Stress corrosion cracking

Stress corrosion cracking and hydrogen cracking are two important forms of environmental cracking which can cause serious failures in the oil and gas, pipeline, process and many other industries. Both forms of cracking result in normally ductile metals failing in a brittle manner. In many if not most cases, analysis clearly shows which of these failure modes has occurred. However, there can be situations where the two can be confused. This paper reviews the differences and similarities between the two modes, concentrating on the metals and environments, especially steels in sulphide environments, where confusion can arise. It looks at the various forms of cracking, the nature of cracking, its causes and how it is prevented. Sulphide stress cracking can be especially troublesome as, unlike other forms of hydrogen damage, the cracking initiates at the surface. A review of the mechanism of sulphide stress cracking is given, along with its relationship to stepwise cracking. There can also be confusion between these two forms of damage, as a case study from the literature shows.

Hydrogen evolution and permeation occur during electroplating, corrosion, and cathodic protection. Hydrogen accumulates in areas of high stress and may reach a critical concentration, potentially causing fractures and catastrophic damage. This chapter describes hydrogen permeation and hydrogen-induced damage in metals and alloys. It also discusses hydrogen evolution kinetics, theoretical diffusion solutions, and basic hydrogen permeation models. Models are used as a diagnostic tool to determine the effectiveness of various metals and alloys as hydrogen permeation inhibitors. It then explains experimental atomic hydrogen permeation transient determination and hydrogen absorption rate constants and diffusivity evaluation into metals using case studies. Hydrogen embrittlement, hydrogen-induced cracking, hydrogen blistering, and hydrogen stress cracking are also discussed to show the relationship to hydrogen permeation and hydrogen-induced cracking mechanisms. Finally, various techniques used to prevent and control hydrogen damage of metals and alloys are described.

The aircraft aluminium alloys generally present low weldability by traditional fusion welding process. The development of the friction stir welding has provided an alternative improved way of satisfactorily producing aluminium joints, in a faster and reliable manner. In this present work, the influence of process and tool parameters on tensile strength properties of AA7075-T 6 joints produced by friction stir welding was analysed. Square butt joints were fabricated by varying process parameters and tool parameters. Strength properties of the joints were evaluated and correlated with the microstructure, microhardness of weld nugget. From this investigation it is found that the joint fabricated at a tool rotational speed of 1400 rpm, welding speed of 60 mm/min, axial force of 8 kN, using the tool with 15 mm shoulder diameter, 5 mm pin diameter, 45 HRc tool hardness yielded higher strength properties compared to other joints.

Many efforts have been made to understand the effects of hydrogen on steels, resulting in an abundance of theoretical models and papers. However, a fully developed and practically applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen damage. In practice, it is observed that different types of damages to industrial boiler components have been associated with the presence and localization of hydrogen in metals. In this paper, a damaged boiler tube made of grade 20 – St.20 (or 20G, equivalent to AISI 1020) was investigated. The experimental research was conducted in two distinctive phases: failure analysis of the boiler evaporator tube sample and subsequent postmortem analysis of the viable hydrogen embrittlement mechanisms (HE) in St.20 steel. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack (HTHA) during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, tube wall thickness measurement, tensile testing, hardness measurement, impact strength testing (on in-strumented Charpy machine), analysis of the chemical composition of corrosion products – deposit and the microstructural characterization by optical and scanning electron microscopy – SEM/EDX. The HTHA damage mechanism is a primary cause of boiler tube fracture. Based on the multi-scale special model, applied in subsequent postmortem investigations, the results indicate a simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. The model is based on the correlation of mechanical properties to the SEM fractography analysis of fracture surfaces.

Corrosion is an unavoidable but a controllable process. Due to the issues of toxicity of substances like chromate inhibitors, there is an increasing interest in exploration and utilization of eco-friendly inhibitors, which are also known as green inhibitors. This review briefly discusses some of the interesting features of the green inhibitors reported during the last decade.

