Stress corrosion cracking (SCC) susceptibility of the phosphorus de-oxidized copper has been evaluated in synthetic seawater polluted by sulfides using slow strain rate test (SSRT). The effect of concentration of sulfide, temperature, and applied cathodic and anodic potentials on the final strain values and maximum stress were also studied. No cracks were found under the tested conditions. The final strain and maximum stress values decreased but not significantly, with increase in the temperature, applied anodic potential and sulfide concentration. The observed effect is due to the section reduction by uniform corrosion. Lateral cross section and microscopic examination of the fractured specimen ruled out the existence of the localized corrosion. Electrochemical measurements showed that the Cu2S film is not a protective film and also exhibits a mass transfer limitation to the inward diffusion of the sulfides. Based on these results the reasons for the absence of cracking are also discussed.

Stress corrosion cracking (SCC) susceptibility of the phosphorus de-oxidized copper has been evaluated in synthetic seawater polluted by sulfides using slow strain rate test (SSRT). The effect of concentration of sulfide, temperature, and applied cathodic and anodic potentials on the final strain values and maximum stress were also studied. No cracks were found under the tested conditions. The final strain and maximum stress values decreased but not significantly, with increase in the temperature, applied anodic potential and sulfide concentration. The observed effect is due to the section reduction by uniform corrosion. Lateral cross section and microscopic examination of the fractured specimen ruled out the existence of the localized corrosion. Electrochemical measurements showed that the Cu2S film is not a protective film and also exhibits a mass transfer limitation to the inward diffusion of the sulfides. Based on these results the reasons for the absence of cracking are also discussed.

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L'exploitation des champs de pétrole et de gaz sous haute pression (HP) et haute température (HT) a augmenté ces dernières années, nécessitant de réévaluer les risques de corrosion dans ces milieux de plus en plus sévères. Afin de contribuer à une meilleure évaluation des risques de rupture différée des aciers en présence d'H2S (SSC, Sulfide Stress Cracking) dans ces conditions, trois axes de recherche ont été suivis. Nous avons d'abord identifié un besoin d'amélioration de prédiction des conditions corrosives sous haute pression et haute température, et en particulier pour le calcul du pH in situ. Un modèle a été développé ; il prend en compte le comportement non-idéal des phases en équilibre, et permet un calcul plus précis du pH et de la fugacité des gaz acides à haute pression et haute température. Dans un deuxième temps, nous avons étudié l'effet de la pression partielle de CO2 (PCO2) sur les réactions de surface et sur les risques de SSC. Cette étude, réalisée à l'aide de mesures électrochimiques en l’absence d’un film de sulfure de fer, a permis de montrer que la présence de CO2 augmente sensiblement les cinétiques des réactions cathodiques à la surface de l'acier ainsi que le chargement en hydrogène, en particulier lorsque la pression partielle en H2S (PH2S) est faible. Enfin, des essais SSC ont été mis en œuvre dans des conditions fixes de pH et de PH2S, en faisant varier PCO2 entre zéro et 100 bar. L'objectif était de vérifier que la présence de CO2 sous forte pression augmentait bien les risques de fissuration, comme prévu par les résultats des essais électrochimiques. Les difficultés liées à la mise en œuvre d'essais en autoclave sous pression n'ont pas permis d'apporter une conclusion définitive. Néanmoins, ces travaux montrent qu'il peut exister un risque de sous-estimation de la sévérité des milieux dans les pratiques conventionnelles, lorsque PCO2 est significativement plus élevée que PH2S. Dans ces conditions spécifiques, les résultats de ce travail peuvent servir à améliorer les critères de choix de matériaux pour les milieux HP/HT. / The production of high pressure (HP) and high temperature (HT) wells has considerably increased in the last decade. It is therefore needed to reassess the risks of corrosion in always more severe environments. This work was three fold to better assess the risk of Sulfide Stress Cracking (SSC) in these environments. Firstly, there was a need to improve prediction methods for the evaluation of HP/HT environments severity, especially the in situ pH calculation. A model was which taking into account the non-ideal behaviour of gas and liquid phases in equilibrium. The determination of the in situ pH and the acid gas fugacity at high pressure and high temperature is more accurate. In a second part of the work, the impact of CO2 partial pressure (PCO2) on surface reactions and hence on the risk of SSC was examined. Electrochemical and hydrogen permeation measurements in the absence of an iron sulphide film showed that CO2 induces an increase of both cathodic reactions kinetics and hydrogen charging in the steel, especially at low H2S partial pressure (PH2S). In the last part of this work, SSC tests were performed at constant pH and constant PH2S, with various PCO2 from 0 to 100 bar. The objective was to experimentally confirm that increasing PCO2 increases the SSC risk, as inferred from the electrochemical study. Unfortunately, experimental artefacts linked with autoclave test conditions did not lead to clear conclusions on this point. However, this work shows that conventional tools might lead to underestimate SSC risks at high PCO2 and low PH2S. In these specific conditions, the new results presented in this report may contribute to improve materials selection criteria for high pressure and high temperature conditions.

