O2 contamination in SSC / HIC test environments. Impact on test results and discussion on acceptable limits for high H 2 S content

International audienceIt is a well admitted fact that oxygen contamination shall be avoided during H 2 S cracking tests of low alloy steels. In the 2016 revisions of NACE TM0177 and NACE TM0284 documents, quantitative limits of O 2 contamination were included with thresholds at 10 and 50 ppb of dissolved oxygen depending on the considered mechanical properties of the tested grade. However, the scientific basis of these values are not well established and there is still a lack of experimental data to illustrate the potential impacts of an oxygen pollution. In addition, while the revised test methods explicitly address initial contamination of the test solution before H 2 S introduction, they do not consider a continuous oxygen supply during testing. Yet, continuous contamination is extremely difficult to be completely eliminated. In order to better understand the impact of O 2 contamination on H 2 S cracking, a 3-years Joint Industrial Project was launched at the end of 2015. The objectives were to evaluate if O 2 contamination can affect H 2 S cracking test results. A range of steel grades covering different types of O&G applications for High H 2 S content were used. SSC (uniaxial tensile tests as well as 4 point-bend) and HIC tests were conducted, with well controlled and continuous O 2 contamination. Three levels of O 2 partial pressures in the gas feed corresponding to 300 ppb, 50 ppb and less than 10 ppb dissolved O 2 were used. 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 project for high H 2 S content

Modeling gas turbine materials’ hot corrosion degradation in combustion environments from H2‐rich syngas

Components of gas turbines (such as blades, vanes, combustor cans) exposed to combustion environments at high temperature are susceptible to hot corrosion attack. To successfully plan maintenance and to determine whether to operate in novel combustion modes (e.g., in integrated gasification combined cycles that incorporate pre-combustion carbon capture) predictions of hot corrosion component life must be made. In this paper, hot corrosion datasets relating to two alloys, MarM 509 (a cobalt-based superalloy), and Rene 80 (a nickel-based superalloy) form the basis of hot corrosion predictive lifetime models. The model framework is based on the two stages of incubation and propagation, with the transitions from incubation to propagation around the samples being based on Weibull statistics. The impact of a range of temperatures (including 700 and 900 °C), gas compositions (simulating the combustion of natural gas, H2-rich syngas, or partially cleaned syngas), and deposit chemistries/fluxes have been assessed. Predictions have been made including the expected damage spread for a range of different exposure conditions

Corrosion of Pure iron and Hydrogen Permeation in the Presence of H 2 S with O 2 contamination

International audienceThis paper examines the influence of traces of oxygen on corrosion and hydrogen charging of steel in an H 2 S containing environment. It is well known that H 2 S promotes hydrogen entry into steels, that may result in many types of steel failures such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC), and Stress-Oriented Hydrogen Induced Cracking (SOHIC). Since it is a huge concern for oil and gas industries, standard test methods have been developed and published as NACE technical methods (e.g. NACE TM0284 and NACE TM0177). Though it is recognized that oxygen pollution should be avoided during H 2 S cracking tests, there is still a lack of experimental data to illustrate the potential impacts of a small oxygen pollution. The aim of the present study is to check if oxygen traces can modify the mechanisms of corrosion and hydrogen charging of steel in H 2 S containing medium. Experiments consisted of hydrogen permeation measurements through a thin pure iron membrane. They were performed at free potential circuit in order to ensure more realistic environmental conditions. The corrosion rate was also evaluated and test solutions analyzed

Electrochemical study of oxygen impact on corrosion and hydrogen permeation of Armco iron in the presence of H 2 S

International audienceH 2 S corrosion of mild steel is a recurrent issue in the oil and gas industry. Many studies related to the corrosion and hydrogen permeation of steel in an H 2 S containing environment have been made during the past decades with the intent of improving the knowledge and the prevention of economic loss. Since H 2 S is also a hydrogen entry promoter, lots of studies are also dedicated to the understanding of H 2 S cracking. Although it is generally accepted to avoid oxygen contamination in such a medium, there is a lack of research concerning its effect on the corrosion and hydrogen charging of steel. In this study, the effect of oxygen on corrosion and hydrogen charging of steels in an H 2 S containing environment is studied using Electrochemical Impedance Spectroscopy (EIS). An equivalent electrical circuit has been built according to SEM observations, literature research and experimental results. Using this equivalent electrical circuit, experimental data was analyzed and the average corrosion rates were deduced and found to be in good agreement with corrosion rates obtained by weight loss measurements. Furthermore, the evolution of fitting parameters (double layer capacity, charge transfer resistance, diffusion impedance, etc.) was found to be in good agreement with the real physical meaning of such parameters in the given conditions. This research contributes to the explanation of the mechanism behind the high corrosion rate observed in an H 2 S environment polluted with traces of oxygen

EIS study of iron and steel corrosion in aqueous solutions at various concentrations of dissolved H2S : impact of oxygen contamination.

