The research reported in this PhD thesis provides a novel approach to estimate 3D pitting corrosion kinetics of austenitic stainless steel with exposure to chloride-containing aqueous environments. A quasi-in-situ X-ray computed tomography (X-ray CT) approach was developed, with the aim of providing an experimental methodology to estimate 3D pitting corrosion kinetics under different exposure conditions. The first part summarises a set of preliminary investigations to identify the pitting corrosion behaviour of three austenitic stainless steels (type 303 bar, type 304 plate and type 304L wire) with different inclusion contents. All observed pit densities were related to the inclusion contents, providing confidence in moving to the next stage of the project, for conducting in-situ corrosion studies using X-ray CT. The second section describes the construction of an in-situ electrochemical cell for X-ray CT studies, the aim being to provide an experimental methodology to estimate 3D pitting corrosion kinetics. Pit growth kinetics of individual pits were estimated from segmented 3D X-ray CT data. The evolution of pit current densities, associated pit stability products, and diffusivity parameters over time were obtained. The study also showed that the kinetics of multiple pits could be estimated using this novel approach, based on separating the current response of each pit over time. This was obtained by electrochemical polarisation control and measuring the total current evolution. The third section discusses the effect of plastic strain on 3D pitting corrosion kinetics. Several in-situ X-ray CT experiments were conducted, with a focus on obtaining 3D pit growth, passivation, and re-activation kinetics, to elucidate the effect of applied strain on pit stability and growth. This section explains a possible mechanism for the re-activation of pre-existing corrosion pits, showing that pits grew more rapidly during reactivation than those grown before plastic strain was applied. A marked difference in pit morphology with fractured lacy metal covers was observed with the application of strain. The implications of this observation are discussed in light of stress corrosion crack nucleation mechanisms.

Le metal dusting est un type de corrosion catastrophique des alliages à base de fer, de nickel ou de cobalt. Il se caractérise par une dégradation de ces matériaux en une fine poussière de particules métalliques et de carbone graphitique, appelée « coke », pouvant également contenir des carbures et des oxydes. Ce phénomène a lieu lorsque le mélange gazeux est sursaturé en carbone (ac>>1), à des températures comprises entre 400°C et 800°C. Cinq alliages commerciaux austénitiques (800HT, HR120, Inconel 625, Inconel 690 et Inconel 693) et deux matériaux modèles fabriqués par SPS (NiFeCr et NiFeCr+Cu) sont testés dans deux environnements de metal dusting à 570°C. Le premier test est effectué sous pression atmosphérique dans un mélange CO-H2-H2O, le second dans une atmosphère CO-H2-CO2-CH4-H2O sous 21 bars de pression. La première composition est ajustée pour obtenir une activité en carbone et une pression partielle en dioxygène proches de celles de l’environnement sous haute pression. Après plus de 14 000 h heures d’exposition, l’alliage 625 n’est pas dégradé. Il présente une précipitation d’aiguilles de γ’’-Ni3Nb, le niobium migrant vers la surface suite à l’appauvrissement en chrome par oxydation. Le matériau NiFeCr+Cu présente une évolution microstructurale proche, le cuivre formant une couche continue à l’interface métal/oxyde. Le cuivre étant non-catalytique pour la formation de carbone, sa ségrégation en surface améliore la résistance du matériau. L’alliage 690 présente une carburation homogène sur toute la surface qui n’évolue pas dans le temps. L’alliage 693 présente au contraire une carburation très importante, de plus en plus profonde avec la durée d’exposition. Celle-ci s’explique par la formation d’une couche continue d’alumine de transition, métastable. Sa transformation en alumine α, stable, s’accompagne d’une contraction de la maille, fissurant la couche d’oxyde. L’atmosphère accède alors directement à la surface métallique, carburant l’alliage. La bonne tenue de cet alliage, malgré la fissuration de l’oxyde, s’explique par sa forte teneur en chrome et par la faible cinétique de la transformation allotropique à 570°C. Les alliages 800HT, HR120 et NiFeCr sont corrodés par piqûration. Pour l’alliage 800HT, celle-ci est simulée en surface par un modèle de germination-croissance dépendant du temps d’incubation des piqûres, de leur croissance et de la densité de piqûres. La prise en compte du volume des piqûres pour reproduire les pertes de masses enregistrées est concluante sous haute pression mais pas à pression atmosphérique. Cela met en exergue l’influence de la géométrie de l’échantillon (les échantillons testés à pression atmosphériques étant très attaqués sur les bords), et donc l’intérêt d’étudier la piqûration. Sous pression atmosphérique, la croissance latérale des piqûres se fait par oxydation des carbures