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel. #

Transport of corrosion products into pores and cracks in concrete must be considered when predicting corrosion induced cracking in reinforced concrete structures, since this transport significantly delays the onset of cracking and spalling by reducing the amount radial displacement displacement imposed on the concrete at the steel/concrete interface. We aim to model this process by means of a transport-structural approach, whereby the transport part is driven by a pressure gradient generated by the volumetric expansion due to the transformation of steel into corrosion products. This pressure driven transport was introduced in an analytical axisymmetric thickwalled cylinder model and a numerical network approach. The influence of cracking and permeability on corrosion induced cracking process with increasing inner displacement is investigated with these two approaches.

Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.

Austenitic stainless steels with their good weldability, superior corrosion resistance and excellent performances in higher temperatures are an important material for engineering applications in industrial plants. Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels is a critical failure mechanism where cracking can result from sensitisation of certain grain boundaries after heat treatment (e.g. post-weld stress relief) or fast neutron irradiation in nuclear plant. Sensitisation is a decrease in the local resistance to stress corrosion, to a degree that depends on the grain boundary structure. Developments of predictive models for stress corrosion crack nucleation require more information about the effect of several external parameters (e.g. stress and time) on the likely extent of crack growth. Understanding is also required about how the grain boundary crystallography and the orientations of grain boundary plane and its surrounding grains affect crack propagation.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.

Thick walled components such as high pressure (HP) steam turbine casings operating under high parameter conditions are subjected to a complex stress state. As a result of that stress state, some parts of HP turbine casing undergo to the creep fatigue caused by the combination of thermal fatigue resulted from repeated start/stop operation and the creep which occurs during long-term operation at high temperature and high-pressure. It is well known that domestic thermal power plants have been in use over 100000 h which means that significant cost is required not only for maintenance, but often for renewal of equipment. Based on comprehensive investigation, the results of residual life assessment of one high pressure steam turbine casing, which belongs to the older turbine generation, taking into account simultaneous action of thermal fatigue and creep, are presented in this paper. Also, the critical flaw crack size of HP turbine casing is determined because this parameter has a strong influence on casing integrity and residual life. The results of residual life assessment provide not only a basis for further maintenance, but also estimated time for reparation or renewal.

This study aims to understand the mechanism of increased SCC susceptibility of machined 304L stainless steel in chloride environment. Austenitic stainless steel grade 304L was surface machined up to a depth of 0.5 mm from the surface. In depth characterization was carried out by optical, scanning electron microscopic technique, hardness measurement and by EBSD and XRD studies. The stress corrosion cracking (SCC) susceptibility was estimated by exposing constant strained samples made up of machined and unmachined stainless steel to 5 M H2SO4 + 0.5 M NaCl solution at room temperature (28 °C) until cracking. In addition strips of machined and unmachined stainless steel were exposed to boiling MgCl2 solution as per ASTM G36 to understand the effect of residual stress and strain generated due to machining on the SCC susceptibility. The study reveals that surface machining results in extensive grain refinement, strain induced martensite transformation and high magnitude of plastic deformation near the surface.

Concrete is a homogenous mixture and cracks in concrete are inevitable so there is a need for repair which affects the economic life of any structure. To overcome this problem an inherent biomaterial is developed, a self-repairing material which can remediate the cracks in concrete. Bacterial concrete is a technique which is highly desirable because the calcium precipitation is induced as a result of microbial activities. This helps in increasing the strength and durability of concrete. As per the results, it is clearly observed that there is increase in compressive strength, tensile strength and durability in bacterial concrete as compared with normal concrete. This is the main objective of the bacterial concrete for which it was introduced. Various tests which are carried out to study these properties of concrete are compressive strength test, Split tensile test. Scanning Electron Microscope (S.E.M) is used to study the growth of bacteria in the concrete. It is observed that for bacterial proportion 10 5 cells (24 ml of bacteria in 1000ml), there is significant increase in compressive strength of the bacterial concrete i.e. around 25% increase in strength as compared with normal concrete. For this purpose bacteria used is Bacillus Subtilis.

Este trabajo presenta los resultados del análisis de las causas de falla por corrosión bajo tensión de los compensadores de dilatación de la tubería de acero inoxidable de un intercambiador de calor que opera a temperaturas entre 70 oC y 900 oC, donde se presentó una falla en algunos de los compensadores que se manifestó por agrietamiento de los mismos. Se realizaron ensayos de composición química, icroscopía óptica y SEM. Con base en los resultados obtenidos se concluyó que la falla por agrietamiento se debió al fenómeno de corrosión bajo tensión, que a su vez se produjo por la sensibilización del acero inoxidable, debido a la exposición a las altas temperaturas de trabajo y su fluctuación a otras más bajas. Como resultado del análisis para prevenir otra posible falla de este tipo, se recomienda el empleo de aceros inoxidables de menor contenido de Carbono y con estabilizadores de carburos como el Niobio o Titanio.

