The discovery of new oil and gas reserves, at remote and hard to reach locations, makes imperative the development of new technologies to ensure effective exploitation of these resources. This exploitation is often performed at highly corrosive environments and equipment such as pipelines should have special mechanical and corrosion properties to guarantee safety levels in service. High-Strength Low Alloy (HSLA) steels are used in pipelines for transporting gas and oil. These steels are in constant exposure to acid environments containing hydrogen sulfide (H 2 S) and water, that can cause pipeline failures due to Hydrogen-Induced Cracking-HIC. The literature reports that Hydrogen-Induced Cracking in steels is normally tested in solutions containing acetic acid and/or, salts (sodium chloride and others) with addition of H 2 S. Chemical composition, steel refining processes and metal forming processes are responsible for the final microstructure of the steel and have effect on the hydrogen embrittlement resistance. The purpose of this work is to analyze and compare the corrosion resistance and HIC resistance, and compare of four materials: pipeline steel API 5L X65 for sour service, its welded junctions, pipeline steel API 5L X65 for non-sour service and pipeline steel plate API 5L X65. The materials were submitted to linear polarization test (Rp) and HIC resistance test according to NACE TM0284-2003 standard. Both tests were carried out with two different electrolytes: the solution A (acetic acid and sodium chloride) and solution B (synthetic seawater). Subsequently; the surface of the steels were evaluated by optical microscope and scanning electron microscopy in order to characterize the cracking modes and corrosion morphology. The Rp tests showed that the solution A is more aggressive than solution B, behavior attributed to the pH difference between solutions. Steel API 5L X65 for sour service had the highest corrosion resistance and pipeline steel API 5L X65 for non-sour service had the lowest. The HIC test and the surface examination revealed that in both solutions, pipeline steel API 5L X65 for sour service, the welded junctions and the pipeline steel plate API 5L X65 showed no cracks. On the other hand, pipeline steel API 5L X65 for non-sour service presented cracking mainly in the central region. The tests revealed that the cracks nucleated at the intergranular cementite in the banded structure. The good performance of the pipeline steel API 5L X65 for sour service was discussed in terms of the microstructure, formed by polygonal ferrite, acicular ferrite and M/A microconstituent. The performance of steel plate (for pipeline API 5L X65) was different. This material did not exhibit cracks in the matrix in spite of its banded microstructure. This result was discussed taking into account that the plate studied had a small amount of intergranular cementite. The results also showed that the solution B, as in the case of corrosion resistance tests, was less aggressive than solution A, because the cracks produced...
ResumoOs aços inoxidáveis supermartensíticos (AISM) foram projetados a partir dos aços inoxidáveis martensíticos de mais alto teor carbono, o seu desenvolvimento começou na década de 90. As características principais que os definem como supermartensíticos são: o baixo teor de carbono (C<0,03%) e adição de alguns elementos de liga, especialmente Ni e Mo, essas mudanças na composição química junto com os tratamentos térmicos adequados (têmpera e revenimento) fornecem excelentes propriedades mecânicas e resistência à corrosão melhorada, comparados com os aços inoxidáveis martensíticos UNS S41000 e S42000. Neste trabalho estudou-se o efeito da adição de Nb e da temperatura de revenimento (550°C, 575°C e 600°C) sobre a resistência à corrosão por pite e sobre o grau de sensitização através de dois AISM, um denominado SM2MoNb contendo 0,11% Nb e outro SM2Mo sem adição de Nb. Os resultados mostraram que o aço SM2MoNb apresentou, nas três temperaturas estudadas de revenimento, melhor resistência à corrosão por pite e menor grau de sensitização(GS), ou seja menor tendência à corrosão intergranular. Sabe-se que o Nb ajuda a diminuir o GS devido à inibição da precipitação de Cr23C6 por estabilização de carbono formando NbC em temperaturas mais altas, mas, na literatura especializada, não foram encontradas explicações quanto ao comportamento da corrosão por pite. Palavras-chave: Corrosão por pite; Grau de Sensitização, Aço Inoxidável supermartensítico; Temperatura de Revenimento EFFECT OF THE Nb ADDITION AND THE TEMPERING TEMPERATURE ON THE PITTING CORROSION RESISTANCE AND THE DEGREE OF SENSITIZATION OF SUPERMARTENSITIC STAINLESS STEEL 13Cr5Ni2Mo AbstractSupermartensitic stainless steels were developed from martensitic stainless steels with high C content, the production of Super Martensitic Stainless Steel (SMSS) started at the middle of 1990's. The main characteristic that defines them as supermartensitics are: The low carbon content (C<0,03%) and the addition of alloys, particularly Ni and Mo. Those chemical changes and the appropriate heat treatments (quenching and tempering) provide excellent mechanical and corrosion properties, when compared with martensitic stainless steels UNS S41000 and UNS S42000. In this work was investigated the effect of Nb and tempering temperatures (550°C, 575°C e 600°C) on the pitting corrosion resistance (Ep) and the degree of sensitization (DOS) of two supermartensitic stainless steels, SM2MoNb containing 0,11% Nb and SM2Mo without Nb. The results showed that the SM2MoNb steel exhibited, in the three temperatures, better corrosion resistance and low degree of sensitization, that means minor tendency to intergranular corrosion. It is known that Nb alloy decrease the DOS due to inhibition of Cr23C6 precipitation by carbon stabilization precipitating NbC in high temperatures, but , in the bibliography used were not found explications about the pitting corrosion behavior in these materials.