tandis que la croissance en profondeur se fait par un mécanisme de graphitisation accélérée, lorsque le flux de carbone est suffisamment grand devant le flux d’oxygène. La graphitisation accélérée n’a lieu qu’en fond de fissure du fait du faible renouvellement de l’atmosphère. Les fissures se forment lors du cyclage thermique effectué toutes les 500 h pour caractériser les échantillons. Cela conduit à un faciès de corrosion constitué d’une oxydation interne fine et discontinue exposant directement à l’atmosphère l’alliage carburé, qui est alors graphitisé. Il en résulte l’apparition d’une succession d’anneaux de corrosion, un sur deux croissant en profondeur. La morphologie issue du mécanisme de graphitisation favorisée est visible sur toute la circonférence des piqûres formées sous haute pression. Le même mécanisme a donc lieu, globalement cette fois, le flux de carbone étant suffisamment grand devant le flux d’oxygène dès l’introduction dans le banc de corrosion. Les morphologies observées sont donc liées aux conditions expérimentales (température, atmosphère, débit) et à la procédure de suivi (retraits). / “Metal dusting” is a catastrophic corrosion phenomenon of Fe-, Ni- and Co-based alloys. It is characterised by the degraded of these materials into a dust of fine metallic particles and graphitic carbon, named “coke”, which can also contain oxides and carbides, depending on the alloy. This phenomenon occurs when the gas mixture is oversaturated in carbon (ac>>1), for temperatures between 400°C and 800°C. Five commercial austenitic alloys (800HT, HR120, Inconel 625, Inconel 690 and Inconel 693) and two model alloys fabricated by SPS (NiFeCr et NiFeCr+Cu) are tested under two metal dusting environments at 570°C. The first test is carried out under atmospheric pressure in a CO-H2-H2O gas mixture, while the second is performed at 21 bar in a CO-H2-CO2-CH4-H2O atmosphere. The first environment is adjusted to obtain a carbon activity and a dioxygen partial pressure similar to the ones of the second environment. After 14 000 h of exposure, 625 alloy is not degraded. Chromium depletion stemmed from oxide scale formation induces niobium diffusion to the surface and precipitation of needle-like γ’’-Ni3Nb precipitates below the alloy surface. NiFeCr+Cu alloy presents a close microstructural evolution, as copper forms a continuous scale at the metal/oxide interface. Its segregation induces an improved resistance of the material, copper being non-catalytic to carbon formation. 690 alloy presents an homogeneous carburation under its whole surface which does not deepen during further exposure. 693 alloy, however, presents an important carburisation, which increases with the exposure time. This can be explained by the formation of a continuous oxide scale composed of a metastable transient alumina. Allotropic transformation of this alumina in its stable form, α-alumina, induces lattice contraction. The oxide scale undergoes tensile stress, and cracks form. The atmosphere can then accede directly to the catalytic surface and carburise the material. Despite this, the macroscopically good behaviour of 693 alloy can be explained by its high chromium level and the low kinetics of the allotropic transformation at 570°C. 800HT, HR120 and NiFeCr alloys are degraded by pitting. Pitting on 800HT is modelled using a nucleation-growth model. It depends on pit incubation time, pit growth kinetics and pit density. Taking into account the volume of the pits to model the mass losses undergone by the alloys is concluding for the specimens tested under high pressure but not for those tested at atmospheric pressure. This shows the influence of the sample geometry (samples tested at atmospheric pressure are more attacked on the edges), hence the interest to study corrosion via pitting. For tests at atmospheric pressure, pit lateral growth occurs by oxidation of internal carbides while pit inward growth stems from an enhanced graphitisation mechanism, when the carbon flux is high enough compared to the oxygen flux. Enhanced graphitisation takes place at the bottom of cracks formed through the internal oxidation zone due to the sample cooling performed every 500h for characterisation. The atmosphere is hardly renewed at the bottom of the crack. It leads to a thin, discontinuous oxidation layer exposing directly to the atmosphere the carburised alloy, which is then graphitised. This results in a succession of corrosion rings, one from two being deep, due to the combination of cracking and enhanced graphitisation. The morphology observed under atmospheric pressure due to enhanced graphitisation is also visible under high pressure, but on the entire pit circumference. It reveals that the same mechanisms takes place but on the entire pit, the carbon flux being high enough compared to the oxygen flux, right from its introduction in the corrosion rig. The two observed pit morphologies are then strongly linked to experimental conditions (temperature, gas mixture, gas flow) and the experimental procedure (thermal cycling induced by regular withdrawals).