This paper considers our recent research on rock bolt stress corrosion cracking (SCC) which studied the influence of metallurgy using a range of (1) existing rock bolt steels and (2) commercial steels. The chemical composition, mechanical properties and microstructures of these steels varied considerably. The aim is to understand this failure mechanism and in particular to apply the knowledge gained from the study of SCC and hydrogen embrittlement (HE) of steels. The resistance of 1008, X65, X70 and 4145H to SCC/HE was similar, an effect which could be hardly expected as these steels have different strengths, microstructures and H-trap distributions. Different steel microstructures (pearlitic-ferritic and tempered martensite) had similar resistance to SCC and HE in the most aggressive conditions (pH 2.1 and À1500 mV SCE ) and had the same dimple rupture fracture surface.

Hydrogen induced damages in metallic components of fossil fuel power plants are usually followed by unpredicted events and very intensive. In the course of clarifying a very large complex of problems associated with hydrogen damage, it is necessary to apply a multidisciplinary approach. Much of this work is based on the theory and, particularly, on experimental tests performed on boiler water wall tube samples, taken from a domestic 210 MW power plant, that had experience hydrogen damages during service. Possible mechanisms for metal–hydrogen interaction are considered from material point of view; fluid hydrodynamics in furnace wall tubes; and from the complexity of structural – exploitation plant characteristics. The aim of this paper is to present a contribution to the methodology of hydrogen damaging of boiler water wall tube based on formulation of the model of damage – influenced by the "micro-departure from nucleate boiling" (micro-DNB) of working fluid.

Until a few years ago there had been no record of stress corrosion cracking (SCC) as a main cause of failures in Argentine pipelines, but as the pipeline system became of a certain age this mechanism started to have an important impact on reliability. This study analyzes three blowouts attributed to high pH SCC in different oil and natural gas transmission pipelines, which occurred by the sudden propagation of longitudinal cracks at the outer surface of the pipes. #

Intergranular stress-corrosion cracking (IGSCC) on a sensitized type AISI 304 stainless steel specimen was monitored simultaneously by acoustic emission, elongation measurements, electrochemical noise, and a digital imaging system. In the presented experiment the specimen was exposed to an aqueous sodium thiosulphate solution in combination with a constant load. It was established that before the final fracture two large cracks and numerous smaller cracks were developed. The detection and characterisation of stress-corrosion processes that generated these cracks are discussed. The presented results confirmed the conclusions from our previous research, where the correlations between various parameters obtained by the implemented techniques and IGSCC processes were established. Moreover, the presented results verified the wider validity of these correlations. In addition, based on analysis of AE signals a clear differentiation of crack related and crack non- related AE signals was made. The relationship between the cracks length calculated by means of digital image correlation analysis and characteristics of electrochemical current noise parameters was also established.