International audienceMildly acidic water containing dissolved H 2 S presents a strong risk in the cracking of low-carbon steels. Several studies on H 2 S cracking mechanisms have shown that the main driving force is linked to the ability of H 2 S to promote hydrogen entry into the bulk material. Standard test methods have been developed and published as NACE technical standards (e.g. NACE TM0284 and NACE TM0177) to aid materials selection in the oil and gas sector. Though it is recognized that oxygen pollution should be avoided during H 2 S cracking tests, there is a lack of experimental data to illustrate the effects of a small oxygen pollution. Dissolved oxygen concentrations greater than the recommended upper limit (50 parts per billion) can easily be obtained in the case of poor laboratory practices. This paper will focus on the interactions between oxygen and H 2 S on electrochemical behavior of unalloyed steel. A continuous O 2 injection at a level corresponding to 500 ppb is applied, together with H 2 S bubbling in our test solutions, for periods lasting the same order as SSC standard tests. Steel surface reaction phenomena/corrosion rates in H 2 S saturated solution, with or without oxygen pollution, are studied using electrochemical impedance spectroscopy. The evolution of corrosion rates obtained from impedance analysis was compared to two other independent methods: i/ weight loss measurements and, ii/ hydrogen permeation. Without O 2 pollution, a permeation efficiency of 100% was obtained, as expected. Permeation current density was thus found to match precisely with the corrosion current density determined by impedance analysis at different times. On the other hand, when a continuous O 2 pollution was added in the system, significantly higher corrosion rates were observed, associated with test solution acidification. At the same time, permeation efficiency was decreased by up to one order of magnitude

Etude du comportement de colonies de fissures courtes de Corrosion Sous Contrainte par Corrélation d'Images Numériques (DIC)

Dans cette étude, un système expérimental est développé pour l'étude par corrélation d'images numériques (DIC) de colonies de fissures courtes de Corrosion Sous Contrainte (CSC). Un alliage d'Inconel 600est étudié par DIC, puis par des techniques plus conventionnelles (émission acoustique, bruit électrochimique), dans une solution de tétrathionate de potassium à 0.01 M et acidifié à pH 3. Il a été aussi développé une méthode permettant d'identifier les fissures en fonction du temps, ainsi que la classification des fissures selon leur activité dynamique (actives ou dormantes). La même méthode a aussi permis le suivi du processus et l'identification de coalescences de fissures

Corrosion par piqûre et par crevasse. Eléments sur les mécanismes, appliqués à la sélection et l’utilisation des aciers inoxydables

Les corrosions par piqûre et par crevasse font partie des formes de corrosion les plus dommageables pour les aciers inoxydables lorsqu’ils sont utilisés dans des solutions aqueuses contenant des ions chlorures. Le caractère localisé de ces formes de corrosion les rend difficiles à détecter pour la piqûre, difficile à éviter pour la crevasse. Cet article, s’adressant à des non spécialistes, n’a pas pour objet une revue exhaustive des mécanismes de corrosion par piqûre et par crevasse, sujets toujours d’actualité, tant pour des travaux fondamentaux qu’appliqués. En rappelant les mécanismes de base de ces deux formes de corrosion, le but poursuivi est de donner des éléments simples, utiles pour la sélection et l’utilisation des aciers inoxydables. L’influence des principaux paramètres sur les étapes d’amorçage et de propagation est résumée, en donnant des exemples choisis, issus de la littérature et de l’expérience. En particulier, l’effet des facteurs composition, état de surface (pollutions, biofilms), potentiel redox de la solution, est discuté. Les types d’essais, normalisés et conventionnels, utiles pour caractériser le comportement des matériaux vis-à-vis de la corrosion par piqûre et par crevasse sont reportés. Enfin des approches plus spécifiques, permettant de prévoir à long terme le comportement des matériaux sont présentées : l’étude des piqûres métastables et la notion de potentiel de repassivation pour la corrosion par piqûre, la définition de critères basés sur des paramètres géométriques (ouverture et profondeur de la crevasse), permettant de déterminer des conditions critiques de stabilité de la crevasse, à l’aide de modèles de plus en plus raffinés

Stress Corrosion Cracking of ferrito-pearlitic steel in aqueous environment containing dissolved CO2

A confined aqueous environment is defined by a very low water-volume to exposed steel-area ratio. In such media containing dissolved CO2, siderite is formed and acts as a protective film. An addition of applied stress and/or environmental fluctuation can disturb the balance between the steel and this protective film, causing the fracture of the latter and leading to Stress Corrosion Cracking (SCC). The material studied is a cold drawn and rolled high strength steel composed of ferrite and spheroidized pearlite and has a strong microstructural anisotropy due to the specific cold work process. To investigate its susceptibility to SCC, Slow Strain Rate Tests (SSRT) were carried out on smooth and notched specimens allowing to separate crack initiation and crack propagation. The environment is an aqueous chloride solution saturated in CO2 at pH around 6. Tests were performed under open circuit potential (OCP) and at cathodic potential. Under OCP, localized anodic dissolution in shear bands is responsible for crack initiation. Under cathodic potential, crack initiation is delayed due to the absence of critical defect on the surface. Both fractographic analyses and results obtained on notched specimens showed that hydrogen plays an important role in crack propagation

Use of Pure Metals to Analyse Hydrogen Electrochemical Permeation in Steels

The electrochemical permeation method is often used to study hydrogen diffusion and trapping in steels ; unfortunately, there is great scatter in the experimental results. To identify the reason for this scattering, experiments were carried out usiug pure iron and pure palladium specimens. It is clearly shown that on the iron samples, surface phenomena are not controlled. In particular, the passive layer on the exit side induces variation of the hydrogen concentration on this side with time. On palladium samples, such phenomena can be controlled and stationary conditions as well as good reproducibility can be obtained. The use of a palladium coating on the exit side of ferrous samples can help to better control the surface phenomena during permeation experiments and so to obtain meaningful measurements