ResumoAços Inoxidáveis Supermartensíticos surgiram na indústria petroquímica como um material alternativo para confecção de dutos, devido principalmente ao sinergismo de suas propriedades mecânicas e de corrosão. Com esta nova demanda, tornou-se necessária a caracterização deste material em suas diversas propriedades, destacando-se neste trabalho o efeito da microestrutura na resistência à corrosão. Objetivou-se, portanto, o estudo da resistência à corrosão por pite de um aço inoxidável supermartensítico 13Cr-5Ni-2Mo-Nb em função da fração volumétrica de ferrita-delta (δ) presente na microestrutura. A variação da ferrita-delta foi alcançada através de tratamentos de recozimento, a 1.050°C, durante diferentes tempos (de 0,5h a 48h). As amostras foram submetidas a ensaios de polarização potenciodinâmica em eletrólito contendo 3,5% NaCl, naturalmente aerado. Quantificou-se, ainda, a porcentagem de ferrita em cada uma das condições através de ataque eletrolítico com solução 20% NaOH. Os resultados obtidos evidenciaram a diminuição desta fase com o aumento do tempo de recozimento, chegando praticamente à sua eliminação com tratamento de 48h. Paralelamente, a resistência à corrosão por pite deste aço foi melhorada de modo proporcional à diminuição da fase ferrita, resultando em um aumento de aproximadamente 115 mV da pior para a melhor condição, comprovando que, de fato, a presença de ferrita-delta prejudica as propriedades de resistência à corrosão localizada. Palavras-chave: Aços Inoxidáveis Supermartensíticos; Corrosão por Pite; Ferrita-Delta. Abstract Supermartensitic Stainless Steels have appeared at the petrochemical industry as an alternative material for pipelines production, due to the synergism of its mechanical and corrosion properties. With this new demand, a characterization of the different properties of this steel became necessary, among them the microstructure effect on the corrosion resistance. So, the aim of this work was study the pitting corrosion resistance of a 13Cr-5Ni-2Mo-Nb supermartensitic stainless steel as a function of delta-ferrite phase percentage at the microstructure. The changes of deltaferrite volume fraction was achieved through annealing treatments (1,050°C) with different times, by 4h up to 48h. The samples were submitted for potentiodynamic polarization, using a 3,5% NaCl electrolyte, with no deaeration. The delta-ferrite was quantified by optical microscopy, using electrolytic etching (20% NaOH) to reveal this phase. Obtained results showed a decrease of delta-ferrite percentage with increasing annealing time, reaching an almost free delta ferrite microstructure for 48h treatment. At the same time, the pitting corrosion resistance of this steel was improved proportionally to the delta-ferrite elimination, showing an average of 115 mV by the worst to the best condition, evidencing that the delta-ferrite phase present in the microstructure is detrimental to localized corrosion properties.
Once artículos publicados por el autor en la revista Engineering Failure Analysis fueron seleccionados para hacer parte de este libro. Los artículos fueron agrupados de acuerdo con sus respectivos sectores económicos: Transportes, Salud y Energía. La crítica de los artículos mostró la importancia de la publicación de casos de análisis de falla y de la existencia de una buena documentación microscópica, permitiendo explorar nuevas interpretaciones de los resultados y discutir detalladamente los mecanismos de fractura. El análisis del conjunto de fallas de cada sector permitió realizar algunos comentarios sobre la actuación de las respectivas agencias reguladoras en el Brasil. Se indicó que la publicación de casos de análisis de falla puede ayudar tanto en la construcción de un dialogo entre las partes involucradas en las fallas de componentes de ingeniería, como en la creación de un ambiente más seguro para los sectores industriales estratégicos de los países de América Latina.
ResumoAços ARBL são utilizados na fabricação de tubos para transporte de gás natural e petróleo e, devido ao ambiente agressivo, rico em H2S, em que são aplicados, devem resistir à fragilização por hidrogênio. No presente trabalho, verificou-se qual a influência do alongamento longitudinal dos grãos de um aço API X65, com baixo Mn, sobre a permeabilidade de hidrogênio. O material foi caracterizado com a utilização dos microscópios óptico e eletrônico de varredura e os ensaios de permeabilidade foram conduzidos com base na norma ASTM G 148-97. A microestrutura do aço era composta por ferrita e perlita, com baixo teor de inclusões arredondadas de Al e Ca e sem bandeamento. Com os resultados obtidos a partir dos ensaios de permeabilidade, calcularam-se os coeficientes de difusão efetiva do hidrogênio nas seções longitudinal e transversal do aço, e o valor do primeiro se mostrou levemente superior ao do segundo. Isto indica que pode haver influência do alongamento longitudinal dos grãos sobre a permeabilidade de hidrogênio. Palavras-chave: Aços ARBL; Baixo Mn; Permeabilidade de hidrogênio. GRAIN ELONGATION INFLUENCE ON HYDROGEN PERMEATION OF STEELFOR API X65 SOUR PIPE WITH LOW Mn CONTENT Abstract HSLA steels are used as pipelines for crude oil and natural gas transportation and, due to the sour environment in which they are applied, the hydrogen embrittlement resistance is a necessary property. In the present work, the influence of grain elongation on hydrogen permeation of steel for API X65 sour pipe with low Mn content was analyzed. Optical and Scanning Electron Microscopy techniques were used to characterize the material and the hydrogen permeation tests were conducted based on the ASTM G 148-97 standard. The steel has a microstructure composed of ferrite and pearlite, low content of rounded shape Al and Ca inclusions and do not show microstructure banding. The effective hydrogen diffusion coefficient was calculated for the longitudinal and transversal directions of the steel sheet, the former being slightly higher than the latter. This result indicates that the hydrogen permeation may be influenced by the grain alignment.
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