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Role of H2S In Localized Corrosion and Cracking of CRAs in Upstream Oil and Gas Applications

Picture for Role of H2S In Localized Corrosion and Cracking of CRAs in Upstream Oil and Gas Applications
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Materials qualification testing of corrosion resistant alloys (CRAs) typically involves the use of simple pass/fail tests. Modification of existing standards is recommended for environments in which pit initiation is statistically improbable but pit propagation is rapid, e.g. low chloride/high H2S.

Product Number: 51317--9463-SG
ISBN: 9463 2017 CP
Author: Gareth Hinds
Publication Date: 2017
Industry: Oil and Gas Production
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$20.00
$20.00

The more aggressive environments encountered in upstream oil and gas applications due to exploitation of deeper oilfields at elevated temperature and higher partial pressures of hydrogen sulphide (H2S) and carbon dioxide (CO­2) have meant that corrosion resistant alloys (CRAs) are increasingly the containment material of choice. Selection of the appropriate alloy is determined primarily by the trade-off between cost and performance which is typically assessed via standard laboratory test methods such as the four-point bend and proof ring tests. Localized corrosion is often the precursor to cracking and there is ongoing debate within the industry regarding the critical pit size that can be tolerated in such tests in the absence of any cracks. This generates considerable uncertainty in the validity of extrapolating the results of relatively short term laboratory tests to predict service performance over a period of several decades.Here we present a detailed investigation of the influence of dissolved H2S on pit initiation pit propagation and the pit-to-crack transition for different CRAs under a range of representative service conditions. The effect of H2S concentration on the pitting and repassivation potential of CRAs is evaluated using a novel electrochemical test method that employs cooling rather than dilution of the specimen seal to prevent crevice corrosion during anodic polarization. These electrochemical measurements are correlated with the results of four-point bend tests under similar conditions. Statistical analysis of pit morphology is carried out using a combination of confocal microscopy and X-ray computer tomography. Scanning electron microscopy and electron backscatter diffraction are used to explore the influence of the sub-surface microstructure and residual stress on pit and crack growth. The results are discussed in terms of current understanding of pit-to-crack transition mechanisms and the implications for sour testing programmes are highlighted.

Key words: localized corrosion, pitting, sulfide stress corrosion cracking, corrosion resistant alloys, hydrogen sulfide, four-point bend testing

 

 

The more aggressive environments encountered in upstream oil and gas applications due to exploitation of deeper oilfields at elevated temperature and higher partial pressures of hydrogen sulphide (H2S) and carbon dioxide (CO­2) have meant that corrosion resistant alloys (CRAs) are increasingly the containment material of choice. Selection of the appropriate alloy is determined primarily by the trade-off between cost and performance which is typically assessed via standard laboratory test methods such as the four-point bend and proof ring tests. Localized corrosion is often the precursor to cracking and there is ongoing debate within the industry regarding the critical pit size that can be tolerated in such tests in the absence of any cracks. This generates considerable uncertainty in the validity of extrapolating the results of relatively short term laboratory tests to predict service performance over a period of several decades.Here we present a detailed investigation of the influence of dissolved H2S on pit initiation pit propagation and the pit-to-crack transition for different CRAs under a range of representative service conditions. The effect of H2S concentration on the pitting and repassivation potential of CRAs is evaluated using a novel electrochemical test method that employs cooling rather than dilution of the specimen seal to prevent crevice corrosion during anodic polarization. These electrochemical measurements are correlated with the results of four-point bend tests under similar conditions. Statistical analysis of pit morphology is carried out using a combination of confocal microscopy and X-ray computer tomography. Scanning electron microscopy and electron backscatter diffraction are used to explore the influence of the sub-surface microstructure and residual stress on pit and crack growth. The results are discussed in terms of current understanding of pit-to-crack transition mechanisms and the implications for sour testing programmes are highlighted.

Key words: localized corrosion, pitting, sulfide stress corrosion cracking, corrosion resistant alloys, hydrogen sulfide, four-point bend testing

 

 

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