The research reported in this doctoral thesis involves constructing physiochemical models that reproduce the transport behaviour of aqueous chemical species present in environments relevant to the oil and gas industry to gain an improved insight into the local electrochemistry near the electroactive surface (uniform corrosion) or inside the pit (pitting corrosion). The first part of the project involved constructing physiochemical models with one dimensional geometry with aqueous chemical species and chemical and electrochemical processes observed in oxygen (O2) containing brine environments to determine the changes in the local electrolyte composition and the potential within an initiated pit for a variety of external physical and chemical conditions. It was determined that the bottom of the pit suffers greatly from the effects of iR drop (Ohmic drop) if the pit geometry is taken to be macroscopic. The model was extended to include additional aqueous chemical species in conjunction with the chemical and electrochemical processes observed in carbon dioxide (CO2) rich environment to investigate the effects of CO2 on the local electrolyte chemistry at the bottom of the pit. It was found that the proton reduction electrochemical process on its own was incapable of supplying the high currents experimentally measured in CO2 environments via the buffering effect. The second part of the project was to investigate the influence of different experimental conditions on the polarisation behaviour of near static carbon steels in CO2 saturated brine electrolyte via multiple electrochemical measurement techniques. The key observation from this study was the presence of two distinct mass transport limited regions on the cathodic polarisation curve at natural pH (3.775). From the physiochemical model fitted to the experimental cathodic curve, the first mass transport limited region, occurring at lower cathodic potentials, was identified to be the direct reduction of carbonic acid while the second wave, occurring at slightly higher cathodic potentials, was shown to be the direct reduction of aqueous carbon dioxide. Based on the polarisation scans under forced convection, the rate of the direct reduction of carbon dioxide was determined to be under neither potential nor mass transport control. The third part of the project involved extending the existing one dimensional models to include the precipitation of salt films (iron chloride – FeCl2(s) and iron carbonate – FeCO3(s)) in O2 and CO2 saturated brine electrolyte respectively along with the capability to track their respective thickness. Furthermore, the ability of the underlying metal to undergo a change in its state from active to passive is implemented in the model via a set of rules based on the Pourbaix diagram. It was determined that the precipitation of salt films is greatly influenced by the mass transport with no or minimal thickness observed under even natural convection conditions. Furthermore the successful precipitation of salt film was determined to be a precursor step to the metal attaining passivation.

Os aÃos inoxidÃveis austenÃticos (AIAs) sÃo largamente aplicados nas indÃstrias de alimentos, transportes nuclear, petroquÃmica devido à adequada combinaÃÃo de resistÃncia mecÃnica, conformabilidade e resistÃncia à corrosÃo. Dentre estes tipos de aÃo, destaca-se o AISI 301 por sua resistÃncia mecÃnica superior. Entretanto, este tipo de aÃo apresenta um dos piores desempenhos em termos de corrosÃo. Toda a resistÃncia à corrosÃo dos AIAs se baseia em sua camada de filme passivo contendo Cr203 que pode facilmente ser destruÃda em ambientes contendo cloreto. Neste trabalho, estabeleceu-se a meta de estudar os aÃos AISI 301 LN e 316L em respeito Ãs mudanÃas na microestrutura por imposiÃÃo de deformaÃÃo e seu efeito na corrosÃo por pites, visto que o AISI 301LN foi escolhido como material base dos vagÃes que servirÃo o sistema de transporte metropolitano de Fortaleza. Empregou-se difraÃÃo de raios âX, metalografia quantitativa, microscopias Ãtica, eletrÃnica de varredura e de forÃa atÃmica para caracterizar a microestrutura, textura cristalogrÃfica, caracterizaÃÃo magnÃtica, microdureza e ensaio de imersÃo em FeCl3 6H2O para caracterizar o comportamento dos dois aÃos em corrosÃo por pites. A deformaÃÃo provocou a formaÃÃo de martensita â no aÃo 301LN e encruamento da austenita. Isto provocou o mais baixo desempenho em corrosÃo por pites. A textura cristalogrÃfica forneceu indÃcios para inferir que a transformaÃÃo austenita-ferrita se deu obedecendo a relaÃÃo de KURDJUMOV-SACHS.