Made available in DSpace on 2016-06-02T20:34:19Z (GMT). No. of bitstreams: 1 3075.pdf: 8688023 bytes, checksum: 800e01878f1f0186b361618d6115dccd (MD5) Previous issue date: 2009-09-08 / Universidade Federal de Sao Carlos / This work presents a study of the 1020 and 1040 carbon steel corrosion in aqueous solution containing dissolved H2S in order to investigate the effect of H2Saq and HSaq species. Some different electrolytes were used: acetic acid buffer and phosphate buffer in presence or absence of 3.5 wt% NaCl solution containing 10, 25, 50, 100, 200, 500, 1000 and 2000 ppm of Na2S. The corrosion steel was described by means electrochemical impedance spectroscopy (EIS), potential polarization curves (PC), open circuit potential (Eoc) and in situ optical microscopy. Other experiments were carried out seeking to understand the steel corrosion process in sulfide solution. Thus, the influence of the 4 factors together (sulfide species, concentration, presence of chloride and steel type) was assessed by the interpretation of interactions and main effects in an experiments using factorial design. The characterization of the films was performed by electrochemical impedance spectroscopy in situ and ex situ techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The EIS measurements showed two distinct regions, high and low frequencies and the data analysis as a combination of diffusion and charge transference resistance, capacitances and constant phase elements (CPE). This parameters allow studying the corrosion of iron in presence of different sulfide species. The influence of total H2S concentration as well as the effect of immersion time on the corrosion was investigated too. In presence of the H2S the Rct values increase initially with immersion time but then decrease gradually. We concluded that there is formation of a protective film that reaches its maximum thickness after 3 h, and this layer is later dissolved to grow back again until reaching an equilibrium value at immersion time over 10 h. If the sulfide films has many porous, the capacitance presents high values to the corrosion process. In this case the number of defects of the sulfide films is proportional to increasing of corrosion. For the factorial design it was concluded that the species of sulfide is the most relevant factor for the corrosion process, followed by concentration and the presence of chloride in the solution. In the presence of H2S happens to active dissolution of iron and in the presence of HS- can occur the formation of a protective film. By images of in situ optical microscopy, carried out during the electrochemical measurements, one can observe the dissolution of ferrite in the pearlitic region and a formation of iron sulfide films with different colors. Morphological analysis and classification of surface corrosion damage, carried out by digital processing of images, show that happens grain-boundaries corrosion, pitting corrosion and formation of sulfide film initially over pearlitic grains. / Este trabalho apresenta o estudo da corrosão dos aços carbono 1020 e 1040 em solução aquosa contendo H2S dissolvido, em diferentes pHs de modo que se tivesse em solução as espécies H2Saq e HSaq. Os eletrólitos usados foram: tampão acetato e fosfato, na presença e ausência de 3,5% de NaCl em uma solução contendo 10, 25, 50, 100, 200, 500, 1000 e 2000 ppm de Na2S. A corrosão do aço foi estudada por meio de espectroscopia de impedância eletroquímica (EIE), curvas de polarização (CP), medidas de potencial de circuito aberto (Eca) e microscopia óptica in situ. Outros experimentos foram realizados buscando entender o processo de corrosão do aço em meio de sulfeto. Assim, a influência dos 4 fatores (espécie de sulfeto, concentração, presença de cloreto e tipo de aço) foi avaliada pela interpretação das interações e efeitos principais dentro de experimentos usando planejamento fatorial. A caracterização dos filmes foi realizada por espectroscopia de impedância eletroquímica in situ e por técnicas ex situ, como difração de raios X (DRX), microscopia eletrônica de varredura (MEV) e espectroscopia de energia dispersiva (EDS). As medidas EIE apresentaram duas regiões distintas, em alta e baixa frequências e a interpretação dos resultados envolveu uma combinação de parâmetros de circuitos equivalentes relacionados à difusão de espécies através do filme, resistências à transferência de carga, capacitâncias e elementos de fase constante (CPE). Esses elementos foram utilizados procurando descrever o mecanismo de corrosão do ferro na presença das diferentes espécies de sulfeto. A influência da concentração de H2S e o efeito do tempo de imersão da amostra de aço no meio corrosivo foi investigado. Na presença do H2S os valores Rct apresentam um aumento inicial e depois diminuem com tempo de imersão. Conclui-se que há a formação de uma película protetora que atinge uma espessura máxima em 3 h e esta camada então se dissolve e volta a crescer novamente, até atingir um valor de equilíbrio com o tempo de imersão de 10 h. Valores elevados de capacitância foram obtidos para filmes porosos e que apresentaram um grande número de defeitos levando a um aumento da corrosão. Pelo planejamento fatorial concluiu-se que a espécie de sulfeto é o fator mais relevante para o processo de corrosão, seguido da concentração e da presença de cloreto no meio. Na presença de H2S ocorre a dissolução ativa do ferro e na presença de HS- pode haver a formação de um filme protetor. Pelas imagens de microscopia óptica in situ, realizadas durante as medidas eletroquímicas, pode-se observar a dissolução da ferrita na região da perlita e a formação dos filmes de sulfeto de ferro com diferentes cores. A análise morfológica e classificação dos tipos de corrosão de superfície, realizadas por processamento digital das imagens, mostraram que ocorre a corrosão intergranular e por pites e que a formação do filme ocorre inicialmente sobre uma das fases do aço carbono, os grãos perlíticos.

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