The spontaneous evolution of nanoporous sponges during dealloying is a fascinating process in nanotechnology. Originally
mostly studied as a (homogeneous) corrosion process, [ 1 ] dealloying has been early-on employed to produce Raney nickel. [ 2 ] Nanoporous gold (npAu) and other metals have been meanwhile proposed for applications from high-strength material, [ 3 ] catalysis, [ 4 ] and actuators [ 5 ] to energy storage [ 6 ] or biosensors. [ 7 ] Naturally, it is very important to understand the mechanical behavior and integrity of npAu and related nanometer-scaled functional materials. [ 8 ] Dealloying is often involved in localized corrosion and stress corrosion cracking, which is a major concern for the stability of metallic materials in general. [ 9 ] A localized dealloying mode can be triggered, for example, by a local breakdown of protective molecular-thin organic inhibitor fi lms. Due to the volume decrease in npAu, [ 10 ] the localized dealloying leads to stress that results in initiation and propagation of cracks. [ 11 ] Local changes in structure and morphology of the surface can thus decisively infl uence the fate of materials. However, a satisfying understanding of initial crack nucleation is still lacking. [ 12 ] Clarifi cation of the mechanisms of the incipient crack initiation and ensuing propagation implies unraveling the interplay and infl uence of both the substrate and the organic fi lm properties. Corrosion inhibition as well as functionalization
are often achieved by self-assembled monolayers (SAMs) employing molecules such as phosphonic acids, silanes, thiols, or selenols. [ 13 ] Apart from possibly preventing catastrophic cracking, a better understanding of the initial surface processes will also improve structural control during formation of (functional) nanoporosity. [ 14 ] Here, we offer a new approach to monitor crack initiation and propagation correlated to defined surface states. We control the localized dealloying on well-defi ned inhibitor fi lm–covered Cu 3 Au binary alloy surfaces by an applied electrochemical potential. The atomically smooth starting surfaces were prepared by sputter-annealing cycles in ultrahigh vacuum (UHV). We employed (1) defi ned surface orientations, which offer a variation of interatomic distances along the substrate surfaces, as well as (2) inhibition by different (molecularly ordered) SAMs based on cross-linking and noncross-linking thiol and selenol precursors. The observed density of initial defects and resulting cracks critically depends on the surface orientation. To our surprise we also fi nd a crystallographic nature of the cleavage planes within the opening microcracks. The detailed crack morphology also indicates a dependence on the chosen SAM precursor. We connect thus the understanding of well-ordered molecular interfaces on the atomic scale with a dangerous but fascinating macroscopic corrosion phenomenon.

Intergranular stress-corrosion cracking (IGSCC) on a sensitised type AISI 304 stainless steel specimen was monitored simultaneously by acoustic emission, electrochemical noise, elongation measurements and a digital imaging system. The specimen was exposed to an aqueous sodium thiosulphate solution in combination with a constant load. It was established that before the final fracture two large cracks and numerous smaller cracks had developed. Detection and characterisation of the stress-corrosion processes which generated these cracks are discussed. The results confirm and generalise previously established correlations between various parameters obtained by the implemented characterisation methods and IGSCC processes. Additionally, a clear differentiation between crack related and crack non-related AE signals was made based on an analysis of the AE signals. The relationship between the crack lengths calculated by means of digital image correlation analysis and the electrochemical current noise was also established.

Since serious corrosion problems that have plagued the light water reactor (LWR) industry for decades. The complex corrosion mechanisms involved and the development of practical engineering solutions for their mitigation will discussed. After a brief discussion of the basic designs of the boiling water reactor (BWR) and pressurized water reactor (PWR), emphasis will be placed on the general corrosion of LWR containments, flow-accelerated corrosion of carbon steel components, intergranular stress corrosion cracking (IGSCC) in BWRs, primary water stress corrosion cracking (PWSCC) in PWRs and irradiation assisted stress corrosion cracking (IASCC) in both systems. Finally, the corrosion future of both plants will be discussed as plants extend their period of operation for an additional 20 to 40 years.

15-5 precipitation hardened (PH) stainless steel, having high strength and good weldability is used in aerospace industry. Welding of these stainless steels is known to be detrimental to various forms of corrosion, including environmental assisted cracking EAC). Not much work has been done on the effect of welding on environmental assistant cracking (EAC) behavior of this alloy. Hence such a study was undertaken in the present work. 6 and 12 millimeter thick plates of 15-5 PH stainless steel were welded using tungsten inert gas welding. The welded sheets were examined for its microstructure, hardness and EAC in 3.5 wt. % NaCl solution.

This paper deals with determination of the failures causes of supporting tubes in a 350 MW fossil fuel power plant. Due to frequent fractures of supporting pipes occurring always at the same location and the ensuing reduction of the plant availability as well as substation financial losses, there was an urgent need to resolve the problem [K. Takahashi, M. Yokouchi, S.K. Lee, K. Ando, J. Am. Ceram. Soc., 86-12, 2003]-[3]. This work describes the test results and the conclusions drawn from the results.