Using ultrasonic guided waves can achieve long range inspection along the pipeline rapidly. The presence of defect or other features on the pipe were identified by analyzing the reflected echoes as well as mode conversion phenomena. However, it is difficult for guided wave to find a minor corrosion, such as pitting. Therefore, a study of the reflection of torsional T(0,1) mode from pits on the pipe has been carried out and an advanced signal processing method wavelet transform is adopted to process the reflected echoes in this study. In order to understand that characteristic of the reflected echoes of pits, the propagation of guided wave T(0,1) through pits was simulated by the finite element method. The frequency response of the signal reflected from the pits with different sizes was discussed both by finite element method and experimental method. Then, we discuss two types of pitting including regular- distributed pitting and the random-distributed pitting. We not only discuss the relation between the axial length of regular pitting and wave length of the T(0,1) mode, but also the reflected singal of four random pittings. The experiments were performed on 3 inch carbon steel pipe for measuring the reflected signals from different pittings with different frequencies. The results of the simulation, indicate that the wave was easily scattered by pitting because the shape of geometry. It is the reason of reducing the amplitude of reflected signals. To receive a dominate signal reflected from pitting, the excitation with higher frequency was choosen within the frequency range of interest. The experimental results indicate that the signals would be too weak to be detected by guided waves when the estimated cross sectional loss of the pitting is less than 2 percent. However, the results after wavelet transform showed the feasibility of improving the abilities of detecting minor pitting. In the case of regular pitting, the maximu value of the reflected signal appeared when the axial length of the pitting equals to the 66 % of the wavelength. It is because the constructive interference. The mode conversion phenomena is another behavior of the reflected signal cased by the non-axissymetric geometry of the pitting. As for the random pitting, The reflected echo shows different behavior with the regular pitting. The amplitude of the signal is bigger with lower frequency we use. The different level of random pitting on the pipe were also identified successfully by wavelet transform. Understanding the phenomena of interaction between the guided wave and the pitting is helpful to the guided wave inspection.

No description available.

The fatigue of materials and structures is a phenomenon in which surface condition plays an important role; specially in initiating the micro-cracks. Moreover, the structural components are also subjected to aggressive environments and hence are susceptible to corrosion phenomenon. Therefore, it is necessary to protect them against the corrosion phenomenon. For aluminium alloys used in aeronautical applications, anodization is widely used surface treatment to increase the corrosion resistance. Despite the benefits obtained in terms of enhanced corrosion resistance, the anodizing process has a damaging effect on the fatigue performance of the base material. One of the reasons for this decrease in fatigue life is associated to the degradation of surface condition during the anodizing process. In this work, we have studied the influence of machining and anodization on fatigue behaviour of aluminium alloys 2214 and 7050. We showed experimentally that for 2214 alloy the process of anodization-sealing reduced the fatigue life considerably while for 7050 alloy the process of pickling is the major cause in decreasing the fatigue life. We have also developed life prediction model for anodized alloys. The model is based on determination of concentration of stress ‘Kt’ generated by the surface condition. The developed model integrates the aspects of multi-site initiation, coalescence and regime of short crack propagation.

