The selection of materials for application in the oil and gas industry in environments containing hydrogen sulphide (H 2 S) is an important matter for the safety of equipments. In the presence of H 2 S, hydrogen atoms originating from the cathodic corrosion reaction can diffuse into the steel, resulting in cracking mechanisms such as hydrogen-induced cracking (HIC) or sulphide stress cracking (SSC). For the SSC cracking mode, a diagram defining different regions of environmental severity was established in the 1990's. The requirements for SSC testing of steels in a given environment are well defined and accepted by the industry. On the other hand, fit-for-purpose (FFP) testing for HIC remains a subject of debate. The most common practice consists in 96 hours exposure under 1 bar H 2 S. But this test environment is extremely severe, and is often considered to be too conservative for the selection of steels for use in mildly sour environments. However, a universally applicable FFP test method for HIC has not yet been established. In addition to pH and H 2 S partial pressure, test duration is another major parameter for HIC testing. Therefore, this experimental study was launched to investigate the impact of time of exposure on HIC. Experiments consisted of HIC exposure tests based on the NACE TM0284-2003 standard method. Experiments were conducted in different nodes of the pH-pH 2 S diagram respectively between pH 6.5 and 3.5, and between 10 mbar and 1 bar of H 2 S. For each condition, tests with different times of exposure were realised, from only a few hours and up to three months. Characterisation of HIC consisted in ultrasonic testing. Permeation experiments were also performed in selected pH-pH 2 S conditions. Permeation transients were analysed for calculation of diffusion coefficients. Modelling of hydrogen profiles in HIC specimens was then realised, and compared with the results of HIC immersion tests. For each test condition, it was possible to draw the evolution with time of the extent of HIC. Depending on the environment, the minimum duration necessary to initiate HIC was extremely variable, from a few hours in the most severe conditions to several weeks in low severity conditions. The extent of cracking at equilibrium also seemed to vary with pH and H 2 S. A good correlation was found between HIC and permeation results, confirming that HIC is strongly linked with the increase of hydrogen concentration in the steel. These results confirm that low severity conditions require longer exposures to reach equilibrium levels of internal hydrogen. This must be taken into account for FFP HIC testing in mildly sour environment. With that aim, permeation experiments could represent an interesting technique to assess the time to reach a certain level of hydrogen in the material.

Résumé -Étapes limitantes dans la biodégradation du MTBE -La voie métabolique de dégra-dation du méthyl tertio butyl éther ou MTBE chez Mycobacterium austroafricanum IFP 2012 a été partiellement élucidée par identification des intermédiaires de dégradation. Elle nécessite l'induction de différentes activités enzymatiques. Au cours des premières étapes de la dégradation du MTBE en tertio butyl alcool (TBA), une même monooxygénase est responsable de l'oxydation du MTBE et du TBA avec une faible affinité pour le TBA (Km = 1,1 mM) et une estérase est impliquée dans l'hydrolyse du tertio butyl formiate (TBF). La lenteur de la dégradation du MTBE chez M. austroafricanum IFP 2012 semble due à un processus complexe combinant principalement un effet négatif du TBF formé au cours de l'oxydation du MTBE sur la MTBE/TBA monooxygénase et l'absence de la dégradation du TBA par la monooxygénase en présence de MTBE. Dans les étapes ultérieures de la dégradation, un besoin spécifique en cations Co ++ intervient au cours de la dégradation de l'acide 2-hydroxyisobutyrique (HIBA), la croissance sur HIBA étant très faible en absence de cette supplémentation. Abstract -Limitations in MTBE Biodegradation

Mycobacterium austroafricanum IFP 2012 is a Gram-positive strain able to grow on methyl tert-butyl ether (MTBE) as a sole carbon and energy source. The effect of two downstream metabolites of MTBE, tert-butyl formate (TBF) and tert-butyl alcohol (TBA) on MTBE degradation was investigated using resting cells. The addition of low concentrations of TBF decreased the MTBE degradation rate by about 30%. In contrast, the addition of TBA did not have a significant effect on MTBE degradation rate, even at high concentrations; and it was also shown that TBA degradation occurred only once MTBE was exhausted. At neutral pH, TBF hydrolysis involved mainly an esterase-type activity regulated by the presence of TBA. The TBF degradation rate was about four times lower than the MTBE degradation rate. Furthermore, acetone was identified as an intermediate during TBA degradation. An acetone mono-oxygenase activity, inhibited by methimazole but not by acetylene, was suggested. It was different from the MTBE/TBA mono-oxygenase and, thus, acetone did not appear to compete with MTBE and TBA for the same enzyme. These new results show that the metabolic regulation of the early steps of MTBE degradation by M. austroafricanum IFP 2012 is complex, involving inhibition and competition phenomena.

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