Incidents of failure due to corrosion/stress corrosion cracking of high-pressure gas pipelines in Pakistan have been observed to occur after about 15-20 years of service. The present paper constitutes the failure analysis of an 18-inch diameter electric resistance-welded gas pipeline. The failure was characterized, on the basis of all the available evidence and the metallurgical examination carried out on the ruptured pipe, as a stress corrosion failure that had initiated at a longitudinal 'stress raiser'. This stress raiser, which was essentially a manufacturing defect, constituted a longitudinal 'step' on the pipe surface that had resulted from the faulty trimming/shaving of the weld flash. The findings of this study, thus, emphasize the need for the care that must be taken during the shaving-off of the weld flash.

We have examined the influence of mechanical surface finishing on the development of residual stresses, and on the subsequent formation of stress corrosion cracks, in 316Ti austenitic stainless steel after exposure to boiling magnesium chloride. The surface residual stresses of as-received plate, prior to machining, were found to be biaxial and compressive. However, abrasive grinding produced significant compressive stresses in the machining direction but much lower perpendicular stresses. On the other hand, milling produced high biaxial tensile stresses (approaching the ultimate tensile strength, UTS, of the material), which were found to be relatively insensitive to cut depth but to vary as a function of feed rate. On the milled surfaces a distinctive pattern of stress corrosion cracking was evident with longer primary cracks nucleating along the milling direction and secondary, shorter, cracks nucleating perpendicularly. As the surface tensile stress was lower perpendicular to the milling direction, we postulate that the nuclea- tion of primary cracks parallel to machining must be driven by the surface profile after machining (and associated micro-stresses) as much as by the macroscopic residual stresses.

This paper presents an experimental study of the stress corrosion cracking (SCC) process on 8-mm-diameter wires which are used industrially in precast concrete prestressed by pre-tension. The service life of steel wires under accelerated SCC and the reduction of their mechanical performance are studied. A dynamic analysis to detect the damage to corroded wire due to SCC before brittle failure and the influence of internal defects on the service life of stress corroded wire are also presented. The study shows that stress corrosion cracking is characterized by an evolution to SCC from pitting corrosion attacks that result in the development of both micro-cracking and micro-voids in the steel bulk. The stress level does not influence the composition of corrosion products. It is a major factor of SCC development, leading to a considerable reduction in the ultimate strain and thus to brittle failure of the corroded wires. Local defects on the steel surface increase the SCC effect due to stress corrosion concentration. A reduction in the elastic modulus and the elastic limit, which may reach 25% and 15%, respectively, can be expected due to steel micro-cracking. No damage detection through mechanical analysis seems possible before the brittle failure occurs as the corrosion is very localized and so does not globally reduce the tension in the wires.

The 22%Cr - 5%Ni, ferritic-austenitic duplex stainless steels (DSS) have potential to offer trouble free service in a variety of corrosive media, which, combined with their high strength-to-mass ratio, allows for savings to be made both in terms of structural weight and maintenance costs.  However, a number of service failures have been reported, and there remains a cloud over the mechanism of stress corrosion cracking (SCC), which is the most prominent form of failure in offshore applications.  In this study, the mechanism of stress corrosion cracking of S2205 duplex stainless steel was investigated in neutral artificial 33.33 %wt sea-salt solution at strain rate of 1.5 x 10-6 s-1, between the ambient temperature and 80oC.  Slow strain rate tests were carried out at open circuit potential and applied austenite-passive potentials at respective temperatures.  The fractured surfaces were examined and the test data were analyzed to verify the susceptibility, as well as understanding the mechanism of stress corrosion cracking.  Fractographical examination revealed that pitting corrosion assisted the initiation and selective dissolution was involved in the propagation of SCC of cold-drawn S2205 DSS in solution temperatures above 50oC.