Neste trabalho foram desenvolvidos procedimentos de soldagem pelo processo GMAW (CCC e Pulsado) para o aço inoxidável superduplex UNS S32760. Esta pesquisa justifica-se pelo fato de que o desempenho dos aços duplex/superduplex pode ser severamente afetado pelas alterações metalúrgicas causadas pelos ciclos térmicos impostos durante a soldagem. Sendo assim, é constante a busca pelo desenvolvimento de processos de soldagem e procedimentos que mantenham as propriedades mecânicas e de resistência à corrosão em patamares que atendam os requisitos cada vez maiores da indústria petroquímica. O objetivo deste trabalho é avaliar a resistência à corrosão do aço inoxidável superduplex UNS S32760 soldado pelo processo GMAW, empregando transferência metálica por Curto Circuito Controlado na raiz e Pulsada nos passes de enchimento e acabamento, sob três diferentes misturas gasosas (Ar-He-CO2, Ar-N2-CO2 e Ar puro). Também foi analisada a influência de diferentes condições de acabamento na solda de raiz (raiz como soldada, escovada, passivada e usinada) no ensaio de corrosão por pite. Foram realizados ensaios de corrosão segundo a norma ASTM G-48 e ensaios para a obtenção da Temperatura Crítica de Pite (CPT) segundo a norma ASTM G-150. Para melhor avaliar e discutir os resultados dos ensaios de corrosão foi realizado a quantificação de fases (contagem de ferrita/austenita), por microscopia ótica, além de avaliação da tenacidade por meio de ensaio Charpy. Os resultados mostraram que o processo GMAW não convencional, apesar do seu uso ainda estar bastante incipiente na indústria petroquímica, pode suprir a necessidade de um processo de alta produtividade com baixa energia de soldagem capaz de produzir juntas metalurgicamente adequadas. A preparação da superfície afeta diretamente os resultados no ensaio de corrosão, sendo o melhor acabamento (usinado) mais benéfico à resistência à corrosão. / In this work, welding procedures were developed using the GMAW (CCC and Pulsed) process for the UNS S32760 superduplex stainless steel. This research is justified by the fact that the performance of the duplex/superduplex steels can be severely affected by metallurgical changes caused by thermal cycling imposed during welding. So, its constant the search of the development of process and procedures to maintain the mechanical properties and corrosion resistance at levels that meet the increasing requirements of the petrochemical industry. The objective of this job was the evaluate the corrosion resistance of superduplex stainless steel UNS S32760 welded the GMAW process, using controlled short circuit metal transfer for the root and pulsed arc for the filling and capping runs under three different shielding gás (Ar-He-CO2, Ar-N2-CO2 and Ar). Also analyzed the influence of different finishing conditions of the weld root (as welded, brushed, pickled and machined) in the pitting corrosion test. Corrosion tests were performed according to ASTM G-48 and testing for the obtaining of critical pitting temperature (CPT) according to ASTM G-150. To better evaluate and discuss the results of corrosion tests was performed to quantify phases (counting the ferrite / austenite), by optical microscopy, and valuation of toughness by Charpy test. The results showed that non conventional GMAW process, although its use still to be quite incipient in petrochemical industry, it can supply the need of a process of high productivity with low heat input capable to produce committees appropriate metallurgy. The preparation of the surface directly affects the results in a corrosion test, being the best finish (machined) better resistance to corrosion.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Com o objetivo de investigar a influência que o molibdênio exerce nas propriedades das camadas passivas das armaduras de aços austeníticos, barras redondas dos aços UNS S30400 e 31600 foram submetidas à ação de soluções que simulam as que são encontradas nos poros de concreto. Para caracterizar o efeito do molibdênio na resistência daqueles aços à corrosão por pites, foram conduzidos ensaios de circuito aberto, polarização anódica, espectroscopia de impedância eletroquímica, análise de Mott-Schottky, tração em baixa taxa de deformação e análise das superfícies por microscopias óptica e eletrônica de varredura. Os resultados obtidos demonstram que, de uma forma geral, o aço UNS S30400 (sem molibdênio) apresentou maior resistência à corrosão localizada que o UNS S31600 (2% de Mo) nas soluções alcalinas cloretadas naturalmente aeradas, comportamento este inverso ao que se observa nos mesmos materiais quando submetidos a soluções cloretadas neutras ou ácidas. / Aiming to investigate the influence that molybdenum has on properties of passive layers of austenitic steels, round bars of UNS S30400 and S31600 steels were subjected to the action of solutions that simulate those found in the concrete pores. To characterize the effect of molybdenum on the resistance of those steels to pitting corrosion, tests were conducted in open circuit, anodic polarization curves, electrochemical impedance spectroscopy, Mott- Schottky analysis, slow strain rate test and surface analysis by optical microscopy and scanning electron microscopy. The obtained results showed that, grosso modo, the stainless steel UNS S30400 (Mo free) has higher corrosion resistance than UNS S31600 (2% Mo) in naturally aerated alkaline solutions containing chloride ions, which characterizes an inverse behaviour in relation to the same materiais in neutral or acidic solutions.

