevice corrosion tests were carried out for 316L austenitic stainless steel and 25%Cr super duplex stainless steel at different levels of the DO and chloride content (Cl-). The DO levels examined were 20 50 and 100ppb in a range of chloride ion concentrations (up to 152g/L Cl-) at temperatures of 50 and 60°C. The test results are reported in this paper.

Key words: oxygen control, pitting corrosion, UNS S31603, UNS S32750.

It is well established that the corrosion rate of carbon/low alloy steels in aqueous solutions (e.g. seawater) is affected by the concentration of dissolved oxygen (DO) along with other environmental factors including temperature chloride ion concentration pH etc. Several models have been developed and used to predict the corrosion rate of carbon steels at different DO levels. However carbon/low alloy steels cannot offer sufficient corrosion resistance in saline solutions at high temperatures (e.g. produced water systems in the oil field) leading to excessive metal loss due to the high corrosion rates. In such cases corrosion resistant alloys (CRAs) may offer a more cost effective/long-term solution. However there have been cases that CRAs were used with the premise that they were immune to corrosion whereas CRAs suffer from localised corrosion in oxygen-containing saline solutions.Depending on the grade of CRAs employed the tolerance level of the DO may vary e.g. a lower level for lower grades of stainless steels (e.g. 316/316L austenitic stainless steel (ASS) and a slightly higher level for higher grades (e.g. 25%Cr superduplex stainless steel (SDSS). However there is a lack of defined DO limits on CRA applications where the materials are considered to be ‘safe’ (free of corrosion) in the operation system.In order to establish the DO limits pitting and crevice corrosion tests were carried out for 316L austenitic stainless steel and 25%Cr super duplex stainless steel at different levels of the DO and chloride content (Cl-). The DO levels examined were 20 50 and 100ppb in a range of chloride ion concentrations (up to 152g/L Cl-) at temperatures of 50 and 60°C. The test results are reported in this paper.

Key words: oxygen control, pitting corrosion, UNS S31603, UNS S32750.

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