There were three consecutive occurrences of bellows failure in a particular pressure safety valve (PSV) of a petroleum refinery within a time span of one week. The bellows were made of 316L grade austenitic stainless steel, and the PSV was mounted on one of the vessels of vacuum gas oil service in a hydrocracker unit. Metallurgical analysis of the failed bellows revealed that the failure had occurred by stress corrosion cracking (SCC). It was found that the SCC was promoted in the bellows due to the presence of high amount of chloride ions in the operating environment. Studies confirmed that SCC had initiated from the outer surface of the bellows and propagated inwards, resulting in leakage of hydrocarbon from the PSV. The source of chlorine in the environment was identified. It was discovered that SCC in
the bellows was caused due to a previous failure in the heavy polynuclear aromatics (HPNA) absorption bed located upstream the process flow line. This failure was due to the presence of high concentrations of chlorine in the granulated activated carbon that was used in the HPNA absorption bed. During the repair of the HPNA bed, there was deposition of carbon soot on the body of the PSV. This carbon soot was the source of chloride ions for SCC to occur in the bellows. Generally, in chloride SCC, crack propagation in 316L SS takes place by
transgranular mode. In the present case, however, the crack propagation was predominantly by intergranular mode. The metallurgical factors responsible for this change in micro-mechanism of crack propagation during SCC have been discussed.


Studies were carried out to evaluate the stress corrosion cracking (SCC) behavior of a X-70 microalloyed pipeline steel, with different microstructures by using the slow strain rate testing (SSRT) technique at 50°C, in NaHCO 3 solutions. Both anodic and cathodic potentials were applied. Additionally, experiments using the SSRT technique but with pre-charged hydrogen samples and potentiodynamic polarization curves at different sweep rates were also carried out to elucidate hydrogen effects. The results showed that the different microstructures in conjunction with the anodic applied potentials shift the cracking susceptibility of the steel. In diluted NaHCO 3 solutions cathodic potentials close to their rest potential values decreased the SCC susceptibility regardless the microstructure, whereas higher cathodic potentials promote SCC in all steel conditions. Certain microstructures are more susceptible to present anodic dissolution corrosion mechanism. Meanwhile concentrated solution did not promotes brittle fracture.

Cyclic voltammetry, hydrogen permeation tests and electrochemical impedance spectroscopy measurements were combined to study the mechanism for hydrogen evolution reaction on X-70 pipe steel in near-neutral pH solution. It is found that hydrogen evolution reaction is dominated by the reduction of water molecules, followed by either an electrochemical hydrogen recombination reaction or a hydrogen absorption reaction. The near-neutral pH environment is capable of generating catalytic surface effect on hydrogen evolution on the pipe steel. The increasing dissolution of the cathodically pre-polarized steel could be due to the enhanced activation of the steel, rather than the increasing amount of hydrogen atoms in the steel. These results provide mechanistic information to understand the nearneutral pH stress corrosion cracking of pipelines.

We study the corrosion resistance of St3S steel under loading and its susceptibility to corrosion and hydrogen-induced cracking in bottom water. Sections of a tank are distinguished according to the character of the media interacting with the metal of the inner surface in the process of operation. It is shown that bottom water is characterized by high levels of corrosion activity and that the degrees of in-service degradation of different sections of the tank are different. The worst corrosion and stress-corrosion resistance are exhibited by steel operating in contact with bottom water. Significant levels of plastic strains intensify the process of corrosion in steel and make the rates of corrosion in different sections of the tank closer to each other. The in-service degradation of steel can not only intensify the process of corrosion of the inner surface of the tank but also promote the brittle fracture of the material characterized by the elevated susceptibility to hydrogen-induced cracking.

Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels occurs at susceptible grain boundaries after sensitisation. In this study, the effects of test duration, static stress (applied and residual) and microstructure orientation on the developed populations of short crack nuclei are reported for a sensitised type 304 austenitic stainless steel in an acidified potassium tetrathionate (K2S4O6) solution. The crack populations were analysed using the Gumbel distribution method, showing an increase in the characteristic crack lengths with increasing time and grain size. There is a weak, but measurable effect of stress on crack length. Tensile stress increases crack growth and compressive residual stresses introduced by surface machining are shown to be beneficial. A significant dependency on sample orientation is observed and this cannot be explained in terms of the bulk microstructure properties or characteristics, which showed no significant variations.