Neste trabalho foram desenvolvidos procedimentos de soldagem pelo processo GMAW (CCC e Pulsado) para o aço inoxidável superduplex UNS S32760. Esta pesquisa justifica-se pelo fato de que o desempenho dos aços duplex/superduplex pode ser severamente afetado pelas alterações metalúrgicas causadas pelos ciclos térmicos impostos durante a soldagem. Sendo assim, é constante a busca pelo desenvolvimento de processos de soldagem e procedimentos que mantenham as propriedades mecânicas e de resistência à corrosão em patamares que atendam os requisitos cada vez maiores da indústria petroquímica. O objetivo deste trabalho é avaliar a resistência à corrosão do aço inoxidável superduplex UNS S32760 soldado pelo processo GMAW, empregando transferência metálica por Curto Circuito Controlado na raiz e Pulsada nos passes de enchimento e acabamento, sob três diferentes misturas gasosas (Ar-He-CO2, Ar-N2-CO2 e Ar puro). Também foi analisada a influência de diferentes condições de acabamento na solda de raiz (raiz como soldada, escovada, passivada e usinada) no ensaio de corrosão por pite. Foram realizados ensaios de corrosão segundo a norma ASTM G-48 e ensaios para a obtenção da Temperatura Crítica de Pite (CPT) segundo a norma ASTM G-150. Para melhor avaliar e discutir os resultados dos ensaios de corrosão foi realizado a quantificação de fases (contagem de ferrita/austenita), por microscopia ótica, além de avaliação da tenacidade por meio de ensaio Charpy. Os resultados mostraram que o processo GMAW não convencional, apesar do seu uso ainda estar bastante incipiente na indústria petroquímica, pode suprir a necessidade de um processo de alta produtividade com baixa energia de soldagem capaz de produzir juntas metalurgicamente adequadas. A preparação da superfície afeta diretamente os resultados no ensaio de corrosão, sendo o melhor acabamento (usinado) mais benéfico à resistência à corrosão. / In this work, welding procedures were developed using the GMAW (CCC and Pulsed) process for the UNS S32760 superduplex stainless steel. This research is justified by the fact that the performance of the duplex/superduplex steels can be severely affected by metallurgical changes caused by thermal cycling imposed during welding. So, its constant the search of the development of process and procedures to maintain the mechanical properties and corrosion resistance at levels that meet the increasing requirements of the petrochemical industry. The objective of this job was the evaluate the corrosion resistance of superduplex stainless steel UNS S32760 welded the GMAW process, using controlled short circuit metal transfer for the root and pulsed arc for the filling and capping runs under three different shielding gás (Ar-He-CO2, Ar-N2-CO2 and Ar). Also analyzed the influence of different finishing conditions of the weld root (as welded, brushed, pickled and machined) in the pitting corrosion test. Corrosion tests were performed according to ASTM G-48 and testing for the obtaining of critical pitting temperature (CPT) according to ASTM G-150. To better evaluate and discuss the results of corrosion tests was performed to quantify phases (counting the ferrite / austenite), by optical microscopy, and valuation of toughness by Charpy test. The results showed that non conventional GMAW process, although its use still to be quite incipient in petrochemical industry, it can supply the need of a process of high productivity with low heat input capable to produce committees appropriate metallurgy. The preparation of the surface directly affects the results in a corrosion test, being the best finish (machined) better resistance to corrosion.