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O2 Contamination in Ssc / Hic Qualification Test Environments – Impact on Test Results and Discussion on Acceptable Limits

Picture for O2 Contamination in Ssc / Hic Qualification Test Environments – Impact on Test Results and Discussion on Acceptable Limits
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It is a well-admitted fact that oxygen contamination shall be avoided during standard H2S cracking tests. Until 2016 NACE technical methods related to H2S cracking resistance evaluation (e.g. NACE TM0284 and NACE TM0177) only gave general suggestions about O2 pollution. For instance it was recommended that ‘tests vessels shall be capable of being purged to remove O2’ and also that ‘O2 contamination is evident by a cloudy appearance’. In the 2016 revisions of NACE TM0177 and NACE TM0284 documents quantitative limits of O2 contamination were included:- Test method must ensure that the test solution contains less than 50 mass. ppb dissolved O2 before the introduction of H2S.- A more stringent limit of 10 ppb max. is imposed when testing Corrosion Resistant Alloys (CRA) or high strength low alloy steels (> 80 ksi).However the scientific basis of these values have not been well-established yet and there is still a lack of available experimental data to illustrate the potential impacts of small dissolved oxygen contents on the cracking resistance of different materials. In addition while the revised test methods explicitly address initial contamination of the test solution before H2S introduction they do not consider a continuous oxygen supply during testing. This possible continuous O2 contamination is extremely difficult to eliminate and control for example in case of poor laboratory practices oxygen ingress may arise by permeation through polymer tubings used for the tests or in case of a lack of tightness of gaskets.In order to better understand the impact of O2 contamination on H2S cracking a 3-years Joint Industrial Project (JIP) was launched at the end of 2015. The objectives were to evaluate if continuous O2 contamination can affect H2S cracking test results. A range of steel grades covering different types of O&G applications such as line pipe OCTG and flexible wires were used. Sulfide Stress Cracking (SSC uniaxial tensile as well as 4 point-bend tests) and Hydrogen Induced Cracking (HIC) tests were conducted with well-controlled and continuous O2 contamination. Three levels of O2 partial pressures in the gas feed corresponding to 300 ppb 50 ppb and less than 10 ppb dissolved O2 were used. These levels were selected to simulate poor deaeration and the current limits specified in the last standard revisions respectively.In parallel to the standard qualification tests hydrogen permeation and weight-loss corrosion experiments were performed with the same test matrix covering all regions of the SSC severity diagram. This paper aims at sharing the main results of this JIP. 

Product Number: 51319-12894-SG
Author: Christophe Mendibide
Publication Date: 2019
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Impact of O2 Content on Corrosion Behavior of X65 Mild Steel in Gaseous, Liquid and Supercritical CO2 environments

Product Number: 51320-14433-SG
Author: Xiu Jiang, Dingrong Qu , Xiaoliang Song
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CO2 stream in CCS system usually contains impurities, such as water, O2, SO2, NO2, H2S, and other trace substances, which could pose a threat to internal corrosion and integrity of CO2 transportation pipelines. The general and localized corrosion behavior of API 5L X65 mild steel were evaluated using an autoclave both in water-saturated CO2 and CO2-saturated water environments in the presence of varying concentrations of O2. Experiments were performed at 25 °C and 35 °C, 8 MPa and 35 °C, 4 MPa to simulate the conditions encountered during dense, supercritical and gaseous CO2 transport. General corrosion rates were obtained by weight-loss method. The surface morphology of the coupons was examined by scanning electron microscopy (SEM). Results indicated that general corrosion rates at each O2 concentration in CO2-saturated water environment were much higher than those in water-saturated CO2 environment. The corrosion rates did not increase with increasing O2 concentration from 0 to 2000 ppm; instead the corrosion rate reached a maximum with 1000 ppm O2 at 25 °C, 8 MPa and 50 ppm O2 at 35 °C, 8 MPa in water-saturated CO2 environment and 50 ppm at 25 °C, 8 MPa and 100 ppm at 35 °C, 8 MPa in CO2-saturated water environment. However, the change trend of general corrosion rate with O2 content at 35 °C, 4 MPa was different from that in 25 °C and 35 °C, 8 MPa both in water-saturated CO2 and CO2-saturated water environments. Localized corrosion or general corrosion rate of over 0.1 mm/y was identified at each test condition both in a water-saturated CO2 and CO2-saturated water environments. When O2 was added, coupon surfaces were covered by a more porous corrosion product scale. A final series of tests conducted with the addition of 100 ppm and 2000 ppm O2 in CO2 environment with 60% relative humidity (RH) and 80% RH revealed that no localized corrosion was observed and the general corrosion rates were lower than 0.1 mm/y at 25 °C and 35 °C, 8 
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