Supercritical water (SCW) has attracted increasing attention since SCW boiler power plants were implemented to increase the efficiency of fossil-based power plants. The SCW reactor (SCWR) design has been selected as one of the Generation IV reactor concepts because of its higher thermal efficiency and plant simplification as compared to current light water reactors (LWRs). Reactor operating conditions call for a core coolant temperature between 280°C and 620°C at a pressure of 25 MPa and maximum expected neutron damage levels to any replaceable or permanent core component of 15 dpa (thermal reactor design) and 100 dpa (fast reactor design). Irradiation-induced changes in microstructure (swelling, radiation-induced segregation (RIS), hardening, phase stability) and mechanical properties (strength, thermal and irradiation-induced creep, fatigue) are also major concerns. Throughout the core, corrosion, stress corrosion cracking, and the effect of irradiation on these degradation modes are critical issues. This paper reviews the current understanding of the response of candidate materials for SCWR systems, focusing on the corrosion and stress corrosion cracking response, and highlights the design trade-offs associated with certain alloy systems. Ferritic-martensitic steels generally have the best resistance to stress corrosion cracking, but suffer from the worst oxidation. Austenitic stainless steels and Ni-base alloys have better oxidation resistance but are more susceptible to stress corrosion cracking. The promise of grain boundary engineering and surface modification in addressing corrosion and stress corrosion cracking performance is discussed.

The development of an intergranular stress corrosion crack initiation site in thermally sensitized type 304 austenitic stainless steel has been observed in situ in high temperature oxygenated water using digital image correlation of time-resolved optical observations.  The grain boundary normal stresses were calculated using the Schmid-Modified Grain Boundary Stress (SMGBS) model of Was et al, applying three-dimensional data for the grain boundary planes and grain orientations.  The initiation site coincided with the most highly stressed sensitised boundary, demonstrating the importance of the combined contributions to crack initiation of grain boundary structure and plastic strain incompatibility.

Sherritt International Corporation experienced corrosion failures with the 316L stainless steel tubing of the high pressure still condenser employed for ammonia recovery. A detailed failure analysis was conducted on the condenser tubing to determine the mode and the root cause of failure. The analysis included both optical and scanning electron microscopy (SEM) of the inner and outer surfaces of the tube and also, characterization of the corrosion products using Energy-Dispersive X-ray spectroscopy (EDX). Results revealed that corrosion attack was confined to the first ~ 100 mm of the tubing at the inlet where the tube is connected to the top tubesheet. The tube suffered both external stress corrosion cracking (SCC) and crevice corrosion from the shell side (water side), and wall thinning of inner surface (the tube side) due to erosion corrosion. It was evident that failure of one of the tubes occurred due to SCC that penetrated the whole wall thickness and resulted in a leak failure. Some prevention measures are proposed to avoid this type of corrosion attack in the future.

We present a model of hydrogen embrittlement based upon: (i) a cohesive law dependent on impurity coverage that is calculated from first principles; (ii) a stress-assisted diffusion equation with appropriate boundary conditions accounting for the environment; (iii) a static continuum analysis of crack growth including plasticity; and (iv) the Langmuir relation determining the impurity coverage from its bulk concentration. We consider the effect of the following parameters: yield strength, stress intensity factor, hydrogen concentration in the environment, and temperature. The calculations reproduce the following experimental trends: (i) time to initiation and its dependence on yield strength and stress intensity factor; (ii) finite crack jump at initiation; (iii) intermittent crack growth; (iv) stages I and II of crack growth and their dependence on yield strength; (v) the effect of the environmental impurity concentration on the threshold stress intensity factor; and (vi) the effect of temperature on stage II crack velocity in the low-temperature range. In addition, the theoretically and experimentally observed intermittent cracking may be understood as being due to a time lag in the diffusion of hydrogen towards the cohesive zone, since a buildup of hydrogen is necessary in order for the crack to advance. The predictions of the model are in good quantitative agreement with available measurements, suggesting that hydrogen-induced degradation of cohesion is a likely mechanism for hydrogen-assisted cracking.

In order to appraise the in~uence of s!phase on the behaviour of duplex stainless steels\ two tests are performed[ The _rst one is the double loop electrochemical potentiodynamic reac! tivation "DLEPR# test that indicates the degree of sensitisation to intergranular corrosion[ The second one is the slow strain rate test "SSRT# that enables us to relate the degree of sensitisation to stress corrosion cracking[ A metallurgical study is also performed on two duplex stainless steels of grade UNS S20792[ Sensitisation is carried out by several heat treatments at 564>C or 899>C[ The in~uence of s!phase is clearly shown on the mechanical properties and the corrosion resistance[ Þ 0888 Elsevier Science Ltd[ All rights reserved[